Determination of N epsilon-(carboxymethyl)lysine in exhaled breath condensate using isotope dilution liquid chromatography/electrospray ionization tandem mass spectrometry

In order to identify new biomarkers for pulmonary diseases in exhaled breath condensate (EBC) it was the aim of this study to develop an analytical method for the identification and quantification of N epsilon-(carboxymethyl)lysine (CML) in EBC. As detection by liquid chromatography with positive electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) offers the advantage of structurally related detection with the necessary specificity required for the identification of a substance, it was the method chosen for the determination of the non-volatile compound. Specific mass transitions and comparison of retention times with standards under given conditions were used for the unequivocal identification of CML in EBC of healthy subjects. Synthesis of isotopically labelled CML was performed and used as an internal standard for an accurate determination. It was possible to identify the advanced glycation end-product CML in 8 out of 10 healthy subjects. The concentration range determined in the quantifiable examined samples ranged between 35 and 110 pg/mL. EBC samples from 11 patients with different diseases such as diabetes and chronic obstructive pulmonary disease were also measured. In one patient with pneumonia a concentration of 1509 pg CML/mL EBC could be detected. This is the first time that CML has been identified and determined in EBC. The developed LC/ESI-MS/MS method could be used to address the utility of CML as a biomarker in pulmonary diseases.