Abstract:
Mass spectrometry systems and methods are disclosed. Briefly described, one embodiment of the mass spectrometry system, among others, includes a mass spectrometer, a laser generator, and a sample substrate. The mass spectrometer includes a first end and a second end and the laser generator produces a laser radiation beam that travels along a first path. The sample substrate holds a sample at the first end of the mass spectrometer, where the sample produces a plurality of ions that travel along a second path that is substantially parallel the first path but towards the second end of the mass spectrometer.

Description:
BACKGROUND 
     A mass spectrometry system is an analytical system used for quantitative and qualitative determination of the compounds of a material (e.g., chemical mixtures and biological samples). In general, a mass spectrometry system uses an ion source to produce electrically charged particles (e.g., molecular or polyatomic ions) from the material to be analyzed. Once produced, the electrically charged particles are introduced to the mass spectrometer and separated by a mass analyzer based on their respective mass-to-charge ratios. The abundance of the separated electrically charged particles are then detected and a mass spectrum of the material is produced. The mass spectrum is analogous to a fingerprint of the sample material being analyzed. The mass spectrum provides information about the mass-to-charge ratio of a particular compound in a mixture sample and, in some cases, molecular structure of that component in the mixture. 
     The molecular weight of a compound is often determined by the use of a mass spectrometry system having a single mass analyzer. The mass analyzer may include a quadrupole (Q) m ass analyzer, a time-of-flight mass analyzer (TOF-MS), an ion trap mass analyzer (IT-MS), etc. Tandem mass spectrometer s (i.e., tandem-MS or MS/MS) are often needed to analyze samples having complicated molecules. Tandem mass analyzers typically include two mass analyzers of the same or different type (e.g., TOF-TOF MS and Q-TOF MS). 
     In a tandem mass spectrometry analysis, electrically charged particles are transmitted to the first mass analyzer and an ion of particular interest is selected. The selected ion is transmitted to a dissociation cell where the selected ion is fragmented. The ionic fragments of the dissociated ion are transmitted to the second mass analyzer for mass analysis. The fragmentation pattern obtained from the second mass analyzer is then analyzed to determine the structure of the corresponding molecule. 
     Matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is of particular importance for the analysis of biological samples, which was introduced by Karas et.al. (Karas, M.; Hillenkamp, F.,  Anal Chem . 60, p. 2299, 1988) and Tanaka et.al. (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T.,  Rapid Commun. Mass Spectrom . 2, p.151, 1988) and has become established as a method of mass spectrometry used to analyze substances such as polypeptides, polynucleotides, proteins, DNA fragments, biopolymers, and other large molecules. 
     MALDI-MS allows for desorption and ionization of non-volatile samples (e.g., biological samples) from a solid-state phase directly into the gas phase without charring, fragmentation, or chemical degradation. The sample (analyte) is suspended or dissolved in a matrix. Matrices are small organic compounds that are co-crystallized with the analyte. 
     Laser radiation (i.e., a laser beam) produced by a laser generator, serves as a desorption and ionization source in MALDI-MS. The matrix absorbs the laser energy and causes part of the illuminated sample to vaporize. A rapidly expanding matrix plume carries some of the analyte ions into the vacuum of the mass spectrometer. The matrix molecules absorb most of the incident laser energy minimizing sample damage and ion fragmentation (i.e., soft ionization). Laser generators of various wavelengths (e.g., ultraviolet and infrared) have been used in MALDI-MS. 
     Conventionally, most laser desorption/ionization sources are performed in a vacuum (vacuum MALDI). In this regard, a sample substrate is under pressure conditions that are similar to the pressure in the mass analyzer (i.e., 1×10 −7  Torr). However, MALDI performed at atmospheric pressure (AP-MALDI) (as described in U.S. Pat. No. 5,965,884, which is included herein by reference) has also become available commercially (AP/MALDI™, available from MassTech Inc. of, Burtonsville, Md. is one example). The advantages of AP-MALDI over vacuum MALDI include simple sample handling and reduced analysis time. 
     Thus, there is a need in the industry for an MALDI-MS system that requires minimal alignment of the laser desorption/ionization source, while also having a high ion throughput. In addition, there is a need in the industry for a MALDI-MS system that can obtain sample ions from different points on a sample surface without having to realign the laser (i.e., profiling a sample), while also having a high ion throughput and being cost effective. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide mass spectrometry systems and methods for matrix assisted laser dissociation ionization of a sample. Briefly described, one embodiment of the mass spectrometry system, among others, includes a mass spectrometer having a first end and a second end opposite the first end. In addition, the mass spectrometry system includes a laser generator that produces a laser radiation beam that travels along a first path. Furthermore, the mass spectrometry system includes a sample substrate for holding a sample at the first end of the mass spectrometer. The sample produces a plurality of ions that travels along a second path towards the second end of the mass spectrometer that is substantially parallel the first path. 
     Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 is a block diagram that illustrates a representative schematic diagram of an ILL-MALDI-MS system. 
     FIG. 2 is a block diagram illustrating another representative schematic diagram of an ILL-MALDI-MS system. 
     FIG. 3 is a block diagram illustrating still another representative schematic diagram of an ILL-MALDI-MS system. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide for an in-line laser matrix assisted laser desorption/ionization mass spectrometry system. Such a system will be referred to using the acronym ILL-MALDI-MS. The ELL-MALDI-MS system has the laser system aligned in-line with the sampling system (i.e., sample), so that a line-of-sight through at least a portion of the mass spectrometer (which will depend upon the mass spectrometer selected for the ILL-MALDI-MS) to the sampling system is obtained. In other words, when the laser system emits laser radiation (i.e., a laser beam), the laser radiation passes th rough the m ass spectrometer an d impinges upon the sample at a angle substantially perpendicular (i.e., about 90 degree) to the sample. 
     In-line alignment of the laser system facilitates symmetric focusing of the laser radiation on the sample and provides better alignment between the sampe, laser radiation, and sampling orifice. In addition, in-line alignment produces precise sampling position and stable sample signals as well as increases ion collection efficiency. Therefore, detection sensitivity is improved using in-line alignment. 
     In addition, in-line alignment of the laser system al lows for precise profiling of samples. Another benefit of aligning the laser system in-line with the sampling substrate is that simple and cost effective optic systems can be used to focus the laser radiation. 
     Further, the position of the sampling substrate (i.e., sample) relative to the sampling orifice can be adjusted in three dimensions (i.e., x-, y-, and z-directions) without substantially changing the focus of the laser radiation on the sample. This decreases problems associated with realignment of the laser system and/or sampling substrate. In-line arrangement of the laser system allows the position of the sample to be adjusted without substantially affecting the focus of the laser. An additional benefit of the ILL-MALDI-MS is that the signal from the analyte ion can be optimized for a wide range of samples and matrices by adjusting the position of the sampling substrate relative to the sampling orifice. 
     FIG. 1 is a block diagram that illustrates a representative ILL-MALDI-MS system  30 . The ILL-MALDI-MS system  30  includes a laser system  32 , a mass spectrometer  34 , and a sampling system  36 . The laser system  32  and the sampling system  36  are located at opposite ends of the ILL-MALDI-MS system  30 . The mass spectrometer  34  includes mass analyzer/detection system  42 , and an ion optic system  46 . The laser system  32  is located near the mass analyzer/detection system  42 , while the ion optic system  46  is located near the sampling system  36 . 
     The laser system  32  emits laser radiation  38  that passes through an optic window  40  positioned on the mass analyzer/detection system  42 . The laser radiation  38  passes through mass analyzer/detection system  42 , and the ion optic system  46 , as shown in FIG.  1 . The laser radiation  38  exits the ion optic system  46  by passing through the sampling orifice  50 . Subsequently, the laser radiation  38  impinges substantially perpendicular (i.e., 90 degree) to a major axis of a sample (not shown) located upon the sampling substrate  48  of the sampling system  36 . 
     The sample absorbs the laser energy, which causes part of the laser illuminated portion of the sample to vaporize. The rapidly expanding matrix plume carries some of the analyte ions into the vacuum of the ion optic system  46  The ions may take the form of a packet of ions (i.e., generated with laser radiation pulse) or a constant stream of ions (i.e., generated with a constant beam of laser radiation). Once the sample molecules are vaporized and ionized, the ions are transferred via ion optic system  46  into the mass analyzer/detection system  42 . The ions are separated in the mass analyzer/detection system  42  according to their mass-to-charge ratio and detected by a detector based on their relative abundance. 
     This process can be performed a plurality of times for a particular position upon the sample. Optionally, the sample can be repositioned by moving the sampling substrate  48  in the x-, y-, and/or z-axis to obtain ions from various positions of the sample (e.g., profiling). However, in most circumstances, the laser system  32  will not need to be repositioned. Then after the sampling substrate  48  is repositioned, the process can be repeated. 
     The ion optic system  46  transmits and/or manipulates ions and can include electrostatic elements, radio frequency (RF) elements, or a combination thereof. A condition of the ion beam (i.e., energy, phase space, etc.) can be determined using ion optic system  46 . In general, the sampling orifice  50  serves as an ion optic element to extract ions into the mass spectrometer  34 . In addition, the sampling orifice  50  acts as a vacuum conducting plate to separate the mass spectrometer  34  from atmospheric pressure. Further, the ion optic system  46  includes other vacuum system components and electronic system components, as are known in the art. 
     The sampling orifice  50  is usually a circular hole with a diameter of typically about 0.1 to 3 mm, but more typically of about 0.3 to 1.5 millimeters. The sampling orifice  50  usually facilitates the passage of ions into the mass spectrometer  34  and also separates the mass spectrometer  34  from atmospheric pressure. Since the laser radiation  38  travels through the sampling orifice  50  the sampling orifice  50  can be used as an aperture for the laser radiation  38  (i.e., the sampling orifice  50  limits the cross section of laser radiation  38 ). 
     Embodiments of the present invention can include one or more ion optic configurations in the ion optic system  46  of the ILL-MALDI-MS system  30 . Preferably, the ion optic system  46  is a combination of electrostatic and RF ion optic elements. 
     The mass analyzer/detection system  42  can include a mass analyzer such as, for example, a time-of-flight (TOF) mass analyzer, an ion trap mass analyzer (IT-MS), a quadrupole (Q) mass analyzer, a magnetic sector mass analyzer, or an ion cyclotron resonance (ICR) mass analyzer. In one embodiment, because it can be used to separate ions having very high masses, the mass analyzer is a TOF mass analyzer. 
     In addition, the mass analyzer/detection system  42  includes an ion detector. A detector is a device for recording the number of ions that are subjected to an arrival time or position in a mass spectrometry system, as is known in the art. Ion detectors can include, for example, a microchannel plate multiplier detector, an electron multiplier detector, or a combination thereof In addition, the mass analyzer/detection system  42  includes vacuum system components and electronic system components, as are known in the art. 
     The optic window  40 , located on the mass analyzer/detection system  42  can include an optic lens through which laser radiation  38  can pass. In accordance to the laser generator chosen for the ionization, the optic window  40  usually has high transmission efficiency for the particular wavelength. The optic window  40  can be a flat plate for transmitting the laser radiation, or it can be a curved surface that functions as a lens to focus the laser radiation  38 . In addition, the optic window  40  is vacuum-sealed so that a low pressure (on the order of 1×10 −7  Torr) in the mass analyzer/detection system  42  is maintained. 
     The laser system  32  includes a laser generator and a laser optic system. The laser generator can include a laser generator such as, for example, a Nd:YAG laser having an output wavelength of 1060 nanometers and a harmonic wavelength of 533 nanometers, 353 nanometers and 256 nanometers, a nitrogen laser having an output wavelength 337 nanometers, or a CO 2  laser having an output wavelength 10.6 micrometers, a tunable laser, or another laser having a wavelength ranging from ultraviolet wavelengths to infrared wavelengths, as are known in the art. The laser used in a laser desorption/ionization source is usually a pulsed laser with a repetition rate of 1 to 100 Hertz, typically 10 to 20 Hertz. Other laser optic systems, known in the art, typically used in conjunction with laser generators can be used in the laser system  32 . 
     The sampling system  36  includes a sampling substrate  48 . The sampling system  36  facilitates the movement (i.e., x-y-z directions) of the sampling substrate  48  manually or through the use of a computer system. Sample substrate systems  36  are known in the art and will not be discussed in detail. 
     The sampling system  36  shown in FIG. 1 is at atmospheric pressure (i.e., atmospheric pressure (AP) MALDI), however, another embodiment of the present invention can be configured to include the sampling system  36  in a vacuum (i.e., vacuum MALDI). In vacuum MALDI, the mass spectrometer is in a first vacuum, while the sampling system is in a second vacuum. 
     Preferably, the sampling substrate  48  is mounted substantially perpendicular to the optical axis of the sampling orifice  50 , and, therefore, substantially perpendicular to the laser radiation. In case of an imperfection of the alignment, the sampling substrate  48  can be tilted about 10 degrees or about 5 degrees to align the laser radiation and the sampling substrate  48  so that they are substantially perpendicular. Preferably, the tilt angle is less than about 1 degree. 
     In the embodiment illustrated in FIG. 1, the laser radiation  38  is generated in a first region at atmospheric pressure. Then, the laser radiation  38  travels through the optic window  40  into the mass analyzer/detection system  42 , and the ion optic lens system  46 , which are in a second region and under a vacuum (1×10 −2  Torr to about 1×10−7 Torr). Subsequently, the laser radiation  38  exits the vacuum through the sampling orifice  50  and impinges upon the sample, which is in a third region at atmospheric pressure. 
     FIG. 2 is a block diagram that illustrates an ILL-MALDI-MS system  60  that includes a laser system  62 , a mass spectrometer  64 , and a sampling system  66 . The laser system  62  and the sampling system  66  are located on opposite ends of the ILL-MALDI-MS system  60 . The mass spectrometer  64  includes a first ion optic system  82 , a first mass analyzer  80 , a collision/fragmentation system  78 , a second ion optic system  76 , and a second mass analyzer/detection system  72 . The first optic system  82  is located near the sampling system  66 , while the second mass analyzer/detection system  72  is located near the laser system  62 . 
     The second mass analyzer/detection system  72 , the optic window  70 , and the laser system  62  are analogous to the mass analyzer/detection system  42 , the optic window  40 , and the laser system  32  discussed in reference to FIG.  1 . The first mass analyzer  80  is analogous to the mass analyzer discussed in reference to FIG.  1 . However, the first mass analyzer  80  and the mass analyzer of the second mass analyzer/detection system  72  can be different mass analyzers (i.e., quadrupole mass analyzer and a TOF mass analyzer, respectively). The first optic system  82  and the second optic system  76  are analogous to the optic system  46  discussed in reference to FIG.  1 . However, the first optic system  82  and the second optic system  76  can have different configurations. The sampling substrate  86  is analogous to the sampling substrate  50 . 
     The laser system  62  emits laser radiation  68  that passes through an optic window  70 . The laser radiation  68  passes through the second mass analyzer/detection system  72 , the second ion optic system  76 , the collision/fragmentation system  78 , the first mass analyzer  80 , and the first ion optic system  82 . The laser radiation  68  exits the first ion optic system  82  by passing through the sampling orifice  86 . Subsequently, the laser radiation  68  impinges substantially perpendicular to a major axis of a sample (not shown) located upon the sampling substrate  84  of the sampling system  66 . 
     The sample absorbs the laser energy, which causes the laser illuminated portion of the sample to vaporize. The rapidly expanding matrix plume carries some of the analyte ions into the vacuum of the first ion optic system  82 . Once the sample molecules are vaporized and ionized they are transferred into the first mass analyzer  80  (e.g., time-of-flight mass spectrometer (TOF-MS)) where a particular analyte ion is selected for further analysis. 
     After the selected analyte ion passes through the first mass analyzer  80 , the selected analyte ion can be dissociated via collision-induced dissociation or photodissociation in the collision/dissociation system  78 . Subsequently, the resulting ions (fragments) are focused with a second ion optic system  76  into the second mass analyzer/detection system  72 , and then detected by a detector based on their relative abundance. 
     The collision/fragmentation system  78  functions to dissociate the selected ion. The collision/fragmentation system  78  can include an ion optic system, a fragment confining system, and gas inlets. Alternatively, collision/fragmentation system  78  includes a photon source that provides photons that are used to photodissociate the selected ion. However, collision/fragmentation systems  78  are known in the art and will not be discussed in detail. 
     In another embodiment, if the first mass analyzer  80  and the second mass analyzer of the second mass analyzer/detection system  72  are TOF mass analyzers then the ions from the sample will change direction (generally a 90° turn). In this situation, the laser system  62  and optic window  70  would be located adjacent the first mass analyzer  80  rather than the second mass analyzer/detection system  72  to achieve in-line alignment. This situation may also occur when the first mass analyzer  80  is another type of mass analyzer. Thus, alternative configuration of the mass spectroscopy systems are contemplated to be within the scope of this disclosure in cases where the location of the laser system is altered to accomplish in-line alignment of the laser system and the sampling system. 
     In still another embodiment the ILL-MALDI-MS system may include, for example, a triple quadrupole mass analyzer configuration, a quadrupole-TOF mass analyzer configuration, or a combination of an ion trap with a TOF-MS. 
     FIG. 3 is a block diagram that illustrates a representative ILL-MALDI-MS system  90 . The ILL-MALDI-MS system  90  that includes a laser system  92 , a mass spectrometer  94 , and a sampling system  96 . The laser system  92  and the sampling system  96  are located on opposite ends of the ILL-MALDI-MS system  90 . The mass spectrometer  94  includes a mass analyzer/detection system  104 , and an ion optic system  108 . The mass analyzer/detection system  104  is located near the laser system  92 , while the ion optic system  108  is located near the sampling system  96 . 
     The laser system  92 , mass analyzer/detection system  104 , the ion optic system  108 , and the sampling system  96  are analogous to the laser system  32 , the mass analyzer/detection system  42 , the ion optic system  46 , and the sampling system  36  discussed in reference to FIG.  1 . 
     The laser system  92  is coupled to the detection system  104  via a section of optical fiber  98 . The optical fiber  98  guides the laser radiation  102  into the mass analyzer/detection system  104  through the optic window  100 . The laser radiation  102  passes through the mass analyzer/detection system  104  and the ion optic system  108 , as shown in FIG.  3 . The laser radiation  102  exits the ion optic system  108  by passing through the sampling orifice  112 . Subsequently, the laser radiation  102  impinges substantially perpendicular to a major axis of a sample (not shown) located upon the sampling substrate  110 . 
     The sample absorbs the laser radiation  102 , which causes part of the laser illuminated sample to vaporize. The rapidly expanding matrix plume carries some of the analyte ions into the vacuum of the ion optic system  108 . Once the sample molecules are vaporized and ionized the ions are transferred into the mass analyzer/detection system  104  (e.g., time-of-flight mass spectrometer (TOF-MS)), where the ions are separated and detected with a detector based on their relative abundance. 
     The ILL-MALDI-MS system  90  incorporates an optical fiber  98 , which is known in the art, to guide the laser radiation  102  into the mass analyzer/detection system  104  through optic window  100 . In this embodiment, the optical fiber  98  can be used instead of or in conjunction with the laser lens system of the laser system  92 . 
     In the embodiment illustrated in FIG. 3, the laser radiation  102  travels through the optical fiber  98  and into the mass analyzer/detection system  104 , and the ion optic system  108 , which are under vacuum (about 1×10 −2  Torr to about 1×10 −7  Torr). Subsequently, the laser radiation  102  exits the vacuum through the sampling orifice  112  and impinges upon the sample, which is at atmospheric pressure. 
     It should be emphasized that the above-described embodiments of the present invention, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.