A mass spectrometer in every hand

Abstract

Portable mass spectrometers have been around for decades but have never quite achieved their potential for several reasons. Though the size and weight of these instruments have been inching steadily downward, only recently have the spectrometers reached a weight at which a typical scientist might be able to lift one (<10 kg). In addition, the amount of power consumed by previous models has made it difficult to run the instruments off of a battery for any length of time, and the types of samples that can be analyzed have been limited because only electron impact ionization sources could be coupled to portable mass spectrometers. Now, in a recent AC paper (2008, DOI 10.1021/ac801275x), Graham Cooks, Zheng Ouyang, and colleagues at Purdue University have introduced the smallest, lightest, and least-energy-consuming portable, autonomous mass spectrometer to date. And, as an added bonus, the new instrument is compatible with many ionization sources, including ESI and several atmospheric-pressure ionization (API) sources. Ouyang is a former student of Cooks at Purdue (and is now a professor of biomedical engineering at the same institution), so he has been working on the portable mass spectrometer project for years. “I remember at the beginning it started because Graham saw this need for a personal mass spec,” says Ouyang. “A small mass spec can be used for multiple applications, because MS technology is obviously so useful for general purposes.” He adds that applications specific to portable mass spectrometers include homeland security screening, environmental and quality-control monitoring, and some biomedical applications like analysis of tissues and biofluids. The current instrument, which the researchers call the Mini 11, weighs in at just 9 lb, with dimensions of 22 × 12 × 18 cm. That’s a far cry from the Mini 5, the original portable instrument developed by the researchers several years ago; it weighed >100 lb. “You could barely call it ‘portable’,” says Ouyang. “It was portable for Olympic weightlifting athletes!” The new instrument consumes just 35 W of power, in contrast with the 150 W required for the Mini 5. The most recent model, the Mini 10, was introduced in 2006; it weighed 22 lb and consumed ~70 W of power (Anal. Chem.2006, 78, 5994-6002). But perhaps the most significant achievement of the Mini 11 is its discontinuous atmosphere-pressure interface (DAPI). On previous miniature instruments, ions were introduced into the analyzer via a multiple-pumping-stage interface, in which pressure is reduced stepwise from ambient to the millitorr levels required for mass analysis. “The pumping system is the most expensive, most heavy, and the most-power-consuming part of the whole system,” says Ouyang. In designing the DAPI system, “what we did instead of letting the ions go through the multiple vacuum stages [is,] we let them jump from the air directly into the vacuum.” Rather than leaving the entry channel open all the time, as in the old design, a DAPI interface opens for ~20 ms and funnels the ions directly into the ion trap. After the interface closes, a small pumping system reduces the pressure to the millitorr range in ~200 ms. Besides reducing the size and power requirements of the instrument, the DAPI interface allows the researchers to couple ESI and API ion sources, for example, DESI, to the Mini 11. With the previous generation of miniaturized mass spectrometers, the researchers were limited to analyzing volatile organic molecules by using electron impact ion sources; now, the applications can be expanded to the analysis of larger nonvolatile molecules like peptides and proteins. “It’s worth noting that while one has to be prepared to give up some performance [with a miniature instrument], we’ve been very surprised at how little we’ve had to give up,” says Cooks. “That includes being able to look at proteins. We really didn’t expect to be able to get reasonable protein spectra on these tiny instruments.” Although Cooks and Ouyang are happy with the Mini 11, they say that it could be even better. “We need to improve the performance of some of the ionization methods... We’ve got nanogram and subnanogram sensitivity in particular cases, but we would really like to get the performance improved in terms of lowering the detection limits,” says Cooks. “We don’t think that there’s any rational reason why you would stop where we are right now.”