title
stringlengths 13
247
| url
stringlengths 35
578
| text
stringlengths 197
217k
| __index_level_0__
int64 1
8.68k
|
|---|---|---|---|
InfoPage
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/00%3A_Front_Matter/02%3A_InfoPage | This text is disseminated via the Open Education Resource (OER) LibreTexts Project and like the hundreds of other texts available within this powerful platform, it is freely available for reading, printing and "consuming." Most, but not all, pages in the library have licenses that may allow individuals to make changes, save, and print this book. Carefully consult the applicable license(s) before pursuing such effects.Instructors can adopt existing LibreTexts texts or Remix them to quickly build course-specific resources to meet the needs of their students. Unlike traditional textbooks, LibreTexts’ web based origins allow powerful integration of advanced features and new technologies to support learning. The LibreTexts mission is to unite students, faculty and scholars in a cooperative effort to develop an easy-to-use online platform for the construction, customization, and dissemination of OER content to reduce the burdens of unreasonable textbook costs to our students and society. The LibreTexts project is a multi-institutional collaborative venture to develop the next generation of open-access texts to improve postsecondary education at all levels of higher learning by developing an Open Access Resource environment. The project currently consists of 14 independently operating and interconnected libraries that are constantly being optimized by students, faculty, and outside experts to supplant conventional paper-based books. These free textbook alternatives are organized within a central environment that is both vertically (from advance to basic level) and horizontally (across different fields) integrated.The LibreTexts libraries are Powered by NICE CXOne and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. This material is based upon work supported by the National Science Foundation under Grant No. 1246120, 1525057, and 1413739.Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation nor the US Department of Education.Have questions or comments? For information about adoptions or adaptions contact More information on our activities can be found via Facebook , Twitter , or our blog .This text was compiled on 07/05/2023 | 1 |
2.1: Direct Vapor Inlet
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/02%3A_SAMPLE_INTRODUCTION/2.01%3A_Direct_Vapor_Inlet | Direct Vapor Inlet. The simplest sample introduction method is a direct vapor inlet. The gas phase analyte is introduced directly into the source region of the mass spectrometer through a needle valve. Pump out lines are usually included to remove air from the sample. This inlet works well for gases, liquids, or solids with a high vapor pressure. Samples with low vapor pressure are heated to increase the vapor pressure. Since this inlet is limited to stable compounds and modest temperatures, it only works for some samples. | 5 |
2.2: Gas Chromatography
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/02%3A_SAMPLE_INTRODUCTION/2.02%3A_Gas_Chromatography | Gas chromatography is probably the most common technique for introducing samples into a mass spectrometer. Complex mixtures are routinely separated by gas chromatography and mass spectrometry is used to identify and quantitate the individual components. Several different interface designs are used to connect these two instruments. The most significant characteristics of the inlets are the amount of GC carrier gas that enters the mass spectrometer and the amount of analyte that enters the mass spectrometer. If a large flow of GC carrier gas enters the mass spectrometer it will increase the pressure in the source region.Probably the most common GC/MS interface uses a capillary GC column. Since the carrier gas flow rate is very small for these columns, the end of the capillary is inserted directly into the source region of the mass spectrometer. The entire flow from the GC enters the mass spectrometer. Since capillary columns are now very common, this inlet is widely used. However this design is not well suited for experiments with wide bore capillaries and packed GC columns which have higher flow rates. The increase in the flow rate significantly increases the pressure in the mass spectrometer and maintaining the required source pressure will require larger and more expensive vacuum pumps. Several inlet designs are available to reduce the gas flow into the source. The simplest design splits the GC effluent so that only a small portion of the total flow enters the mass spectrometer. Although this inlet reduces the gas load on the vacuum system, it also reduces the amount of analyte and thus the sensitivity. Effusive separators and membrane inlets are more selective and transport a higher fraction of the analyte into the source region. Each of these methods has efficiency and resolution drawbacks but they are necessary for some experiments. | 6 |
2.3: Liquid Chromatography
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/02%3A_SAMPLE_INTRODUCTION/2.03%3A_Liquid_Chromatography | Liquid Chromatography. Liquid chromatography inlets are used to introduce thermally labile compounds not easily separated by gas chromatography. These inlets have undergone considerable development and LC/MS is now fairly routine. Because these inlets are used for temperature sensitive compounds, the sample is ionized directly from the condensed phase. These inlets are discussed in greater detail in the section on ionization techniques. | 7 |
2.4: Direct Insertion Probe
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/02%3A_SAMPLE_INTRODUCTION/2.04%3A_Direct_Insertion_Probe | Direct Insertion Probe. The Direct Insertion Probe (DIP) is widely used to introduce low vapor pressure liquids and solids into the mass spectrometer. The sample is loaded into a short capillary tube at the end of a heated sleeve. This sleeve is then inserted through a vacuum lock so the sample is inside the source region. After the probe is positioned, the temperature of the capillary tube is increased to vaporize the sample. This probe design allows higher temperatures than are possible with a direct vapor inlet. In addition, the sample is under vacuum and located close to the source so that lower temperatures are required for analysis. This is important for analyzing temperature sensitive compounds. Although the direct insertion probe is more cumbersome than the direct vapor inlet, it is useful for a wider range of samples. | 8 |
2.5: Direct Ionization of Sample
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/02%3A_SAMPLE_INTRODUCTION/2.05%3A_Direct_Ionization_of_Sample | Direct Ionization of Sample. Unfortunately, some compounds either decompose when heated or have no significant vapor pressure. These samples may be introduced to the mass spectrometer by direct ionization from the condensed phase. These direct ionization techniques include electrospray, matrix assisted laser desorption (MALDI), glow discharge mass spectrometry, fast atom bombardment and laser ablation. The development of new ionization techniques is an active research area and these techniques are rapidly evolving. Direct ionization is discussed in greater detail in the next section. | 9 |
3.1: Electron Ionization
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.01%3A_Electron_Ionization | Electron Ionization (EI) is the most common ionization technique used for mass spectrometry.* EI works well for many gas phase molecules, but it does have some limitations. Although the mass spectra are very reproducible and are widely used for spectral libraries, EI causes extensive fragmentation so that the molecular ion is not observed for many compounds. The fragmentation is useful because it provides structural information for interpreting unknown spectra. Fragmentation patterns are discussed in more detail in the chapter on Interpretation.: Electron Ionization Source
The electrons used for ionization are produced by passing a current through a wire filament ). The amount of current controls the number of electrons emitted by the filament. An electric field accelerates these electrons across the source region to produce a beam of high energy electrons. When an analyte molecule passes through this electron beam, a valence shell electron can be removed from the molecule to produce an ion.
Ionization does not occur by electron capture, which is highly dependent upon molecular structure. Instead, EI produces positive ions by knocking a valence electron off the analyte molecule ). As the electron passes close to the molecule the negative charge of the electron repels and distorts the electron cloud surrounding the molecule. This distortion transfers kinetic energy from the fast-moving electron to the electron cloud of the molecule. If enough energy is transferred by the process, the molecule will eject a valence electron and form a radical cation M•+.Since the ionization is produced by a single electron that is accelerated to 70 V, this is commonly referred to as 70 eV EI.** This is enough energy to cause extensive fragmentation, and at this level small changes in the electron energy do not significantly effect the fragmentation patterns. The amount of energy transferred during this process depends up on how fast the electron is traveling and how close it passes to the molecule. In most 70 eV EI experiments, approximately 1400 kJ (15 eV) of energy is transferred during the ionization process. There is, however, a distribution of energy and as much as 2800 kJ (30 eV) is transferred to some molecules. Since approximately 960 kJ/mole (10 eV) of energy is required to ionize most organic compounds and a typical chemical bond energy is 290 kJ/mole (3 eV), extensive fragmentation is often observed in 70 eV EI mass spectra. The distribution of energy transferred during ionization and the large number of fragmentation pathways results in a variety of products for a given analyte. Other electron voltages may be used to vary the amount of fragmentation produced during ionization. For most organic compounds the threshold energy for EI is about 20 eV.Because a mass spectrum is produced by ionizing many molecules, the spectrum is a distribution of the possible product ions. Intact molecular ions are observed from ions produced with little excess energy. Other molecular ions are formed with more energy and undergo fragmentation in the source region. The abundance of the resulting fragments, often called product ions, is determined by the kinetics of the fragmentation pathways and the ionization energy. Changing the ionization energy changes the observed distribution of fragment ions. This distribution provides the structural information for interpreting mass spectra and is discussed in detail in the section on interpretation.* Some older literature will refer to EI as electron impact, but this term is not considered accurate. Electron Ionization is the currently accepted term.** The SI unit for energy is the Joule. The energetics of chemical reactions are typically expressed in kilojoules per mole. In many gas phase experiments (like mass spectrometry), the mole is not a convenient unit. The electron volt is frequently used as an energy unit for single molecules or atoms. 1 eV = 1.60217733 x 10-19 J. So that: 1 eV (per molecule or atom) = 96.4152206 kJ/mole. | 12 |
3.2: Chemical Ionization
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.02%3A_Chemical_Ionization | Chemical Ionization (CI) is a soft ionization technique that produces ions with little excess energy. As a result, less fragmentation is observed in the mass spectrum. Since this increases the abundance of the molecular ion, the technique is complimentary to 70 eV EI. CI is often used to verify the molecular mass of an unknown. Only slight modifications of an EI source region are required for CI experiments.In Chemical Ionization the source is enclosed in a small cell with openings for the electron beam, the reagent gas and the sample. The reagent gas is added to this cell at a pressure of approximately 10 Pa (0.1 torr). This is higher than the pressure of 10-3 Pa (10-5 torr) typical for a mass spectrometer source. At 10-3 Pa the mean free path between collisions is approximately 2 meters and ion-molecule reactions are unlikely. In the CI source, however, the mean free path between collisions is only 10-4 meters and analyte molecules undergo many collisions with the reagent gas. The reagent gas in the CI source is ionized with an electron beam to produce a cloud of ions. The reagent gas ions in this cloud react and produce adduct ions like \(\mathrm{CH}_{5}^{+}\) ), which are excellent proton donors.When analyte molecules are introduced to a source region with this cloud of ions, the reagent gas ions donate a proton to the analyte molecule and produce adduct ions, [M+H]+. The energetics of the proton transfer is controlled by using different reagent gases. The most common reagent gases are methane, isobutane and ammonia. Methane is the strongest proton donor commonly used with a proton affinity (PA) of 5.7 eV. For softer ionization, isobutane (PA 8.5 eV) and ammonia (PA 9.0 eV) are frequently used. Acid base chemistry useful for describing these chemical ionization reactions. The reagent gas must be a strong enough Brønsted acid to transfer a proton to the analyte. Fragmentation is minimized in CI by reducing the amount of excess energy produced by the reaction. Because the adduct ion have little excess energy and are relatively stable, CI is very useful for molecular mass determination. Some typical reactions in a CI source are shown in .\[\begin{gathered} \mathrm{CH}_{4}+\mathrm{e}^{-} → \mathrm{CH}_{4}^{+} + 2 \mathrm{e}^{-} \end{gathered} \nonumber \]B) Reaction of reagent gas ions to form adducts:\[\begin{gathered} \mathrm{CH}_{4}^{+}+\mathrm{CH}_{4} → \mathrm{CH}_{3} + \mathrm{CH}_{5}^{+} \\ \mathrm{OR} \\ \mathrm{CH}_{4}^{+} → \mathrm{CH}_{3}^{+} + \mathrm{H} \\ \mathrm{CH}_{3}^{+}+\mathrm{CH}_{4} → \mathrm{C}_{2} \mathrm{H}_{5}^{+}+\mathrm{H}_{2} \end{gathered} \nonumber \]C) Reaction of Reagent Gas Ions with analyte molecules:\[\begin{gathered} \mathrm{CH}_{5}^{+}+\mathrm{M} → \mathrm{CH}_{4} + \mathrm{MH}^{+} \\ \mathrm{C}_{2} \mathrm{H}_{5}^{+}+\mathrm{M} → \mathrm{C}_{2} \mathrm{H}_{4}+\mathrm{MH}^{+} \\ \mathrm{CH}_{3}^{+}+\mathrm{M} → \mathrm{CH}_{4} + (\mathrm{M-H})^{+} \end{gathered} \nonumber \] | 13 |
3.3: Atmospheric Pressure Ionization and Electrospray Ionization
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.03%3A_Atmospheric_Pressure_Ionization_and_Electrospray_Ionization | Atmospheric Pressure Ionization (API) sources ionize the sample at atmospheric pressure and then transfer the ions into the mass spectrometer. These techniques are used to ionize thermally labile samples such as peptides, proteins and polymers directly from the condensed phase. The sample is dissolved in an appropriate solvent and this solution is introduced into the mass spectrometer. With conventional inlets the solvent increases the pressure in the source region of the mass spectrometer. In addition, Joule-Thompson cooling of the liquid as it enters the vacuum causes the solvent droplets to freeze. The frozen clusters trap analyte molecules and reduce the sensitivity of the experiment. No matrix is used and the ionizing beam is focused directly on the sample. Although this makes sampling more difficult, it is useful for studying surface chemistry.API sources introduce the sample through a series of differentially pumped stages. This maintains the large pressure difference between the ion source and the mass spectrometer ) without using extremely large vacuum pumps. In addition a drying gas is used to break up the clusters that form as the solvent evaporates. Because the analyte molecules have more momentum than the solvent and air molecules, they travel through the pumping stages to the mass analyzer.ElectroSpray Ionization (ESI) is the most common API application. It has undergone remarkable growth in recent y ears and is frequently used for LC/MS of thermally labile and high molecular weight compounds. The electrospray is created by apply ing a large potential between the metal inlet needle and the first skimmer in an API source ). The mechanism for the ionization process is not well understood and there are several different theories that explain this ionization process. One theory is that as the liquid leaves the nozzle, the electric field induces a net charge on the small droplets. As the solvent evaporates, the droplet shrinks and the charge density at the surface of the droplet increases. The droplet finally reaches a point where the coulombic repulsion from this electric charge is greater than the surface tension holding it together. This causes the droplet to explode and produce multiply charged analyte ions. A typical ESI spectrum shows a distribution of molecular ions with different charge numbers.Because electrospray produces multiply charged ions, high molecular weight compounds are observed at lower m/z value. This increases the mass range of the analyzer so that higher molecular weight compounds may be analyzed with a less expensive mass spectrometer. An ion with a mass of 5000 u and a charge of \(+10\) is observed at 500 m/z and is easily analyzed with an inexpensive quadrupole analyzer.API Sources are also used for Inductively Coupled Plasma Mass Spectrometry (ICP/MS) and glow discharge experiments. In ICP/MS a nebulizer is used to introduce liquid samples into a high temperature plasma. The temperature of the plasma is high enough to efficiently ionize most elements. These ions are introduced to the mass spectrometer using an series of differentially pumped regions similar to the electrospray source discussed above. Glow discharge experiments are similar, but used for solid samples. The high sensitivity and selectivity of the mass spectrometer provides rapid multi-element detection at very low levels. Because the high temperature of the plasma destroys any chemical bonds, these techniques are used for elemental analysis. | 14 |
3.4: Matrix Assisted Laser Desorption/Ionization
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.04%3A_Matrix_Assisted_Laser_Desorption_Ionization | Matrix Assisted Laser Desorption/Ionization (MALDI) is used to analyze extremely large molecules. This technique directly ionizes and vaporizes the analyte from the condensed phase. MALDI is often used for the analysis of synthetic and natural polymers, proteins, and peptides. Analysis of compounds with molecular weights up to 200,000 dalton is possible and this high mass limit is continually increasing.In MALDI, both desorption and ionization are induced by a single laser pulse ). The sample is prepared by mixing the analyte and a matrix compound chosen to absorb the laser wavelength. This is placed on a probe tip and dried. A vacuum lock is used to insert the probe into the source region of the mass spectrometer. A laser beam is then focused on this dried mixture and the energy from a laser pulse is absorbed by the matrix. This energy ejects analyte ions from the surface so that a mass spectrum is acquired for each laser pulse. The mechanism for this process is not well understood and is the subject of much controversy in the literature. This technique is more universal (works with more compounds) than other laser ionization techniques because the matrix absorbs the laser pulse. With other laser ionization techniques, the analyte must absorb at the laser wavelength. Typical MALDI spectra include the molecular ion, some multiply charged ions, and very few fragments. : MALDI Ionization Target with Sample and MatrixOther Ionization Methods. There are several other ionization methods used for mass spectrometry and interested readers are referred to the chemical literature for additional information about other techniques. Field Desorption was used for ionization and vaporization of moderate sized molecules before the development of FAB, electrospray, and MALDI. It is still an important technique for some analysis and is typically used for non-polar polymers and petroleum samples. Plasma Desorption (PD) is a technique used to analyze high molecular weight compounds before the development of MALDI and electrospray. However, it is very complex and has not found widespread application. Resonance Ionization Mass Spectrometry (RIMS) is used for selective atomic and molecular ionization. Photoionization with lasers, lamps, and synchrotron sources is used to study the photochemistry and energetics of many compounds. Lasers are used to ionize surface samples with Laser Microprobe Mass Analysis (LAMMA). | 15 |
3.5: Fast Atom Bombardment and Secondary Ion Mass Spectrometry
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.05%3A_Fast_Atom_Bombardment_and_Secondary_Ion_Mass_Spectrometry | Fast Atom Bombardment (FAB) and Secondary Ion Mass Spectrometry (SIMS) both use high energy atoms to sputter and ionize the sample in a single step. In these techniques, a beam of rare gas neutrals (FAB) or ions (SIMS) is focused on the liquid or solid sample. The impact of this high energy beam causes the analyte molecules to sputter into the gas phase and ionize in a single step : Fast Atom Bombardment Source. ). The exact mechanism of this process is not well understood, but these techniques work well for compounds with molecular weights up to a few thousand dalton. Since no heating is required, sputtering techniques (especially FAB) are useful for study ing thermally labile compounds that decompose in conventional inlets.The most significant difference between FAB and SIMS is the sample preparation. In FAB the analyte is dissolved in a liquid matrix. A drop of the sample/matrix mixture is placed at the end of an insertion probe and introduced to the source region. The fast atom beam is focused on this droplet to produce analyte ions. Glycerol or similar low vapor pressure liquids are typically used for the matrix. Ideally, the analyte is soluble in the liquid matrix and a monolayer of analyte forms on the surface of the droplet. According to one theory, this monolayer concentrates the analyte while the dissolved sample provides a reservoir to replenish the monolayer as the analyte is depleted. Without this constant replenishment from the bulk solution, the ionizing beam will rapidly deplete the analyte and the signal is difficult to observe.SIMS experiments are used to study surface species and solid samples. Liquid SIMS (LSIMS) is very similar to FAB except cesium ions are used for higher energy collisions. No matrix is used and the ionizing beam is focused directly on the sample. Although this makes sampling more difficult, it is useful for studying surface chemistry. High resolution chemical maps are produced by scanning a tightly focused ionizing beam across the surface and depth profiles are produced by probing a single location. Although SIMS is a very sensitive and powerful technique for surface chemistry and materials analysis, the results are often difficult to quantitate. | 16 |
3.6: Inductively Coupled Plasma
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.06%3A_Inductively_Coupled_Plasma | In addition to use for atomic emission spectroscopy, Inductively Coupled Plasma (ICP) is also used as an ionization method for elemental analysis. A liquid or slurry sample is introduced into an inductively coupled plasma torch and the ions produced are extracted and analyzed by mass spectrometry. These instruments are capable of extremely low detection limits and simultaneous detection of multiple elements.3.6: Inductively Coupled Plasma is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. | 17 |
3.7: Self-Test #1
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/03%3A_IONIZATION_TECHNIQUES/3.07%3A_Self-Test_1 | What ionization technique would be appropriate for analyzing the following compounds:1) Gasoline fractions. Since these are very volatile, EI would be very easy to use and would provide abundant fragment information. CI may help to identify the molecular ions.2) Pesticide residue. These are usually volatile enough to use with EI. Once again CI may provide some useful information that would compliment the fragmentation in the EI spectrum. If the pesticide is thermally labile it may be appropriate to use electrospray to avoid sample decomposition.3) Ibuprofen and acetaminophen. These pharmaceutics are often analyzed by liquid chromatography, so electrospray would be an ideal interface for ionization.4) Insulin. This is a large protein molecule. MALDI is probably required.5) Tripeptides. These are generally small enough to be readily ionized by FAB.6) Heavy metals in water. Atmospheric pressure ionization in a ICP torch will provide very low limits of detection. | 18 |
4.1: Quadrupole
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.01%3A_Quadrupole | The quadrupole mass spectrometer is the most common mass analyzer. Its compact size, fast scan rate, high transmission efficiency,* and modest vacuum requirements are ideal for small inexpensive instruments. Most quadrupole instruments are limited to unit m/z resolution** and have a mass range of 1000 m/z . Many bench-top instruments have a mass range of 500 m/z but research instruments are available with mass range up to 4000 m/z.In the mass spectrometer, an electric field accelerates ions out of the source region and into the quadrupole analyzer. The analyzer consists of four rods or electrodes arranged across from each other ( ). As the ions travel through the quadrupole they are filtered according to their m/z value so that only a single m/z value ion can strike the detector. The m/z value transmitted by the quadrupole is determined by the Radio Frequency (RF) and Direct Current (DC) voltages applied to the electrodes. These voltages produce an oscillating electric field that functions as a bandpass filter to transmit the selected mass to charge value.The RF voltage rejects or transmits ions according to their m/z value by alternately focusing them in different planes. The four electrodes are connected in pairs and the RF potential is applied between these two pairs of electrodes. During the first part of the RF cycle the top and bottom rods are at a positive potential and the left and right rods are at a negative potential. This squeezes positive ions into the horizontal plane. During the second half of the RF cy cle the polarity of the rods is reversed. This changes the electric field and focuses the ions in the vertical plane. The quadrupole field continues to alternate as the ions travel through the mass analyzer. This causes the ions to undergo a complex set of motions that produces a three-dimensional wave.The quadrupole field transmits selected ions because the amplitude of this three dimensional wave dep ends up on the m/z value of the ion, the potentials applied, and the RF frequency. By selecting an appropriate RF frequency and potential, the quadrupole acts like a high pass filter, transmitting high m/z ions and rejecting low m/z ions. The low m/z ions have a greater acceleration rate so the wave for these ions has a greater amplitude. If this amplitude is great enough the ions will collide with the electrodes and can not reach the detector. The low m/z value cutoff of the quadrupole is changed by adjusting the RF potential or the RF frequency. Any ions with a m/z greater than this cutoff are transmitted by the quadrupole.A DC voltage is also applied across the rods of the analyzer. This potential combined with the RF potential acts like a low pass filter to reject high m/z ions. Because they respond quickly to the changing RF field the motion of the low m/z ions is dominated by the RF potential. This motion stabilizes their trajectory by refocusing each time the RF potential changes polarity. Because low m/z ions are quickly refocused, the DC potential does not affect these ions. High m/z ions, however, do not refocus as quickly during the RF cycle. The DC potential has a greater influence on their trajectory and they slowly drift away from the center of the quadrupole. At the end of the analyzer, they are too far off-axis to strike the detector.The combination of high and low pass filters produced by the RF and DC potentials is adjusted to only transmit the selected m/z value. All ions above or below the set m/z value are rejected by the quadrupole filter. The RF and DC fields are scanned (either by potential or frequency) to collect a complete mass spectrum. Quadrupole mass analyzers are often called mass filters because of the similarity between m/z selection by a quadrupole and wavelength selection by an optical filter or frequency selection by an electronic filter.*Transmission efficiency refers to how many of the ions produced in the source region actually reach the detector. This is an important measure of sensitivity for mass spectrometers.
**Unit resolution (or low resolution) mass spectra distinguish between ions separated by \(1 \mathrm{~m} / \mathrm{z}\) unit. The spectra, like those presented here, are commonly displayed as histograms. This is a common method for presenting spectra because it results in much smaller data file size. Some mass analyzers can obtain spectra at much higher resolution. This is discussed in detail in the interpretation section.References | 19 |
4.2: Magnetic Sector
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.02%3A_Magnetic_Sector | Magnetic Sector. The first mass spectrometer, built by J.J. Thompson in 1897 , used a magnet to measure the m/z value of an electron. Magnetic sector instruments have evolved from this same concept. Sector instruments have higher resolution and greater mass range than quadrupole instruments, but they require larger vacuum pumps and often scan more slowly. The typical mass range is to 5000 m/z, but this may be extended to 30,000 m/z. Magnetic sector instruments are often used in series with an electric sector, described below, for high resolution and tandem mass spectrometry experiments.Magnetic sector instruments ) separate ions in a magnetic field according to the momentum and charge of the ion. Ions are accelerated from the source region into the magnetic sector by a 1 to 10 kV electric field. This acceleration is significantly greater than the 100 V acceleration typical for a quadrupole instrument. Since the ions are charged, as they move through the magnetic sector, the magnetic field bends the ion beam in an arc. This is the same principal that causes electric motors to turn. The radius of this arc (r) depends upon the momentum of the ion \(\mathrm{\mu}\), the charge of the ion (C) and the magnetic field strength (B) according to Equation \(\PageIndex{1}\).\[r=\frac{\mu}{C \times B} \nonumber \]Ions with greater momentum will follow an arc with a larger radius. This separates ions according to their momentum, so magnetic sectors are often called momentum analyzers. The momentum of the ion is the product of the mass \((m)\) and the velocity \((v)\). The charge of the ion is the product of the charge number of the ion (z) and the charge of an electron (e). Substituting these variables into Equation \(\PageIndex{1}\) yields:\[r=\frac{m / z \times v}{\mathrm{~B} \times \mathrm{e}} \nonumber \]The velocity of an ion is determined by the acceleration voltage in the source region (V) and the mass to charge ratio (m/z) of the ion. Equation \(\PageIndex{2}\) rearranges to give the m/z ion transmitted for a given radius, magnetic field, and acceleration voltage as: \[m / z=\frac{r^{2} \mathrm{~B}^{2} \mathrm{e}}{2 \mathrm{~V}} \nonumber \]Only one m/z value will satisfy Equation \(\PageIndex{3}\) for a given radius, magnetic field, and acceleration voltage. Other m/z ions will travel a different radius in the magnetic sector.Older magnetic sector instruments use a photographic plate to simultaneously detect ions at different radii. Since each m/z has a different radius, they strike the photographic plate at a different location. Modern instruments have a set of slits at a fixed radius to transmit a single m/z to the detector. The mass spectrum is scanned by changing the magnetic field or the acceleration voltage to transmit different m/z ions. Some new instruments use multichannel diode array detectors to simultaneously detect ions over a range of m/z values. | 20 |
4.3: Electric Sector/Double Focusing Mass Spectrometers
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.03%3A_Electric_Sector_Double_Focusing_Mass_Spectrometers | Electric Sector/Double Focusing Mass Spectrometers. An electric sector consists of two concentric curved plates. A voltage is applied across these plates to bend the ion beam as it travels through the analyzer. The voltage is set so that the beam follows the curve of the analyzer. The radius of the ion trajectory (r) depends up on the kinetic energy of the ion (V) and the potential field (E) applied across the plates.\[r=\frac{2 V}{E} \nonumber \]Equation \(\PageIndex{1}\) shows that an electric sector will not separate ions accelerated to a uniform kinetic energy. The radius of the ion beam is indep endent of the ion’s mass to charge ratio so the electric sector is not useful as a standalone mass analyzer.* An electric sector is, however, useful in series with a magnetic sector. The mass resolution of a magnetic sector is limited by the kinetic energy distribution ( \(\mathrm{V}\) ) of the ion beam. This kinetic energy distribution results from variations in the acceleration of ions produced at different locations in the source and from the initial kinetic energy distribution of the molecules. An electric sector significantly imp roves the resolution of the magnetic sector by reducing the kinetic energy distribution of the ions**. These high resolutions experiments are discussed in the section on mass spectral interpretation. The effect of the electric sector is shown in for a reverse geometry (BE) instrument with the magnetic sector \((B)\) located before the electric sector \((\mathrm{E})\).*The electric sector is a kinetic energy analyzer. In the source region of the mass spectrometer all ions are accelerated to the same kinetic energy. Because they have the same kinetic energy, they are not separated by an electric sector. A magnetic sector will resolve different mass ions accelerated to a uniform kinetic energy because it separates ions based upon their momentum (See 4.2: Magnetic Sector).**Ion optics are complex and interested readers are referred to the literature for more detail. The model presented here provides a framework for understanding many high resolution and tandem mass spectrometry experiments. The article by Nier provides an excellent introduction, a historical perspective, and many references for the development and theory of these instruments. | 21 |
4.4: Time-of-Flight
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.04%3A_Time-of-Flight | Time-of-Flight. The time-of-flight (TOF) mass analyzer separates ions in time as they travel down a flight tube ). This is a very simple mass spectrometer that uses fixed voltages and does not require a magnetic field. The greatest drawback is that TOF instruments have poor mass resolution, usually less than 500 . These instruments have high transmission efficiency, no upper m/z limit, very low detection limits, and fast scan rates. For some applications these advantages outweigh the low resolution. Recent developments in pulsed ionization techniques and new instrument designs with improved resolution have renewed interest in TOF-MS.In the source of a TOF analyzer, a packet of ions is formed by a very fast (ns) ionization pulse. These ions are accelerated into the flight tube by an electric field (typically 2-25 kV) applied between the backing plate and the acceleration grid. Since all the ions are accelerated across the same distance by the same force, they have the same kinetic energy. Because velocity \((v)\) is dependent upon the kinetic energy, Equation \(\PageIndex{1}\) shows \(\left(\mathrm{E}_{\text {kinetic }}\right)\) and mass \((m)\) lighter ions will travel faster.\[\mathrm{E}_{\text {kinetic }}=\frac{1}{2} m v^{2} \nonumber \]\(\mathrm{E}_{\text {kinetic }}\) is determined by the acceleration voltage of the instrument \((\mathrm{V}\) ) and the charge of the ion (e \(\times z)\). Equation \(\PageIndex{2}\) rearranges to give the velocity of an ion \((v)\) as a function of acceleration voltage and m/z value.\[v=\sqrt{\frac{2 \mathrm{~V} \times \mathrm{e}}{m / z}} \nonumber \]After the ions accelerate, they enter a 1 to 2 meter flight tube. The ions drift through this field free region at the velocity reached during acceleration. At the end of the flight tube they strike a detector. The time delay ( \(\mathrm{t}\) ) from the formation of the ions to the time they reach the detector dependents up on the length of the drift region (L), the mass to charge ratio of the ion, and the acceleration voltage in the source.\[\mathrm{t}=\frac{\mathrm{L}}{\sqrt{\sqrt{2 \times \mathrm{C} \times}}} \sqrt{m / z} \nonumber \]Equation \(\PageIndex{3}\) shows that low m/z ions will reach the detector first. The mass spectrum is obtained by measuring the detector signal as a function of time for each pulse of ions produced in the source region. Because all the ions are detected, TOF instruments have very high transmission efficiency which increases the \(\mathrm{S} / \mathrm{N}\) level.References | 22 |
4.5: Quadrupole Ion Trap
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.05%3A_Quadrupole_Ion_Trap | The Quadrupole ion storage trap mass spectrometer (QUISTOR) is a recently developed mass analyzer with some special capabilities. Several commercial instruments are available and this analyzer is becoming more popular. QUISTORs are very sensitive, relatively inexpensive, and scan fast enough for GC/MS exp eriments. The sensitivity of the QUISTOR results from trapping and then analyzing all the ions produced in the source. Since all the ions are detected, the \(\mathrm{S} / \mathrm{N}\) is high.The QUISTOR consists of a doughnut shaped ring electrode and two endcap electrodes. A cutaway view of this arrangement is shown in . A combination of RF and DC voltages is applied to the electrodes to create a quadrup ole electric field similar to the electric field for the quadrupole mass analyzer. This electric field traps ions in a potential energy well at the center of the analyzer. The mass spectrum is acquired by scanning the RF and DC fields to destabilize low mass to charge ions. These destabilized ions are ejected through a hole in one endcap electrode and strike a detector. The mass spectrum is generated by scanning the fields so that ions of increasing \(m / z\) value are ejected from the cell and detected. The trap is then refilled with a new batch of ions to acquire the next mass spectrum. The mass resolution of the ion trap is increased by adding a small amount \(0.1 \mathrm{~Pa}\left(10^{-3}\right.\) torr \()\) of Helium as a bath gas. Collisions between the analyte ions and the inert bath gas dampen the motion of the ions and increases the trapping efficiency of the analyzer. | 23 |
4.6: Ion Cyclotron Resonance
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.06%3A_Ion_Cyclotron_Resonance | Ion Cyclotron Resonance. The Ion Cy clotron Resonance (ICR) mass spectrometer uses a superconducting magnet to trap ions in a small sample cell. This type of mass analyzer has extremely high mass resolution (ca. \(10^{9}\) ) and is also useful for tandem mass spectrometry experiments. These instruments are very expensive and are typically used for specialized research applications. The ICR traps ions in a magnetic field that causes ions travel in a circular path ). This is similar to the path of an ion in a magnetic sector, but the ions are not traveling as fast and the magnetic field is stronger. As a result the ions are contained in the small volume of the trap.The ion’s cyclotron frequency \((\omega)\), is the angular frequency* of an ion’s orbit. Equation \(\PageIndex{1}\) shows this frequency is determined by the magnetic field strength \((B)\) and the m/z value of the ion.\[\omega=\frac{\mathrm{B} \times \mathrm{e}}{m / z} \nonumber \]After ions are trapped in this cell they are detected by measuring the signal at this cyclotron frequency. This signal is measured by placing electrodes on each side of the ions circular orbit. An RF voltage is applied to the transmitter electrodes at the cyclotron frequency of the ion of interest. This RF energy moves ions at the applied frequency to a larger orbit. As these ions travel around the ICR cell they are close enough to the receiver electrodes to induce a capacitive current. This capacitive current oscillates at the cyclotron frequency and is detected as the signal.The ICR is also used as a Fourier Transform Mass Spectrometer (FT-MS). Instead of using a single excitation frequency, a fast RF pulse is applied to the transmitter electrodes. This simultaneously excites all the ions and produces a signal at the cyclotron frequency of each m/z ion present. This signal is similar to the Free Induction Decay (FID) produced in an FT-NMR experiment. A complete mass spectrum is obtained by using the Fourier transform to convert this signal from the time domain to the frequency domain.*The angular frequency \((\omega)\) is in radians per second. The unit Hertz (Hz) is in cycles per second where there are \(2 \mathrm{\pi}\) radians per cycle. | 24 |
4.7: Self-Test #2
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/04%3A_MASS_ANALYZERS/4.07%3A_Self-Test_2 | Self-Test #2: Which mass analyzer is appropriate for the following analysis:Self-Test #2: Which mass analyzer would be appropriate for the following analysis:1) Routine analysis of drug testing samples. A quadrupole mass analyzer would provide the necessary mass range and resolution. It is also fast enough for use with high resolution chromatography.2) Analysis of small, 2000 dalton, proteins. This will push the limits of a quadrupole (unless electrospray ionization is used to create multiply charged ions). A sector instrument with FAB ionization would work well.3) Analysis of polymers up to 50,000 dalton. The value of singly charged ions is probably to high for a sector instrument (It might work with electrospray ionization to form multiply charged ions). A TOF analyzer does not have any mass limit so it would be ideal for this analysis.4) High sensitivity testing for chemical warfare agents. For this experiment the high sensitivity of a QUISTOR would be beneficial.5) High resolution analysis. This is usually done with a double focusing sector instrument, although even higher resolution is possible with an ICR. | 25 |
5.1: Vacuum System
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/05%3A_MASS_SPECTROMETER_SYSTEMS/5.01%3A_Vacuum_System | All mass spectrometers operate at very low pressure (high vacuum). This reduces the chance of ions colliding with other molecules in the mass analyzer. Any collision can cause the ions to react, neutralize, scatter, or fragment. All these processes will interfere with the mass spectrum. To minimize collisions, experiments are conducted under high vacuum conditions, typically 10−2 to 10−5 Pa (10−4 to 10−7 torr) depending up on the geometry of the instrument. This high vacuum requires two pumping stages. The first stage is a mechanical pump that provides rough vacuum down to 0.1 Pa (10−3 torr). The second stage uses diffusion pumps or turbomolecular pump s to provide high vacuum. ICR instruments have even higher vacuum requirements and often include a cryogenic pump for a third pumping stage. The pumping system is an important part of any mass spectrometer and the control software will allow the user to turn the pumps off and on and monitor the pressure in different parts of the spectrometer. The pumpdown sequence for turning on a spectrometer starts by operating the roughing pumps to establish the initial vacuum and check for major leaks. After the roughing pumps get the system down to a pressure of approximately 0.1 Pa the high vacuum pumps are turned on to establish operating pressure. This sequence is more important with diffusion pumps for the high vacuum system because they do not tolerate atmospheric pressure.The vacuum system will also include different types of gauges for measuring pressure in different parts of the system. Thermocouple or convectron gauges are used with the roughing pumps to measure pressure down to 0.01 Pa. Ion gauges are used to measure high vacuum down to 10-8 Pa but they cannot be used above 0.1 Pa. To protect the ion gauges and other high voltage electronics the instrument will typically include an interlock system that does not allow power to these components until the roughing pumps have reduced the pressure below a certain threshold. If there is a leak or loss of vacuum the interlock will also turn off power to these systems to protect the components. The thermocouple gauges are normally located at the entrance to any mechanical pumps and ion gauges are normally located in the source and analyzer regions. Depending on the ionization method additional pressure gauges may also be used to monitor the ionization system or any collision regions.5.1: Vacuum System is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. | 26 |
5.2: Source Region Control
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/05%3A_MASS_SPECTROMETER_SYSTEMS/5.02%3A_Source_Region_Control | The mass spectrometer system will also include controls for the source region. These controls will vary depending upon the ionization technique being used for analysis. For GC/MS and LC/MS systems this software will also control the chromatography system. In general, this requires setting parameters that control the temperature of the sample inlet, determine the ionization energy and efficiency along with parameters that control the efficiency of extracting ions from the source region and transferring them into the analyzer region. These parameters often interact with each other so acquiring spectra with good signal to noise levels requires careful optimization. Typically, this is done using a reference sample and an automated tuning program. The automated tuning program allow the user to set some parameters, like the ionization energy, and the software then varies the other parameters, including voltages on the ion extraction lens systems, to get the best signal possible. The source region for an electrospray mass spectrometer is shown in The reference sample is often a fluorinated compound used for calibration and tuning since fluorine has a single isotope, which simplifies the spectra, and they have relatively high vapor pressure for their mass. Perfluorotributylamine and prefluorokerosene are two common reference standards for gas phase samples. Ultramark 1621, a mixture of fluorinated compounds, is often used for electrospray and FAB. 5.2: Source Region Control is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. | 27 |
5.3: Mass Analyzer Control
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/05%3A_MASS_SPECTROMETER_SYSTEMS/5.03%3A_Mass_Analyzer_Control | Control of the mass analyzer requires adjustment of voltages and currents described for the operation of the analyzer and control over voltages on the ion lenses that focus and direct the ions to the detector. The user will need to set the mass range for the analysis. The mass range is limited by the design of the instrument and is an important specification. The range scanned and the speed of the scan are typically adjusted for an experiment. The mass range should cover the expected range for the analyte but scanning a larger range than needed should be avoided. Scanning outside the range needed will either slow down the analysis or just results in the collection of background noise. The scanning speed should be adjusted to balance the speed of the experiment with the signal to noise. Scanning too fast may distort the spectra and reduce S/N levels. Scanning too slow is a significant problem with chromatography or other experiments where the signal is transient. For GC/MS in particular the chromatography peak may only be several seconds wide. The mass spectrum needs to be acquired quickly enough that the analyte concentration is constant for the entire scan. If the concentration varies during the scan it will distort the relative intensity of ions collected at different times.The mass analyzer needs to be tuned to optimize the efficiency of ion transmission and calibrated to determine the mass scale. This is typically done using a reference sample as part of the tuning process described for the source region. 5.3: Mass Analyzer Control is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. | 28 |
5.4: Detector Control
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/05%3A_MASS_SPECTROMETER_SYSTEMS/5.04%3A_Detector_Control | Detection of ions is based up on their charge or momentum. For large signals a faraday cup is used to collect ions and measure the current. Older instruments used photographic plates to measure the ion abundance at each mass to charge ratio. Most detectors currently used amplify the ion signal using a collector similar to a photomultiplier tube. These amplifying detectors include: electron multipliers (shown in ), channeltrons and multichannel plates. The gain is controlled by changing the high voltage applied to the detector. A detector is selected for it’s speed, dynamic range, gain, and geometry. Some detectors are sensitive enough to detect single ions. The mass spectrometer system will include controls for the gain of the detector. Typically, the gain is adjusted by changing the potential applied to the detector. These voltages are controlled by the software and care should be taken to balance the sensitivity required for the analysis. If the gain is set too low, signal will not be detected, if the gain is set too high the signal will include a lot of noise, the response may not be linear, and the detector life will be shortened.For GC/MS systems it is typical to use a solvent delay so that the detector is turned off at the start of a run. After the solvent has gone through the system the detector is turned on. This protects the detector from being overloaded by the signal from the solvent but set the gain high enough to see analytes at very low concentration. 5.4: Detector Control is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. | 29 |
5.5: Data System
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/05%3A_MASS_SPECTROMETER_SYSTEMS/5.05%3A_Data_System | The final component of a mass spectrometer is the data system. This part of the instrument has undergone revolutionary changes. It has evolved from photographic plates and strip chart recorders to data systems that control the instrument, acquire hundreds of spectra in a minute and search tens of thousands of reference spectra to identify an unknown. Important features of the data system include control over data acquisition and effective data processing.Critical features for data processing include averaging, subtracting, and deconvolution of spectra. shows the background spectra for a mass spectrometer using 70 eV electron ionization. The peak at 44 m/z corresponds to carbon dioxide. Water (18 m/z), nitrogen (28 m/z), and oxygen (32 m/z) are normally observed if they are included in the mass range scanned by the spectrometer. The peak at 207 m/z is from a siloxane compound that is commonly observed in mass spectra and is likely caused by the GC septum used for injection. Other background peaks may be carryover from previous experiments or from the vacuum pump oil. It is a good idea to be familiar with the background peaks and levels for an instrument since changes in the background often indicate possible problems with the instrument. The data processing software for the system can be used to reduce the background signal in mass spectrum. shows two mass spectra from the same time in the chromatogram. The top spectrum is the raw data from the spectrometer. In the bottom spectrum the background signal was subtracted. The background peaks at 77 m/z and 207 m/z are removed and a large number of smaller peaks are also eliminated. Another important data processing feature is shown in . This figure shows data for the analysis of caffeine by GC/MS. The top trace is the total ion chromatogram – the sum of the intensity for all masses as a function of time. The bottom trace is the extracted ion chromatograph that only shows the intensity of the 194 m/z signal as a function of time. Since caffeine is the only compound with an ion observed at 194 m/z this is the only peak in the chromatogram. This chromatogram shows a significant reduction in the background noise. This chromatogram was extracted from a full scan at each time in the chromatogram. It is also possible to set up the spectrometer to only monitor a single ion, this is called selective ion monitoring and the technique can significantly enhance the sensitivity of a mass spectrum analysis. : Comparison of total ion chromatagram (top) and the extracted ion chromatagram for 194 m/z (bottom). (Copyright; Van Bramer via source) The final data processing technique for discussion here is database searching. shows the library search results using NIST MS Search 2.0 for the caffeine peak at 8.54 minutes in the chromatogram. After sending the mass spectrum to the search routine, the program displays likely matches and shows the reference spectra for easy comparison. Search routines like this make it possible to compare an unknown with a large database of target compounds for quick identification. 5.5: Data System is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. | 30 |
6.1: Molecular Ion
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/06%3A_INTERPRETATION/6.01%3A_Molecular_Ion | The molecular ion provides the molecular mass of the analyte and is the first clue used to interpret a mass spectrum. The mass to charge ratio of the molecular ion is based up on the mass of the most abundant isotope for each element in the molecule. This is not the relative atomic mass from the periodic table. Since many mass spectrometers have unit mass resolution, the isotope mass is normally rounded to the nearest whole number, this is called the nominal mass. For example the molecular ion for CHBr3 is observed at 250 m/z;(12 + 1 + 3 \(\times\) 79)=250), not at the relative molecular weight of 253. The mass of the molecular ion is based upon the mass of the isotope with the highest natural abundance. The most common bromine isotope is 79Br. Do not use the weighted average atomic weight for Br (79.9) which is based upon the natural abundance of 79Br and 81Br. The mass spectrum of CHBr3 includes ions for all the naturally occurring isotopes and the intensity of each peak depends upon the probability for that combination of isotopes. These patterns are discussed in detail in the section on isotope abundance.In many mass spectra, the molecular ion is easily identified as the ion with the highest mass to charge ratio. However, this assignment should be made with caution because the highest mass to charge ion may be an impurity, an isotope of the molecular ion, or a fragment. Many compounds fragment easily and no molecular ion is observed in the 70 eV EI spectrum. It is important to clarify that the molecular ion IS NOT necessarily the ion with the greatest abundance, the ion with the greatest abundance is called the base peak. The base peak is the peak with the greatest abundance. The mass spectrum is usually normalized so that this peak has an intensity of 100.A list of molecular ion characteristics are in Table \(\PageIndex{1}\) to help you identify them in a mass spectrum. Low energy EI, where the ionization energy is reduced, often increases in intensity of the molecular ion. Chemical Ionization, CI, is also useful for identifying the molecular ion since the the adduct ion is often more stable than the radical cation produced by electron ionization. The adduct ion is often formed by protonating the analyte to form \((\mathrm{M}+\mathrm{H})\) and is observed at a mass to charge ratio of M+1. shows the mass spectrum of acetone (CH3COCH3). The molecular ion is clearly shown at 58 m/z (12 x 3 + 6 x 1 + 16 = 58). The base peak is at 43 m/z and corresponds to loss of 15 m/z from the intact molecule, this is caused by breaking a C-C bond for loss of a CH3• radical to give CH3CO+ at 43 m/z (12 x 2 + 3 x 1 + 16 = 43). The mass spectrum also includes several other minor peaks - the peak at 59 m/z is caused by the small abundance of C-13 that gives a small fraction of the acetone molecules a mass of 59; the peak at 15 m/z is caused by the CH3 fragment retaining the charge when the C-C bond breaks. These fragmentation and isotope patterns are discussed in more detail in the following sections. | 31 |
6.2: Fragmentation
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/06%3A_INTERPRETATION/6.02%3A_Fragmentation | Although the molecular ion is useful for identification, it does not provide any structural information about an unknown. The structural information is obtained from the fragmentation patterns of the mass spectrum. Identifying an unknown without analyzing the fragmentation patterns is like putting together a jigsaw puzzle without the picture. Fragmentation patterns are often complex, but they fit together like pieces of the puzzle to identify the structure of the molecule.After a molecule is ionized, the molecular ion retains the excess ionization energy. If this excess energy is greater than the energy required to break a chemical bond, the molecule can fragment. The fragmentation processes are typically categorized as direct cleavage where a single bond is broken or rearrangement where bonds are broken and created simultaneously ).The molecular ion formed by electron ionization is an odd electron ion, a radical species with an unpaired electron. These ions are formed by removing a lone pair electron or a bonding electron from a molecule during ionization. For example, water is ionized by removing a non-bonding electron from oxygen to produces \(\mathrm{H}_{2} \mathrm{O}^{+1}\). This is an example of an odd electron ion. Odd electron ions have an even mass to charge value. The exception to this is if the ion has an odd number of nitrogen atoms. Calculate the mass to charge value for some molecular ions to verify this statement.When a molecular ion fragments by direct cleavage a single bond is broken to produce two fragments. This usually separates the charge and the radical of the molecular ion. Direct cleavage produces an even electron ion, AB+, and a neutral odd electron radical, CD•. The even electron ion is detected at an odd mass to charge value (assuming no nitrogen) and since the radical is a neutral fragment it is not observed in the mass spectrum. Even electron ions have all paired electrons. An example of this was shown in the mass spectrum of acetone where the molecular ion, CH3–CO–CH3+•, fragments to form CO–CH3+ - an even electron ion observed at an 43 m/z. The radical CH3• has an odd mass but since it is neutral this fragment is not observed in the mass spectrum. It is possible for the charge and radical species to switch. As a result, cleavage of CH3–CO–CH3+• can also form CH3+ which is an even electron ion observed at 15 m/z, an odd mass. The radical formed by this cleavage, CO–CH3•, would not be observed. In the mass spectrum of acetone the 43 m/z peak is much larger than the 15 m/z peak, so the formation of CO–CH3+ at 43 m/z is clearly favored. When you are interpreting mass spectra look for possible cleavage fragments but keep in mind that either or both of the fragments may be observed in the mass spectrum.Rearrangements are more complex reactions that involve both making and breaking bonds. These reactions are thermodynamically favorable because they require less energy. However they also require a concerted mechanism that is not as kinetically favorable when compared to a simple cleavage reaction. Rearrangement ions are easily identified because they are observed as odd electron ions with an even m/z value. These fragments often provide important clues about the location and identity of certain functional groups. Rearrangements are discussed in more detail in the next section.The mass spectra of 4 different \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) isomers are shown in and see what you can find.For 1-butanol the molecular ion should be observed at 74 m/z (4x12 + 10x1 + 1x16 = 74). There is a very small peak at this location, which is not unusual alcohols - and many other compound classes. If you look at the mass spectra for a large number of alcohols you will notice that they often show little or no molecular ion intensity. This makes interpreting their spectra challenging and IR spectra - which very clearly show OH functional groups - compliment mass spectra by helping to identify these functional groups. If you know from the IR that a compound is an alcohol you can be careful about identifying the molecular ion, knowing that it may not be observed. Next look at possible cleavage fragments from the molecular ion. One possibility is loss of a hydrogen to give 73 m/z. There is a small peak at 73 m/z in the mass spectrum - that indicates that this fragmentation is possible but it is is not common. The same is also true for loss of OH• - which is observed with a small peak at 57 m/z (74 - 17). The next loss is alpha-clevage, breaking the C-C bond next to the OH functional group, to form CH2OH+, observed at 31 m/z (CH2OH+) or the compliment, CH3CH2CH2+, observed at 43 m/z. Since the charge could be retained by either fragment both are observed in the spectrum. Alpha-cleavage is a common fragmentation pattern for alcohols, so observing a peak at 31 m/z is useful for identifying primary alcohols.The 1-butanol spectrum also has a major peak at 56 m/z. This is an even mass ion so it is not formed by breaking a single bond. Looking at the loss from the molecular ion to this fragment (74 - 56 = 18) is a clue to the identity. Alcohols often undergo loss of water (H2O - 18 m/z, so 56 m/z is a likely peak for 1-butanol. This rearrangement is favorable because water is very stable and the resulting radical ion, CH2=CH-CH2-CH3+, has the same structure as an alkene. Rearrangements are much more likely when they create a stable species. The other significant peak in this mass spectrum is at 41 m/z. It is not possible to get this mass from breaking a single bond so it must also involve some sort of rearrangement. It is not unusual for fragmentation and loss of H2 to occur so this ion could be formed by alpha-cleavage followed by H2 loss. Since H2 is an intact molecule, this fragmentation is energetically favorable although it also requires some rearrangement.The next spectrum to examine is 2-butanol. Before looking at the mass spectrum draw the Lewis dot structure for 2-butanol and determine the mass of the possible alpha-cleavage fragments. Then compare your results with the spectrum in .Since 2-butanol has the same molecular formula as 1-butanol, C4H10O, it also has the same molecular ion at 74 m/z. The molecular ion is not seen in but there are several very informative fragment ions observed in the mass spectrum. From the analysis of 1-butanol, it is reasonable to look for alpha cleavage fragments. Since this is a secondary alcohol, there are two possible alpha-cleavage locations for 2-butanol. Alpha-cleavage could result in loss of CH3• or C2H5• to produce ions observed at 59 m/z (74 - 15) and 45 m/z (74 - 29) respectively. Both of these peaks are observed in and their high intensity clearly distinguish this mass spectrum from 1-butanol. The compliment ions, CH3+ or C2H5+ are observed at 15 m/z and 29 m/z but are not particularly useful for identification since they are present in almost all organic mass spectra.The next spectrum to examine is 2-methyl-1-propanol. Before looking at the mass spectrum in , draw the Lewis dot structure for 2-methyl-1-propanol and determine the mass of the possible alpha cleavage fragments. Then compare what you find with the spectrum below.Since 2-methyl-1-propanol has the same molecular formula as 1-butanol and 2-butanol, C4H10O, it also has the same molecular ion at 74 m/z. Although the molecular ion at 74 m/z was not readily observed in the previous two spectra, it is clearly seen for 2-methyl-1-propanol in Based on the discussion of the previous two spectra we should also look for alpha cleavage fragments. Based on the structure for 2-methyl-1-propanol alpha-cleavage would result in loss of C3H7• to form CH2OH+ which is observed at 31 m/z and is characteristic of a primary alcohol. The complement ion C3H7+ at 43 m/z is also observed in Since the C3H7+ at 43 m/z is a secondary carbocation it is more stable than the C3H7+ ion formed from the fragmentation of 1-butanol. As a result the peak at 43 m/z is the base peak in the spectum of 2-methyl-1-propanol. The relative intensity of peaks like this is very important for distinguishing the mass spectra of similar compounds. You can also compare the relative intensity of the peaks at 31 m/z and 43 m/z in the spectra of 1-butanol and 2-methyl-1-propanol.The next spectrum to examine is 2-methyl-2-propanol. Before looking at the mass spectrum draw the Lewis dot structure for 2-methyl-2-propanol and determine the mass of the possible alpha cleavage fragments. Then compare what you find with the spectrum in .The molecular ion is not observed for 2-methyl-1-propanol in However, the alpha-cleavage peak showing loss of CH3• at 59 m/z is the base peak and is far more abundant than any other ion in the spectrum. There are two reasons for this, the first is that there are three different locations in the structure where alpha-cleavage results in loss of CH3•. This alone would increase the probability of forming 59 m/z but the additional consideration is that the C3H7O+ carbocation produced by alpha-cleavage is a tertiary carbocation. As a result it is much more stable and therefore less likely to undergo further fragmentation. It is clear from the spectra shown in . Given the stability of aromatic compounds it should not be surprising that the molecular ion at 92 m/z has a high intensity. The base peak observed at 91 m/z is interesting because loss of H• is not typically this intense. It turns out that the tropylium ion, C7H7+, is also aromatic and so this fragment is very stable and often has a high intensity.
| 32 |
6.3: Rearangement
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/06%3A_INTERPRETATION/6.03%3A_Rearangement | The mass spectrum of heptane is shown in . This mass spectrum is consistent with the fragmentation patterns discussed in the previous section. The molecular ion, C7H16•+ is observed at 100 m/z and a series of cleavage peaks are observed for loss of CH3• (M - 15), C2H5• (M - 29), and C3H7• (M - 43). These peaks are observed at 85 m/z , 71 m/z , and 57 m/z respectively. This fragmentation is characteristic for a linear hydrocarbon.Some functional groups, however, can undergo very different fragmentation processes than the direct cleavage discussed so far. One common example is the McLafferty rearrangement ) which results in formation of an intact neutral molecule and a radical ion, both with an even mass to charge ratio. Since the most intense direct cleavage fragments have odd mass to charge ratios, this fragmentation pattern is very useful for identifying carbonyl compounds and for determining their structure. The McLafferty rearrangement is often observed for carbonyl compounds that contain a linear alkyl chain. If this alkyl chain is long enough, a six-membered ring forms from the carbonyl oxygen to the hydrogen on the fourth carbon. This spacing allows the hydrogen to transfer to the carbonyl oxygen via a six membered ring. This is followed by a rearrangement of the electrons to break the beta C-C bond, the second bond from the carbonyl carbon, to form an alkene and resonance stabilized radical with the carbonyl group. The McLafferty rearrangement is energetically favorable because it results in loss of a neutral alkene and formation of a resonance stabilized radical. Both these fragments may be observed in the mass spectrum, depending upon which fragment retains the charge. shows the charge on the resonance stabilized radical, this is the McLafferty ion. The alkene is referred to as the McLafferty compliment.The products from the McLafferty rearrangement are observed in the mass spectra of C-7 carbonyl compounds shown in - \(\PageIndex{7}\).The mass spectrum of heptanal shown in contains two even mass ions. C2H4O+ m/z 44 is produced by the McLafferty rearrangement of an aldehyde and is a characteristic peak that is very useful for interpretation of aldehydes. The McLafferty compliment, C5H10+, is observed at 70 m/z . The McLafferty compliment is produced when the charge is transferred to the alkene fragment during the rearrangement.The mass spectrum of 2-hepanone shown in is easily distinguished from heptanal because the McLafferty rearrangement breaks the C-C bond between C-3 and C-4. This results in loss C4H8 to give the McLafferty ion for a 2-ketone, C3H6O+, at 58 m/z. The McLafferty compliment, C4H8+ (56 m/z) is not observed for 2-heptanone. The mass spectrum of 3-hepanone in is easily distinguished from heptanal and 2-heptanone because the McLafferty rearrangement breaks the C-C bond between C-4 and C-5. This results in loss C3H6 to give the McLafferty ion for a 3-ketone, C4H8O+, at 72 m/z. The McLafferty compliment, C3H6+ (42 m/z) is not observed for 3-heptanone. The mass spectrum of 4-hepanone shown in is easily distinguished from heptanal, 2-heptanone, and 3-heptanone. The McLafferty rearrangement would break the C-C bond between C-2 and C-3. This results in loss C2H4 to give the McLafferty ion for a 4-ketone, C5H10O+, at 86 m/z - which has a very low intensity in the mass spectrum of 4-heptanone shown in The two major peaks in this spectrum 43 m/z and 71 m/z correspond to alpha cleavage to produce C3H7 and C4H7O which would be observed at 43 m/z and 71 m/z respectively. In this molecule the direct cleavage is highly favored over the McLafferty rearrangement. In this case the faster kinetics of the direct cleavage are favored over the concerted mechanism required for the rearrangement.The mass spectrum of heptanoic acid shown in is easily distinguished from heptanal, 2-heptanone, 3-heptanone, and 4-heptanone because the McLafferty rearrangement produces C2H4O2+ observed at 60 m/z and characteristic of a carboxylic acid. In this case the McLafferty compliment, C5H8+, is not observed in the mass spectrum. Based up on the discussion so far you should be able to identify many of the other fragments in these three mass spectra. Spend some time with a piece of scratch paper and see what you come up with. | 33 |
6.4: Isotope Abundance
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/06%3A_INTERPRETATION/6.04%3A_Isotope_Abundance | The existence of isotopes was first observed by Aston using a mass spectrometer to study neon ions. When interpreting mass spectra it is important to remember that the relative atomic mass or atomic weight of an element is a weighted average of the naturally occurring isotopes. Mass spectrometers separate these isotopes and they are each observed at their respective mass to charge ratio. The relative abundance used to determine the relative atomic mass is determined using mass spectrometry. Although this complicates the mass spectrum, it also provides useful information for identifying the elements in an ion. Chlorine is an excellent example of how isotope distributions are useful for interpretation. The molecular weight of chlorine is \(35.45 \mathrm{u}\). This is calculated from the natural abundance of \({ }^{35} \mathrm{Cl}(75 \%)\) and \({ }^{37} \mathrm{Cl}(25 \%)\). To avoid ambiguity the molecular ion is defined as the ion with the most commonly occurring isotopes. For \(\mathrm{CH}_{3} \mathrm{Cl}\) the molecular ion is \({ }^{12} \mathrm{C}^{1} \mathrm{H}_{3}{ }^{35} \mathrm{Cl}\) at 50 m/z .The natural abundance of these two isotopes is observed in the mass spectrum as two peaks separated by 2 m/z with a relative intensity of \(3: 1\). The mass spectrum of chlorobenzene C6H5Cl in clearly shows the chlorine isotope distribution at 112 m/z and 114 m/z . These peaks correspond to the molecular ion - the molecular ion has the most abundant isotope for each element - at 112 m/z (6x12 + 5x1 + 35) and the 37Cl isotope peak at 114 m/z (6x12 + 5x1 + 37) and the relative intensity is determined by the natural abundance of the 37Cl isotope. The other major peak in this spectrum at 77 m/z corresponds to the loss of chlorine from the molecular ion or the 37Cl isotope peak to give C6H5+ (112 - 35 = 77 OR 114 - 37 = 77).If more than one chlorine atom is present, the isotope abundance is more complex. An ion with two chlorine atoms has three possible isotope combinations. This pattern is apparent in the mass spectrum of CH2Cl2 shown in . Ions are observed for \(\mathrm{CH}_{2}{ }^{35} \mathrm{Cl}_{2}^{+}\) at 84 m/z, \(\mathrm{CH}_{2}{ }^{35} \mathrm{Cl}^{37} \mathrm{Cl}^{+}\) at 86 m/z , and \(\mathrm{CH}_{2}{ }^{37} \mathrm{Cl}_{2}^{+}\) at 88 m/z . Based up on the probability of each combination of isotopes, the relative intensity of these peaks is \(10: 6: 1\). The \(3: 1\) isotope ratio for an ion with a single chlorine atom is observed at 49 m/z and 51 m/z . This corresponds to \(\mathrm{CH}_{2}{ }^{35} \mathrm{Cl}^{+}\)and \(\mathrm{CH}_{2}{ }^{37} \mathrm{Cl}^{+}\)fragments formed by loss of \(\mathrm{Cl}\) from the molecular ion. Careful examination of the spectrum also shows ions produced by loss of H• and \(\mathrm{H}_{2}\).Bromine also has two naturally occurring isotopes, 79Br is the most abundant and 81Br has a relative abundance of 98% which results in a relative intensity for these two peaks of 1:1. This is observed in the mass spectrum of bromobenzene shown in . The bromine isotope pattern is seen in the peaks at 156 m/z and 158 m/z which have the 1:1 relative abundance characteristic of bromine. These two peaks correspond to the molecular ion C6H579Br at 156 m/z and C6H581Br at 158 m/z . The base peak in this spectrum is from loss of Br to form C6H5 observed at 77 m/z .The \(1.1 \%\) of natural abundance of \({ }^{13} \mathrm{C}\) is another useful tool for interpreting mass spectra. The abundance of a peak one m/z value higher, where a single \({ }^{12} C\) is replaced by a \({ }^{13} C\), is determined by the number of carbons in the ion. The rule of thumb for small compounds is that each carbon atom in the ion increases the abundance of the \(M+1\) peak by \(1 \%\). This effect is seen in all the spectra discussed in this paper. For example, in the \(n\)-decane mass spectrum ) compare the peak for \({ }^{12} \mathrm{C}_{9}{ }^{13} \mathrm{C}^{1} \mathrm{H}_{22}\) at 143 m/z (0.38 % relative abundance) to the peak for \({ }^{12} \mathrm{C}_{10}{ }^{1} \mathrm{H}_{22}\) at 142 m/z (3.96% relative abundance). The abundance of the \(13 \mathrm{C}\) peak is \(10 \%\) the abundance of the \({ }^{12} \mathrm{C}\) peak, consistent with a compound containing 10 carbon atoms. Now look at some previous spectra to find more examples of this pattern. Be aware that for compounds with low molecular ion abundances the uncertainty in measuring this ratio may be +/- several carbon atoms.Because all atoms have several naturally occurring isotopes, the patterns discussed here become more complex. Fortunately, most elements common in organic mass spectrometry have one predominant isotope. The high abundance of the two chlorine isotopes is unusual, so they are easy to identify. The relative abundances for isotopes of frequently encountered elements are given in Table \(\PageIndex{1}\). For molecules with more complex isotope patterns there are a number of programs and websites available for modeling the distributions. The calculator provided by Scientific Instrument Services is available at: //www.sisweb.com/mstools/isotope.htm. | 34 |
6.5: Amine Fragmentation
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/06%3A_INTERPRETATION/6.05%3A_Amine_Fragmentation | Functional groups can have a significant effect the fragmentation patterns observed in mass spectrometry and textbooks on mass spectrometry cover a large range of common fragmentation patters for different functional groups. For a detailed discussion of this, interested readers are encouraged to look at any of the following books:
As one final example aliphatic amines often undergo cleavage at the \(\alpha { } \mathrm{C}-\mathrm{C}\) bond to produce a relatively stable \(\mathrm{CH}_{2} \mathrm{NH}_{2}{ }^{+}\)ion ). The resulting fragments distinguish primary, secondary, and tertiary amines.
: \(\alpha\)-Cleavage fragmentation of an amine.This fragmentation is useful for distinguishing mass spectra of the three different C4H11N isomers. Draw the structure of 1-butanamine, 2-butanamine, 2-methyl-1-propanamine, and 2-methyl-2-propanamine. Determine the mass to charge ratio for the molecular ion, identify the site for alpha-cleavage for each molecule, and determine the mass to charge ratio for the expected fragments. After you have done this, look up the mass spectra for these four compounds in the NIST Chemistry WebBook which contains mass spectra for a large number of compounds.All four compounds have the same molecular formula, C4H11N with an odd number of nitrogen atoms so the molecular ion has an odd mass to charge ratio. The molecular ion is observed for all four compounds at 73 m/z .1-butanamine. The \(\alpha\)-cleavage fragment for 1-butanamine produces CH2NH2+ at 30 m/z and C3H7•. The C3H7 fragment has a very low intensity in the mass spectrum because since the charge is retained by the nitrogen containing fragment. See NIST Webbook for the mass spectrum of 1-butanamine.2-butanamine. There are two \(\alpha\)-cleavage sites for 2-butanamine. Loss of \(\mathrm{CH}_{3}^{\prime}\) produces \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{NH}_{2}{ }^{+}\) (58 m/z) and loss of \(\mathrm{C}_{2} \mathrm{H}_{5}^{\prime}\) produces \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{NH}_{2}{ }^{+}\) (44 m/z). Both of these ions are observed but the greater abundance of the 44 m/z signal indicates that loss of \(\mathrm{C}_{2} \mathrm{H}_{5}^{\prime \prime}\) is favored. See NIST Webbook for the mass spectrum of 2-butanamine.2-methyl-1-propanaimne. The \(\alpha\)-cleavage fragment for 2-methyl-1-propanamine produces CH2NH2+ at 30 m/z and C3H7•. The C3H7 fragment has a very low intensity in the mass spectrum because since the charge is retained by the nitrogen containing fragment. The resulting mass spectrum is very similar to 1-butanamine and distinguishing these two isomers by mass spectrometry will depend on careful comparison of the relative intensity of the molecular ion and other fragments observed in the mass spectrum. The See NIST Webbook for the mass spectrum of 2-methyl-1-propanamine.2-methyl-2-propanaimne. The \(\alpha\)-cleavage fragment for 2-methyl-2-propanaimne produces C3H6NH2+ at 58 m/z and CH3•. The CH3 fragment has a very low intensity in the mass spectrum because since the charge is retained by the nitrogen containing fragment. See NIST Webbook for the mass spectrum of 1-butanamine. | 35 |
6.6: Exact Mass
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/An_Introduction_to_Mass_Spectrometry_(Van_Bramer)/06%3A_INTERPRETATION/6.06%3A_Exact_Mass | Exact Mass. In most mass spectrometry experiments the nominal mass is used and the mass to charge ratio of an ion is rounded to the nearest whole number. High resolution instruments, including double focusing and FT-ICR mass spectrometers, are capable of determining the "exact mass" of an ion. This is useful for interpretation because each element has a slightly different mass defect. This "mass defect" is the difference between the mass of the isotope and the nominal mass (which is equivalent to the number of protons and neutrons).Recall that the atomic mass scale is defined by carbon-12 with a mass of exactly \(12.0000\) u. The exact mass of a specific isotope is determined relative to \({ }^{12} \mathrm{C}\) by high resolution mass spectrometry (see Table \(\PageIndex{1}\)). High resolution mass spectrometry can distinguish compounds with the same nominal mass but different exact mass caused by different elemental composition.For example, \(\mathrm{C}_{2} \mathrm{H}_{6}, \mathrm{CH}_{2} \mathrm{O}\), and \(\mathrm{NO}\) all have a nominal mass of 30 u. Because they have the same nominal mass, a mass spectrometer with unit mass resolution can not distinguish these three ions. However, the exact masses for \(\mathrm{C}_{2} \mathrm{H}_{6}(30.04695039), \mathrm{CH}_{2} \mathrm{O}(30.01056487)\) and \(\mathrm{NO}^{2}\) (29.99798882) are different and a high resolution mass spectrometer can distinguish these three compounds.Table \(\PageIndex{1}\) lists the exact mass for the most abundant isotopes of several common elements. The Isotope Distribution Calculator on the SIS website will also calculate the exact mass for any chemical formula. This is available online at: //www.sisweb.com/mstools/isotope.htmValues in parentheses indicate error in last digit.This section is only an introduction to the interpretation of mass spectra. A full analysis of fragmentation patterns is beyond the scope of this text but with practice interpretation becomes much easier. Several excellent references include McLafferty’s book and the ACOL book on mass spectrometry. These contain additional information on mass spectral interpretation and many more practice problems. | 36 |
InfoPage
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/00%3A_Front_Matter/02%3A_InfoPage | This text is disseminated via the Open Education Resource (OER) LibreTexts Project and like the hundreds of other texts available within this powerful platform, it is freely available for reading, printing and "consuming." Most, but not all, pages in the library have licenses that may allow individuals to make changes, save, and print this book. Carefully consult the applicable license(s) before pursuing such effects.Instructors can adopt existing LibreTexts texts or Remix them to quickly build course-specific resources to meet the needs of their students. Unlike traditional textbooks, LibreTexts’ web based origins allow powerful integration of advanced features and new technologies to support learning. The LibreTexts mission is to unite students, faculty and scholars in a cooperative effort to develop an easy-to-use online platform for the construction, customization, and dissemination of OER content to reduce the burdens of unreasonable textbook costs to our students and society. The LibreTexts project is a multi-institutional collaborative venture to develop the next generation of open-access texts to improve postsecondary education at all levels of higher learning by developing an Open Access Resource environment. The project currently consists of 14 independently operating and interconnected libraries that are constantly being optimized by students, faculty, and outside experts to supplant conventional paper-based books. These free textbook alternatives are organized within a central environment that is both vertically (from advance to basic level) and horizontally (across different fields) integrated.The LibreTexts libraries are Powered by NICE CXOne and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. This material is based upon work supported by the National Science Foundation under Grant No. 1246120, 1525057, and 1413739.Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation nor the US Department of Education.Have questions or comments? For information about adoptions or adaptions contact More information on our activities can be found via Facebook , Twitter , or our blog .This text was compiled on 07/05/2023 | 38 |
1.1: What is Analytical Chemistry
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/01%3A_Introduction_to_Analytical_Chemistry/1.01%3A_What_is_Analytical_Chemistry | This quote is attributed to C. N. Reilly on receipt of the 1965 Fisher Award in Analytical Chemistry. Reilly, who was a professor of chemistry at the University of North Carolina at Chapel Hill, was one of the most influential analytical chemists of the last half of the twentieth century.For another view of what constitutes analytical chemistry, see the article “Quo Vadis, Analytical Chemistry?”, the full reference for which is Valcárcel, M. Anal. Bioanal. Chem. 2016, 408, 13-21.Let’s begin with a deceptively simple question: What is analytical chemistry? Like all areas of chemistry, analytical chemistry is so broad in scope and so much in flux that it is difficult to find a simple definition more revealing than that quoted above. In this chapter we will try to expand upon this simple definition by saying a little about what analytical chemistry is, as well as a little about what analytical chemistry is not.Analytical chemistry often is described as the area of chemistry responsible for characterizing the composition of matter, both qualitatively (Is there lead in this paint chip?) and quantitatively (How much lead is in this paint chip?). As we shall see, this description is misleading.Most chemists routinely make qualitative and quantitative measurements. For this reason, some scientists suggest that analytical chemistry is not a separate branch of chemistry, but simply the application of chemical knowledge [Ravey, M. Spectroscopy, 1990, 5, 11]. In fact, you probably have performed many such quantitative and qualitative analyses in other chemistry courses.You might, for example, have determined the concentration of acetic acid in vinegar using an acid–base titration, or used a qual scheme to identify which of several metal ions are in an aqueous sample.Defining analytical chemistry as the application of chemical knowledge ignores the unique perspective that an analytical chemist bring to the study of chemistry. The craft of analytical chemistry is found not in performing a routine analysis on a routine sample—a task we appropriately call chemical analysis—but in improving established analytical methods, in extending these analytical methods to new types of samples, and in developing new analytical methods to measure chemical phenomena [de Haseth, J. Spectroscopy, 1990, 5, 11].Here is one example of the distinction between analytical chemistry and chemical analysis. A mining engineers evaluates an ore by comparing the cost of removing the ore from the earth with the value of its contents, which they estimate by analyzing a sample of the ore. The challenge of developing and validating a quantitative analytical method is the analytical chemist’s responsibility; the routine, daily application of the analytical method is the job of the chemical analyst.Steps 1–3 and 5 are the province of analytical chemistry; step 4 is the realm of chemical analysis.The seven stages of an analytical method listed here are modified from Fassel, V. A. Fresenius’ Z. Anal. Chem. 1986, 324, 511–518 and Hieftje, G. M. J. Chem. Educ. 2000, 77, 577–583.Another difference between analytical chemistry and chemical analysis is that an analytical chemist works to improve and to extend established analytical methods. For example, several factors complicate the quantitative analysis of nickel in ores, including nickel’s unequal distribution within the ore, the ore’s complex matrix of silicates and oxides, and the presence of other metals that may interfere with the analysis. Figure 1.1.1
outlines one standard analytical method in use during the late nineteenth century [Fresenius. C. R. A System of Instruction in Quantitative Chemical Analysis; John Wiley and Sons: New York, 1881]. The need for many reactions, digestions, and filtrations makes this analytical method both time-consuming and difficult to perform accurately.The discovery, in 1905, that dimethylglyoxime (dmg) selectively precipitates Ni2+ and Pd2+ led to an improved analytical method for the quantitative analysis of nickel [Kolthoff, I. M.; Sandell, E. B. Textbook of Quantitative Inorganic Analysis, 3rd Ed., The Macmillan Company: New York, 1952]. The resulting analysis, which is outlined in Figure 1.1.2
, requires fewer manipulations and less time. By the 1970s, flame atomic absorption spectrometry replaced gravimetry as the standard method for analyzing nickel in ores, resulting in an even more rapid analysis [Van Loon, J. C. Analytical Atomic Absorption Spectroscopy, Academic Press: New York, 1980]. Today, the standard analytical method utilizes an inductively coupled plasma optical emission spectrometer.Perhaps a more appropriate description of analytical chemistry is “the science of inventing and applying the concepts, principles, and...strategies for measuring the characteristics of chemical systems” [Murray, R. W. Anal. Chem. 1991, 63, 271A]. Analytical chemists often work at the extreme edges of analysis, extending and improving the ability of all chemists to make meaningful measurements on smaller samples, on more complex samples, on shorter time scales, and on species present at lower concentrations. Throughout its history, analytical chemistry has provided many of the tools and methods necessary for research in other traditional areas of chemistry, as well as fostering multidisciplinary research in, to name a few, medicinal chemistry, clinical chemistry, toxicology, forensic chemistry, materials science, geochemistry, and environmental chemistry.To an analytical chemist, the process of making a useful measurement is critical; if the measurement is not of central importance to the work, then it is not analytical chemistry.You will come across numerous examples of analytical methods in this textbook, most of which are routine examples of chemical analysis. It is important to remember, however, that nonroutine problems prompted analytical chemists to develop these methods.An editorial in Analytical Chemistry entitled “Some Words about Categories of Manuscripts” highlights nicely what makes a research endeavor relevant to modern analytical chemistry. The full citation is Murray, R. W. Anal. Chem. 2008, 80, 4775; for a more recent editorial, see “The Scope of Analytical Chemistry” by Sweedler, J. V. et. al. Anal. Chem. 2015, 87, 6425.This page titled 1.1: What is Analytical Chemistry is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey. | 41 |
1.2: The Analytical Perspective
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/01%3A_Introduction_to_Analytical_Chemistry/1.02%3A_The_Analytical_Perspective | Having noted that each area of chemistry brings a unique perspective to the study of chemistry, let’s ask a second deceptively simple question: What is the analytical perspective? Many analytical chemists describe this perspective as an analytical approach to solving problems.For different viewpoints on the analytical approach see (a) Beilby, A. L. J. Chem. Educ. 1970, 47, 237-238; (b) Lucchesi, C. A. Am. Lab. 1980, October, 112-119; (c) Atkinson, G. F. J. Chem. Educ. 1982, 59, 201-202; (d) Pardue, H. L.; Woo, J. J. Chem. Educ. 1984, 61, 409-412; (e) Guarnieri, M. J. Chem. Educ. 1988, 65, 201-203, (f) Strobel, H. A. Am. Lab. 1990, October, 17-24.Although there likely are as many descriptions of the analytical approach as there are analytical chemists, it is convenient to define it as the five-step process shown in Figure 1.2.1
.Three general features of this approach deserve our attention. First, in steps 1 and 5 analytical chemists have the opportunity to collaborate with individuals outside the realm of analytical chemistry. In fact, many problems on which analytical chemists work originate in other fields. Second, the heart of the analytical approach is a feedback loop (steps 2, 3, and 4) in which the result of one step requires that we reevaluate the other steps. Finally, the solution to one problem often suggests a new problem.Analytical chemistry begins with a problem, examples of which include evaluating the amount of dust and soil ingested by children as an indicator of environmental exposure to particulate based pollutants, resolving contradictory evidence regarding the toxicity of perfluoro polymers during combustion, and developing rapid and sensitive detectors for chemical and biological weapons. At this point the analytical approach involves a collaboration between the analytical chemist and the individual or agency working on the problem. Together they determine what information is needed and clarify how the problem relates to broader research goals or policy issues, both essential to the design of an appropriate experimental procedure.These examples are taken from a series of articles, entitled the “Analytical Approach,” which for many years was a regular feature of the journal Analytical Chemistry.To design the experimental procedure the analytical chemist considers criteria, such as the required accuracy, precision, sensitivity, and detection limit, the urgency with which results are needed, the cost of a single analysis, the number of samples to analyze, and the amount of sample available for analysis. Finding an appropriate balance between these criteria frequently is complicated by their interdependence. For example, improving precision may require a larger amount of sample than is available. Consideration also is given to how to collect, store, and prepare samples, and to whether chemical or physical interferences will affect the analysis. Finally a good experimental procedure may yield useless information if there is no method for validating the results.The most visible part of the analytical approach occurs in the laboratory. As part of the validation process, appropriate chemical and physical standards are used to calibrate equipment and to standardize reagents.The data collected during the experiment are then analyzed. Frequently the data first is reduced or transformed to a more readily analyzable form and then a statistical treatment of the data is used to evaluate accuracy and precision, and to validate the procedure. Results are compared to the original design criteria and the experimental design is reconsidered, additional trials are run, or a solution to the problem is proposed. When a solution is proposed, the results are subject to an external evaluation that may result in a new problem and the beginning of a new cycle.Chapter 3 introduces you to the language of analytical chemistry. You will find terms such accuracy, precision, and sensitivity defined there. Chapter 4 introduces the statistical analysis of data. Calibration and standardization methods, including a discussion of linear regression, are covered in Chapter 5. See Chapter 7 for a discussion of how to collect, store, and prepare samples for analysis. See Chapter 14 for a discussion about how to validate an analytical method.As noted earlier some scientists question whether the analytical approach is unique to analytical chemistry. Here, again, it helps to distinguish between a chemical analysis and analytical chemistry. For an analytically-oriented scientist, such as a physical organic chemist or a public health officer, the primary emphasis is how the analysis supports larger research goals that involve fundamental studies of chemical or physical processes, or that improve access to medical care. The essence of analytical chemistry, however, is in developing new tools for solving problems, and in defining the type and quality of information available to other scientists.As an exercise, let’s adapt our model of the analytical approach to the development of a simple, inexpensive, portable device for completing bioassays in the field. Before continuing, locate and read the article“Simple Telemedicine for Developing Regions: Camera Phones and Paper-Based Microfluidic Devices for Real-Time, Off-Site Diagnosis”by Andres W. Martinez, Scott T. Phillips, Emanuel Carriho, Samuel W. Thomas III, Hayat Sindi, and George M. Whitesides. You will find it on pages 3699-3707 in Volume 80 of the journal Analytical Chemistry, which was published in 2008. As you read the article, pay particular attention to how it emulates the analytical approach and consider the following questions:Don’t let the technical details in the paper overwhelm you; if you skim over these you will find the paper both well-written and accessible.What is the analytical problem and why is it important? A medical diagnoses often relies on the results of a clinical analysis. When you visit a doctor, they may draw a sample of your blood and send it to the lab for analysis. In some cases the result of the analysis is available in 10-15 minutes. What is possible in a developed country, such as the United States, may not be feasible in a country with less access to expensive lab equipment and with fewer trained personnel available to run the tests and to interpret the results. The problem addressed in this paper, therefore, is the development of a reliable device for rapidly performing a clinical assay under less than ideal circumstances.What criteria did the authors consider in designing their experiments? In considering a solution to this problem, the authors identify seven important criteria for the analytical method: it must be inexpensive; it must operate without the need for much electricity, so that it can be used in remote locations; it must be adaptable to many types of assays; its must not require a highly skilled technician; it must be quantitative; it must be accurate; and it must produce results rapidly.What is the basic experimental procedure? The authors describe how they developed a paper-based microfluidic device that allows anyone to run an analysis simply by dipping the device into a sample (synthetic urine, in this case). The sample moves by capillary action into test zones containing reagents that react with specific species (glucose and protein, for this prototype device). The reagents react to produce a color whose intensity is proportional to the species’ concentration. A digital photograph of the microfluidic device is taken using a cell phone camera and sent to an off-site physician who uses image editing software to analyze the photograph and to interpret the assay’s result.What interferences were considered and how did they overcome them? In developing this analytical method the authors considered several chemical or physical interferences. One concern was the possibility of non-specific interactions between the paper and the glucose or protein, which might lead to non-uniform image in the test zones. A careful analysis of the distribution of glucose and protein in the text zones showed that this was not a problem. A second concern was the possibility that particulate materials in the sample might interfere with the analyses. Paper is a natural filter for particulate materials and the authors found that samples containing dust, sawdust, and pollen do not interfere with the analysis for glucose. Pollen, however, is an interferent for the protein analysis, presumably because it, too, contains protein.How did the author’s calibrate the assay? To calibrate the device the authors analyzed a series of standard solutions that contained known concentrations of glucose and protein. Because an image’s intensity depends upon the available light, a standard sample is run with the test samples, which allows a single calibration curve to be used for samples collected under different lighting conditions.How did the author’s validate their experimental method? The test device contains two test zones for each analyte, which allows for duplicate analyses and provides one level of experimental validation. To further validate the device, the authors completed 12 analyses at each of three known concentrations of glucose and protein, obtaining acceptable accuracy and precision in all cases.Is there any evidence of repeating steps 2, 3, and 4 in Figure 1.2.1? Developing this analytical method required several cycles through steps 2, 3, and 4 of the analytical approach. Examples of this feedback loop include optimizing the shape of the test zones and evaluating the importance of sample size.Was there a successful conclusion to the analytical problem? Yes. The authors were successful in meeting their goals by developing and testing an inexpensive, portable, and easy-to-use device for running clinical samples in developing countries.This exercise provides you with an opportunity to think about the analytical approach in the context of a real analytical problem. Practice exercises such as this provide you with a variety of challenges ranging from simple review problems to more open-ended exercises. You will find answers to practice exercises at the end of each chapter.Use this link to access the article’s abstract from the journal’s web site. If your institution has an on-line subscription you also will be able to download a PDF version of the article.This page titled 1.2: The Analytical Perspective is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey. | 42 |
1.3: Common Analytical Problems
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/01%3A_Introduction_to_Analytical_Chemistry/1.03%3A_Common_Analytical_Problems | Many problems in analytical chemistry begin with the need to identify what is present in a sample. This is the scope of a qualitative analysis, examples of which include identifying the products of a chemical reaction, screening an athlete’s urine for a performance-enhancing drug, or determining the spatial distribution of Pb on the surface of an airborne particulate. An early challenge for analytical chemists was developing simple chemical tests to identify inorganic ions and organic functional groups. The classical laboratory courses in inorganic and organic qualitative analysis, still taught at some schools, are based on this work.See, for example, the following laboratory textbooks: (a) Sorum, C. H.; Lagowski, J. J. Introduction to Semimicro Qualitative Analysis, 5th Ed.; Prentice-Hall: Englewood, NJ, 1977; (b) Shriner, R. L.; Fuson, R. C.; Curtin, D. Y. The Systematic Identification of Organic Compounds, 5th Ed.; John Wiley and Sons: New York, 1964.Modern methods for qualitative analysis rely on instrumental techniques, such as infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and mass spectrometry (MS). Because these qualitative applications are covered adequately elsewhere in the undergraduate curriculum, typically in organic chemistry, they receive no further consideration in this text.Perhaps the most common analytical problem is a quantitative analysis, examples of which include the elemental analysis of a newly synthesized compound, measuring the concentration of glucose in blood, or determining the difference between the bulk and the surface concentrations of Cr in steel. Much of the analytical work in clinical, pharmaceutical, environmental, and industrial labs involves developing new quantitative methods to detect trace amounts of chemical species in complex samples. Most of the examples in this text are of quantitative analyses.Another important area of analytical chemistry, which receives some attention in this text, are methods for characterizing physical and chemical properties. The determination of chemical structure, of equilibrium constants, of particle size, and of surface structure are examples of a characterization analysis.The purpose of a qualitative, a quantitative, or a characterization analysis is to solve a problem associated with a particular sample. The purpose of a fundamental analysis, on the other hand, is to improve our understanding of the theory that supports an analytical method and to understand better an analytical method’s limitations.A good resource for current examples of qualitative, quantitative, characterization, and fundamental analyses is Analytical Chemistry’s annual review issue that highlights fundamental and applied research in analytical chemistry. Examples of review articles in the 2015 issue include “Analytical Chemistry in Archaeological Research,” “Recent Developments in Paper-Based Microfluidic Devices,” and “Vibrational Spectroscopy: Recent Developments to Revolutionize Forensic Science.”1.3: Common Analytical Problems is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts. | 43 |
1.5: Additional Resources
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/01%3A_Introduction_to_Analytical_Chemistry/1.05%3A_Additional_Resources | The role of analytical chemistry within the broader discipline of chemistry has been discussed by many prominent analytical chemists; several notable examples are listed here.For additional discussion of clinical assays based on paper-based microfluidic devices, see the following papers.This textbook provides one introduction to the discipline of analytical chemistry. There are other textbooks for introductory courses in analytical chemistry and you may find it useful to consult them when you encounter a difficult concept; often a fresh perspective will help crystallize your understanding. The textbooks listed here are excellent resources.To explore the practice of modern analytical chemistry there is no better resource than the primary literature. The following journals publish broadly in the area of analytical chemistry.This page titled 1.5: Additional Resources is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey. | 45 |
1.6: Chapter Summary and Key Terms
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/01%3A_Introduction_to_Analytical_Chemistry/1.06%3A_Chapter_Summary_and_Key_Terms | Analytical chemists work to improve the ability of chemists and other scientists to make meaningful measurements. The need to work with smaller samples, with more complex materials, with processes occurring on shorter time scales, and with species present at lower concentrations challenges analytical chemists to improve existing analytical methods and to develop new ones.Typical problems on which analytical chemists work include qualitative analyses (What is present?), quantitative analyses (How much is present?), characterization analyses (What are the sample’s chemical and physical properties?), and fundamental analyses (How does this method work and how can it be improved?).This page titled 1.6: Chapter Summary and Key Terms is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey. | 46 |
10.1: Overview of Spectroscopy
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/10%3A_Spectroscopic_Methods/10.01%3A_Overview_of_Spectroscopy | The focus of this chapter is on the interaction of ultraviolet, visible, and infrared radiation with matter. Because these techniques use optical materials to disperse and focus the radiation, they often are identified as optical spectroscopies. For convenience we will use the simpler term spectroscopy in place of optical spectroscopy; however, you should understand we will consider only a limited piece of what is a much broader area of analytical techniques.Despite the difference in instrumentation, all spectroscopic techniques share several common features. Before we consider individual examples in greater detail, let’s take a moment to consider some of these similarities. As you work through the chapter, this overview will help you focus on the similarities between different spectroscopic methods of analysis. You will find it easier to understand a new analytical method when you can see its relationship to other similar methods.Electromagnetic radiation—light—is a form of energy whose behavior is described by the properties of both waves and particles. Some properties of electromagnetic radiation, such as its refraction when it passes from one medium to another (Figure 10.1.1
), are explained best when we describe light as a wave. Other properties, such as absorption and emission, are better described by treating light as a particle. The exact nature of electromagnetic radiation remains unclear, as it has since the development of quantum mechanics in the first quarter of the 20th century [Home, D.; Gribbin, J. New Scientist 1991, 2 Nov. 30–33]. Nevertheless, this dual model of wave and particle behavior provide a useful description for electromagnetic radiation.Electromagnetic radiation consists of oscillating electric and magnetic fields that propagate through space along a linear path and with a constant velocity. In a vacuum, electromagnetic radiation travels at the speed of light, c, which is \(2.99792 \times 10^8\) m/s. When electromagnetic radiation moves through a medium other than a vacuum, its velocity, v, is less than the speed of light in a vacuum. The difference between v and c is sufficiently small (<0.1%) that the speed of light to three significant figures, \(3.00 \times 10^8\) m/s, is accurate enough for most purposes.The oscillations in the electric field and the magnetic field are perpendicular to each other and to the direction of the wave’s propagation. Figure 10.1.2
shows an example of plane-polarized electromagnetic radiation, which consists of a single oscillating electric field and a single oscillating magnetic field.An electromagnetic wave is characterized by several fundamental properties, including its velocity, amplitude, frequency, phase angle, polarization, and direction of propagation [Ball, D. W. Spectroscopy 1994, 9, 24–25]. For example, the amplitude of the oscillating electric field at any point along the propagating wave is\[A_{t}=A_{e} \sin (2 \pi \nu t+\Phi) \nonumber\]where At is the magnitude of the electric field at time t, Ae is the electric field’s maximum amplitude, \(\nu\) is the wave’s frequency—the number of oscillations in the electric field per unit time—and \(\Phi\) is a phase angle that accounts for the fact that At need not have a value of zero at t = 0. The identical equation for the magnetic field is\[A_{t}=A_{m} \sin (2 \pi \nu t+\Phi) \nonumber\]where Am is the magnetic field’s maximum amplitude.Other properties also are useful for characterizing the wave behavior of electromagnetic radiation. The wavelength, \(\lambda\), is defined as the distance between successive maxima (see Figure 10.1.2
). For ultraviolet and visible electromagnetic radiation the wavelength usually is expressed in nanometers (1 nm = 10–9 m), and for infrared radiation it is expressed in microns (1 mm = 10–6 m). The relationship between wavelength and frequency is\[\lambda = \frac {c} {\nu} \nonumber\]Another unit useful unit is the wavenumber, \(\overline{\nu}\), which is the reciprocal of wavelength\[\overline{\nu} = \frac {1} {\lambda} \nonumber\]Wavenumbers frequently are used to characterize infrared radiation, with the units given in cm–1.When electromagnetic radiation moves between different media—for example, when it moves from air into water—its frequency, \(\nu\), remains constant. Because its velocity depends upon the medium in which it is traveling, the electromagnetic radiation’s wavelength, \(\lambda\), changes. If we replace the speed of light in a vacuum, c, with its speed in the medium, \(v\), then the wavelength is\[\lambda = \frac {v} {\nu} \nonumber\]This change in wavelength as light passes between two media explains the refraction of electromagnetic radiation shown in Figure 10.1.1
.In 1817, Josef Fraunhofer studied the spectrum of solar radiation, observing a continuous spectrum with numerous dark lines. Fraunhofer labeled the most prominent of the dark lines with letters. In 1859, Gustav Kirchhoff showed that the D line in the sun’s spectrum was due to the absorption of solar radiation by sodium atoms. The wavelength of the sodium D line is 589 nm. What are the frequency and the wavenumber for this line?SolutionThe frequency and wavenumber of the sodium D line are\[\nu=\frac{c}{\lambda}=\frac{3.00 \times 10^{8} \ \mathrm{m} / \mathrm{s}}{589 \times 10^{-9} \ \mathrm{m}}=5.09 \times 10^{14} \ \mathrm{s}^{-1} \nonumber\]\[\overline{\nu}=\frac{1}{\lambda}=\frac{1}{589 \times 10^{-9} \ \mathrm{m}} \times \frac{1 \ \mathrm{m}}{100 \ \mathrm{cm}}=1.70 \times 10^{4} \ \mathrm{cm}^{-1} \nonumber\]Another historically important series of spectral lines is the Balmer series of emission lines from hydrogen. One of its lines has a wavelength of 656.3 nm. What are the frequency and the wavenumber for this line?The frequency and wavenumber for the line are\[\nu=\frac{c}{\lambda}=\frac{3.00 \times 10^{8} \ \mathrm{m} / \mathrm{s}}{656.3 \times 10^{-9} \ \mathrm{m}}=4.57 \times 10^{14} \ \mathrm{s}^{-1} \nonumber\]\[\overline{\nu}=\frac{1}{\lambda}=\frac{1}{656.3 \times 10^{-9} \ \mathrm{m}} \times \frac{1 \ \mathrm{m}}{100 \ \mathrm{cm}}=1.524 \times 10^{4} \ \mathrm{cm}^{-1} \nonumber\]When matter absorbs electromagnetic radiation it undergoes a change in energy. The interaction between matter and electromagnetic radiation is easiest to understand if we assume that radiation consists of a beam of energetic particles called photons. When a photon is absorbed by a sample it is “destroyed” and its energy acquired by the sample [Ball, D. W. Spectroscopy 1994, 9 20–21]. The energy of a photon, in joules, is related to its frequency, wavelength, and wavenumber by the following equalities\[E=h \nu=\frac{h c}{\lambda}=h c \overline{\nu} \nonumber\]where h is Planck’s constant, which has a value of \(6.626 \times 10^{-34}\) Js.What is the energy of a photon from the sodium D line at 589 nm?SolutionThe photon’s energy is\[E=\frac{h c}{\lambda}=\frac{\left(6.626 \times 10^{-34} \ \mathrm{Js}\right)\left(3.00 \times 10^{8} \ \mathrm{m} / \mathrm{s}\right)}{589 \times 10^{-7} \ \mathrm{m}}=3.37 \times 10^{-19} \ \mathrm{J} \nonumber\]What is the energy of a photon for the Balmer line at a wavelength of 656.3 nm?The photon’s energy is\[E=\frac{h c}{\lambda}=\frac{\left(6.626 \times 10^{-34} \ \mathrm{Js}\right)\left(3.00 \times 10^{8} \ \mathrm{m} / \mathrm{s}\right)}{656.3 \times 10^{-9} \ \mathrm{m}}=3.03 \times 10^{-19} \ \mathrm{J} \nonumber\]The frequency and the wavelength of electromagnetic radiation vary over many orders of magnitude. For convenience, we divide electromagnetic radiation into different regions—the electromagnetic spectrum—based on the type of atomic or molecular transitions that gives rise to the absorption or emission of photons (Figure 10.1.3
). The boundaries between the regions of the electromagnetic spectrum are not rigid and overlap between spectral regions is possible.In the previous section we defined several characteristic properties of electromagnetic radiation, including its energy, velocity, amplitude, frequency, phase angle, polarization, and direction of propagation. A spectroscopic measurement is possible only if the photon’s interaction with the sample leads to a change in one or more of these characteristic properties.We will divide spectroscopy into two broad classes of techniques. In one class of techniques there is a transfer of energy between the photon and the sample. Table 10.1.1
provides a list of several representative examples.electron spin resonancenuclear magnetic resonancefluorescence spectroscopyphosphorescence spectroscopyatomic fluorescence spectroscopyIn absorption spectroscopy a photon is absorbed by an atom or molecule, which undergoes a transition from a lower-energy state to a higher-energy, or excited state (Figure 10.1.4
). The type of transition depends on the photon’s energy. The electromagnetic spectrum in Figure 10.1.3
, for example, shows that absorbing a photon of visible light promotes one of the atom’s or molecule’s valence electrons to a higher-energy level. When an molecule absorbs infrared radiation, on the other hand, one of its chemical bonds experiences a change in vibrational energy.When it absorbs electromagnetic radiation the number of photons passing through a sample decreases. The measurement of this decrease in photons, which we call absorbance, is a useful analytical signal. Note that each energy level in Figure 10.1.4
has a well-defined value because each is quantized. Absorption occurs only when the photon’s energy, \(h \nu\), matches the difference in energy, \(\Delta E\), between two energy levels. A plot of absorbance as a function of the photon’s energy is called an absorbance spectrum. Figure 10.1.5
, for example, shows the absorbance spectrum of cranberry juice.When an atom or molecule in an excited state returns to a lower energy state, the excess energy often is released as a photon, a process we call emission (Figure 10.1.4
). There are several ways in which an atom or a molecule may end up in an excited state, including thermal energy, absorption of a photon, or as the result of a chemical reaction. Emission following the absorption of a photon is also called photoluminescence, and that following a chemical reaction is called chemiluminescence. A typical emission spectrum is shown in Figure 10.1.6
.Molecules also can release energy in the form of heat. We will return to this point later in the chapter.In the second broad class of spectroscopic techniques, the electromagnetic radiation undergoes a change in amplitude, phase angle, polarization, or direction of propagation as a result of its refraction, reflection, scattering, diffraction, or dispersion by the sample. Several representative spectroscopic techniques are listed in Table 10.1.2
.nephelometryturbidimetryThe spectroscopic techniques in Table 10.1.1
and Table 10.1.2
use instruments that share several common basic components, including a source of energy, a means for isolating a narrow range of wavelengths, a detector for measuring the signal, and a signal processor that displays the signal in a form convenient for the analyst. In this section we introduce these basic components. Specific instrument designs are considered in later sections.You will find a more detailed treatment of these components in the additional resources for this chapter.All forms of spectroscopy require a source of energy. In absorption and scattering spectroscopy this energy is supplied by photons. Emission and photoluminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a suitable excited state.Sources of Electromagnetic Radiation. A source of electromagnetic radiation must provide an output that is both intense and stable. Sources of electromagnetic radiation are classified as either continuum or line sources. A continuum source emits radiation over a broad range of wavelengths, with a relatively smooth variation in intensity (Figure 10.1.7
). A line source, on the other hand, emits radiation at selected wavelengths (Figure 10.1.8
). Table 10.1.3
provides a list of the most common sources of electromagnetic radiation.Sources of Thermal Radiation. The most common sources of thermal energy are flames and plasmas. A flame source uses a combustion of a fuel and an oxidant to achieve temperatures of 2000–3400 K. Plasmas, which are hot, ionized gases, provide temperatures of 6000–10000 K.Chemical Sources of Energy. Exothermic reactions also may serve as a source of energy. In chemiluminescence the analyte is raised to a higher-energy state by means of a chemical reaction, emitting characteristic radiation when it returns to a lower-energy state. When the chemical reaction results from a biological or enzymatic reaction, the emission of radiation is called bioluminescence. Commercially available “light sticks” and the flash of light from a firefly are examples of chemiluminescence and bioluminescence.In Nessler’s original colorimetric method for ammonia, which was described at the beginning of the chapter, the sample and several standard solutions of ammonia are placed in separate tall, flat-bottomed tubes. As shown in Figure 10.1.9
, after adding the reagents and allowing the color to develop, the analyst evaluates the color by passing ambient light through the bottom of the tubes and looking down through the solutions. By matching the sample’s color to that of a standard, the analyst is able to determine the concentration of ammonia in the sample.In Figure 10.1.9
every wavelength of light from the source passes through the sample. This is not a problem if there is only one absorbing species in the sample. If the sample contains two components, then a quantitative analysis using Nessler’s original method is impossible unless the standards contains the second component at the same concentration it has in the sample.To overcome this problem, we want to select a wavelength that only the analyte absorbs. Unfortunately, we can not isolate a single wavelength of radiation from a continuum source, although we can narrow the range of wavelengths that reach the sample. As seen in Figure 10.1.10
, a wavelength selector always passes a narrow band of radiation characterized by a nominal wavelength, an effective bandwidth, and a maximum throughput of radiation. The effective bandwidth is defined as the width of the radiation at half of its maximum throughput.The ideal wavelength selector has a high throughput of radiation and a narrow effective bandwidth. A high throughput is desirable because the more photons that pass through the wavelength selector, the stronger the signal and the smaller the background noise. A narrow effective bandwidth provides a higher resolution, with spectral features separated by more than twice the effective bandwidth being resolved. As shown in Figure 10.1.11
, these two features of a wavelength selector often are in opposition. A larger effective bandwidth favors a higher throughput of radiation, but provide less resolution. Decreasing the effective bandwidth improves resolution, but at the cost of a noisier signal [Jiang, S.; Parker, G. A. Am. Lab. 1981, October, 38–43]. For a qualitative analysis, resolution usually is more important than noise and a smaller effective bandwidth is desirable; however, in a quantitative analysis less noise usually is desirable.Wavelength Selection Using Filters. The simplest method for isolating a narrow band of radiation is to use an absorption or interference filter. Absorption filters work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum. Interference filters use constructive and destructive interference to isolate a narrow range of wavelengths. A simple example of an absorption filter is a piece of colored glass. A purple filter, for example, removes the complementary color green from 500–560 nm.Commercially available absorption filters provide effective bandwidths of 30–250 nm, although the throughput at the low end of this range often is only 10% of the source’s emission intensity. Interference filters are more expensive than absorption filters, but have narrower effective bandwidths, typically 10–20 nm, with maximum throughputs of at least 40%.Wavelength Selection Using Monochromators. A filter has one significant limitation—because a filter has a fixed nominal wavelength, if we need to make measurements at two different wavelengths, then we must use two different filters. A monochromator is an alternative method for selecting a narrow band of radiation that also allows us to continuously adjust the band’s nominal wavelength.The construction of a typical monochromator is shown in Figure 10.1.12
. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with a large number of parallel grooves (see insert to Figure 10.1.12
). The diffraction grating disperses the radiation and a second mirror focuses the radiation onto a planar surface that contains an exit slit. In some monochromators a prism is used in place of the diffraction grating.Radiation exits the monochromator and passes to the detector. As shown in Figure 10.1.12
, a monochromator converts a polychromatic source of radiation at the entrance slit to a monochromatic source of finite effective bandwidth at the exit slit. The choice of which wavelength exits the monochromator is determined by rotating the diffraction grating. A narrower exit slit provides a smaller effective bandwidth and better resolution than does a wider exit slit, but at the cost of a smaller throughput of radiation.Polychromatic means many colored. Polychromatic radiation contains many different wavelengths of light. Monochromatic means one color, or one wavelength. Although the light exiting a monochromator is not strictly of a single wavelength, its narrow effective bandwidth allows us to think of it as monochromatic.Monochromators are classified as either fixed-wavelength or scanning. In a fixed-wavelength monochromator we manually select the wavelength by rotating the grating. Normally a fixed-wavelength monochromator is used for a quantitative analysis where measurements are made at one or two wavelengths. A scanning monochromator includes a drive mechanism that continuously rotates the grating, which allows successive wavelengths of light to exit from the monochromator. A scanning monochromator is used to acquire spectra, and, when operated in a fixed-wavelength mode, for a quantitative analysis.Interferometers. An interferometer provides an alternative approach for wavelength selection. Instead of filtering or dispersing the electromagnetic radiation, an interferometer allows source radiation of all wavelengths to reach the detector simultaneously (Figure 10.1.13
). Radiation from the source is focused on a beam splitter that reflects half of the radiation to a fixed mirror and transmits the other half to a moving mirror. The radiation recombines at the beam splitter, where constructive and destructive interference determines, for each wavelength, the intensity of light that reaches the detector. As the moving mirror changes position, the wavelength of light that experiences maximum constructive interference and maximum destructive interference also changes. The signal at the detector shows intensity as a function of the moving mirror’s position, expressed in units of distance or time. The result is called an interferogram or a time domain spectrum. The time domain spectrum is converted mathematically, by a process called a Fourier transform, to a spectrum (a frequency domain spectrum) that shows intensity as a function of the radiation’s energy.The mathematical details of the Fourier transform are beyond the level of this textbook. You can consult the chapter’s additional resources for additional information.In comparison to a monochromator, an interferometer has two significant advantages. The first advantage, which is termed Jacquinot’s advantage, is the greater throughput of source radiation. Because an interferometer does not use slits and has fewer optical components from which radiation is scattered and lost, the throughput of radiation reaching the detector is \(80-200 \times\) greater than that for a monochromator. The result is less noise. The second advantage, which is called Fellgett’s advantage, is a savings in the time needed to obtain a spectrum. Because the detector monitors all frequencies simultaneously, a spectrum takes approximately one second to record, as compared to 10–15 minutes when using a scanning monochromator.In Nessler’s original method for determining ammonia (Figure 10.1.9
) the analyst’s eye serves as the detector, matching the sample’s color to that of a standard. The human eye, of course, has a poor range—it responds only to visible light—and it is not particularly sensitive or accurate. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector’s signal, S, is a linear function of the electromagnetic radiation’s power, P,\[S=k P+D \nonumber\]where k is the detector’s sensitivity, and D is the detector’s dark current, or the background current when we prevent the source’s radiation from reaching the detector.There are two broad classes of spectroscopic transducers: thermal transducers and photon transducers. Table 10.1.4
provides several representative examples of each class of transducers.Transducer is a general term that refers to any device that converts a chemical or a physical property into an easily measured electrical signal. The retina in your eye, for example, is a transducer that converts photons into an electrical nerve impulse; your eardrum is a transducer that converts sound waves into a different electrical nerve impulse.Photon Transducers. Phototubes and photomultipliers use a photosensitive surface that absorbs radiation in the ultraviolet, visible, or near IR to produce an electrical current that is proportional to the number of photons reaching the transducer (Figure 10.1.14
). Other photon detectors use a semiconductor as the photosensitive surface. When the semiconductor absorbs photons, valence electrons move to the semiconductor’s conduction band, producing a measurable current. One advantage of the Si photodiode is that it is easy to miniaturize. Groups of photodiodes are gathered together in a linear array that contains 64–4096 individual photodiodes. With a width of 25 μm per diode, a linear array of 2048 photodiodes requires only 51.2 mm of linear space. By placing a photodiode array along the monochromator’s focal plane, it is possible to monitor simultaneously an entire range of wavelengths.Thermal Transducers. Infrared photons do not have enough energy to produce a measurable current with a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons increases a thermal transducer’s temperature, changing one or more of its characteristic properties. A pneumatic transducer, for example, is a small tube of xenon gas with an IR transparent window at one end and a flexible membrane at the other end. Photons enter the tube and are absorbed by a blackened surface, increasing the temperature of the gas. As the temperature inside the tube fluctuates, the gas expands and contracts and the flexible membrane moves in and out. Monitoring the membrane’s displacement produces an electrical signal.A transducer’s electrical signal is sent to a signal processor where it is displayed in a form that is more convenient for the analyst. Examples of signal processors include analog or digital meters, recorders, and computers equipped with digital acquisition boards. A signal processor also is used to calibrate the detector’s response, to amplify the transducer’s signal, to remove noise by filtering, or to mathematically transform the signal.If the retina in your eye and the eardrum in your ear are transducers, then your brain is the signal processor.This page titled 10.1: Overview of Spectroscopy is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey. | 47 |
10.2: Spectroscopy Based on Absorption
| https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/10%3A_Spectroscopic_Methods/10.02%3A_Spectroscopy_Based_on_Absorption | In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation passes through the sample without a loss in intensity. At selected wavelengths, however, the radiation’s intensity is attenuated. This process of attenuation is called absorption.There are two general requirements for an analyte’s absorption of electromagnetic radiation. First, there must be a mechanism by which the radiation’s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, absorption of a photon changes the energy of the analyte’s valence electrons. A bond’s vibrational energy is altered by the absorption of infrared radiation., must exactly equal the difference in energy, \(\Delta E\), between two of the analyte’s quantized energy states. Figure 10.1.4 shows a simplified view of a photon’s absorption, which is useful because it emphasizes that the photon’s energy must match the difference in energy between a lower-energy state and a higher-energy state. What is missing, however, is information about what types of energy states are involved, which transitions between energy states are likely to occur, and the appearance of the resulting spectrum.We can use the energy level diagram in Figure 10.2.1
to explain an absorbance spectrum. The lines labeled E0 and E1 represent the analyte’s ground (lowest) electronic state and its first electronic excited state. Superimposed on each electronic energy level is a series of lines representing vibrational energy levels.The energy of infrared radiation produces a change in a molecule’s or a polyatomic ion’s vibrational energy, but is not sufficient to effect a change in its electronic energy. As shown in Figure 10.2.1
, vibrational energy levels are quantized; that is, a molecule or polyatomic ion has only certain, discrete vibrational energies. The energy for an allowed vibrational mode, \(E_{\nu}\), is\[E_{\nu}=\nu+\frac{1}{2} h \nu_{0} \nonumber\]where \(\nu\) is the vibrational quantum number, which has values of 0, 1, 2, ..., and \(\nu_0\) is the bond’s fundamental vibrational frequency. The value of \(\nu_0\), which is determined by the bond’s strength and by the mass at each end of the bond, is a characteristic property of a bond. For example, a carbon-carbon single bond (C–C) absorbs infrared radiation at a lower energy than a carbon-carbon double bond (C=C) because a single bond is weaker than a double bond.At room temperature most molecules are in their ground vibrational state (\(\nu = 0\)) . A transition from the ground vibrational state to the first vibrational excited state (\(\nu = 1\)) requires absorption of a photon with an energy of \(h \nu_0\). Transitions in which \(\Delta \nu = \pm 1\) give rise to the fundamental absorption lines. Weaker absorption lines, called overtones, result from transitions in which \(\Delta \nu\) is ±2 or ±3. The number of possible normal vibrational modes for a linear molecule is 3N – 5, and for a non-linear molecule is 3N – 6, where N is the number of atoms in the molecule. Not surprisingly, infrared spectra often show a considerable number of absorption bands. Even a relatively simple molecule, such as ethanol (C2H6O), for example, has \(3 \times 9 - 6\), or 21 possible normal modes of vibration, although not all of these vibrational modes give rise to an absorption. The IR spectrum for ethanol is shown in Figure 10.2.2
.Why does a non-linear molecule have 3N – 6 vibrational modes? Consider a molecule of methane, CH4. Each of methane’s five atoms can move in one of three directions (x, y, and z) for a total of \(5 \times 3 = 15\) different ways in which the molecule’s atoms can move. A molecule can move in three ways: it can move from one place to another, which we call translational motion; it can rotate around an axis, which we call rotational motion; and its bonds can stretch and bend, which we call vibrational motion. Because the entire molecule can move in the x, y, and z directions, three of methane’s 15 different motions are translational. In addition, the molecule can rotate about its x, y, and z axes, accounting for three additional forms of motion. This leaves 15 – 3 – 3 = 9 vibrational modes. A linear molecule, such as CO2, has 3N – 5 vibrational modes because it can rotate around only two axes.The valence electrons in organic molecules and polyatomic ions, such as \(\text{CO}_3^{2-}\), occupy quantized sigma bonding (\(\sigma\)), pi bonding (\(\pi\)), and non-bonding (n) molecular orbitals (MOs). Unoccupied sigma antibonding (\(\sigma^*\)) and pi antibonding (\(\pi^*\)) molecular orbitals are slightly higher in energy. Because the difference in energy between the highest-energy occupied MOs and the lowest-energy unoccupied MOs corresponds to ultraviolet and visible radiation, absorption of a photon is possible.Four types of transitions between quantized energy levels account for most molecular UV/Vis spectra. Table 10.2.1
lists the approximate wavelength ranges for these transitions, as well as a partial list of bonds, functional groups, or molecules responsible for these transitions. Of these transitions, the most important are \(n \rightarrow \pi^*\) and \(\pi \rightarrow \pi^*\) because they involve important functional groups that are characteristic of many analytes and because the wavelengths are easily accessible. The bonds and functional groups that give rise to the absorption of ultraviolet and visible radiation are called chromophores.Many transition metal ions, such as Cu2+ and Co2+, form colorful solutions because the metal ion absorbs visible light. The transitions that give rise to this absorption are valence electrons in the metal ion’s d-orbitals. For a free metal ion, the five d-orbitals are of equal energy. In the presence of a complexing ligand or solvent molecule, however, the d-orbitals split into two or more groups that differ in energy. For example, in an octahedral complex of \(\text{Cu(H}_2\text{O)}_6^{2+}\) the six water molecules perturb the d-orbitals into the two groups shown in Figure 10.2.3
. The resulting \(d \rightarrow d\) transitions for transition metal ions are relatively weak.A more important source of UV/Vis absorption for inorganic metal–ligand complexes is charge transfer, in which absorption of a photon produces an excited state in which there is transfer of an electron from the metal, M, to the ligand, L.\[M-L+h \nu \rightarrow\left(M^{+}-L^{-}\right)^{*} \nonumber\]Charge-transfer absorption is important because it produces very large absorbances. One important example of a charge-transfer complex is that of o-phenanthroline with Fe2+, the UV/Vis spectrum for which is shown in Figure 10.2.4
. Charge-transfer absorption in which an electron moves from the ligand to the metal also is possible.Why is a larger absorbance desirable? An analytical method is more sensitive if a smaller concentration of analyte gives a larger signal.Comparing the IR spectrum in Figure 10.2.2
to the UV/Vis spectrum in Figure 10.2.4
shows us that UV/Vis absorption bands are often significantly broader than those for IR absorption. We can use Figure 10.2.1
to explain why this is true. When a species absorbs UV/Vis radiation, the transition between electronic energy levels may also include a transition between vibrational energy levels. The result is a number of closely spaced absorption bands that merge together to form a single broad absorption band.The energy of ultraviolet and visible electromagnetic radiation is sufficient to cause a change in an atom’s valence electron configuration. Sodium, for example, has a single valence electron in its 3s atomic orbital. As shown in Figure 10.2.5
, unoccupied, higher energy atomic orbitals also exist.The valence shell energy level diagram in Figure 10.2.5
might strike you as odd because it shows that the 3p orbitals are split into two groups of slightly different energy. The reasons for this splitting are unimportant in the context of our treatment of atomic absorption. For further information about the reasons for this splitting, consult the chapter’s additional resources.Absorption of a photon is accompanied by the excitation of an electron from a lower-energy atomic orbital to an atomic orbital of higher energy. Not all possible transitions between atomic orbitals are allowed. For sodium the only allowed transitions are those in which there is a change of ±1 in the orbital quantum number (l); thus transitions from \(s \rightarrow p\) orbitals are allowed, but transitions from \(s \rightarrow s\) and from \(s \rightarrow d\) orbitals are forbidden.The atomic absorption spectrum for Na is shown in Figure 10.2.6
, and is typical of that found for most atoms. The most obvious feature of this spectrum is that it consists of a small number of discrete absorption lines that correspond to transitions between the ground state (the 3s atomic orbital) and the 3p and the 4p atomic orbitals. Absorption from excited states, such as the \(3p \rightarrow 4s\) and the \(3p \rightarrow 3d\) transitions included in Figure 10.2.5
, are too weak to detect. Because an excited state’s lifetime is short—an excited state atom typically returns to a lower energy state in 10–7 to 10–8 seconds—an atom in the exited state is likely to return to the ground state before it has an opportunity to absorb a photon.Another feature of the atomic absorption spectrum in Figure 10.2.6
is the narrow width of the absorption lines, which is a consequence of the fixed difference in energy between the ground state and the excited state, and the lack of vibrational and rotational energy levels. Natural line widths for atomic absorption, which are governed by the uncertainty principle, are approximately 10–5 nm. Other contributions to broadening increase this line width to approximately 10–3 nm.As light passes through a sample, its power decreases as some of it is absorbed. This attenuation of radiation is described quantitatively by two separate, but related terms: transmittance and absorbance. As shown in Figure 10.2.7
a, transmittance is the ratio of the source radiation’s power as it exits the sample, PT, to that incident on the sample, P0.\[T=\frac{P_{\mathrm{T}}}{P_{0}} \label{10.1}\]Multiplying the transmittance by 100 gives the percent transmittance, %T, which varies between 100% (no absorption) and 0% (complete absorption). All methods of detecting photons—including the human eye and modern photoelectric transducers—measure the transmittance of electromagnetic radiation.Equation \ref{10.1} does not distinguish between different mechanisms that prevent a photon emitted by the source from reaching the detector. In addition to absorption by the analyte, several additional phenomena contribute to the attenuation of radiation, including reflection and absorption by the sample’s container, absorption by other components in the sample’s matrix, and the scattering of radiation. To compensate for this loss of the radiation’s power, we use a method blank. As shown in Figure 10.2.7
b, we redefine P0 as the power exiting the method blank.An alternative method for expressing the attenuation of electromagnetic radiation is absorbance, A, which we define as\[A=-\log T=-\log \frac{P_{\mathrm{T}}}{P_{0}} \label{10.2}\]Absorbance is the more common unit for expressing the attenuation of radiation because it is a linear function of the analyte’s concentration.We will show that this is true in the next section when we introduce Beer’s law.A sample has a percent transmittance of 50%. What is its absorbance?SolutionA percent transmittance of 50.0% is the same as a transmittance of 0.500. Substituting into Equation \ref{10.2} gives\[A=-\log T=-\log (0.500)=0.301 \nonumber\]What is the %T for a sample if its absorbance is 1.27?To find the transmittance, T, we begin by noting that\[A=1.27=-\log T \nonumber\]Solving for T \[\begin{array}{c}{-1.27=\log T} \\ {10^{-1.27}=T}\end{array} \nonumber\]gives a transmittance of 0.054, or a %T of 5.4%.Equation \ref{10.1} has an important consequence for atomic absorption. As we learned from Figure 10.2.6
, atomic absorption lines are very narrow. Even with a high quality monochromator, the effective bandwidth for a continuum source is \(100-1000 \times\) greater than the width of an atomic absorption line. As a result, little radiation from a continuum source is absorbed when it passes through a sample of atoms; because P0 ≈ PT the measured absorbance effectively is zero. For this reason, atomic absorption requires that we use a line source instead of a continuum source.When monochromatic electromagnetic radiation passes through an infinitesimally thin layer of sample of thickness dx, it experiences a decrease in its power of dP (Figure 10.2.8
).This fractional decrease in power is proportional to the sample’s thickness and to the analyte’s concentration, C; thus\[-\frac{d P}{P}=\alpha C d x \label{10.3}\]where P is the power incident on the thin layer of sample and \(\alpha\) is a proportionality constant. Integrating the left side of Equation \ref{10.3} over the sample’s full thickness\[-\int_{P=P_0}^{P=P_t} \frac{d P}{P}=\alpha C \int_{x=0}^{x=b} d x \nonumber\]\[\ln \frac{P_{0}}{P_T}=\alpha b C \nonumber\]converting from ln to log, and substituting into Equation \ref{10.2}, gives\[A=a b C \label{10.4}\]where a is the analyte’s absorptivity with units of cm–1 conc–1. If we express the concentration using molarity, then we replace a with the molar absorptivity, \(\varepsilon\), which has units of cm–1 M–1.\[A=\varepsilon b C \label{10.5}\]The absorptivity and the molar absorptivity are proportional to the probability that the analyte absorbs a photon of a given energy. As a result, values for both a and \(\varepsilon\) depend on the wavelength of the absorbed photon.A \(5.00 \times 10^{-4}\) M solution of analyte is placed in a sample cell that has a pathlength of 1.00 cm. At a wavelength of 490 nm, the solution’s absorbance is 0.338. What is the analyte’s molar absorptivity at this wavelength?SolutionSolving Equation \ref{10.5} for \(\epsilon\) and making appropriate substitutions gives\[\varepsilon=\frac{A}{b C}=\frac{0.338}{(1.00 \ \mathrm{cm})\left(5.00 \times 10^{-4} \ \mathrm{M}\right)}=676 \ \mathrm{cm}^{-1} \ \mathrm{M}^{-1} \nonumber\]A solution of the analyte from Example 10.2.2
has an absorbance of 0.228 in a 1.00-cm sample cell. What is the analyte’s concentration?Making appropriate substitutions into Beer’s law\[A=0.228=\varepsilon b C=\left(676 \ \mathrm{M}^{-1} \ \mathrm{cm}^{-1}\right)(1 \ \mathrm{cm}) C \nonumber\]and solving for C gives a concentration of \(3.37 \times 10^{-4}\) M.Equation \ref{10.4} and Equation \ref{10.5}, which establish the linear relationship between absorbance and concentration, are known as Beer’s law. Calibration curves based on Beer’s law are common in quantitative analyses.As is often the case, the formulation of a law is more complicated than its name suggests. This is the case, for example, with Beer’s law, which also is known as the Beer-Lambert law or the Beer-Lambert-Bouguer law. Pierre Bouguer, in 1729, and Johann Lambert, in 1760, noted that the transmittance of light decreases exponentially with an increase in the sample’s thickness.\[T \propto e^{-b} \nonumber\]Later, in 1852, August Beer noted that the transmittance of light decreases exponentially as the concentration of the absorbing species increases.\[T \propto e^{-C} \nonumber\]Together, and when written in terms of absorbance instead of transmittance, these two relationships make up what we know as Beer’s law.We can extend Beer’s law to a sample that contains several absorbing components. If there are no interactions between the components, then the individual absorbances, Ai, are additive. For a two-component mixture of analyte’s X and Y, the total absorbance, Atot, is\[A_{tot}=A_{X}+A_{Y}=\varepsilon_{X} b C_{X}+\varepsilon_{Y} b C_{Y} \nonumber\]Generalizing, the absorbance for a mixture of n components, Amix, is\[A_{m i x}=\sum_{i=1}^{n} A_{i}=\sum_{i=1}^{n} \varepsilon_{i} b C_{i} \label{10.6}\]Beer’s law suggests that a plot of absorbance vs. concentration—we will call this a Beer’s law plot—is a straight line with a y-intercept of zero and a slope of ab or \(\varepsilon b\). In some cases a Beer’s law plot deviates from this ideal behavior (see Figure 10.2.9
), and such deviations from linearity are divided into three categories: fundamental, chemical, and instrumental.Beer’s law is a limiting law that is valid only for low concentrations of analyte. There are two contributions to this fundamental limitation to Beer’s law. At higher concentrations the individual particles of analyte no longer are independent of each other. The resulting interaction between particles of analyte may change the analyte’s absorptivity. A second contribution is that an analyte’s absorptivity depends on the solution’s refractive index. Because a solution’s refractive index varies with the analyte’s concentration, values of a and \(\varepsilon\) may change. For sufficiently low concentrations of analyte, the refractive index essentially is constant and a Beer’s law plot is linear.A chemical deviation from Beer’s law may occur if the analyte is involved in an equilibrium reaction. Consider, for example, the weak acid, HA. To construct a Beer’s law plot we prepare a series of standard solutions—each of which contains a known total concentration of HA—and then measure each solution’s absorbance at the same wavelength. Because HA is a weak acid, it is in equilibrium with its conjugate weak base, A–.In the equations that follow, the conjugate weak base A– is written as A as it is easy to mistake the symbol for anionic charge as a minus sign.\[\mathrm{HA}(a q)+\mathrm{H}_{2} \mathrm{O}(l)\rightleftharpoons\mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{A}^{-}(a q) \nonumber\]If both HA and A– absorb at the selected wavelength, then Beer’s law is\[A=\varepsilon_{\mathrm{HA}} b C_{\mathrm{HA}}+\varepsilon_{\mathrm{A}} b C_{\mathrm{A}} \label{10.7}\]Because the weak acid’s total concentration, Ctotal, is\[C_{\mathrm{total}}=C_{\mathrm{HA}}+C_{\mathrm{A}} \nonumber\]we can write the concentrations of HA and A– as\[C_{\mathrm{HA}}=\alpha_{\mathrm{HA}} C_{\mathrm{total}} \label{10.8}\]\[C_{\text{A}} = (1 - \alpha_\text{HA})C_\text{total} \label{10.9}\]where \(\alpha_\text{HA}\) is the fraction of weak acid present as HA. Substituting Equation \ref{10.8} and Equation \ref{10.9} into Equation \ref{10.7} and rearranging, gives\[A=\left(\varepsilon_{\mathrm{HA}} \alpha_{\mathrm{HA}}+\varepsilon_{\mathrm{A}}-\varepsilon_{\mathrm{A}} \alpha_{\mathrm{A}}\right) b C_{\mathrm{total}} \label{10.10}\]To obtain a linear Beer’s law plot, we must satisfy one of two conditions. If \(\varepsilon_\text{HA}\) and \(\varepsilon_{\text{A}}\) have the same value at the selected wavelength, then Equation \ref{10.10} simplifies to\[A = \varepsilon_{\text{A}}bC_\text{total} = \varepsilon_\text{HA}bC_\text{total} \nonumber\]Alternatively, if \(\alpha_\text{HA}\) has the same value for all standard solutions, then each term within the parentheses of Equation \ref{10.10} is constant—which we replace with k—and a linear calibration curve is obtained at any wavelength.\[A=k b C_{\mathrm{total}} \nonumber\]Because HA is a weak acid, the value of \(\alpha_\text{HA}\) varies with pH. To hold \(\alpha_\text{HA}\) constant we buffer each standard solution to the same pH. Depending on the relative values of \(\alpha_\text{HA}\) and \(\alpha_{\text{A}}\), the calibration curve has a positive or a negative deviation from Beer’s law if we do not buffer the standards to the same pH.There are two principal instrumental limitations to Beer’s law. The first limitation is that Beer’s law assumes that radiation reaching the sample is of a single wavelength—that is, it assumes a purely monochromatic source of radiation. As shown in essentially is constant over the wavelength range passed by the wavelength selector. For this reason, as shown in Figure 10.2.10
, it is better to make absorbance measurements at the top of a broad absorption peak. In addition, the deviation from Beer’s law is less serious if the source’s effective bandwidth is less than one-tenth of the absorbing species’ natural bandwidth [(a) Strong, F. C., III Anal. Chem. 1984, 56, 16A–34A; Gilbert, D. D. J. Chem. Educ. 1991, 68, A278–A281]. When measurements must be made on a slope, linearity is improved by using a narrower effective bandwidth.Stray radiation is the second contribution to instrumental deviations from Beer’s law. Stray radiation arises from imperfections in the wavelength selector that allow light to enter the instrument and to reach the detector without passing through the sample. Stray radiation adds an additional contribution, Pstray, to the radiant power that reaches the detector; thus\[A=-\log \frac{P_{\mathrm{T}}+P_{\text { stray }}}{P_{0}+P_{\text { stray }}} \nonumber\]For a small concentration of analyte, Pstray is significantly smaller than P0 and PT, and the absorbance is unaffected by the stray radiation. For higher concentrations of analyte, less light passes through the sample and PT and Pstray become similar in magnitude. This results is an absorbance that is smaller than expected, and a negative deviation from Beer’s law.This page titled 10.2: Spectroscopy Based on Absorption is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey. | 48 |
End of preview. Expand
in Dataset Viewer.
- Downloads last month
- 1