Courtesy: Human Genome Program, U.S. Department of Energy, Genomics and Its Impact on Science and Society: A 2008 Primer, 2008. (Original version 1992, revised 2001 and 2008.) http://genomicscience.energy.gov.
MALDI, ESI, and the Explosion of Proteomics
For years larger biomolecules were too fragile for existing ionization methods. But with a few key improvements biologists and mass spectrometry were inching closer to the dawn of proteomics. Soon biology would be able to do what it had wanted to do, with the help of innovation and better instrumentation. The first steps were taken early in the 1980s when Marvin Vestal’s lab at the University of Houston developed the thermospray technique, a particularly useful technique in LC-MS in which ions were transferred from the liquid to the gas phase. Vestal noted, “We believed that if we could just heat this liquid really quickly and minimal contact with surface, maybe we could get these things into the gas phase,” without destroying the liquid (55). In his oral history Vestal described the process of creating the technique:
Hear Marvin Vestal: We thought, well, how could we do that? Well, you know, powerful lasers were available at that time, so we got a 50-watt CO2 laser, which is enough energy to vaporize a milliliter of water if you get it all absorbed. It worked with molecular beams and with this crossbeam machine, so we knew a lot about how to do it. So we basically built a beam apparatus with the laser beam crossing a liquid jet, and it was going to vaporize the liquid jet, and shoot it into the mass [spectrometer]. We had differential pumping. So it sounded pretty good. As I said, I thought it was a reasonable proposal. And it was, I suppose, in a way except that, what we soon discovered was that when you try to heat a liquid jet with a laser, it’s not so simple. Basically what we made were beautiful ice crystals and stalactites. Look in the window and you’d look in and see these things growing, and [it] didn’t matter if you had the laser on or off. You still see them. Well, of course, what happens is the damn things won’t sit still. You hit them with a laser and they move. They vaporize on one side and go flying away. So you get a little energy, but no way [do] you get any significant amount of energy absorbed, and so that didn’t work. But, as I showed in the talk, we very quickly realized the way to do it is just . . . I think we did it intentionally, or might have accidentally. We moved the laser over to where it just heated the end of the metal tube. Then we could get vaporization. Then we could also get relatively nonvolatile molecules to fly with EI [electron ionization] and CI [chemical ionization], and we had this enormous apparatus with multi diffusion pumps and fore pumps and the biggest diffusion pump I had ever seen, although John [B.] Fenn had bigger ones. We got it working, and we actually met most of the criteria we had proposed. (55)
John Fenn's electrospray on display at the CHF Museum.
Thermospray was a better method to get those tricky nonvolatile molecules—like proteins—to ionize, which is necessary for mass-spectrometry analysis. Thermospray was so highly regarded that other, alternative techniques were initially ignored by the mass-spectrometry community, even if those techniques could improve ionization of molecules like proteins further. John Fenn, from the field of molecular-beam research rather than mass spectrometry, had worked on an electrospray-ion source for combined LC-MS, an efficient method of transferring compounds from the liquid chromatograph to the mass spectrometer for analysis. But his initial proposals were rejected, mostly due to thermospray’s success. Still Fenn persisted, joining the American Society for Mass Spectrometry and having his group members present papers at the ASMS annual conference. After resubmitting the proposal to the 1988 conference the power of Fenn’s electrospray-ion source for combined LC-MS was recognized by conference attendees. Vestal explains that he knew Fenn’s method had merit on hearing the proposal:
But in 1988, I realized the handwriting was on the wall, because once I heard Fenn’s paper on the high-molecular-weight electrospray [at the 1988 ASMS conference] and the MALDI [matrix-assisted laser desorption/ionization] paper by Karas and Hillenkamp [at the 11th International Mass Spectrometry Conference in Bordeaux, France, in 1988], and also, shortly thereafter, the work by [R. C.] Beavis and [B. T.] Chait on MALDI/ToF [time of flight]. (63)
Besides providing an efficient method of transferring compounds from the liquid chromatograph to the mass spectrometer for analysis, electrospray ionization (ESI) had the added advantage of creating highly multiply charged ions when analyzing proteins. Since the mass analyzer separates ions according to their mass-to-charge ratio, the long sought-after high-mass proteins could be analyzed with an instrument of normal mass range. The pharmaceutical industry in particular saw immense possibilities for the new technique, and mass spectrometers have been prevalent ever since. By the mid-1990s analysis of biological compounds with combined LC-MS ESI instruments dominated biological applications of mass spectrometry.
Bendix Time of Flight Mass Spectrometer (TOFMS) MA-17 Nude ion source on MA-2. CHF Collections.
Concurrently, a group of researchers coined the term MALDI—matrix-assisted laser desorption/ionization—and by the late 1980s MALDI had been used to ionize a protein. MALDI employs a chemical matrix that ionizes molecules with a laser and is commonly paired with a ToF mass spectrometer. Developed in the mid-1950s, ToF was an instrument that had failed to live up to its potential of unlimited mass range. Improvements in 1970s and newer ionization methods breathed new life into this instrument. The interest in these most recent ionization techniques led to a resurgence in the development of ToF mass analyzers by instrument companies.