Correction of Temperature-Induced Spectral Variation by Continuous Piecewise Direct Standardization

Abstract

In process analytical applications it is not always possible to keep the measurement conditions constant. However, fluctuations in external variables such as temperature can have a strong influence on measurement results. For example, nonlinear temperature effects on near-infrared (NIR) spectra may lead to a strongly biased prediction result from multivariate calibration models such as PLS. A new method, called Continuous Piecewise Direct Standardization (CPDS) has been developed for the correction of such external influences. It represents a generalization of the discrete PDS calibration transfer method and is able to adjust for continuous nonlinear influences such as the temperature effects on spectra. It was applied to shortwave NIR spectra of ethanol/water/2-propanol mixtures measured at different temperatures in the range 30-70 degrees C. The method was able to remove, almost completely, the temperature effects on the spectra, and prediction of the mole fractions of the chemical components was close to the results obtained at constant temperature.

The impact of temperature variations on spectroscopic calibration modelling: a comparative study

“…As PDS is only suitable for standardization under a discrete set of conditions, continuous piecewise direct standardization (CPDS) [23] was proposed to account for the variation in the spectra resulting from continuous external factors, such as temperature.…”

“…Apart from the number of latent variables to include in the PLS calibration model for the reference temperature, there are three additional parameters to be determined: the width of the band in ) ( k t Q (2w+1); the number of latent variables in the PLS model to obtain ) ( k t Q ; and the order of the polynomial. It was proposed that these tuning parameters are determined by using cross-validation [23] [27]. Although the polynomial is typically taken to be second order by appealing to the established findings in the literature [24][28] [29], the application of cross-validation to attain the other tuning parameters is a time-consuming process.…”

“…To extend the concept of PDS to continuous external factors, such as temperature as observed in an online or at-line industrial application, the continuous piecewise direct standardization (CPDS) algorithm was proposed [23]. More recently, loading space standardization (LSS) [24] was proposed where the standardization is implemented in the loading space derived from the data, as opposed to the original data space.…”

“…In 2000, Wülfert et al [23] undertook a comparison of the predictive performance of a number of temperature correction techniques against which they benchmarked their proposed CPDS method. A more comprehensive study on temperature robust calibration was carried out by Hageman et al [8].…”

Transferência de calibração em métodos multivariados

“…Algumas modificações do algoritmo original já foram propostas, dentre elas a correção de diferenças aditivas entre espectros 12 , que é semelhante a incluir um termo de desvio sistemático ("offset") na equação do PDS, e a correção de variações contínuas de temperatura 3 . No entanto, o PDS apresenta problemas para estimar os postos locais dos modelos PCR/PLS, pois o número de autovalores retidos é pré-determinado.…”