Field-portable mass spectrometers for onsite analytics: What's next?

International News on Fats, Oils and Related Materials : INFORM, Oct 2009 by Mulligan, Christopher C

Editor's note: The following article is adapted from the author's Hot Topic presentation delivered at the 1 00th AOCS Annual Meeting & Expo, held May 3-6, 2009, in Orlando, Florida, USA.

The need for rapid, onsite chemical analyses is as apparent as it has ever been, especially for compounds detrimental to health or to the environment, such as toxic industrial species, explosives, chemical warfare agents, and environmental toxins. Onsite analysis can allow early warning of a harmful release, saving time and resources and increasing safety compared with lab analysis of field samples. The utility of afield-portable, analytical device goes well beyond security applications, though. Many chemical analyses would benefit from being performed at the original location or native environment of the sample of interest.

Several analytical technologies with varying levels of performance and feasibility have emerged as candidates for continued development and evaluation as portable analytical instruments. Of these technologies, several exhibit significant drawbacks, including long analysis times, high false positive and negative rates, high limits of detection, and narrow applicability. Mass spectrometry (MS) has the potential to fulfill the major criteria for field-portable analytical instruments. Of the general-purpose methods of chemical analysis, MS has proven to be one of the most sensitive techniques, and detection of ultra- trace quantities of specific compounds has been demonstrated, even from complex mixtures (as little as 1 part in 1012, even ca. 1O-15 g in favorable cases). The high specificity of MS comes from tandem mass spectrometric analysis (two or more coupled stages of mass analysis), which allows both molecular weight and structural data to be gathered.

Recent advances in vacuum and electronic technologies have led to the miniaturization of MS instruments, thus making portability feasible, and commercialization of portable MS systems has grown rapidly. Most of the commercial equipment currently available uses membrane introduction or gas chromatography (GC) in combination with MS. Although this offers an added degree of separation, it also increases the time required for analysis. In a typical GC/MS analysis, preconcentration, followed by thermal separation of components on a capillary column, may increase the analysis time to several minutes or hours, with the actual MS analysis taking only a small percentage of the total analysis time.

Several research groups have designed and reported handheld MS prototypes (Ouyang et al., 2009), and while these instruments typically require as little as 50 ms to acquire a complete mass spectrum, conventional sample preparation typically can take several hours, depending on the sample matrix in question and the desired analyte. While a significant savings in time is realized by performing analyses onsite, these instruments still require extensive preparation of samples before their introduction into the GC interface and lack the versatility in analysis and breadth of applicable samples that is desired for field-portable instrumentation.

Ionization of analyte outside of the instrumental vacuum system can be advantageous compared with traditional analytical methods that require extensive sample preparation and ionization in vacuo. Electrospray ionization (ESI), for which John Fenn was awarded the Nobel Prize in Chemistry in 2002, readily allows for the analysis of chemicals in solution and has revolutionized the analysis of biological macromolecules and thermally labile compounds. Atmospheric pressure chemical ionization (APCI) has allowed rapid analysis of volatile and semi volatile chemical species in complex gas matrices without preconcentration. Techniques such as ESI and APCI have allowed analysis of liquid and gas phase ana- lytes, but there are still substantial limitations to these ionization methods. Using ESI requires that pure liquid analytes be dissolved into organic solution before spraying and that natural aqueous solu- tions (e.g., contaminated groundwater) undergo extensive prepara- tion, including filtration, desalting, and addition of organic solution, in order to attain acceptable ionization efficiencies. There is also one glaring constraint: the lack of surface analysis ability.

The ability to directly analyze untreated condensed phases and surface-bound species at ambient conditions had not been investigated until recently. New ambient mass spec tro metric methods (Cooks et al., 2006), in which ionization takes place both at atmospheric pressure and directly from the native sample without prior preparation, have rapidly been developed, and several interesting applications have come from their implementation. As recently proposed by Venter et al. (2008), these newly developed methods can be distinguished by the processes used for desorbing analytes from the sampling surface (momentum, laser, or thermal desorption) and the subsequent ionization (ESI or APCI) of these molecules.