Many of the advantages of a research-grade Fourier transform infrared (FT-IR) instrument arise as a consequence of the powerful minicomputer available as part of the instrument. This computer allows for enhanced treatment of the digital spectroscopic data in order to improve the analytical capabilities. These enhanced treatment methods can be used for improved qualitative analysis (for example, by spectral searching) or quantitative analysis.
Enhanced data treatment techniques routinely used in our laboratory will be discussed, including the advantages and limitations of each method. One particularly useful approach of obtaining quantitative information involves the use of variable ratio spectral subtraction. In this method, the absorbance spectrum of a compound is interactively subtracted from the spectrum of a mixture. The value of the ratio necessary to null out the absorption of the component gives the concentration. “Spectral stripping” involves sequentially subtracting several component spectra from a sample spectrum of a multicomponent mixture. Examples of these two techniques will be shown.
When small concentrations of a component are present in a diluent (for example, oil additives) a 1:1 subtraction of the diluent spectrum from the mixture spectrum yields a difference spectrum of the microcomponent from which quantitative information can be obtained.
Another procedure, useful when examining spectra with overlapping band contours, involves resolution enhancement (deconvolution) of the spectral data prior to quantitation. Using this technique the concentration of carboxylic acid salts have been measured in aromatic oils even though there are overlapping absorptions in the 1600 cm−1 region.