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MEETING NEWS 16th International Mass Spectrometry Conference—Edinburgh, Scotland
Fragmentation in a drift tube? When David Clemmer at Indiana University first heard the news from his postdoc Steve Valentine and his graduate student Stormy Koeniger, he didn’t believe it. “I actually said [to Valentine], ‘I don’t believe ions should fragment there,’ and I tried to suggest that Steve and Stormy work on some of their other experiments,” recalls Clemmer. But his colleagues were persistent and convinced him that they were seeing ions fragment inside their ion mobility drift tube, induced by a second applied field. Valentine says he discovered the phenomenon accidentally. He and Koeniger were trying to improve the resolution of their ion mobility spectrometer. “I noticed that when I turned up the voltages at the back of the drift tube, the resolution improved,” he says. “At the same time, I noticed that the +3 charge states of the peptides were beginning to fragment.” Ions are typically fragmented in MS/ MS experiments in a separate collision cell
Low field
Differentially pumped region
Drift rings
Schematic of a new split-field drift tube. 452 A
ions, but the researchers haven’t yet observed fragmentation of +1 ions. They report that the fragmentation patterns are highly reproducible. The researchers are currently using the device to analyze entire proteomes. According to Clemmer, the next big question is how to handle the deluge of data obtained from these experiments. “We’ve been dealing with that question now for about nine months, and it’s quite a challenge,” he says. (Anal. Chem. 2003, 10.1021/ac030111r)
positioned between two mass spectrometers. But in the past few years, Clemmer and others have been using instruments that separate ions by ion mobility spectrometry (IMS), followed by fragmentation in a collision-induced dissociation (CID) cell and MS. Clemmer explains that an advantage of IMS/CID-MS is that all parent ions are fragmented, whereas in MS/MS experiments, researchers typically select only the largest peaks for analysis after the initial MS step. Ions traveling through the new splitfield drift tube invented by Clemmer’s group are first separated by their mobility through a low-drift field. Once the ions reach the last 0.5 cm of the drift tube, they experience a high-drift field that fragments them. The new technology not only obviates the need for a separate collision cell but also improves ion transmission to the mass spectrometer and gives researchers more control over the fragmentation process, says Clemmer. The high field at the back of the drift tube can be modulated to specifically fragment +3 or +2
A N A LY T I C A L C H E M I S T R Y / N O V E M B E R 1 , 2 0 0 3
Low field
Drift rings
Studying large complexes by ESI-GEMMA
High Differentially field pumped region
COURTESY OF DAVID CLEMMER
Katie Cottingham reports from the
Joseph Loo and colleagues at the University of California, Los Angeles (UCLA); MDS-Sciex (Canada); and TSI, Inc., think big—as in megadaltons. The researchers electrospray large protein complexes and analyze them by MS/MS and gas-phase electrophoretic mobility molecular analysis (GEMMA). That such large complexes could be electrosprayed was unexpected. “I think just from the sheer size of some of the complexes, we’re pleasantly surprised that these [noncovalent] forces are still intact and we can make measurements for proteins as large as 14 million molecular weight,” he says. The researchers initially analyzed the 20S proteasome, a cellular component that degrades proteins, by electrospray ionization MS and MS/MS. The proteasome complex was then electrosprayed into the GEMMA device, which is similar to an ion mobility spectrometer. “The molecular weight from the mass spectrometry gives us the confidence that what we’re spraying into the GEMMA is a proteasome,” he explains. The team found that the diameter of the proteasome was consistent with the dimensions deduced from its crystal structure. “It’s a little bit surprising,” says Loo, because they really didn’t know what to expect once water was removed from the complexes.
