Determination of 18 Amino Acids in Water by Solid Phase Extraction-Ultra Performance Liquid Chromatography-Tandem Triple Quadrupole Mass Spectrometry

Amino acids (AAs) are dissolved organic nitrogen, which are widely found in water bodies. During chlorine disinfection, AAs can be converted to potentially biotoxic disinfection byproducts such as haloacetonitrile, halogenated nitroalkanes and haloamides. During the process of chlorine disinfection, chlorine rapidly reacts with the vast majority of free amino acids, particularly α-amino acids, forming organic monochloramines or dichloramines. These organic monochloramines undergo decarboxylation and dechlorination reactions, resulting in the formation of aldehydes and other odorous disinfection by products. Odor has been an important problem and challenge in the water supply industry, which can cause residents to worry about the safety of tap water, and the rapid warning and the traceability of tap water odor are an effective strategy to deal with the odor of the tap water. Therefore, it is necessary to determine the concentration of AAs in water bodies, especially the tap water. However, the concentration of AAs in water bodies is trace amounts, mostly in the range of nanograms to micrograms per liter, which puts more demand on the development of analytical method. In this work, a method of solid-phase extraction-ultra performance liquid chromatography tandem triple quadrupole mass spectrometry (UPLC-MS/MS) was developed for the determination of 18 amino acids in water bodies, and the sample pretreatment conditions and instrument conditions of UPLC were optimized. The optimal pH value of water sample is 2.8, the eluent of the drip washing is methanol-acetic acid solution (1:9, V/V), the eluent of the purge is ammonia-methanol solution (1:9, V/V), and the mobile phases are 0.2% formic acid in water and 0.2% formic acid in methanol-acetonitrile solution (1:9, V/V). The results showed that the standard curves of this method for 18 AAs have good linearity with the correlation coefficient (R2) larger than 0.9913, the limits of detection and the limits of quantification are 0.06-0.23, 0.19-0.76 nmol/L, respectively. The recoveries are 60.1%-123.2%, the relative standard deviations (RSDs) are 1.7%-18.7%, and the inter-day and intra-day precisions are 3.7%-7.0% and 3.3%-7.1%, respectively. This method was used to determine the source water in a drinking water treatment plant, the total concentrations of 18 amino acid are 300-500 nmol/L, in which the concentration contribution ratios of alanine, valine, aspartic acid and asparagine are relative high.