Rapid and unambiguous identification of melamine in contaminated pet food based on mass spectrometry with four degrees of confirmation

Abstract

A method for analyzing pet food without sample processing is described for rapid identification of melamine based on mass spectrometry (MS) using soft ionization by direct analysis in real time (DART) to provide accurate measurement of mass and isotope-peak intensities, in-source collisionally activated dissociation (CAD) fragmentation, and determination of active hydrogens. Usually, MS analyses based on other than electron ionization (EI) spectra can be suspect because of the limited amount of information provided by a single mass spectral peak (or very few peaks). In such cases, additional degrees of confirmation are desirable to increase confidence in the experimental results. Chromatographic retention time can provide a degree of confidence; however, this requires time and, in some cases, detailed sample processing. Currently, the United States Food and Drug Administration uses a gas chromatography-EI-MS technique for the determination of melamine in pet food that involves sample extraction and derivatization prior to a lengthy chromatographic separation. In the method described here, identification is also confirmed through a determination of the number of active hydrogen atoms in the analyte molecule achieved by hydrogen/deuterium (H/D) exchange by treatment with deuterium oxide (D2O) at the initial stage of analysis. Cross-correlation of these four experimental data provides an unambiguous identification of melamine in contaminated pet food without the need for any sample preparation or chromatography. Limits of detection and the validity of the H/D exchange method as a confirmatory technique are also presented.