Direct comparison of near-infrared absorbance spectroscopy with Raman scattering spectroscopy for the quantitative analysis of xylene isomer mixtures

In order to overcome instrument problems associated with moving parts, such as optical misalignment and/or mechanical breakdown, we have developed two solid-state (no moving parts) spectrometers suitable for many industrial process monitoring applications. The first instrument utilizes near-infrared absorbance spectroscopy with a 1024-element platinum-silicide linear photodiode array detector, and the other employs Raman scattering spectroscopy with a 1024×1024 element charge-coupled device (CCD) detector. In order to demonstrate the utility of solid-state instrumentation for industrial process monitoring analysis, both instruments were used for the simultaneous quantitative analysis of individual components of xylene isomer mixtures. The xylene isomer mixture samples prepared for this study contained approximately 75-86, 0.6-5, and 0.1-14% w/v ortho-, meta-, and para-xylene, respectively, to reflect compositions of xylene raw materials used by specialty chemical manufacturers. Each spectroscopic system provides a means for fast (seconds), nondestructive data acquisition with no sample preparation. With the use of the chemometric data treatment of partial least-squares (PLS) regression, the absolute accuracies at 95% confidence for each isomer were found to be ±0.05, ±0.12, and ±0.09% w/v with near-infrared spectroscopy and ±0.08, ±0.04, and ±0.07% w/v with Raman spectroscopy for ortho-, meta-, and para-xylene, respectively.