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FABMS: Still FABulous? How quickly fashions come and go. A brief survey of mass spectrometry (MS) manufacturers would lead one to believe that fast-atom bombardment (FAB), one of the oldest of the "soft" ionization techniques, has fallen from favor as the market is dominated by electrospray and matrixassisted laser desorption/ionization (MALDI). Only a few manufacturers still provide FAB as an optton, and only rarely do they sell it However, the exodus of suppliers from this area fails to tell the entire story, because FAB still has a place in the MS laboratory. Table 1 lists sepresentative sources A question of semantics? "When you say 'FAB', in the back of your mind, you're talking about biological and organic compound analysis," says Kenneth Busch, a professor at the Georgia Institute of Technology. A similar technique, secondary ion MS (SIMS), is generally considered to be a surface-analysis S0003-2700(97)97903-0 CCC: $14.0( © 1997 American Chemical Society
FAB is still finding uses in the shadow ofMALDI and electrospray. technique. (For more information on SIMS instruments, see Anal. Chem. 1996,68,683A-687A) When SIMS is used for organic and biochemical molecular analysis in solution, it is called liquid SIMS (LSIMS)) FAB is remarkably similar to LSIMS; in fact, some might argue that they are minor variations of the same technique. In both methods, the sample is bombarded with a primary beam that sputters ions from a viscous solvent matrix into the gas phase. The difference is in the composi-
tion of the beam. In FAB, the primary beam consists of neutral atoms, typically xenon or argon; whereas the LSIMS primary beam contains ions such as cesium. Originally, the two terms distinguished the techniques, but today the terms are used interchangeably. "LSIMS and FAB are names that came from the days when people didn't really understand how they worked," says Richard Caprioli of the University of Texas Medical School. Andrew Tyler, director of the MS facility in the chemistry department at Harvard University and one of the developers of FAB, agrees that the distinction between the two techniques is largely historical. "The practical results of the experiments, especially for low-mass species are essentially the same. I've never seen anything that would convince a large audience otherwise " Therefore "FAB" no longer implies only a neutral beam The neutral atom beams are generated by subjecting an ion beam to a charge-
Analytical Chemistry News & Features, October 1, 1997 6 2 5 A
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Table 1 . Summary of representative m o a n d u i m i suppliers. Company Address
Bruker Daltonics 15 Fortune Dr. Manning Park Billerica, MA 01821 508-667-9580
Finnigan Hiden Analytical 355 River Oaks Pkwy 420 Europa Blvd. San Jose, CA Gemini Business 95134-1991 Park Warrington 408-433-4800 WA5 5UN England 44-1925-445225
JEOL 11 Dearborn Rd. Peabody, MA 01961 508-535-5900
Micromass 100 Cummings Center Suite 407N Beverly, MA 01915-6101 800-390-4660
URL General product description
www.bruker.com Ar or Xe FAB gun with 10-kV power supply; Cs+ gun (25 kV max.) with focusing; flowing FAB interface; MCI source; available for the APEX series of FT-ICR instruments
www.finnigan.com FAB and continuousflow FAB are available for the TSQ triple quadrupole and the MAT series magnetic sector mass spectrometers
www.hiden.co.uk HAL IFG200 FAB/ Ion Gun (O or inert gases); compatible with Hiden EQS SIMS system
www.jeol.com FAB gun (Xe) with separate ionization chamber; Cs+ source; frit-FAB; robot probe for analyzing up to 40 FAB probes automatically (GCmate only); available for the RSVP, AX505, MStation, HX-110, and GCmate magnetic sector instruments
www.micromass.co.uk Cs+ accessory for Cs+ source and static LSIMS and massive cluster dynamic LSIMS impact source screen/wick probe; available for the AutoSpec series of magnetic sector instruments and the Platform II (single) and Quattro II (triple) quadrupole instruments
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exchange process and then removing any ionic species remaining. The energy of the atom beam is usually ~8 keV. Caprioli observes that, in practice, FAB guns don't produce only neutrals. "The fascinating thing about it is that, from the sample's point of view, it doesn't make a difference," says Busch. "The sample's just getting sputtered away, so it really doesn't matter in most situations whether it's an ion or an atom coming at it." Nonetheless, neutral particle beams have several disadvantages relative to ion beams. The atom beam cannot be manipulated; therefore, it cannot be focused. Once the translational energy has been established, that energy cannot be changed. James Yergey of Merck Frosst (Canada) says, "I prefer LSIMS, because it has the ability to give you the focused ion beam and therefore focus right on the target. It gives you higher ion yields and greater sensitivity relative to FAB." Gary Kruppa of Bruker Daltonics agrees. He says that Bruker's LSIMS source is pulsed and can be deflected from the target when ions are not being accumulated in the ICR analyzer cell leading to increased sensitivity and sample lifetime However, says Robert Cody of Jeol, LSIMS sources are hotter and dirtier than FAB guns, and they operate at higher electrical potentials. Because high mass analyses are better carried out with MALDI or electrospray, the advantages of the ion source relative to an efficient atom source are reduced. 626 A
Phrasor Scientific 1536 Highland Ave. Duarte, CA 91010 818-357-3201
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Cody says that Jeol's FAB source of FAB, known as continuous-flow FAB avoids some of the problems associated (CF-FAB), can reduce some of the probwith other FAB systems by using separate lems associated with FAB. chambers for ionization and neutralizaCF-FAB allows repetitive analyses by tion. The xenon ion can be focused and flow injection. Caprioli says, "Whenever accelerated into a neutralization chamber. you remove one sample probe and put anThe resulting fast-atom beam retains most other in, you can never get the probe in of the momentum of the original ion exactly the same place. It is therefore diffibeam. The beam can be better focused cult to compare two samples. .f you flow than those formed with a "saddle-field" inject, the probe position is constant. The discharge source, in which electrons oscil- second big advantage of the flow technique late in an ion-atom plasma between two is that you can connect a liquid chromatocathodes. However the beam still cannot graph or an electrophoretic device to it." be manipulated after it is neutralized. In CF-FAB, the matrix can be added in two ways—to the mobile phase or postFAB and LSIMS are inherently compatible with most if not all mass analyzers. column. Caprioli says, "If your separation can tolerate it [matrix] in the mobile However, FAB seems to be offered as an phase, it's easier to put it there. You don't ionization option mostly for magnetic sechave to have splitting or addition tees. tor and FT-ICR instruments. Lance Nicolaysen of Micromass indicates that FAB is The plumbing is easier [than adding the matrix post-column]." The matrix, usually offered as an option for magnetic sector glycerol, is present at less than 5%. "In and quadrupole (single and triple) instruCF-FAB, you have about a 5% glycerol ments. He notes that FAB is not particusolution that is continuouslyflowingand larly popular and that the source is purwashing the surface, so the background is chased for fewer than 5% of the instruquite lower [than in the probe technique]. ments Micromass sells. The downside is that it's aflowtechnique. FAB is most often used as a probe You have plumbing and a pump to worry technique in which the sample is mixed about so it's just more complex" with a viscous matrix such as glycerol, thioglycerol, or nitrobenzyl alcohol (NBA) Massive cluster impact (MCI) ionizaand placed on a probe in the vacuum tion is a new technique that is similar to chamber of the mass spectrometer. The FAB in that a beam of primary particles matrix is often mixed with water, which bombards a sample in a liquid matrix. evaporates rapidly under vacuum condiHowever, the particles are large clusters tions. Because the sample is at that point (as much as or more than 106 Da) of a in 90-100% glycerol, the background from species such as glycerol that has been the matrix can be quite high. A variation doped with a salt or metal to become
Analytical Chemistry News & &eatures, October 1, 1997
highly conductive. MCI spectra have less chemical noise than FAB and are similar in many respects to the spectra obtained by electrospray. The method was developed by John Mahoney and Julius Perel of Phrasor Scientific and is available from Phrasor and Bruker Daltonics. Picking a matrix
Various materials are used regularly as FAB or LSIMS matrices, ,ncluding such compounds as glycerol, thioglycerol, and NBA. What are the important considerations in a matrix material? Those considerations depend on the intended goal. Busch says, "What you really want is a matrix that doesn't do anything interesting at all or a matrix that does lots of interesting stuff. If it doesn't do anything interesting at all, you've got the perfect matrix for analysis of your target compound. If it does a lot of interesting stuff, you've got a lot of chemistry to explore. What you don't want is for it to be in between." In a service facility such as the one Tyler runs, where throughput is important, the users' suggestions for solvents are a guiding force in choosing a matrix. Polar solvents mean glycerol as the first guess; slightly less polar, NBA If the compound is expected to work well in negative ion mode, ethanolamine would be thefirstchoice. Higher mass compounds would suggest thioglycerol. He describes these as "fairly standard first-go-around choices". Yergey is combining LSIMS with HPLC. "When you're doing LC/MS, [the matrix is] almost always glycerol, because you can't put something like thioglycerol through an LC column wiihout ruining the column." The glycerol (—1.5%) is added to the mobile phase. He says that most of the time the glycerol does not affect the separation. Competing w i t h MALDI and ESI
When FAB was first developed, it was used for the analysis of peptides and small proteins. As one of the earliest "soft" ionization methods—methods that cause very little fragmentation of the analyte— FAB presented the first opportunity to analyze some of these larger molecules by MS. With the subsequent development and proliferation of electrospray and MALDI, it would seem that those techniques have usurped FAB's role. "ESI and MALDI really don't compete with LSIMS at all anymore," says Busch. "They used to compete because FAB was the first of the widely used ionization methods that could start to get you into
the higher mass biological compounds. If you really want to [analyze high mass compounds] now, you're going to use electrospray or MALDI." Caprioli states that electrospray is a better all-around ionization method but he indicates that FAB still has a place. He says, "If I were buying an instrument right now, it's clear that electrospray is a much more dynamic technique. If I were buying one ionization technique, I'd get electrospray without question. Could FAB still be used on that machine and be a useful addition? Yes, it could." According to Caprioli, FAB originally was and continues to be suitable for the analysis of peptides and small proteins. He says that of his eight mass spectrometers, one is permanently configured for FAB and is used every day. "Nothing is simpler than running a FAB [analysis]. You put a small amount of sample on a probe, put it in the mass spectrometer, and you have a spectrum in seconds.
higher molecular weights, hell wait for the electrospray instrument, because FAB is 'less convenient" and "more work". Yergey uses LSIMS primarily for metabolism studies of small molecules (
