i n s t ru m e n t a l s
Tiny ion traps Pushing the limits of MS, researchers microfabricate an array of tiny ion trap analyzers on a chip.
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hen chemist Mike Ramsey was at Oak Ridge National Laboratory (ORNL) a few years ago, he had a problem. He and physicist Bill Whitten, also at ORNL, wanted to construct miniaturized mass spectrometers, but they didn’t have the right tools. Meanwhile, engineer Stanley Pau, who was at the Nanofabrication Laboratory at Bell Laboratories, had the opposite problem. He knew how to fabricate tiny instruments, such as mass spectrometers, but he wasn’t able to test them. “Stanley actually called me up out of the blue saying that he’d read our papers, and it would be nice to microfabricate” mass spectrometers, says Ramsey. “We said that was exactly what we wanted to do!” The result of that collaboration among Ramsey, Whitten, Pau, and colleagues at the University of Arizona, Bell Laboratories, ORNL, and the University of North Carolina, Chapel Hill, is an array of tiny ion trap mass analyzers on a chip (Phys. Rev. Lett. 2006, 96, doi 10.1103/PhysRevLett.96.120801). The radius of each ion trap is 20 µm, and the entire chip is ~25 mm2. According to the researchers, small ion traps could be used in the future as personal chemical sensors in homeland security applications or as clinical diagnostic devices in doctors’ offices. Instead of fabricating a single ion trap on a chip, the researchers created an array of analyzers. “As you make the traps smaller, they can’t store as many ions as the traps we normally use as laboratory instruments,” says Whitten. “To compensate for that, we use an array of traps.” All the traps work in parallel, he explains, so they eject ions simultaneously. These ions are all measured at a single detector, so the individual spectra are summed. Optimal results were obtained with an array of 256 analyzers. Ion traps were chosen over other types of mass analyzers because the properties of ion traps stay the same or 5248
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(a) Scanning electron micrograph of an ion trap analyzer (inset) and the chip array. (b) A size comparison of the chip with a U.S. nickel. (Adapted with permission. Copyright 2006 American Physical Society.)
improve as the devices shrink, says Ramsey. “The rf drive frequency goes up [so] you ought to be able to operate at higher pressures and maintain the same resolving power” as ion trap analyzers are miniaturized, he explains. Operation at high pressures is a big advantage. Currently, commercial ion trap mass spectrometers work at ~10–5 Torr, but as Pau says, operation at 1 atm (760 Torr) is the “holy grail”. At that pressure, pumps are not needed, so the size and power consumption of the entire instrument can be reduced even further. Bob Cotter at the Johns Hopkins University School of Medicine points out that although the microfabricated ion traps reported in the paper produced spectra at 10–4 Torr, they could operate at even higher pressures in theory. Nonetheless, he explains that one advantage of having an ion trap that operates at 10–4 Torr is that expensive, bulky turbo pumps are no longer necessary; only a rough pump is needed. According to Graham Cooks at Purdue University, small laboratory-built instruments often work at 10–4 Torr. He points out that “the limit to pressure is the detector, not the trap itself.” Even if instruments were developed that could work at ~1 atm, compatible detectors
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currently do not exist, says Cooks. Ramsey says, however, that the researchers are considering the use of Faraday cup detectors. These detectors function at high pressure but could result in a reduction of sensitivity compared with electron-multiplier detectors, which are used at low pressures. But even obtaining a spectrum at a pressure of 10–4 Torr with such small instruments is quite an achievement for the field. “The important thing is that [the researchers] have succeeded on this scale,” says Cooks. Although the mass resolution of the chip is not as good as what has been obtained with commercial ion traps or with other miniature traps, it may improve as the researchers continue to advance the technology, Ramsey points out. In the future, the researchers want to push this technology even further. The analyzers have a radius of 20 µm, but the electronics and pumps have not been miniaturized yet. The final size of the mass spectrometer, therefore, is still quite large, so one of the group’s goals is to reduce the overall size of the device, says Whitten. The researchers also plan to develop even smaller analyzers, perhaps with a radius of
