Mass Spectrometry

The power and versatility of mass spectrometry (MS) make it an analytical technique of choice in the laboratory. Participants in this Technology Forum are Kevin A. Schug of the University of Texas at Arlington and Michael P. Balogh of Waters Corporation.

Spectroscopy: Claims made for some additions to the analytical arsenal such as “lab-on-a-chip” and some MS ionization techniques have not played out to the degree expected. Others such as electrospray ionization (ESI) have become the backbone of modern practice in less than 20 years. Where do you think ambient thermal desorption techniques such as desorption electrospray ionization (DESI), direct analysis real time (DART), and atmospheric solids analysis probe (ASAP) best fit?

Schug: Every technique has its advantages and limitations, and these factors invariably control the range of applications for each. If you consider the notion of ambient ionization as a whole, I think you see a range of fairly simple and easily integrated techniques that have quickly moved from the discovery phase into practical use. There are so many choices of ambient ionization techniques out there, so this inherently adds complexity to the area, but the truth is that some of these have real benefit in addressing limitations of mainstream techniques, like ESI. I think they will continue to be integrated into routine analytical use and be a common tool in the toolbox for many researchers in the year to come.

Balogh: Some techniques may show promise but to become commercially successful, they need to score well for cost effectiveness, robust performance, ease of use, and ability to fit existing workflows. DESI bargains off the already well-established acceptance of ESI since the early 1990s, extending the analytical utility of ESI to samples in their final form, or at least not requiring dilution–solvation before injection as in liquid chromatography–mass spectrometry (LC–MS) (although in the execution solvation does occur at the sample surface). DART and the closely related ASAP technique similarly act directly on samples but without dilution–solvation relying on the volatility of the compounds responding to hot gas to release them into the atmosphere in the presence of metastable ions for secondary ionization (where ESI as the name implies is the primary ionization technique). Atmospheric pressure chemical ionization (APCI) has been in play as long as ESI and a technique like ASAP is an atmospheric modernization of the common practice gas chromatography–mass spectrometry (GC–MS) using a probe inserted into the MS vacuum for analysis of solids.

Since ESI has been so successful handling polar compounds in solution with great sensitivity the other techniques are perhaps overlooked. They in essence rely on APCI, also a well-established technique, covering much of the same chemical realm as ESI, even extending coverage beyond the reach of ESI to nonpolar–neutral analytes.

Spectroscopy: Is one technique better suited to a specific application than another? Can you give examples?

Schug: There are definitely different techniques that are suited to different applications. For example, DESI and DART, while demonstrated for liquid systems, is most powerful for analysis of solid samples. Integration of these and other similar setups into high-throughput routine quality control are a real phenomenon. Recently, our group published an overview of the combination between ambient ionization and different separations techniques (Curr. Trends Mass Spectrom., 8–14 [October 2011]). This review shows the plethora of ambient ionization techniques that can handle flowing gaseous and liquid phase samples. I believe that this combination is going to be a growing area of research, as more researchers use ambient ionization to analyze complex mixtures. Additionally, some techniques, such as extractive electrospray ionization (EESI) have shown an impressive capacity for discrimination of matrix suppression effects.

Balogh: Adaptations of DESI to tissue imaging have shown promise beyond the initial work with solid final form samples such as drug tablets and foods where the “APCI alternatives” have shown greater applicability to final form product testing and a variety of uses requiring in situ surface analysis. As practitioners we develop intuition working so successfully with LC–MS as an ESI platform and need to take a step back and reassess how we approach the analysis with these new applications of an old technique. For instance, the samples in ESI need to be more basic than the solvent to ionize so we add acid to the mobile phase; we accept the dilution factor and need for sample prep and deal with ion suppression affecting our quantitation (common in ESI but not in APCI).

Faced with undiluted samples, contrary to previous practice, we need to be careful to not introduce too much sample. The great advance of time-of-flight (TOF) MS technology in recent years allows differentiation by increasingly accurate mass response in “unprepped” samples but will place greater burden on the MS linear dynamic range response to accurately deal with extreme ranges of compounds the materials. Any experiment performed at atmospheric pressure provides an opportunity to preferentially induce adductions enhancing our understanding of elements in a complex mixture while at the same time confounding us with the results since the compounds do exhibit these preferences. Coupling hybrid techniques such as ion mobility with MS to differentiate these mixtures will likely be one of the key enablers of these technologies.