Six potential sampling techniques for combined supercritical fluid chromatography and Fourier transform infrared spectrometry measurements Involving mobile phase elimination are compared. These are conventional (i) transmission and (ii) external reflection spectrometry of the sample deposited on a flat ZnSe plate, (iii) reflection-absorption spectrometry of the sample deposited on a smooth metallic substrate, diffuse reflection spectrometry of the sample deposited on a thin layer of powdered NaCI on either (iv) a metallic or (v) an infrared transmitting (ZnSe) substrate, and (vi) diffuse transmittance spectrometry for samples deposited on the latter substrate. For external reflection measurements of thin films of organic molecules deposited on flat surfaces, the optical constants of the substrate determine the nature of the spectrum. When the film thickness exceeds about a quarter of a wavelength, the effect of diffuse reflectance becomes apparent. The signal-to-noise ratio and base-line flatness of diffuse reflectance or diffuse transmittance spectra of samples deposited on powdered substrates are too poor to allow either technique to be used beneficially for supercritical fluid chromatography/Fourier transform Infrared spectrometry measurements. Conventional transmission spectra of samples on flat infrared windows give the best compromise between high sensitivity, faithful representation of relative band intensities, and adherence to the Beer-Lambert law
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