UC Davis
UC Davis Previously Published Works
Title
LC- and GC-QTOF-MS as Complementary Tools for a Comprehensive Micropollutant Analysis
in Aquatic Systems.
Permalink
https://escholarship.org/uc/item/60q8z91d
Journal
Environmental science & technology, 51(3)
ISSN
0013-936X
Authors
Moschet, Christoph
Lew, Bonny M
Hasenbein, Simone
et al.
Publication Date
2017-02-01
DOI
10.1021/acs.est.6b05352
Peer reviewed
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University of California
LC- and GC-QTOF-MS as Complementary Tools
for a Comprehensive Micropollutant Analysis in
Aquatic Systems
Christoph Moschet
1
, Bonny M. Lew
1
, Simone Hasenbein
1
, Tarun Anumol
2
, and Thomas M.
Young
1*
1
Department of Civil and Environmental Engineering, University of California, One Shields
Ave., Davis, CA, 95616
2
Agilent Technologies, 2850 Centerville Road, Wilmington, DE, USA
*Corresponding author: tyoung@ucdavis.edu; (ph) 530-754-9399; (fax) 530-752-7872
Abstract
Efficient strategies are required to implement comprehensive suspect screening methods using
high-resolution mass spectrometry within environmental monitoring campaigns. In this study,
both liquid and gas chromatography time-of-flight mass spectrometry (LC-QTOF-MS and GC-
QTOF-MS) were used to screen for >5,000 target and suspect compounds in the Sacramento-San
Joaquin River Delta in Northern California. LC-QTOF-MS data were acquired in All-Ions
fragmentation mode in both positive and negative electrospray ionization (ESI). LC suspects
were identified using two accurate mass LC-QTOF-MS/MS libraries containing pesticides,
pharmaceuticals and other environmental contaminants and a custom exact mass database with
predicted transformation products (TPs). The additional fragment information from the All-Ions
acquisition improved the confirmation of the compound identity; with a low false positive rate
(9%). Overall, 25 targets, 73 suspects and 5 TPs were detected. GC-QTOF-MS extracts were run
in negative chemical ionization (NCI) for 21 targets (mainly pyrethroids) at sub-ng/L levels. For
suspect screening, extracts were re-run in electron ionization (EI) mode with a retention time
locked method using a GC-QTOF-MS pesticide library (containing exact mass fragments and
retention times). Sixteen targets and 42 suspects were detected, of which 12 and 17, respectively,
were not identified by LC-ESI-QTOF-MS. The results highlight the importance of analyzing
water samples using multiple separation techniques and in multiple ionization modes to obtain a
comprehensive chemical contaminant profile. The investigated river delta experiences significant
pesticide inputs, leading to environmentally critical concentrations during rain events.
Introduction
The investigation of micropollutants in waste water, surface water and drinking water is an
important component of water quality assessments
1, 2
. Classical monitoring approaches consist of
screening for a defined number of target compounds. However, it has been shown that with a
targeted approach investigating a few compounds, the exposure and risk of pollutants towards
aquatic organisms can be significantly underestimated compared to more comprehensive
screenings
3, 4
. With the use of high-resolution mass spectrometry (HRMS) it is possible to go
beyond target analysis
5-8
. The field of suspect and non-target screening, primarily using liquid
chromatography (LC)-electrospray ionization (ESI)-HRMS, is currently expanding, especially
for emerging contaminants in water. Efficient and practical approaches with quick confirmation
of compound identities are, however, still needed.
Suspect screening employs compound databases containing chemical formulas, accurate
monoisotopic masses and isotope patterns, and, in some instances, MS/MS spectra
5
. This enables
users to presumptively identify compounds without the need for procuring analytical reference
standards. It has proven to be an efficient and successful approach for detecting expected and
unexpected compounds in the water
9-13
. Schymanski et al. (2014)
14
proposed a system for
communicating confidence in unknown assignments depending on the amount of information
available. It ranges from level 1 (confirmed structure by reference standard), level 2 (probable
structure by library spectrum match or diagnostic evidence), level 3 (tentative candidates by
plausible sub-structure or chemical class), level 4 (unequivocal molecular formula by isotope
pattern match) to level 5 (exact mass only). This system is widely accepted by the environmental
non-target community
6
and is used here to describe the findings.
If the molecular formula is the only a priori information about the compound in a suspect
screening
11
, it can initially only be identified with a confidence level 4, because all isomers have
the same exact mass and isotope pattern. As MS/MS libraries become increasingly available
from open sources (e.g., NORMAN MassBank
15
) and vendors (e.g., Agilent Technologies
Personal Compound Database and Library, PCDL), additional fragment information should be
considered when doing suspect screening
16
.
MS/MS information can be acquired by either data-dependent acquisition (DDA, isolating
precursor masses of compounds in the suspect list or using preset intensity triggers) or data-
independent fragmentation (DIA, fragmenting all ions or ions between certain mass ranges
independent of a suspect list or MS data). DIA with a constant, wide mass window is also known
as broadband DIA
17
or All-Ions fragmentation. DDA provides very specific MS/MS spectra
which is very helpful in identifying unknown chemicals from a non-target screening, but scan
speed will not be high enough to trigger all MS/MS scans in large suspect lists. DIA can become
very complex due to co-eluting chemicals in an environmental matrix, and it is difficult to
reconstruct an individual MS/MS spectrum. However, DIA gives additional confidence in
confirmation of a suspect compound with known MS/MS fragments, when the chromatographic
co-elution of library fragments with the molecular ion in the MS full scan is monitored. A
compound with matching isotope pattern and at least one co-eluting fragment can be considered
as level 2 identification
14
.
For compounds missing from MS/MS libraries, such as predicted transformation products,
suspect screening is limited by necessity to the molecular formula. Although a larger effort is
necessary in the subsequent identification, findings of novel relevant TPs are important.