Stable Isotopic Labeling and Nontarget Identification of Nanogram/Liter Amino Contaminants in Water

We report a new combinatorial approach of stable isotopic labeling (SIL)-solid phase extraction (SPE)-liquid chromatography-tandem high-resolution mass spectrometry (LC-HRMS/MS) for identification of amino-containing contaminants at trace levels in source water. The new SIL method requires small amounts of formaldehyde (CH₂O) and deuterated formaldehyde (CD₂O) to efficiently label ng/L amino compounds in 1 L of water and improves SPE recovery, enabling environmental analysis of trace amino-compounds. Isotopically methylated components were confirmed using LC-MS/MS based on their retention times, and characteristic isotope patterns of the molecular and product ions. Using the characteristic isotope patterns, we established a data prioritization process to identify the amino compounds in thousands of mass peaks in raw data. Analysis of a labeled authentic source water detected 8952 m/z peaks and tentatively identified 154 amino compounds. Our SIL-methylation prioritization approach effectively reduced the complexity of data. Manual spectrum interpretation identified 77 of the 154 components as amino acids and peptides. We confirmed 8 of the 77 compounds using commercially available standards to demonstrate the feasibility and reliability of our SIL-SPE-LC-HRMS/MS method for environmental analysis of trace amino-containing contaminants. The method can efficiently identify amino-precursors in source water, enabling other studies of nitrogenous disinfection byproduct formation.