Analysis of advanced glycation endproducts in dairy products by isotope dilution liquid chromatography–electrospray tandem mass spectrometry. The particular case of carboxymethyllysine

A fully validated multiple-transition recording isotope dilution liquid chromatography–electrospray tandem mass spectrometry (LC–MS/MS) method for the simultaneous quantitative determination of Nɛ-carboxymethyllysine (CML) and lysine in dairy products is described. Internal standards were [N-1′,2′-13C2]CML and [1,2,3,4,5,6-13C6-2,6-15N2]lysine, and the method was validated by evaluating the selectivity, linearity, precision (repeatability and reproducibility) and trueness, using both powder and liquid products. For liquid dairy products, the repeatability and reproducibility was 2.79% and 11.0%, while 4.85% and 4.92% were determined for powder dairy products, respectively. The trueness of the method ranged from −9.6% to −3.6% for powder and from −0.99% to 6.8% for liquid dairy products. The limit of detection for CML was estimated to be 8 ng CML per mg protein while the limit of quantification was 27 ng CML per mg protein. The method encompasses a proteolytic cleavage mediated by enzymatic digestion to reach a complete release of the amino acids prior to a sample cleanup based on solid phase extraction, and followed by LC–MS/MS analysis of CML and lysine residues. To ensure a suitable performance of the enzymatic digestion, CML measurements were compared to values obtained with an acid hydrolysis-mediated proteolysis. Finally, the method was employed for the analysis of CML in various dairy products. The values compare well to the data available in the literature when similar methods were used, even if some discrepancies were observed upon comparison with the results obtained by other techniques such as enzyme-linked immunosorbent assay and GC–MS.