Quantitative mineralogical analysis of clay-containing materials using ATR-FT-IR spectroscopy with PLS method

A universal method for quantitative analysis of clay components, in terms of mineral composition, using ATR-FT-IR spectroscopy (in the mid-IR and far-IR regions) combined with partial least squares (PLS) regression technique (ATR-FT-IR-PLS) is reported. For the PLS method development, altogether 222 samples covering natural clay sources and various archaeological/cultural heritage artefacts were used as calibration and validation standards. This is the largest calibration set used for creating an ATR-FT-IR-PLS method for clay minerals. The quantitative compositions of these standards containing combinations of altogether 29 minerals for the PLS method were determined using XRD analysis. The developed ATR-FT-IR with PLS method is quick and easy to use, and enables analysis of very small sample amounts (down to a few mg). This is very important when working with samples from archaeological and cultural heritage objects. The developed ATR-FT-IR-PLS method enables quantifying the contents of 7 main classes of minerals in different clays with a root mean square error of prediction (RMSEP) ranging from 0.9 to 5.1 g/100g. This means that in some cases, depending also on the content of the mineral in the sample, the accuracy is at a semiquantitative level. This quantitative method was successfully applied to 11 cultural heritage case-study samples.