Environmental Engineering Reference
In-Depth Information
(SPME, MAE, SFE, soxhlet or LLE previous sonication) is typically at the
ยต
gL -1 level (0.5-8 ppb) [25, 26, 35, 38].
3.3
Chlorothalonil
The detector most frequently used for chlorinated compound analysis has
been the ECD [23], but MS has become common because of its high sensi-
tivity, versatility and selectivity [14, 26-28, 35]. Thus, selective detectors have
been progressively replaced by GC-MS mainly using EI and CI ionization
techniques. EI has been widely used as the ionization method and most of the
analytical methods published for the determination of Chlorothalonil as an
antifouling compound in water and sediment samples, have been developed
using this ionization technique [14, 26, 28, 35]. However, NCI techniques are
also suitable for the analysis of organochlorine compounds because they offer
higher sensitivity, with methane as the reagent gas, similar to ECD [27]. An
improvement in sensitivity has also been observed when NCI and PCI have
been compared. The enhancement of the signal-to-noise (S
N) ratio has been
estimated at 10-20 times, with respect to PCI or EI [27]. This is well-known
behaviour for chlorinated compounds.
Moreover, the use of IT-MS has allowed for improvements in the selectivity
and the best sensitivity conditions in NCI have been obtained in the MS
/
/
MS
mode. Limits of detection down to sub-low ng L -1
(0.05 ppt) have been ob-
tained in MS
MS mode [27] and lower sensitivity operating in the SIM mode
(approx. 10 times lower than for the MS
/
/
MS mode with DL of 0.5 ppt) [28]
(Table 1).
Using methane as the reagent gas, the predominant peak in the mass spec-
trum under NCI conditions was m
z 266 for [M +2] - , one of the ions from
the cluster generated by the chlorine isotopes (Table 3). This ion is selected
astheprecursoriontobefragmentedinproductions,whichareindica-
tive of the structure of the analyte. A product ion with m
/
/
z 229 was formed,
together with a fragment ion at m
MS
spectrum. These ions have a relative abundance of approximately 100 and
90%, respectively, and correspond to [M - Cl] -
/
z 231, also present in the NCI-MS
/
and [M -2-Cl].Thepar-
ent ion is also present in the MS
MS spectrum with a relative abundance
between 20-30%. MS-MS conditions are usually adjusted to provide unequiv-
ocal identification criteria based on the presence of product and precursor
ions in the spectra. The EI-MS spectrum also offers suitable information for
the identification of Chlorothalonil. However, in environmental samples, such
as sediment samples, a high background level can make difficult the confir-
mation of a compound by EI-MS detection, this is usually eliminated when
MS
/
MS conditions are applied. In the EI-MS spectrum, the base peak is also
the ion m
/
z 266 [M +2] - , and ions corresponding to m
z 264 and 268, which
have a relative abundance of 76 and 49%, respectively. In addition a fragment
/
/
Search WWH ::




Custom Search