COVER STORY
Scientists discuss recent addition to types of
STATIONARY PHASES available for gas chromatography CELIA HENRY ARNAUD, C&EN WASHINGTON
TWO YEARS AGO, room-tempera-
ture ionic liquids became the first new type of commercial stationary phase to hit gas chromatography in more than three decades. Scientists have known about ionic liquids for more than a century, but their usefulness for GC wasn’t proven until 2005. The first commercial ionic liquid stationary phases were introduced by Supelco, a division of Sigma-Aldrich, in 2010. In a Pittcon symposium, separation scientists described the use of ionic liquids as GC stationary phases and as matrices for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Ionic liquids are salts that are liquid at room temperature. They generally consist of a bulky organic cation, FIBER COATING different from those of suchas1-butyl-3-methylimidazolium, Polymeric ionic other stationary phases, can be used paired with an anion, which can range liquids they could play a big role as coatings for solidfrom simple halides to bulky comphase microextraction in multidimensional GC, pounds such as triflate. he said. The separation fibers, as shown in this scanning electron selectivity of ionic liquid Several characteristics of roommicrograph. temperature ionic liquids make media can be tweaked them particularly attractive as GC by changing the cation, stationary phases, said Daniel W. anion, or both. Armstrong, a professor of chemistry and A particular strength of ionic liquid biochemistry at the University of Texas, stationary phases, Armstrong said, is that Arlington, who was the first to show that they can be used for the analysis of water— ionic liquids could be used as stationary not just compounds phases. Foremost among these properties in water, but water in are their high thermal stability and high other solvents. Armviscosity. Ionic liquids are stable over a strong’s group anabroad temperature range, from –40 °C to lyzed the water conmore than 450 °C. Triflate-containing ionic tent of 50 solvents. The liquids have particularly good thermal detection limit using stability, Armstrong said. That thermal GC with ionic liquid stability translates into columns that last columns was 5,000longer before “bleeding” or degrading. fold better than the deTheir high viscosity means they are easy to tection limit of the Karl use as coatings for columns, said Leonard Fischer method, the M. Sidisky, a scientist at Supelco. conventional titration Ionic liquid columns are more polar method for quantifying than any other commercially available GC trace amounts of water. columns, Armstrong said. Because they Luigi Mondello, a provide a separation that has selectivity professor in the analytiWWW.CEN-ONLIN E .ORG
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cal and food chemistry division at the University of Messina, in Italy, has done much to characterize the chromatographic performance of ionic liquid columns. Compared with other columns, ionic liquid columns can be run at higher temperatures, he said. The observed number of theoretical plates of such columns, a measure of column efficiency, is in good agreement with theoretical estimates of those values, he said. Mondello has used ionic liquid columns to separate fatty acid methyl esters. The ability to resolve cis and trans fatty acids is particularly important in food analysis because of labeling requirements for trans fats. With a 100-meter ionic liquid column, Mondello fully separated 20 out of 22 cis/trans fatty acids. COURTESY OF JAR ED AN DERSO N
IONIC LIQUIDS IMPROVE SEPARATIONS
SUPELCO’S SIDISKY showed how
changes in the ionic liquid translate into changes in the separation of a test mixture of compounds. The retention of an aliphatic compound like tridecane shifts relative to that of an aromatic compound like toluene depending on the polarity of the stationary phase, he said. Similarly, increasing the polarity changes the retention times of the BTEX (benzene, toluene, ethylbenzene, and xylene) components of gasoline relative to one another. The better elution control provided by ionic liquids can translate into better separations. In the case of polychlorinated biphenyls, Sidisky said, one of the ionic liquid stationary phases seems to be ideal. Most separations of PCBs are plagued by coelution, particularly of compounds that can’t easily be distinguished in other ways. But the few PCBs that coelute when a particular ionic liquid stationary phase is used can be differentiated by mass. Ionic liquids, including polymeric ionic liquids, can be used not only as GC stationary phases but also as coatings for solid-phase microextraction fibers, according to Jared L. Anderson, a chemistry professor at the University of Toledo, in Ohio. Whereas most ionic liquids used in GC are dications, Anderson uses monocationic linear polymeric ionic liquids.
COU RT ESY O F R ICO DEL S ESTO
structure and extracting the dyes. Because the dyes absorb ultraviolet light from the MALDI laser, the extract can be used as its own MALDI matrix. In one textile, Del Sesto and his colleagues identified the synthetic dye acid red 51 and a mothproofing agent. In another sample, they quantified a natural dye called cota. “One thing is clear, and it is that ionic
Depending on how the polymer is functionalized, such ionic liquids can be used on microextraction fibers to preconcentrate a range of analytes including hydrocarbons, carbon dioxide, genotoxic impurities, and fatty acid methyl esters. The polymeric ionic liquid coatings are quite resistant to damage from complex matrices, Anderson said, making them particularly useful for environmental analysis. Fibers coated with specially designed polymeric ionic liquids canbeusedforhigh-temperatureheadspace extraction as well as for monitoring the enantioselectivity of chiral syntheses, he said. Rico E. Del Sesto and his colleagues at Los Alamos National Laboratory have shown that they can get double duty out of ionic liquids by using them as both an extraction solvent and a MALDI matrix. They found such a combination to be effective for forensic analysis of wool textiles. Many such textiles are dyed in small batches, and the dye signature can help pinpoint the origin of the textile. Wool is tough to break down, and many analytical methods destroy the dye along with the wool. Ionic liquids are ideal for breaking down the keratin in wool while simultaneously extracting the dye. Del Sesto and his colleagues synthesized phosphonium ionic liquids that contained ferulate, α-cyano-4-hydroxycinnamate, or 2,5-dihydroxybenzoate anions. Those anions are often used in conventional MALDI matrices, and the resulting ionic liquids can be used instead of a conventional MALDI matrix. Del Sesto and his colleagues used the ionic liquids to analyze different textiles and dyes, with chloride-based ionic liquids being most effective at disrupting the wool BUILT-IN MATRIX
A chloridecontaining ionic liquid (top left) is used to extract cota dye from wool textile (top right). Because the dye absorbs UV light (bottom), the extract can be used for MALDIMS without an additional matrix.
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liquids are going to play an important role in the future of separations,” Armstrong told C&EN. “This includes everything from sample preparation to high-end analytical instrumental techniques to process-scale purifications. The average person may not realize that many of these techniques and processes are already in place and many more are in the development stage.” ◾
