Environmental Engineering Reference
In-Depth Information
The most commonly used mass spectrometers that allow MS/MS experiments are triple
quadrupole (TQ) and QIT. This is mainly due to their easier operating performance,
better robustness for routine analysis, and relatively low cost compared to TOF or Fourier
Transform-Ion Cyclotron Resonance (FT-ICR) instruments. The TQ is the most frequently
applied mass analyzer in routine pesticide residue analysis. It displays high sensitivity
when working in multiple reaction monitoring (MRM) mode; therefore, it is best suited
for achieving the strict MCLs regulated for various toxic compounds in different matrices.
The QIT is a small, low-cost, easy-to-use, fast, sensitive, and versatile mass analyzer. Its
unique feature is the ability to perform multiple stages of MS (Plomley et al. 2000, 2001).
These characteristics make QIT an attractive option for the detection of pesticides and
other water contaminants, as many studies have shown (Jeannot et al. 2000; Grujic et al.
2008, 2009, 2010). When performing MS/MS, QIT instruments are generally less sensitive
than TQ analyzers, but they have the advantage of working in product-ion scan without
losses in sensitivity and the possibility of performing multiple-stage fragmentation (MS
n
).
The disadvantages of QIT are low resolution, interfering side reactions (because all reac-
tions occur in the same space), and a limited dynamic range.
The procedures for chromatographic separation are very similar and most of them rely
on the use of reversed-phase columns. In most cases, a C18-modified silica stationary
phase has been applied, while methanol and acetonitrile are commonly used as organic
solvents in the mobile phase (
Table 9.8
). In several methods, acetic acid, formic acid, or
ammonium acetate has been added to the mobile phase to improve the separation and
enhance the ionization.
One of the major problems in the quantitative analysis using LC-MS is that matrix com-
ponents coeluting with the analytes from the LC column can interfere with the ionization
process in the electrospray, causing ionization suppression or enhancement. This effect is
very common when working with complex matrices, such as environmental samples, and
is known as the matrix effect (ME) (Niessen et al. 2006). ME can cause significant deteriora-
tion of the analytical method precision and accuracy, although the complex environmental
matrices, such as soil and sludge, are more prone to ME than water samples. To overcome
ME when quantifying, several approaches are available. The use of isotopically labeled
target compounds as internal standards is the best option if they are available (Niessen
et al. 2006). This approach allows signal suppression (or enhancement) to be corrected, as
both labeled and native compounds will undergo the same suppression effect. However,
the availability of isotopically labeled analogs as internal standards is frequently limited.
Quantification by standard addition is another possibility to correct ME, but this method
is not convenient when a high number of samples must be analyzed. Matrix-matched cali-
bration is an additional way to compensate the ME, and it was successfully used in some
recently published papers (Grujic et al. 2005; Martínez et al. 2007).
9.5.2.3 Comparison Between GC and LC Methods
In a recent review paper, GC-MS versus LC-MS/MS have been evaluated for the determi-
nation of 500 high-priority pesticides (Alder et al. 2006). For each of the selected pesticides,
applicability and sensitivity of both methods were compared. LC-MS/MS was shown to
be better or exclusively suited for sulfonyl or benzoyl ureas, carbamates, and triazines
than GC-MS. Furthermore, LC-MS/MS was found to have a wider scope. Only 49 com-
pounds out of 500 exhibited no response when LC-MS/MS was used (mostly organochlo-
rine compounds) compared to 135 pesticides and metabolites that could not be analyzed
by GC-MS. Both GC-MS- and LC-MS-based methods revealed a significant variation in
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