Simultaneous determination of oxalate and citrate in urine and serum of calcium oxalate kidney stone rats by IP-RP LC-MS/MS

Oxalate and citrate in 24 h urine and serum are considered to be associated with the incidence and recurrence risk of calcium oxalate kidney stones. The quantification of oxalate and citrate contributes to understand the pathological metabolism of kidney stones and guide the early diagnosis and recurrence monitoring. Although simultaneous quantification of oxalate and citrate in urine using liquid chromatography tandem mass spectrometry (LC-MS/MS) have been reported, the optimization of chromatographic column, mobile phase and mass spectrometry (MS) parameters has not been performed. In addition, these is a lack of suitable method for simultaneous detection of oxalate and citrate both in serum and urine. Therefore, we developed a method for the simultaneous determination of oxalate and citrate in urine and serum based on ion-pairing reversed-phase (IP-RP) LC-MS/MS. Herein, five ion-pair reagents, namely, triethanolamine, dimethylbutyl amine, diisopropenyl amine, N,N-dimethylcyclohexylamine and tripropylamine, and three ion-pairing reagent (IPR) buffers, namely, acetic acid, hexafluoro-2-isopropanol, and hexafluoro-2-methyl-2-propanol, were compared in regard to their chromatographic peak abundance and separation of oxalate and citrate. Moreover, MS parameters and the multiple reaction monitoring (MRM) conditions were also evaluated and optimized to obtain the maximum peak abundance. After that, the method was validated in the linear range of 0.25-1000 µM, and the correlation coefficient was ≥ 0.99. The precision and accuracy were < 14.70% and < 19.73%, respectively. The extraction recovery was 80.53-108.79%, and the matrix effect was < 8.79%. The quality control samples were stable at room temperature for 4 h, 4 °C for 24 h, and for 3 freeze-thaw cycles. Finally, this method was applied to the determination of oxalate and citrate in the serum and urine of rats with calcium oxalate kidney stones. The establishment of a stable and effective oxalate and citrate detection method is conducive to the diagnosis and monitoring of kidney stones.