Development of Environmentally Compatible Mobile Phases for Separation

     The widespread usage of many organic solvents that are currently relied upon in chemical processes is under increasing restriction due to environmental/hazardous waste concerns and potential toxicity.  Several examples of this can already be noted for solvents being phased out or restricted in their usage.  For example, the once common solvent carbon tetrachloride is now rarely found in many laboratories.  Many other currently used solvents such as dichloromethane and acetonitrile are slated to follow this same trend in the coming years.  In terms of analytical separations, many of these solvents are standard components in countless extraction and/or chromatographic methods that have evolved over time.  Therefore, going forward, novel solvents which are environmentally compatible and non-toxic that can replace these conventional organic solvents are needed.  Our laboratory is working at investigating new mobile phases that meet this criteria and are capable of potentially acting as new solvents for the future.  Moreover, many of these solvents possess unique and beneficial properties in addition to their inherent solvent capabilities.

     One solvent that has interesting potential in this regard is water.  While obviously non-toxic, water is also an inherently polar solvent and frequently used in binary mobile phase mixtures in analytical chemistry.  However, an interesting property of water is that it becomes a considerably non-polar solvent if it is pressurized and heated beyond its boiling point.  As such, this 'sub-critical' state of water can be used to dissolve and mobilize analytes of varying polarity in extractions and chromatography.  The capabilities of water in this regime are impressive.  For example, the solubility of benzene in water increases nearly a million times between room temperature and 200 ^oC.

     We have been working in the area of Subcritical Water Chromatography (SWC), which takes advantage of this property of water.  By placing the column in a simple GC oven and only using water as the mobile phase, temperature programming of the separation column can then change the mobile phase properties in-situ.  Thus, reverse phase type separations can be achieved, which mimic organic solvent gradients by raising the temperature of a purely water mobile phase.  While this arrangement is environmentally compatible, it is also well-suited for using the desirable universal carbon sensing Flame Ionization Detector, which conventional LC cannot use due to the organic solvents present in the mobile phase.  In efforts to make SWC even more applicable to non-polar analyte analysis, we have also been exploring the addition of liquid carbon dioxide to water under SWC conditions.  This has greatly extended the capabilities of SWC in this regard without the need for excessively high temperatures.  Further, it also maintains the environmental and detector compatibility benefits of the mobile phase.  Work continues in this area exploring ways to expand the capabilities and applications of this mobile phase approach.