FTIR Condition Monitoring of In-Service Lubricants: Analytical Role and Quantitative Evolution

Fourier transform infrared (FTIR) spectroscopy is a well-established primary qualitative screening tool for Condition Monitoring (CM) of in-service lubricants, with three key determinative ASTM methods often used to accept their continued use or condemn them. These include Acid Number (AN), Base Number (BN) and moisture (H2O), all tedious and environmentally problematic titrimetric methods. The latter have been developed into FTIR quantitative methods through the use of targeted stoichiometric reactions to produce infrared-active products readily and accurately quantified. FTIR CM is discussed in the context of these methods, their tribological significance and the basic analytical concepts examined and compared. The evolution of new FTIR qualitative and quantitative methodology is reviewed from its origins in edible oil analysis to the high-throughput analytical FTIR systems used by commercial CM laboratories to produce ASTM-comparable data in lieu of titration. Comparisons of chemometric vs. stoichiometric approaches are made, including implementation, applications, and instrumentation. Commercial implementation and performance of proprietary quantitative FTIR/IR systems are discussed with a focus on the stoichiometric approach. Current developments include generic, manual FTIR methods combining simplicity, universality, and practical environmental benefits in being able to serve as alternatives to titrimetric procedures. A new methodological approach that could unify the three deterministic FTIR methods under a single, IR-active reaction product is also presented.