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Mass Spectrometry P1: GTM/GRL P2: GNH Final Pages Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32 Mass Spectrometry Kenneth L. Busch Kennesaw State University I. Instrumentation II. Launch Points for the Next Fifty Years GLOSSARY by a process of surface ablation, and ionized by proton transfer from the acidic matrix molecules. Chemical ionization An ion/molecule reaction that leads Quadrupole mass filter A mass analyzer that uses a stru- to ionization of the neutral gas-phase sample molecule cture of four colinear rods of hyperbolic cross section via its reaction with an ion generated from the reagent (energized with a combination of DC and rf fields) that gas present in excess in the ionization source. creates trajectory oscillations in low-kinetic-energy Electron ionization The direct ionization of a neutral ions passing through the rods. At a particular set of gas-phase sample molecule via loss of an electron in- DC and rf values, only ions of one mass maintain a duced by interaction with a high energy (70 eV) elec- stable trajectory that allows passage through the mass tron emitted from a metal filament. filter. Electron multiplier A transducer for creation of a cas- Sector mass analyzer A mass analyzer that uses either cading electron flow initiated by the initial impact of an a magnetic field, or a combination of magnetic and energetic particle on the front electro-emissive surface. electric fields, to change the direction of travel of high- Electrospray ionization A process through which isola- kinetic energy ions. Ions of different masses are influ- ted and usually highly charged ions of sample mole- enced to different degrees, providing the basis for a cules are created by spraying a fine stream of droplets of mass separation. sample solution from a highly charged needle, followed Time-of-flight mass analyzer A mass analyzer that sep- by progressive loss of solvent molecules. arates a mixture of ions generated in an ionization pulse Ion trap A mass analyzer that traps all ions from a pulse by providing all ions with an equal kinetic energy, and of ionization at the center of a ring and cap electrode then noting the different ion transit times (due to dif- structure energized with DC and radio frequency (rf) ferent velocities for ions of different masses) through fields. Ions of different masses are accelerated out of a flight tube of fixed length. the trap to an external detector by selective addition of energy. Matrix-assisted laser desorption ionization A process MASS SPECTROMETRY (MS) today is certifiably dis- by which sample molecules cocrystallized with a laser- tinct from the analytical method of five, ten, and cer- energy-absorbing matrix are transferred without de- tainly fifty years ago when it was first used as a gen- composition directly from the solid into the gas phase eral purpose analytical tool. Its fundamental principles 145 P1: GTM/GRL P2: GNH Final Pages Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32 146 Mass Spectrometry remain intact, and the supporting instrumentation contin- that “new capabilities catalyze new demands,” and the in- ues its steady and innovative evolution. But applications in tertwined relationship of modern mass spectrometry, its biological mass spectrometry have expanded at an extraor- instrumentation, and its market is aptly described. dinary rate, and fundamental new approaches to the cre- ation and interpretation of mass spectrometric informa- A. Sample Introduction Systems tion are being developed to address new questions. It has been estimated that a billion mass spectra are recorded The mass analysis and ion detection functions of a mass daily (Busch, 2000a). The growth and demand for high- spectrometer are completed in a vacuum, usually of 10−5 throughput mass spectrometry suggests that even that ex- to 10−6 torr (a slightly higher pressure prevails in the traordinary number is an underestimate, if not now, then operation of an ion trap mass spectrometer). The pro- certainly within the next few years. Classically, mass spec- cesses of electron ionization, chemical ionization, and tra were interpreted to provide details of molecular struc- matrix assisted laser desorption ionization (MALDI) also ture revealed through the unimolecular dissociations of operate under vacuum, while the electrospray ionization the molecular ion. Increasingly, however, mass spectral (ESI) source operates at atmospheric pressure, and ESI- data in other forms are being recorded and manipulated. generated ions then pass through a set of differential pres- Mass spectrometry is now used to study interactions be- sure apertures into the mass spectrometer. Mass spectrom- tween complex molecules in the gas phase to provide eters must operate under vacuum so that the ions move clues in combinatorial investigations. Mass spectrometric through the instrument in a controlled manner rather than data is transparently summarized in databases that support being scattered by collisions with residual gas molecules. repetitive high-throughput proteomics analysis. Multidi- The mean free path of the ion should be larger than the mensional MS/MS data is stored in arrays examined with dimensions of the mass spectrometer itself. The issues of chemometric-based routines that mine the data in search vacuum in mass spectrometry are more thoroughly dealt of answers for complex questions of pattern and structure with elsewhere (Busch, 2000b); the many orders of mag- that have just now begun to be formulated. nitude difference between instrument operating pressure This overview covers relevant issues in instrumentation and the atmospheric pressure of the laboratory, or the pres- for modern mass spectrometry, an overview of techniques sure regime of a column-based separations method, is a used in both classical and newer areas of mass spectromet- basic design parameter of the sample introduction system. ric investigation and a preview of growth areas in mass Mass spectrometry is unique among many modern an- spectrometry research and application. Current applica- alytical methods in that sample molecules are physically tions are best reviewed by examination of the manuscripts and irretrievably introduced into the instrument. The sam- published in the current journals of mass spectrometry, ple handling devices must be transport devices that accom- and review of the papers presented at the annual profes- modate a wide dynamic range of sample quantities, effi- sional meetings, especially meetings dealing with biolog- ciently transporting all sample molecules from the outside ical chemistry, and the annual meetings of the American world into the ionization source of the mass spectrometer. Society for Mass Spectrometry. Given the diversity of sample types, there is a concurrent diversity of sample introduction systems. I. INSTRUMENTATION 1. Direct Insertion Probe Francis W. Aston won the Nobel Prize in Chemistry in The direct insertion probe (or direct probe) is a device to 1923 for his prolific work in the discovery of isotopes introduce small amounts of solid or liquid samples into the using mass spectrometers that he had constructed in the ionization source of the mass spectrometer without chro- Cavendish Laboratory. Aston was experienced and skilled matographic separation. The direct probe is appropriate in the construction and maintenance and operation of in- for use when the sample is known to be pure or of lim- struments. But, in his 1942 book “Mass Spectra and Iso- ited complexity, or when a chromatographic separation topes” Aston described his mass spectrometer as an in- is impossible because of sample volatility, or when such strument that “behaves at times in the most capricious and a separation would be too time-consuming. About 10−5 unaccountable manner.” Where would the field of mass to 10−4 torr of sample pressure is sufficient to record a spectrometry be today if this were still the case, if, for ex- mass spectrum in an electron ionization or chemical ion- ample, there had been fewer instrumental developments ization source. This sample pressure can be achieved by of reliable and capable ionization sources, mass analyz- evaporation of the sample into the vacuum at room (in- ers, and ion detectors? A well-known adage states that strument ambient) temperature, or by increasing the tem- “new instrumentation begets new chemistry”; add a codicil perature of the direct insertion probe, which can be heated P1: GTM/GRL P2: GNH Final Pages Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32 Mass Spectrometry 147 electrically to 400◦C. Automation of a direct probe sample different from performing the same specification at levels introduction system has recently been described (Manura of 0.01%. With increased resolution, chromatography is and Manura, 2000). With the automated system, samples better able to isolate components present in mixtures at can be introduced to the mass spectrometer at the rate of lower levels. With increased sensitivity, mass spectrome- 15–20 samples per hour. This is a faster rate of sample try is better able to identify such components. Finally, with analysis than is normally possible than with chromatogra- increased emphasis on the measurement of analytical in- phy coupled with mass spectrometry, in which the column formation for both screening and regulatory purposes, the separation time accounts essentially for all of the time of demand for trace analyses has increased significantly. the analysis. a. Gas chromatography. The combination of gas chromatography with mass spectrometry (GC/MS), itself 2. Chromatographic Columns the topic of an excellent text by McFadden in 1973, is The ability of mass spectrometry to identify a sample com- now realized within an integrated, low-cost, widely avail- pound is maximized when the sample is pure; the combi- able analytical instrument. There is no longer any need to nation of mass spectrometry with a chromatographic inlet review the history of development of GC/MS instrumen- system has therefore become a mainstay of instrumental tation, nor the transition from packed-column to capillary analysis.
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