Infrared quantitative analyses are an everyday requirement in the analytical laboratory. While liquid solutions are commonly analyzed, mixtures of powdered components also exist and often require quantitative analysis without dissolution into a solvent.
This paper outlines the development of quantitative analysis methods for two powdered mixtures.
The traditional infrared analysis method for powdered samples is the collection of a KBr pellet spectrum of an aliquot of the powdered sample. However, preparation of KBr pellets requires skill, and precise weighing of every component for each sample. The Golden Gate™ singlereflection diamond ATR provides a simple and effective alternative, suitable for the infrared analysis of powders.
The powdered sample is simply placed onto the ATR crystal and the sample spectrum is collected. The sapphire anvil ensures that up to 100 lbs load is evenly applied to the sample lying on the diamond micro-ATR crystal during sample analysis. The sample is then cleaned from the crystal surface and the accessory is ready to collect additional spectra. ATR analysis is less complicated than using KBr pellets, is fast and only requires a very small amount of the sample.
To develop the quantitative analysis method, a number of standard powder mixtures of known concentration are prepared and spectra are collected from aliquots of these standards. Specified analyte absorption bands are identified and the peak heights, or areas are calculated for the various standards.
The resulting quantitative data is examined by a least squares analysis based on Beer's Law, A = abc. Where, A = the absorbance value of an analyte band; a = the absorptivity coefficient of the analyte band - a constant; b = the pathlength - generally considered a constant; and c = analyte concentration.
ATR spectra cannot be used with a quantitative method previously developed using transmission spectra due to differences in the relative peak intensity of the absorption bands - this is a result of the internal reflection mechanism of ATR accessories. – as seen in the example, Figure 1, a plot of the transmission and ATR spectra of caffeine. However, a least squares relationship can be drawn for ATR spectra of quantitative samples.