Multivariate Calibration Applied to Infrared Spectroscopy for Quantitative Determination of Crystalline and Amorphous Silica

Abstract

The application of infrared spectroscopy for quantitative microanalysis of silica is hindered by one severe difficulty. There is an almost complete spectral overlap in the region around wavelength 800 cm⁻¹, where the various forms of silica have their main absorption band. Multivariate analysis with the use of plsr techniques has been applied to mixtures of α-quartz and fumed amorphous silica, by the calibration of the wavelength region 900-600 cm⁻¹ at 10-cm⁻¹ intervals. The two silica components could be determined simultaneously in concentrations relevant for occupational hygiene control work. The standard deviation of the method is ±1.8%, whereas the relative prediction error is better than ±5.2%. The plsr multivariate calibration techniques are found to be more robust and more accurate than a combined method of infrared spectroscopy and x-ray diffraction for the quantitative determination of silica mixtures. Thus, multivariate calibration offers a rapid, low-cost, and reliable method for the determination of silica polymorphs in low concentrations by infrared spectroscopy, excluding the use of x-ray diffraction. The ability to extract qualitative spectral information from plsr calibrations is demonstrated, and the principle of matrix specific calibration sets is discussed.