Daniel Armstrong on Ionic Liquids, Capillary Electrophoresis, and Future Research

LCGC recently interviewed Daniel Armstrong, Professor of Chemistry & Biochemistry, and the Robert A. Welch Chair of Chemistry at the University of Texas at Arlington on his research and upcoming presentations at Pittcon.

Please tell us about your recent work with ionic liquid (IL)–based capillary gas chromatography (GC) methods coupled with thermal conductivity detection (TCD) to determine the water content in liquid samples.

Armstrong: Ionic liquids are the first new class of GC stationary phases in many decades. They are mostly orthogonal to conventional molecular stationary phases. We have synthesized ILs to accentuate a variety of desirable properties — from unique selectivity to high stabilities. Recently, we synthesized three IL stationary phases that are completely stable to water and oxygen even at high temperatures. In addition, they were engineered to produce efficient, symmetrical water peaks. This allowed us to quickly and effectively measure water concentrations in all manner of samples and solvents. Conversely, we can easily separate low levels of organics from water and quantify them. This is a modern, more broadly applicable and effective manifestation of a packed column method I used back in the 1970s. Then we used a molecular sieve as the stationary phase. This only worked well for a few specific sample types. Today with ILs, we analyse more diverse samples at any concentration and do it more quickly and accurately. The same columns can be used to analyse the water vapour content of gases. (For more on this topic, please read for the featured article by Dan Armstong et al. at chromatographyonline.com/ArmstrongFeb2012 or in LCGC’s February 2012 print issue.)

Many people consider you the “father” of micelle and cyclodextrin-based separations. What was your first groundbreaking moment? Can you tell us about some of your early research?

Armstrong: I realized three things in the mid-to-late 1970s. One was that you could deliberately use the unique properties of micelles and cyclodextrins to enhance analytical methodologies (both separation and spectral). Second, you could theoretically treat these systems using a pseudophase model (also known as multiphase treatment or complex equilibria). Third, you could not only use separation instruments (chromatography and electrophoresis, for example) to achieve separations, but also as tools for obtaining physicochemical data on dissolved species (such as binding or association constants, diffusion data and some kinetic parameters).

The pseudophase model has been applied to all manner of separations, but had perhaps the greatest analytical impact in micellar capillary electrophoresis (CE). In spectroscopy and spectrometry, we studied and utilized the surfactant effect on aerosols. We used it to explain signal enhancements in atomic absorption, signal suppression in CE–electrospray ionization mass spectrometry (ESI-MS), and recently, the ultrasensitive detection of anions in positive-mode ESI-MS. The latter of these used the multifunctional cations derived from our ionic liquids. It became clear that the practical ramifications of all this work were as important and useful as the theoretical and mechanistic studies. Of course, our early cyclodextrin work lead to the first practical commercial reversed- phase chiral stationary phase in 1983. Subsequently, the field of chiral separations exploded and altered both the federal regulatory environment and the pharmaceutical industry worldwide.

Your work in capillary electrophoresis also has given rise to groundbreaking discoveries. What was the most challenging aspect of that research and what benefits do you hope will come from it?

Armstrong: Using CE for enantiomeric separations and the measurement of affinity constants caught on quickly, but after a while as an area matures, you get bored and look for new avenues to pursue. In the late 1990s, we began looking at whole intact cells (that is, bacteria, yeast and viruses). At that time it was common to analyse the contents of lysed single cells by CE. It also was known that cells could be moved by electro-osmotic flow. However, without some manner of effective separation or other means of selective identification, this was not particularly useful and, therefore, was not being pursued. At this point, we developed a means to effectively separate, identify, and quantify intact bacteria, fungi and other cells with CE. We could tell live from dead cells. We examined the mechanism and then developed a variation of this CE method, which allowed it to be used as a rapid test for sample sterility. We found that our initial papers spurred a substantial increase in research involving microbial analysis by CE and microfluidic systems.

At Pittcon this year you or someone from your research group will be presenting at a number of technical sessions. What session or research are you most excited to discuss with your peers?

Armstrong: We will be making a number of presentations in three of our main research areas. They are not really related to each other except for the fact that we are interested in all of them. To pick one, I believe our symposium “Ionic Liquids in Separations and Mass Spectrometry” will be particularly exciting. All of the people presenting will be discussing forefront work and their latest advances. Interesting applications include the IL extraction and MS analysis of archaeological samples. I’m really looking forward to this session. It will be the first IL symposium ever held at Pittcon.

What is your next big research project?

Armstrong: I can’t give you specific details right now, but we have a new material or support that can be used in separations. It can be used as particles packed in a chromatography column, as a nonshrinking monolithic phase, as an extraction media, or even a chip-based material for microfluidics. It is not based on any of the known and commonly used materials such as silica, polymers, alumina, titania, zironia or graphitic carbon. We believe it could have a major impact on the way future separations are done.

For more on Armstrong’s work, please visit his website at www.uta.edu.