Marbach, Ralf

Teppola, Pekka

A breakthrough in scientific understanding about multivariate calibration &amp; measurement has recently created the method of science-based calibration ("SBC"). Multivariate measurements are not fundamentally different from the univariate case; rather, the two cases correspond one-to-one in every aspect. The significant advantages provided by SBC to the calibration of process analyzers will be shortly reviewed. Following this, the focus will be on the conclusions that can be drawn when applying the scientific understanding generated by SBC to the existing methods of calibration. The results are concerning. A fair estimate is that several ten per cent of PLS and PCR calibrations are not purely measuring what people think they are measuring. (A peer-reviewed publication about these findings is in preparation.) The correct mathematical definitions of sensitivity and specificity are given. Both limits are testable from first principles, i.e., from measurable pieces of spectroscopic data. Applying the limits post factum to today's "statistical" calibration practice reveals why many PLS/PCR results are affected by unspecific correlations (which, unlike spurious correlations, do not disappear when applying PLS or PCR to larger and larger calibration data sets). An overhaul of practices is required. The importance of applying spectroscopic expertise and application knowledge to the calibration is stressed, as is the need to scientifically estimate both the "spectral signal" and the "spectral noise" parts required in calibration. Both estimates are important because proof of specificity is a two-step process. First, the user must prove that the multivariate measurement measures "the right thing;" and then, second, that this "correct" measurement is not affected by any unspecific correlations. Current practices of testing for specificity (ASTM 1655 etc.) are shown to be insufficient and misleading in the case of PLS/PC

Exact definition of "Specificity of Response" in the multivariate case:Severe consequences for today's practice of PLS/PCR

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