Lube-Tech
Chemometric AN by FTIR – Neat Oils Almost all current FTIR AN (and BN) methods are based on the direct analysis of neat-oils using chemometric, Partial Least Squares (PLS) based methods. These require calibrations being available or developed as software add-ons to laboratory CM systems (6) and stand-alone FTIR systems, but are most common for on-site FTIR (7), grating IR (8) and filter Infrared (IR) analysers (9). The latter have built in chemometric calibrations, targeting users with machinery infrastructure requiring local monitoring rather than sending samples to a commercial CM lab for analysis. Such instruments come with generalised Library chemometric calibrations developed by the instrument supplier relating AN/BN titrimetric data to correlating spectral features of in-use oils representative of a particular oil-type, class, or family (8). Many of these systems are cited or listed as following designated ASTM approved protocols (e.g., D7418, D7417, D7889) which provide a veneer of official approval. These protocols, however, relate specifically to operational qualitative attributes rather than quantitative capabilities (e.g., predicting AN or BN) often associated with ASTM approved procedures. As a result, users tend to place undue reliance on the predictive ability of such instrumentation assuming the outputs to be representative of ASTM titrimetric procedures.
Advanced chemometrics can only model so much on a correlation-only basis without having a substantive cause-and-effect spectral signal to work with, lacking in most circumstances aside from being restricted to the oils modeled in the calibration. The veracity of the predictions is heavily reliant on proper selection of the representative oil-type, additive package, and lubricant application to have a reasonable expectation of obtaining rough and ready estimates of AN or BN. Thus, anyone using direct PLS-only approaches and instrumentation must be quite cautious in reporting, relying, and acting on chemometric-based analytical results if they are to be used for critical determinative in-use lubricant assessments.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.134 page 2
Stoichiometric AN by FTIR – Diluted Oils The stoichiometric AN approach, which relies on dilution of a sample containing a basic reagent to react with acids in oils has been limited to specific automated CM systems. Here a spectrally active base in a carrier solvent is added to an oil to elicit a spectral response proportionate to the acidity present in an oil, mimicking ASTM D664. Designed for high throughputs labs, it is also oil-type restricted, involves a chemometric element (1) and is not suited for on-site analysis.
A new manual stoichiometric FTIR AN method has been developed to generically address on-site capabilities as well as overcome the universality limitations associated with oil-specific direct chemometric methods and their associated accuracy and validation issues (2). The manual stoichiometric AN method has some unique benefits, including being independent of oil-type as well as being capable of acid differentiation. It is limited to ~20 samples/h because it uses a split-paired vs. the single-sample approach of the automated version, with both relying on a base to react with acids much like ASTM D664, but determines the reaction “endpoint” spectroscopically.
Concept
In the AN stoichiometric methods, a primary calibration is devised by adding oleic acid to a neutral hydrocarbon matrix and treated with NaPhenolate delivered via a solvent carrier. In the new manual method, the oil sample is split into two equal portions, with a reagent- free Solvent-Diluent added to one portion and a Reagent-Diluent containing NaPhenolate, the IR active base, added to the other and spectrally analysed as a pair; hence the term split-paired analysis. These are scanned sequentially, first as a background scan (Ion followed by a sample scan (In); producing a differential spectrum -Log (In/Ion
) ) = ΔAbsn . This contrasts to the
automated method, where a representative sample of the oil-type undergoing analysis (e.g., mineral based turbine oils) is used as a common reference spectrum
LUBE MAGAZINE NO.163 JUNE 2021 25
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