Environmental Engineering Reference
In-Depth Information
has become the interface more generally applied in the analysis of Diuron,
providing sensitive, robust and accurate methods [31, 46, 47, 52] while few
workshavereportedontheuseofESI[32].Thedegreeofconfidencein
identification can be enhanced by using a single quadrupole combined with
collision-induced dissociation (CID). CID is obtained depending on the LC-
MS system by applying an octapole or an orifice voltage that fragments the
molecules. Under such conditions, the full-scan mode provides structural in-
formation which is generated in the mass spectra from protonated molecules
and fragment ions. To evaluate the best mass spectral data, generally, flow in-
jection analysis is performed at different fragmentor voltages (usually from
20 to 100 V) to find the maximum response under the optimum ESI or APCI
conditions. The base peak observed for Diuron when it is analyzed by APCI
in PI mode, in most of the reports, corresponds, as was previously men-
tioned to the protonated molecule [M + H]
+
at m
/
z 233 [31, 47, 52]. With the
NI mode, the deprotonated [M - H]
-
molecule is the predominant peak in
the spectra, at m
z 231 [46]. Under these conditions, typically low rate frag-
mentation is obtained. For example, the ion at m
/
/
z 235 and fragment ion
at m
z 186 are obtained in both PI and NI modes, respectively [31, 46]. The
fragment ion in NI mode can be assigned to [M - H - HN(CH
3
)
2
]
-
.Onthis
basis, diuron is identified using the retention time and the monitoring of
ions (SIM mode). These criteria have also been applied for the identification
of its degradation products (i.e.: DCPMU m
/
/
z, 221; DCPU m
/
z, 205 + 207;
CPMU m
z, 199) [50].
For trace determinations, SPE is the sample preparation method of choice
for enrichment of pollutants from water samples [19, 30-32, 47, 50-52]. Com-
parable sensitivity has been reported with the SPE-LC-MS (SIM mode) or
SPE-LC-MS
/
MS (MRM mode) methods that have a limit of detection at the
ng L
-1
level (0.1-10 ppt) [32, 52] (Table 3).
In spite of the identification power of LC-MS systems, this technique has
its limitations due to the low rate of ion fragmentation, and in this sense, MS-
MS detection is of great interest. This is particularly true for the identification
of trace amounts of compounds in environmental samples or in the deter-
mination of the structure of degradation products. In LC-ESI-MS analysis,
the predominant ion that corresponds to the protonated molecule [M + H]
+
is used as the precursor ion in the MRM mode. In the fragmentation of Di-
uron, the major product ions observed are at m
/
z 72 and 46. Therefore, in
the analysis of Diuron, two MRM transitions 233
/
46 can
be monitored as confirmatory criteria for identification purposes as well as
the retention time [32]. Figure 3 shows a chromatogram corresponding to
the monitored MRM transitions for Diuron. The first MRM transition was
selected for quantification and the second one for confirmation.
→
72 and 233
→
