Focus will want to avoid trying to determine an ether in an eluent of tetrahydrofuran. • A microcomputer can be used to subtract solvent spectra (acquired beforehand) from sample spectra. Although gradient elution presents a more complex problem, the correction can still be made: Spectra are first obtained from sequential time frames in a blank run (eluent only), and each spectrum is subtracted from the corresponding time frame for the sample run.
• Peter Griffiths of Ohio University has been experimenting with solvent elimination from LC effluents. His technique works well with nonaqueous solvents, but it cannot be used when water is a component of the mobile phase. In addition to the problem of solvent interferences, there are serious sensitivity limitations in LC/FTIR. Nevertheless, the technique has been gaining in popularity for particular applications. Examples include the determination of the distribution of
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Galileo Electro-Optics Corp. CIRCLE 121 ON READER SERVICE CARD 332 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
functional groups of polymers as a function of their molecular weight (size exclusion LC/FTIR), functional group analysis of solvent-refined coal, and pesticide residue analysis. Precolumn and Postcolumn Reactions Chemical reactions have long been used to enhance chromatographic analyses, and work on such techniques for LC continues apace. The basic idea is to change the analyte into something that is more amenable to detection than the original analyte. For instance, derivitization can be used to add on a functional group that either absorbs or fluoresces, making it possible to determine a nonabsorbing and nonfluorescing substance with an absorbance or fluorescence detector. Such a technique is the basis of automated amino acid and amine analyzers that are available commercially. These analyzers employ postcolumn addition of ninhydrin or o-phthalaldehyde reagents. Another example, given by Sj. van der Wal of Technicon Instrument Corporation at the EAS meeting, involves the postcolumn addition of a reagent to samples of body fluid. The reagent reacts with specific corticosteroids in the samples to produce a chromophore that absorbs in the visible region. Detection limits for this technique are in the nanogram range. A third example is a postcolumn procedure recently reported by C. W. Thorpe and A. E. Pohland of the Food and Drug Administration, involving the reversed-phase analysis of corn and peanut product extracts. Water saturated with iodine is added after the separation to enhance the fluorescence of aflatoxins in the samples. This procedure has been used to detect aflatoxins Bi and Gi at levels as low as 2 ng/g. A number of reagents may also be used to form electroactive derivatives susceptible to determination with electrochemical detectors. There are many other promising techniques being investigated for their applicability to LC detection. In fact, just name any analytical technique and chances are it's being considered as an LC detector. This includes chemiluminescence, atomic absorption spectrometry, inductively coupled plasma spectrometry, ion-selective electrodes, Raman spectrometry, and photoacoustic spectroscopy. Many other detection schemes could be included on this list. Perhaps from one of these concepts the ideal LC detector, one that is universal, sensitive, and perhaps even inexpensive, will yet emerge. Stuart A. Borman
