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MEETING NEWS Cheryl M. Harris reports from the —New Orleans, La.
225th ACS National Meeting
Mini MS shows big results Analytical chemists are finding out that shrinking a mass spectrometer down to size doesn’t necessarily mean that all compromises have to be made. Just months after reporting the results of their ~63-kg miniature cylindrical ion trap mass spectrometer (Anal. Chem. 2002, 74, 6145–6162), R. Graham Cooks and his team at Purdue University have announced an even smaller mass spectrometer, the Mini-CIT 7.0. It weighs ~17 kg; is more portable than the older version of the instrument; and offers better mass range, limits of detection, and resolving power. Compared with the 5.0 version, which provides an m/z range of ~250 and uses 200–300 W, the 7.0 version offers an m/z range of ~500 and needs only 136 W, says Leah Riter, a postdoc in Cooks’ laboratory who presented the results. Despite progress on miniaturizing mass spectrometers, researchers realize there’s still more work ahead. “The main problem for all of us is that we don’t yet have miniature pumps that provide the high vacuum needed in time-of-flight instruments,” says Robert Cotter of Johns Hopkins University, whose laboratory has produced miniaturized MALDI-TOF mass spectrometers. “The vacuum required for ion trap mass spectrometers is not as demanding as that required for the time-of-flight.” At Purdue University, Riter says the research group was able to perform MS/ MS/MS with the Mini-CIT 7.0, as well as direct air analysis using membrane introduction MS and ion molecule reactions inside the 8-g analyzer. They have taken the tote-bag-sized Mini-CIT 7.0 around the country and used it indoors and outdoors. They’ve even carried it on commercial airplanes. Now, Cooks’ group is looking to make the mass spectrometer even smaller by removing the complex, fragile turbo pump and replacing it with a diaphragm pump, such as 250 A
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the ones used in fish tanks. At Johns Hopkins, Cotter has noted that TOF mass spectrometers maintain their very high mass ranges when miniaturized, a distinct advantage for biodetection. Cotter and Ben Gardner, a research associate in his laboratory, are working to increase the resolving power of a miniaturized MALDI-TOF mass spectrometer to as high as 3000 or 4000 by combining a temporally dynamic electric field and a spatially nonhomogeneous electric field along the ion flight path. “The idea of a single field in which the ions accelerate from the ion source—from the time they’re born to the time they hit the detector— is a very, very new idea,” says Cotter. Cotter and his team have already developed a 3-in. MALDI-TOF mass spectrometer with a mass range of 70 kDa and a resolving power of ~1000 (m/z 4500) (J. Mass Spectrom. 2002, 37, 1158–1162). Their work has been supported by the Defense Advanced Research Project Agency (DARPA), and they have worked with researchers at Johns Hopkins Applied Physics Laboratory on a system that DARPA is planning to use in the field. To improve resolving power, Cotter and Gardner have focused on an elusive problem inherent to TOF mass spectrometers: Not all of the ions made have the same kinetic energies, explains Cotter. “So, when they are accelerated down the drift tube, ions of the same m/z don’t arrive at the detector precisely at the same time.” The result is peaks with a finite time width that limits the resolving power, he says. The kind of electric field the researchers wish to create can be thought of as a curved skateboard ramp, explains Cotter, with the ions at the top of the ramp and the detector at the bottom. The time dependence is achieved by momentarily flattening the field, making the ions, waiting about a microsecond, and then quickly pulling the “ramp” up to force the ions down.
