Infrared Spectrometric Purity Control of Organic Liquids and Water

For the purpose of purity control, spectral comparisons are performed by regressing the infrared spectrum of a potentially polluted sample on the reference spectrum of the pure compound. The resulting correlation coefficient, R , is a measure of the similarity between both spectra. It is shown that it is advantageous to transform R to Fisher's Z coefficient, especially in the case of relatively small impurities. Following this transformation, a critical value for Z can be determined by reproducibility measurements and if Z falls below this critical value for a certain sample, a contamination of the examined product will be diagnosed. As a first example, the manner in which a certain type of phthalate plasticizer containing small contaminations of another similar type can be analyzed, is described. The purity control of water is discussed as a second example. In this case, potassium hydrogen phthalate is the impurity that can be detected. All spectra are measured by multiple internal reflection using a Diamond Attenuated Total Reflectance (ATR) accessory. Impurities in the per mill range (ca. 0.1–10%) can be detected using this technique.