Baseline technique, infrared spectroscopy

Fourier Transform Infrared (FT-IR) Spectroscopy With the introduction of commercial FT-IR spectrometers, the application of oil analysis by IR became relatively commonplace for production oil analysis laboratories. The mathematically intensive infrared data analysis techniques that were difficult or impossible to perform on the earlier IR systems became easy on these systems. In addition, quantitative analysis measurement techniques such as peak height, peak area, local baselines and more sophisticated matrix methods could be easily employed in the analysis, and the automation of lubricant analysis became commercially viable. 

In principle, all kinds of spectroscopic techniques lend themselves to on-line measurements. Only a very few are practical. Although low-field NMR has been used to measure various material properties by applying empirical relationships, NMR is still not a realistic proposition for on-line measurements. Ironically, ETIR spectroscopy suffers from too much sensitivity. Typically, good spectra can be obtained only from very thin polymeric films (or solutions). Attenuated total reflection (ATR) probes, in which only a fraction of the IR light penetrates a very short distance into the sample, reduce the problem of excessive sensitivity. However, they aggravate the problems of variations in the baseline and nonlinear response. The latter problem also obstructs the use of UV spectrometry for monitoring polymerization reactions. Of the remaining options, near-infrared (NIR) and Raman spectroscopy are the most attractive. 

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