The Transmission Library contains over 1000 spectra of lubricants and oils, including motor oils, greases, gear and hydraulic oils, anticorrosion oils, fuels, and natural oils. The spectra were collected on an FT-IR spectrometer using the thin film transmission technique with either KBr or NaCl windows. The pure samples were measured for this library so no solvent bands are present in the infrared spectra.

Transmission spectra of lubricants normally contain strong, even totally absorbing, hydrocarbon bands in the range between 3050 and 2800 cm^-1 and between 1450 and 1380 cm^-1. As a result, the spectral data in these regions may be distorted or unreadable. However, none of the library spectra have been processed to artificially remove these types of distortions, for example by blanking the data at the baseline or by replacing it with a straight line. This type of processing tends to de-emphasize weaker bands in other regions, which are typically due to refining additives that are characteristic of each product. However, in order to ensure accurate search results, all spectral absorbance above 2.3 absorbance units were eliminated from the spectra. The library spectra have also been normalized so that the data are shown between 0 and 1.0 absorbance units.

A lead spacer was used in some instances to increase the sample pathlength or thickness. This allowed us to optimize the band intensities in the region where petroleum additives normally absorb (between 1600 and 400 cm^-1), often referred to as the "fingerprint region". Since the concentrations of additives present in the samples often vary over a wide range, our tests show that this method produces better search results than can be obtained using library spectra that are measured at the same thickness.

ATR Libraries

There are two versions of the ATR (Attenuated Total Reflectance) library. The only difference in the spectral data between the two versions is that a processing step, called an "ATR Correction," has been applied to the spectra in the library labeled "Corrected." The other ATR library contains the same spectra before the processing step was applied. We’ll explain when each version should be used and point out any disadvantages or benefits to each later in this document.