Study on the identification of an eluate from ion-suppression HPLC by microscopic FT-IR using the pinpoint concentration

Abstract

The identification of an eluate from ion-suppression HPLC using a water/methanol mobile phase containing trifluoroacetic acid was investigated using the pinpoint (PP) concentration and a microscopic reflection FT-IR measurement. Sodium-5-benzoyl-4-hydroxy-2-methoxybenzensulfonate was chosen as the standard, since it has been formulated in many cosmetic products at a fairly low level as a UV absorber. The absorbance of the background spectrum, mainly due to a trifluoroacetic acid residue, could be minimized by using a centrifugal vacuum concentrator with a polypropylene tube for fractionation. An evaluation of the identification limit of the obtained spectrum was carried out using a commercially available IR spectra retrieval system with an attached spectra database and a custom-made database comprising of spectra obtained by the PP concentration ; also the degree of agreement between the database and the sample spectra, were used as an index of the identification limit. By spectral retrieval using a range of wave-numbers from 2000 to 700 cm-1, the identification limit was found to be 500 ng for the HPLC eluent of the standard and 2 μg for that of the cosmetic lotions when retrieved against the PP-standard spectra.