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.
Aspartame (APM), a dipeptide of aspartic acid (ASP) and phenylalanine (PHE), is a widely used artificial sweetener in beverages. It is unclear whether residual chlorine in tap water can react with APM to form disinfection byproducts (DBPs). Therefore, we investigated the formation of DBPs from the reaction of APM with residual chlorine in authentic tap water. APM and a commercial sweetener (CS) packet containing APM were studied under authentic and simulated tap water conditions. Eight chlorinated products of APM were detected using solid-phase extraction (SPE) and high performance liquid chromatography quadrupole time-of-flight mass spectrometry (HPLC-QTOF-MS). These new chloro-products were tentatively identified based on accurate masses, isotopic patterns of 35,37 Cl, and MS/MS spectra. Furthermore, we identified APM as a precursor to 2,6-dichloro-1,4-benzoquinone (DCBQ). DCBQ significantly increased to 2.3−12 ng/L with the addition of APM or CS in tap waters collected from different locations compared to 1.4−1.8 ng/L in the same tap water samples without sweetener. DCBQ and two of the chlorinated transformation products were identified in cold prepared tea containing APM. DCBQ formation was eliminated when the residual chlorine in tap water was reduced by ascorbic acid or boiling prior to the addition of APM or CS. This study found that eight new DBPs and DCBQ were produced by the reactions of residual chlorine with APM and CS. These findings show an unintended exposure source of emerging DBPs via APM sweetened beverages.
N-Nitrosamine disinfection byproducts (DBPs) are
a health concern because they are probable human carcinogens. Complex
organic nitrogenous compounds, nitrosamine precursors, are largely
unidentified in source water. Using stable isotopic labeling-enhanced
nontargeted analysis, we identified a natural product N-heterocyclic amine 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic
acid (MTCCA) in source water. Interestingly, we discovered that chloramination
of MTCCA-containing water could produce four nitrosamines: methylethylnitrosamine, N-nitrosopyrrolidine, N-nitrosoanatabine,
and N-nitrosoanabasine. Computational modeling and
experimental results helped explain potential pathways of nitrosamines
generated from chloramination of MTCCA. Further investigations confirmed
widespread occurrence of MTCCA in source water and wastewater. Its
concentration ranged from high in upstream creeks (23.2–332.2
ng L–1) to low in the river (5.7–37.6 ng
L–1) during the 2020 spring runoffs, indicating
that sources of MTCCA came from creeks around farms. Analysis of wastewater
before and after ultraviolet, as well as microfiltration with subsequent
ozonation treatments, showed increased MTCCA after treatments, demonstrating
a difficulty to degrade and remove MTCCA in water. This study discovered
the extensive presence of MTCCA in source water and wastewater, suggesting
that natural N-heterocyclic compounds may serve as
a new source of nitrosamine precursors.
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