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GC/MS Not the Same Old Combination Some mass spectrometers have become small enough, inexpensive enough, and simple enough that they can serve as routine detectors in a gas chromatographer's arsenal. "Mass spectrometers have gone from being vulnerable and sensitive to the need for repairs to being robust, rugged instruments," says Gary Eiceman of New Mexico State University. "I have undergraduates using GC/MS in my lab on a regular basis. When I was a graduate student, even graduate students didn't get their hands on the instruments." "It used to be that instruments were more specific and morefragilein how they operate," says Charles Lyle of Shimadzu. "They're more robust [now]. If you take the complexity out and leave the important things—like sensitivity and upgradability— but not build in all those pieces to start with, the customer can tailor the final product based on his requirements. They don't get options that they don't need and can break down. You make the whole instrument easier to use. Anything that's simpler is more reliable." Analytical Chemistry last published a product review about GC/MS systems in 1994 (Anal. Chem. 1194, 66,927 A930 A). Much has changed since then— MS/MS capabilities have been introduced to benchtop instruments, chemical ionization has been improved, and many new mass analyzers are now available. 'Ten years ago, MS/MS was so terribly expensive that nobody considered doing it. Negative ion CI [chemical ionization], based on my experience, was often unreliable," says Terry Sheehan of Varian. "But the technology is changing. The fact that you can get Celia M. Henry
GC/MS systemsareon the bench but still in the game. negative ion CI on most of these benchtop transmission quadrupoles is proof of that. And the fact that you have benchtop MS/MSfromVarian and Finnigan is proof of how that technology has come along." Benchtop GC/MS systems are available with transmission quadrupole (mostly single quadrupoles, but there is one triple quadrupole instrument), quadrupole ion
trap, time-of-flight (TOF), and magneticsector mass analyzers. Of these, transmission quadrupoles are the dominant analyzers. The general agreement among company representatives is that transmission quadrupoles comprise 80% of the benchtop GC/MS market. Most of the remaining 20% is composed of quadrupole ion traps, and a small percentage is made up of TOF and
Analytical Chemistry News & Features, June 1, 1999 4 0 1 A
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magnetic-sector instruments. Table 1, which is not intended to be comprehensive, lists the features of representative benchtop GC/MS systems. Companies offering more than one benchtop system with the same type of mass analyzer (e.g., single quadrupoles) have only one of their products listed in the table. Readers are encouraged to contact the companies directly for information on their complete product lines. MS/MS on the bench
"The most interesting evolution in benchtop GC/MS has been the ion trap mass spectrometer," says Richard A. Yost of the University of Florida, "which has made it possible—in a benchtop-sized and benchtop-priced instrument—to do chemical ionization in addition to electron ionization and to do tandem mass spectrometry." Ion trap GC/MS systems are offered by Finnigan and Varian. One interesting feature of ion trap mass spectrometers is that they offer tandem MS, which improves selectivity in the analysis of mixtures and, therefore, can lower detection limits by eliminating or minimizing chemical interferences. Ion traps perform "tandem in time" MS by selectively storing ions within the trap for fragmentation. Following fragmentation, the fragment ions are scanned out of the trap. Both MS stages occur in a single ion trap. This differs from the "tandem in space" arrangement of triple Quadrupole mass spectrometers in which tn
