Determination of urine melamine by validated isotopic ultra-performance liquid chromatography/tandem mass spectrometry

Little is known about melamine (MEL) analysis in children's urine. In this study, an isotopic ultra-performance liquid chromatography/tandem mass spectrometry (UPLC/MS/MS) method was developed and systematically validated for the analysis of MEL in urine. The method is easily performed and comprises acidification, solid-phase extraction (SPE) and UPLC/MS/MS analysis. ¹³C₃N₃(^1SNH₂)3 was used as the internal standard (IS) for calibration. Transition ions m/z 127 > 85 of MEL and m/z 133 > 89 of the IS were used for quantification and m/z 127 > 68 of MEL was used for quantitative confirmation. Recovery and precision were assessed to guarantee the applicability of the method. The limit of quantification (LOQ) was 0.01 μg/mL while the calculated method detection limit was 0.006 μg/mL. The mean recoveries ranged from 96-99%. The method was then applied to analyze urine samples from children who had potentially consumed MEL-tainted dairy products during screening in Taiwan. Ten nephrolithiasis cases and 20 age- and gender-matched controls were selected for this study. Three out of the 10 nephrolithiasis cases had elevated levels of MEL. Comparatively, twenty age- and gender-matched non-nephrolithiasis controls consuming Taiwan brand milk powder all showed MEL levels lower than the detection limit except for two children with background levels of 0.02 μg/mL. The background level in these children urine samples was established by UPLC/MS/MS analysis. Positive results of urine MEL tests might be associated with nephrolithiasis in these candidates. Measurement of urine MEL concentration can be helpful in confirming MEL-related nephrolithiasis, but its clinical application needs further clarification.