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No laser required Instead of using the laser needed for MALDI MS, Renato Zenobi and his coworkers at the Swiss Federal Institute of Technology use the tungsten filament from a flashlight bulb (1.2 V, 0.22 A) to thermally desorb ions in a technique that they call matrix-assisted filament desorption/ionization MS. The method could reduce the cost of matrixassisted desorption techniques. Substances with molecular weights up to 3000 Da could be analyzed. The filament was mounted directly in front of the flight tube of a time-offlight mass spectrometer. Grounding the voltage on one of the leads for 5-10 us caused the filament to rapidly heat up. It reached its maximum temperature—from its color, estimated to be 700-800 K—within a few microseconds, but it cooled much more slowly. The small electric field between the bulb socket and the grid propelled positive ions into the mass spectrometer.
Hybrid mass spectrometer for electrospray
The ions were accelerated to 1500 V, and they drifted through a 16-cm field-free tube. The ions were then detected by a plate electron multiplier.
(A) Experimental setup of a filament desorption/ionization TOF mass spectrometer. (B) Timing scheme of the desorption and extraction pulses. (Adapted with permission. Copyright 1998 American Society for Mass Spectrometry.)
continuous-flow TOFMS, in which the ESI products are analyzed without trapping or fragmentation; trap-TOFMS, in which trapping and pulsing are used to improve the duty cycle; and MS/TOFMS, in which the fragmentation spectra of isolated precursor ions are recorded via TOFMS. The linear ion trap can improve the duty cycle, resolution, and sensitivity of the TOFMS. For high mass resolution
Pulsed extraction helped to eliminate unwanted alkali ions and to refocus spatially distributed ions. Mass spectra of substance P were obtained using the thermal desorption source using 2,5-dihydroxybenzoic acid (2,5-DHB) as a matrix. However, the researchers found that the crystalline 2,5-DHB matrix was difficult to position reproducibly on the filament and that a liquid matrix was easier to use. Mass spectra of valinomycin, mellitin, gramicidin I), and gramicidin S were obtained using a glycerol matrix. Even polyethylene glycol 3000 could be analyzed using the filament desorption method, although individual olicould not be resolved The researchers believe that this device could be used for miniaturized instruments for mass analysis, parallel mass analysis with arrays of filaments, disposable ion sources, and remote sensing applications. (J. Am. Soc. Mass Spectrom. 1998, 9, 1226-28)
however, a trade off occurs between sensitivity and resolution. The performance of the system is demonstrated using ion trapping and pulsing, ion isolation, ion fragmentation, At first glance the continuous nature of and competition between ejection and electrospray ionization and the pulsed fragmentation. Improved duty cycle was operation of time-of-flight (TOF) MS demonstrated using a storage experiment makes them seem incompatible. Orthowith cytochrome c ions. A broadband gonal acceleration into the flight tube waveform was used to isolate the +3 alleviates some of the problems, and it has previously been accomplished charge state of renin substrate from with hybrid instruments, such as reserpine. Although most of the unquadrupole-TOF and threewanted ions were eliminated, some dimensional ion trap-TOF mass spectral features other than the prespectrometers. D. J. Douglas and cursor ion remained. co-workers at the University of BritThe researchers acknowledge that ish Columbia (Canada) describe the current performance is not comparable with either the quadrupole-TOF another hybrid instrument a linear mass spectrometer or the ion trapion trap/iime-of-flight system with TOF mass spectrometer. They sugtandem MS capabilities gest, however, several instrumental The instrument consists of an improvements—including a lowerRF-only linear quadrupole and a pressure and lower-mass collision trapping quadrupole that is coupled gas, an improved broadband waveto a linear TOF mass spectrometer. form and the addition of an axial Timed stopping potentials on the field to the trap and a reflectron to entrance and exit apertures create a the linear TOF mass spectromelinear ion trap, and applying a "tickter wj]] make the instrument comle" voltage to one pair of the quadruSchematic of the linear ion trap and the TOF mass petitive with its two rivals. {Rapid pole rods isolates precursor ions by spectrometer. IQ, the interquad aperture, serves as the Commun Mass Spectrom 1 9 9 8 12 ejecting unwanted ions. The instrutrap entrance and L1 as the trap exit. (Adapted with 1463-74) ment has three modes of operation: permission. Copyright 1998 John Wiley & Sons.) 16 A
Analytical Chemistry News & Features, January 1, 1999
