The effect of analyte concentration range on measurement errors obtained by NIR spectroscopy

Near infrared spectroscopy (NIRS) was employed to quantify five compounds, ammonium, glucose, glutamate, glutamine, and lactate, in conditions similar to those obtained in animal cell cultivations over varying ranges of analyte concentrations. These components represent the primary nutrients and wastes of animal cells for which such noninvasive monitoring schemes are required for development of accurate control schemes. Ideal cultivation conditions involve maintaining concentrations of these components as low as 1 mM each, however, it is not known if measurements of these compounds can be accurately accomplished at such a low level. We have found that NIRS measurements of these analytes over narrow and low (0-1 mM) concentration ranges yield measurement errors of roughly 11% of the concentration range. By contrast, wide concentration ranges (0-30 mM) yield measurement errors of roughly 1.6% of the concentration range. Decreasing the concentration range over which an analyte is quantified in four out of five cases decreases the optimal spectral range by 100 cm(-1) for measurement by partial least squares regression analysis. There appears a similarity in the ratio of (standard error of prediction (SEP)/concentration range) which may provide an estimation of the anticipated SEP to be obtained for measurement over a new concentration range. It was found that for the five analytes evaluated here, the ratio of SEP to concentration range divided by that obtained for a second concentration range is equal to a fairly constant value of 6.6. This relationship was found to be followed reasonably well by an extensive number of measurement results reported in the literature for similar conditions.