The Rise of an Alternative Ion Source

Inductively coupled plasma (ICP) sources have been around for so long that most scientists don’t even question their capability to get the job done. However, an ion source for elemental mass spectrometry (MS) has been developed that offers different benefits compared to ICP sources: the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source, which features small sample volumes, low uptake rates and waste, and low operating power. Primarily developed for work in metal speciation, the LS-APGD source is moving into other areas of research such as laser ablation. R. Kenneth Marcus, a professor in analytical chemistry at Clemson University and the winner of the 2015 Lester W. Strock award, recently spoke to Spectroscopy about his group’s work developing the LS-APGD ionization source and where he sees its use heading in the future.

What are the most important questions or problems that atomic spectroscopy needs to solve in the next 5–10 years?Marcus: It seems to really come down to what chemical and physical systems can benefit from more comprehensive elemental information. But you do not know what you do not know. Once people started to realize that chemical speciation determines aspects of nutrition and environmental transport, the question became what level of quantification and chemical specificity was really required. It's the bane of the analytical chemist’s existence; no one knows what is significant until it can be measured; that is, better methods tend to open Pandora’s box further. The challenges to atomic spectroscopy are driven by our prior successes. So where are we going: smaller samples, higher throughput, greater absolute sensitivity, and greater levels of chemical information. Atomic spectroscopy isn’t just about determining the concentration of lead in drinking water any more, so we need new tools that are developed to explicitly meet new challenges. This is where microplasmas come into play.  

What role does your research play in tackling these areas?Marcus: The LS-APGD microplasma was initially developed in approximately 2001 to address the then burgeoning area of metal speciation (1). There was an inherent incompatibility with the solvent flow rates, composition, and sample sizes that exit the typical reversed-phase and ion-exchange separation columns and conventional atomic spectroscopy sources. A simple liquid chromatograph (LC) coupled to a 1–2 kW, 16-L/min argon flow, 1-mL/min aqueous sample uptake ICP is not a natural match. In fact, Thermo Jarrell Ash tried to commercialize such a system back in the late 1980s. In reality, the major driver was the long known fact that chromatographers will only incorporate detection methods that are of the same operation scale and complexity as the separation itself. The history and evolution of gas chromatography–mass spectrometry (GC–MS) bears this out. We were looking for a plasma that could operate at flow rates below 100 mL/min, perhaps mixed-phase solvents, and volumes of <10 mL. The LS-APGD source meets all of the operational metrics, though it has take a dozen years to realize its potential in atomic spectroscopy: small sample volumes, low uptake rates and waste, diverse solvents, low operating power, and greater chemical information content.  

What benefits does the LS-APGC ionization source offer compared to other laser ablation and elemental MS techniques?Marcus: As I said, we actually started working with the LS-APGD microplasma as an optical emission source almost 15 years ago. Frankly, the device received no traction in terms of general acceptability. That was probably because of the fact that I pitted it against the ICP, as opposed to it being a complementary device. At the same time, manufacturers decided that they did not need alternatives to the veritable ICP, and so they were not interested. In reality, the microplasma does not compete with the ICP doing what an ICP does well. Any benefits come when the analytical parameters I described above (sample size, solvent composition and rate, cost, complexity, and so on) are important. It's a total package to consider, and there will obviously be trade-offs. For example, if a microplasma can yield direct speciation information (such as a molecular mass spectrum) on 10-mL volumes, in mixed solvents, at parts-per-billion levels, is that better than using a method that delivers parts-per-trillion detection limits, but only provides elemental information? It is in this scenario that the LS-APGD source makes pragmatic sense. Beyond the basic operational space, the fact that the microplasma can be coupled to virtually any atmospheric pressure MS interphase (for example, electrospray ionization [ESI] or atmospheric pressure chemical ionization [APCI]) means that it can be used as an alternative source on those (far more numerous) mass analyzers. Put another way, I can do elemental MS using the LS-APGD ionization source on an organic instrument, rather than having a dedicated instrument that only does elemental MS.  

In a recent paper (2), you described using an LS-APGD ionization source to ionize metal particles within a laser ablation aerosol. Can you tell us about that work? Is this one of the primary application areas for the LS-APGD ionization source?Marcus: The coupling of laser ablation sampling with an ICP, while highly successful, is sort of a mismatch as you are introducing micrometer-sized particles into a multiple-milliliter ICP plasma volume.  No doubt that the vaporization is highly efficient, but it is perhaps overkill and there may be actual losses in sensitivity simply because of dilution. As it turns out, the power density of the LS-APGD is much higher than the ICP, with ~50 W dispersed over a single cubic millimeter volume. We do not know yet the absolute efficiency of the overall particle vaporization–ionization processes, but here again, there may be a better match in terms of the size and complexity for this coupling. We have a long way to go to define the efficacy of the laser ablation-to-LS-APGD coupling, but the results to date suggest that the application makes good practical sense. I could see a very simple tabletop package put together, particularly with modern, compact optical emission spectrometers.

Another recent publication (3) described the use of the LS-APGD ionization source in ambient desorption–ionization (ADI) mass spectrometry. How are ambient ionization sources different from common atmospheric pressure ionization sources, and what are the advantages of the LS-APGD source when it is used for ADI?Marcus: There have been a large number of reviews of the general topic of ADI-MS, with probably the greatest point of consensus being that there seem to be as many different designs as there are practitioners. The entire genre is typically divided into devices that use low-power gaseous plasmas as the means of generating the ionizing species and those where ionizing agents are derived from the solution phase. The LS-APGD microplasma is a hybrid of the two. It is too early to say the specific analytical benefits relative to the other designs in terms of the typical applications of small-molecule assays of practical sample surfaces. What is definitely an advantage is that the same ionization can be used for solution analysis, sampling of laser ablation-generated particles, and small molecules on surfaces without any hardware changes. The only change is the pointing of the plasma toward a surface for ADI-MS or toward the MS sampling orifice in the other cases; a change that takes less than 1 min! Add to this, the ability to perform elemental (atomic) analysis, direct speciation of organometallic compounds, and organic molecule MS with the same source and on a single mass spectrometer. So, while the device may not be superior (by whatever definition) to a given device for a singular analysis, the low cost, low complexity, and analytical versatility are definite advantages.

What are the next steps in your research?Marcus: I feel that we are on the cusp of big things on a couple of fronts in terms of applications. Frankly, it's a challenge to not try to throw wet spaghetti against the wall, just to see what sticks. To be clear, any analytical device can only be used at its ultimate utility when you understand the underlying chemical and physical processes. So a large component of our research program is directed studies that many people may see as mundane, but when you are using novel spectrochemical devices they tend to provide incredible insights into new opportunities. Our present funding, through the United States Department of Energy (DOE) and the Defense Threat Reduction Agency (DTRA), focuses on the versatility of LS-APGD as a potential field-deployable approach for elemental, isotopic, and molecular analyses. So, what we are realizing is very much in line with the objectives we projected a dozen years ago. 

References

(1) R. K. Marcus and W. C. Davis, Anal. Chem. 73, 2903 (2001).
(2) A.J. Carado, C.D. Quarles Jr., A.M. Duffin, C.J. Barinag, R.E. Russo, R.K. Marcus, G.C. Eidena, and D.W. Koppenaal, J. Anal. At. Spectrom. 27, 385 (2012).
(3) R.K. Marcus, C.Q. Burdette, B.T. Manard, and L.X. Zhang, Anal. Bioanal. Chem. 405, 8171–8184 (2013).