Comparison of the Use of Volume Fractions with other Measures of Concentration for Quantitative Spectroscopic Calibration Using the Classical Least Squares Method

Abstract

Since the commercial development of modern near-infrared spectroscopy in the 1970s, analysts have almost invariably used units of weight percent as the measure of analyte concentration, due largely to the historical precedent from other analytical methods, including other spectroscopic techniques. The application of the CLS algorithm to a set of binary and ternary liquid mixtures reveals that the spectroscopic measurement sees the sample differently; that the measured absorbance spectrum is in fact sensitive to the volume fraction of the various components of the mixture. Because there is not a one-to-one relationship between volume fraction and other measures of analyte concentration, nor is the relationship linear, this has important implications for the application of both the CLS algorithm and the various other, more conventional, calibration algorithms that are commonly used.

Identifying Hybridisation in Pinus Species Using near Infrared Spectroscopy of Foliage

“…In the Cooman plot, the simulated F 1 hybrid [ Figure 6(B), filled triangles] clustered tightly between the PEE and PCH parents and was close in distance to the model centres of the two pure parents. This is as expected in the case of an unreactive mixture of equal parts of foliage from the pure parents in a manner similar to a mixture of ethanol and water, 48 and indicates that a favourable bias in discrimination is introduced by creating simulated F 1 hybrids by simple mixing of foliage from the parents. By contrast, the actual F 1 hybrid [ Figure 6(A), open circles] showed much more scatter with most individual samples lying at greater distances from the model centres than the simulated hybrids.…”

Quantitative Analysis of Organic Liquid Three-Component Systems: Near-Infrared Transmission versus Raman Spectroscopy, Partial Least Squares versus Classical Least Squares Regression Evaluation and Volume versus Weight Percent Concentration Units

“…From this procedure it was found that the differences in the actual volumes of the mixture samples and the sum of the volumes of the pure components used for sample preparation was l minimal, and the coefficients of variation for the actual volumes of the mixture samples were 0.43% and 0.51% for system 1 and system 2, respectively. Furthermore, in their publications [10,11,12,13] Mark et al claimed, that for NIR calibrations the use of volume percentage—corresponding to the scaled volume fraction concentration unit of Beer’s law—is the better approach than weight percentage. In what follows we will show, that this statement is not supported by the results of the investigated multicomponent systems.…”

“…Thus, in this work, apart from comparing the impact of molecular interactions in liquid multicomponent systems on the results obtained by the two different types of spectroscopies, a further topic of this publication are the effects of these structural phenomena on the performance of two different multivariate evaluation routines (PLS and CLS regression) for the quantitative analysis of the investigated liquid three-component systems. Last but not least—and with reference to previous studies by Mark et al [10,11,12,13]—we have tried to shed light on the consequences of using different concentration units (%V and %W) for the quantitative analysis of the described multicomponent systems.…”

Effect of measurement units on NIR calibrations

“…Nevertheless, discrepancies as large as 10-15% were found to exist between the computed concentrations and the ''known'' concentrations of the samples. The details can be found in Mark et al 2 or in the other above-listed references. A summary of the explanation follows.…”

“…But it seems that very few chemists or spectroscopists (or chemometricians) do. An abbreviated mathematical proof of that fact, however, can be found in Mark et al 2 In modern day spectroscopic analysis, the overwhelmingly common concentration units used are weight percents: 100 times the weight fractions. For chemists, that ordinarily does not matter.…”