For field applications, "miniature" and "rapid" have become almost synonymous, yet these small mass spectrometers are not useful if performance is too severely compromised. (To listen to a podcast about this feature, please go to the Analytical Chemistry website at pubs. acs.org/journal/ancham.)MS is widely regarded as the gold standard for chemical analysis with respect to selectivity, detection limits, and broad applicability. However, mass spectrometers are comparatively delicate instruments, partly a consequence of the need for a vacuum system into which the sample is introduced. There have long been efforts at direct MS analysis of complex mixtures, 1 but extensive sample cleanup and increasingly sophisticated multianalyzers have been involved. These considerations have limited the applications of MS outside the laboratory. It is striking that although progress in many areas of technology is strongly associated with miniaturization, this correlation is barely evident in MS.Nevertheless, for many applications of MS, in situ experiments would have significant advantages over laboratory measurements, even if there are significant losses in analytical performance. Some applications of immediate interest include environmental monitoring (especially surveys that require large numbers of samples), quality control, food safety, forensics, security and public safety, and clinical diagnostics. The objective of this article is to summarize recent progress in developing handheld miniature mass spectrometers. Note that the focus is on complete systems, not particular components, even important ones like the mass analyzer.
ON THE SMALL SIDEInterest in small mass analyzers stretches back several decades. For example, a hyperbolic ion trap analyzer the size of a quarter with a 2.5-mm radius and an m/z range of 70,000 was reported in 1991.2 A Mattauch-Herzog-type, nonscanning, double-focusing sector mass analyzer was reported in 1991, and a very small, double-focusing, crossed electric-and magnetic-field sector analyzer was described in 2001.3,4 Recent work by Ramsey, Austin, Short, and others has advanced small analyzers further, and the emphasis is increasingly turning to microelectromechanical systems (MEMS)-fabricated mass analyzers. [5][6][7][8] However, only in the past decadesand especially in the past 5 yearsshas significant progress been made in the development of small, low-power, portable, autonomous mass spectrometer systems; some have progressed to the point of commercialization. A review in 2000 summarized the mass analyzers used in miniature instruments, showing examples of virtually all the common types, including quadrupole mass filters, quadrupole ion traps, magnetic sector fields, TOF, and FT ion cyclotron resonance instruments. 9 More recent work has seen the commercialization of a novel toroidal ion trap system. 10 A website devoted to small instruments and a conference series on MS in harsh environments prominently features handheld mass spectrometers (www.gcms.