If you shine an infrared (IR) light beam through a thin film of oil, then vary the wavelength of the light beam from one end of the IR spectrum to the other, you can measure the amount of infrared light ‘absorbed’ at particular wavelengths, develop a ‘spectra’ for the entire infrared wavelength range of how much infrared light was absorbed at what wavelength, and compare that against known standards in a library or against a baseline spectra.
For example, the Oxygen-Hydrogen (O-H) single bond in water will absorb IR light in the 3000 – 3500 nanometer wavelength range. It is a broad range spanning nearly 1/6 of the entire IR wavelength range. The amount of IR light absorbed is directly proportional to the number of O-H single bonds available to absorb the IR light.
FTIR analysis or Fourier Transform Infrared analysis is a quicker and more accurate measurement of Infrared analysis than the original IR method. IR analysis is useful qualifying the amount of oxidation by-products present, qualifying the amount of nitration products present, qualifying if glycol is present, qualifying if water is present > 300 ppm in most R&O oils and up to 1000 ppm in AW, EP and engine oils, and indexing the level of soot and fuel present in the lubricant. All these tests are appropriate for gasoline and diesel engine oils, but not for most industrial lubricant applications. Oxidation in industrial lubricants will first be noted by slight increases of the TAN (Total Acid Number) before notable increases in the IR are reported.
The absorbance peaks we look for:
Water — Generally P for positive or > 0.1%, or N for negative.
Glycol — Generally P for positive or N for negative.
Soot — An index that indicates the amount carbon soot that absorbs IR light over basically the entire spectrum. It should change very little over time, but not reliable to quantify percent of soot within 1% unless a calibration curve is developed for each sample, which is cost prohibitive.
Oxidation — A qualifying index that is directly proportional to the amount of oxidation by-products in the lubricant. This applies to gasoline and diesel engine lubricant, but NOT to industrial lubricant applications.
Nitration — A qualifying index that is directly proportional to the amount of nitriles present. Nitriles are formed as a result of a too rich or too lean air/fuel ratio.