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Chapter 8
Liquid Chromatography IR Spectroscopy (LC/IR) Tandem Systems

Infrared spectra, in general, provide more information on molecular structure than UV spectra, and by
comparison with reference spectra confirm solute identity with greater certainty. This is largely because
the IR spectra of the majority of compounds contain far more fine structure than there is present in their
corresponding UV spectra. However, the association of the IR spectrometer with the liquid
chromatograph is far more difficult than the UV spectrometer, due to the inherent IR absorption
characteristics of the solvents normally employed in LC development. In fact, the majority of useful
solvents absorb light in the infrared range, and, more important, those ranges are frequently the most
informative in structure elucidation and sample identity confirmation. This means that, for the practical
association of the liquid chromatograph with the IR spectrometer, either the solvent needs to be
removed before measurement, or an infrared transparent solvent must be employed. Unfortunately, of
the few solvents that are transparent to the IR over the wavelength range that is important, many are not
compatible with the mobile phase requirements of the liquid chromatograph.

The IR spectrometer is generally less sensitive than the UV spectrometer and thus the LC/IR tandem
system is at a further disadvantage. The introduction of the FTIR instrument has partly compensated
for the sensitivity difference but the IR spectrometer is still several orders less sensitive than the mass
spectrometer. Consequently, to obtain an adequate