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Gas Chromatography
Rosa Yu, David Reckhow
CEE772 Instrumental Methods in Environmental Analysis

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Contents• The primary components to a GC system
1. Carrier Gas System (including Gas Clean Filters)• The concept of theoretical plates and van Deemter curves• Selection of proper carrier gas

2. Sample Introduction System• Split & splitless injection

3. Column (most critical component)• Column configurations: packed vs. open tubular/capillary• Stationary phase

4. Detection System/GC Detectors• Types of detectors and their specific applications

5. Computer ChemStation/Integrator
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The Basic Components to a GC SystemInjection PortDetector
Gas Clean FilterCarrier
GasGC Column

Integrator
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Contents• The primary components to a GC system
1. Carrier Gas System (including Gas Clean Filters)• The concept of theoretical plates and van Deemter curves• Selection of proper carrier gas

2. Sample Introduction System• Split & splitless injection

3. Column (most critical component)• Column configurations: packed vs. open tubular/capillary• Stationary phase

4. Detection System/GC Detectors• Types of detectors and their specific applications

5. Computer ChemStation/Integrator
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I. Carrier Gas System• Type of carrier gas effect oncolumn efficiency and resolution
 H2/He/N2• Selection of carrier gas linear
velocity/column flow rate

Pressurized
cylinder/Gasgenerator• Gas clean filter
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Type of Carrier Gas Effect on Column Efficiency
and Resolution

Selection of carrier gas:
H2 > He > N2 (> Argon)
H2 should be applied with safety
precautions
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Optimizing Linear Velocity/Flow Rate for High
Column Efficiency

van Deemter
PlotCEE 772 #167

Optimizing Linear Velocity/Flow Rate for HighColumn Efficiencyvan Deemter
PlotCEE 772 #16

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Gas Clean Filter• Significant damages can bedone to the column if it isheated above 70℃ with eventrace amounts of O2 in thecolumn• Use carrier gas that meets the99.9995% specification (UHPgrade)• Use O2 & moisture traps

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Contents• The primary components to a GC system
1. Carrier Gas System (including Gas Clean Filters)• The concept of theoretical plates and van Deemter curves• Selection of proper carrier gas

2. Sample Introduction System• Split & splitless injection

3. Column (most critical component)• Column configurations: packed vs. open tubular/capillary• Stationary phase

4. Detection System/GC Detectors• Types of detectors and their specific applications

5. Computer ChemStation/Integrator
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II. Sample Introduction• The sample must be of a suitable size (especially forWCOT/capillary columns) and introduced
instantaneously as a PLUG OF VAPOR
Slow injection/oversize causes peak broadening andpoor resolution

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Types of GC samples and injection methods
LiquidGas

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Solid12

Auto Sampler

•
•
•
•

Automation
Up to 150 samples
Instantaneous injection
Same amount ofsample injected everytime

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Injection PortPacked Column Injector
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Split vs. Splitless Injection• Splitless injection
For example, a liquid sample is injected into the port, it is quickly
volatilized at the end of the microsyringe and at the head of the column;
the solutes are then taken by the carrier gas into the column• Split injection
Open tubular/capillary columns usually have a much smaller crosssection area than that of packed columns. This makes them more
subject to extra-column band-broadening, requiring that special low
volume injection techniques be used with them.
plug of solutes

Volumeof injector

Packed bed

open tubular
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Sample Splitter• The sample is first placed
into the injection port
and is vaporized• As the sample leaves theinject port, only a small
portion of the vaporized
samples is applied to thecolumn (usually 1/50 to1/500), with theremainder going towaste (by opening thepurge valve/split vent).

Glass frit/ wool

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* Cold On-Column Injector• Cold on-column injectors
involves direct injection of asample onto a column atlow temperature.
• No heated injection port isused. The low initial column
temperature increases theretention of all solutes andconcentrates them at the
top of the column in anarrow plug. The column
temperature is thenincreased, allowing thesolutes to volatilize and beseparated.
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*Programmed temperature vaporizer (PTV)•A programmed temperature
vaporize involves placing sample
into a cold injection port, where itis then heated and applied tocolumn at any desired
temperature.

•A“universal” injector for opentubular columns since it
temperature program may bechanged so that it can be usedeither in cold injectors, splitless
injectors, or split injectors.

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Contents• The primary components to a GC system
1. Carrier Gas System (including Gas Clean Filters)• The concept of theoretical plates and van Deemter curves• Selection of proper carrier gas

2. Sample Introduction System• Split & splitless injection

3. Column (most critical component)• Column configurations: packed vs. open tubular/capillary• Stationary phase

4. Detection System/GC Detectors• Types of detectors and their specific applications

5. Computer ChemStation/Integrator
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III. GC Column (heart of a GC system)• Column configurations: packed vs. open tubular/capillarycolumns• Stationary phase• Film thickness and column efficiency

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The Heart of GC

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The Heart of GC

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1. Types of GC columns (GLC)

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1. Types of GC columns (GLC)

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Packed GC Columns

Stationary phase• Less ubiquitous application: fixed gas analysis• Lower column efficiency than that of capillary columns(smaller in length)• Larger sample capacity
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Particle Size of Supports

The efficiency of a gas chromatographic column increases rapidly
with decreasing particle diameter of the packing. The pressure
difference (head loss) required to maintain an acceptable flowrate of carrier gas, however, varies inversely as the square of theparticle diameter; the latter relationship has placed lower limits
on the size of particles used in GC because it is not convenient to usepressure differences that are greater than 50 psi.
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Open Tubular/Capillary GC Columns

Stationary Phase• Most widely used• High column efficiency (large number of theoretical plates
due to long column length, up to 100 m)• Small sample capacity (splitCEEsample
inside inlet)
772 #1627

Open Tubular/Capillary GC Columns
Fused silica--pure form of glassthat is very inert but fragile
Polyamide--provides
great mechanicalstrength and flexibility

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Open Tubular/Capillary Columns

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Packed vs. Capillary

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Packed vs. Capillary

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2. Stationary Phase
Important Attributes
1.
2.
3.
4.

Low volatility (boiling point at least 100 ℃ higher than max. column
operating temperature)
Thermo stability (wide temperature operating range)
Chemical inertness (non-reactive to both solutes and carrier gas)
Solvent characteristics (differential solvent for different components)

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Qualitative Guidelines for Stationary Phase
Selection• Sources:
Literature review, Internet search, prior experience, advice from a vendor
of chromatographic equipment and supplies• General rule: “like dissolves like”• “Like” refers to the POLARITIES of the analyte and the immobilizedliquid• The polarity of a molecule, as indicated by its dipole moment, is ameasure of the electric field produced by separation of charge within
the molecule• Polar functional groups: -CN, -CO, -OH, -COOH, -NH2, -CHO, -X, etc.
Nonpolar function groups: saturated alkane –CH, etc.

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Types and Polarities of Stationary PhaseOPolarity

C 3HS iC 3H
1 0"X"- 1
Polydimethyl siloxane backbone
CH3OSiO

Si

CH395%

5%"X"- 5
Phenyl substitution of methylgroups
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Types and Polarities of Stationary Phase

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3. Effect of Film Thickness on Column Efficiency

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3. Effect of Film Thickness on Column Efficiency

Peak Fronting

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Packed vs. Capillary

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Contents• The primary components to a GC system
1. Carrier Gas System (including Gas Clean Filters)• The concept of theoretical plates and van Deemter curves• Selection of proper carrier gas

2. Sample Introduction System• Split & splitless injection

3. Column (most critical component)• Column configurations: packed vs. open tubular/capillary• Stationary phase

4. Detection System/GC Detectors• Types of detectors and their specific applications

5. Computer ChemStation/Integrator
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IV. GC Detection Systems/Detectors

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Characteristics of the Ideal Detector
1.

Adequate sensitivity (application specific, i.e. adequate for certaintasks)

S/N >3
1.
2.
3.
4.
5.
6.
7.

Good stability and reproducibility
A linear response to solutes that extends over several orders ofmagnitude (calibration purposes)
A wide temperature range
A short response time independent of flow rate
High reliability and ease of use (unfortunately, usually not the case )
Similarity in response toward all solutes/most classes of solutes
The detector should be nondestructive
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Typical GC Detectors

Universaldetectors

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1. Flame Ionization Detector (FID)• Most common detector for GC• In an FID, effluent from the column isdirected into a small air-hydrogen flame.
Most carbon atoms (except C=O) produce
radicals (CHO+) in the flame:
CH + O→ CHO+ + e• Electrons are used to neutralize the CHO+atoms and the ions are collected at anelectrode to create a current to bemeasured. This current is proportional to
the number of molecules present.
• The ionization of carbon compounds inthe FID is not fully understood, although
the number of ions produced is roughly
proportional to the number of reduced
carbon atoms in the flame.
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Pros and Cons of FID• Advantages:
1.
2.
3.
4.
5.
6.

universal detector for organics
does not respond to common
inorganic compounds
mobile phase impurities notdetected
carrier gases not detected
limit of detection: FID is 1000x
better than TCD
linear and dynamic range better
than TCD• Disadvantage:
destructive detector
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2. Thermal Conductivity Detector (TCD)
Temp.-sensitiveelement• One of the earliest detectors of GC• The device contains an electricallyheated source whose temperature atconstant electrical power depends on
the thermal conductivity of thesurrounding gas.
• Twin detectors are usually used, one
being located ahead of the sampleinjection chamber and the otherimmediately beyond the column. Thebridge (Wheatstone bridge) circuit isarranged so that the thermal
conductivity of the carrier gas iscanceled.
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2. Thermal Conductivity Detector (TCD)

Twin detectors• The thermal conductivities of helium
and hydrogen (commonly used carriergases for TCD) are roughly 6~10 times
greater than those of most organic
compounds. Thus, even small amountsof organic species cause relatively
large decreases in the thermal
conductivity of the column effluent,which results in a marked rise in thetemperature of the detector.
• Advantages:
Simplicity, large linear dynamicrange, nondestructive• Disadvantages:
Low sensitivity (precludes their use
with WCOT columns with small
amounts of sample)
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3. Electron Capture Detector (ECD)• Radioactive decay-based detector• Selective for compounds containing electronegative atoms, such as halogens,peroxides, quinones, and nitro groups• The sample effluent from a column is passed over a radioactive β emitter,
usually 63Ni. An electron from the emitter causes ionization of the carrier gas(often N2) and the production of a burst of electrons.
• In the absence of organic species, a constant standing current between a pairof electrode results from this ionization process. The current decreases
significantly in the presence of organic molecules containing electron negative
functional groups that tend to capture electrons.
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3. Electron Capture Detector (ECD)•••

Ionization of carrier gases:
N2 + β- → N2+ + eAr2 + β- → Ar2+ + eAdvantages:
 useful for environmental testing
detection of chlorinated pesticides or herbicides; polynuclear
aromatic carcinogens, organometallic compounds
 selective for halogen- (I, Br, Cl, F), nitro-, and sulfur-containing
compounds
 detects polynuclear aromatic compounds, anhydrides andconjugated carbonyl compounds
Disadvantages:
 could be affected by theCEE
flow
rate
772 #1648

4. Thermionic Detector/Nitrogen-Phosphorous
Detector (NPD)• A NPD is based on the same basic
principles as an FID.
• However, a small amount of alkali
metal vapor in the flame, which
greatly enhances the formation ofions from nitrogen and phosphoruscontaining compounds.
• The NPD is about 500-fold moresensitive that an FID in detecting
phosphorous-containing compounds,
and 50-fold more sensitive tonitrogen-containing compounds• Applications:
Organophosphate in pesticides andin drug analysis for determinationof amine-containing or basic drugs

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5. Electrolytic Conductivity Detector• Element-selective detector for
halogen-, sulfur- and nitrogencontaining compounds• Compounds containing halogens,sulfur, or nitrogen are mixed witha reaction gas in a small reactor
tube, usually made of Ni. The
products from the reaction tube
are then dissolved in a liquid,
which produces a conductivesolution. The change inconductivity as a result of theionic species is then measured.
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Other GC Detectors• Photoionization Detector
aromatic hydrocarbons
organosulfur/organophosphorous• Atomic Emission Detector
element-selective detector• Flame Photometric Detector
sulfur and phosphorous containing compounds

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Comparison of GC Detector Sensitivity andDynamic Range

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*Mass Spectrometry Detector (MS)• One of the most powerful detectors for gas chromatography• SAVE FOR LATER

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Contents• The primary components to a GC system
1. Carrier Gas System (including Gas Clean Filters)• The concept of theoretical plates and van Deemter curves• Selection of proper carrier gas

2. Sample Introduction System• Split & splitless injection

3. Column (most critical component)• Column configurations: packed vs. open tubular/capillary• Stationary phase

4. Detection System/GC Detectors• Types of detectors and their specific applications

5. Computer ChemStation/Integrator
CEE 772 #1654

V. Quantitative Chromatographic Analysis• Quantitative Analysis
Based on a comparison of either the height or the area of the analytepeak with that of one or more standards• Peak height vs. Peak area
Peak heights are inversely related to peak width. Thus, accurate results
are obtained with peak heights only if variations in column conditions do
not alter the peak width during the period required to obtainchromatograms for samples and standards.
Peak areas are independent of broadening effects, which are usually thepreferred method of quantitation.
＊most modern chromatographic instruments are equipped withcomputers or digital electronic integrators that permit precise estimationof peak areas
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Questions?

Rosa Yu
Email: victorosa1212@gmail.com

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• To next lecture

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