FTIR spectroscopy as detection principle in aqueous flow analysis

Whereas FTIR spectroscopic detection is routinely used in gas chromatography (B. Erikson, Anal. Chem., 1998, 70, 801A), its use for liquid chromatography (LC) and flow injection analysis (FIA) is a rather exotic exception. The most prominent reason is the strong IR absorption of most of the common solvents, especially water. Hence FTIR spectroscopy is normally not even considered a valuable detection method. This practice neglects that FTIR spectroscopy offers some unique features which now, using modern instrumentation, can be exploited in an advantageous manner. It is the aim of this Highlight article to demonstrate the wide range of possible applications in LC and FIA. To regard FTIR spectroscopy as too exotic for routine use may be a luxury paid for with the neglect of a simple analytical approach. The term flow analysis (FA) will be used to provide a common cover for both LC and FIA because both rely on the injection of a sample into a flowing stream, passage through a modulator and recording of transient peaks. Although the processes taking place in the modulator are different, being chemical reactions in FIA and separations in LC, the same interfaces can be used for a FTIR spectrometer as a detector. The interfaces used can be divided into two categories, flow through cells where the liquid is probed directly, and solvent removal interfaces where the analyte is separated from the carrier liquid prior to detection. It is necessary to emphasize the complementary nature of these techniques (D. E. Pivonka and K. M. Kirkland, Appl. Spectrosc., 1997, 51, 866) and this Highlight will stress their particular strengths and weaknesses. Special focus is laid on aqueous phase systems because of their high importance in biological systems. As water is certainly the most challenging solvent for IR detection, equal or even better performance of the presented approaches can be expected for other solvents. Additionally three developments will be discussed in detail with respect to their prospects for FA-FTIR instruments: the increasing availability of sophisticated chemometric methods, the miniaturization of analytical instruments and applicaton to combinatorial chemistry.