Environmental Engineering Reference
In-Depth Information
Dual head systems also exist which combine ESI and APCI, i.e., heat and charge parameters within
a single ionisation 'head'. These can be adjusted to make that head more akin to an ESI or APCI sys-
tem. In ESI, a strong electric charge is imparted to the nebulised eluent as it emerges from the HPLC
via an ESI 'probe' (i.e., the interface that essentially connects the HPLC and MS systems). Fine elu-
ent droplets are sprayed into a charged heated chamber and the aerosol droplets undergo rapid size
reduction as the solvent (mobile phase) evaporates. Once the droplets have attained suffi cient charge
density, compound ions are ejected from the surface of the droplet (ion evaporation).
Typical ions generated by ESI include: [M
H]
, [M
Na]
, [M
NH
4
]
, [M
K]
in positive ion
mode, [M
H]
or [M
Cl]
in negative ion mode, where M is the relative molecular mass of the
neutral molecule. In APCI, sample and solvent are again nebulised, converted into an aerosol, and
then rapidly heated to a vapour/gas using an APCI probe before entering the ion-source region. The
APCI ion-source differs from the ESI ion-source because a corona discharge pin is also incorporated,
which typically operates with a discharge current of 2μA. Mobile phase molecules react with the ions
generated by the corona discharge and produce stable reagent ions. Analyte molecules introduced
within the mobile phase react with these reagent ions at atmospheric pressure and typically become
protonated (positive ions: [M
H]
) or deprotonated (negative ions: [M
H]
).
Because of their physico-chemical properties, some compounds have a better response in one
ionisation mode than the other. Positive ionisation is usually satisfactory, but there are cases where
negative mode should be used because the response in positive mode lacks the required sensitivity.
1.5.1.4 Further analytical options
A number of innovative techniques and adaptations of pre-existing methods are also being devel-
oped for the simultaneous separation and detection of multiple pesticide residues including car-
bofuran, its metabolites and other carbamates. For example, Science and Advice for Scottish
Agriculture (SASA), have developed a method (see Chapter 6, Section 6.5) whereby sample prepa-
ration and liquid-liquid extraction techniques that separate compounds on the basis of their rela-
tive solubilities in a chosen solvent, are combined with gel permeation chromatography (GPC) in
order to remove any unwanted material from the test sample (e.g., lipids or proteins) that could
interfere with the analysis. The fi nal analytical extract remains a complex mixture, and separation
of the components present in the extract is still achieved using either gas or liquid chromatogra-
phy. Mass spectrometric detection, identifi cation and quantifi cation of compounds of interest with
GC/MS or LC/MS can be achieved at ultra-low concentration levels even in the presence of a com-
plex matrix. In the future, the enhanced selectivity/sensitivity afforded by tandem mass spectrometry
or time-of-fl ight (TOF) mass spectrometry will improve the capacity to screen for, and confi rm,
suspected cases of illegal wildlife poisoning.
1.5.1.5 Carbofuran mass spectrum data
The molecular M
+
of carbofuran has a nominal molecular mass of 221 and, when subjected to elec-
tron impact ionisation (EI) in GC/MS, it yields the positive ion (full-scan) mass spectrum shown in
Figure 1.15.
The main features of the EI mass spectrum shown are the presence of a small molecular ion peak
[M
.
] at mass to charge ratio (m/z) 221, and an intense fragment ion at m/z 164 [C
10
H
12
O
2
]
.
. This
fragment ion is generated by the loss of the [O
C
NCH
3
] group from the molecular ion (refer back
to Figure 1.1), i.e., a mass difference of 57 m/z units (methyl isocyanate).
In contrast, the main feature of the ESI positive ion full-scan mass spectrum (Figure 1.16) for the
LC/MS is the presence of an intense ion at m/z 222 which corresponds to the [M
H]
molecular ion.
The primary fragments at m/z 165 [the phenol C
10
H
13
O
2
]
and m/z 123 [C
7
H
7
O
2
]
are also visible. The
small ion at m/z 244 corresponds to the complementary sodium adduct molecular ion (i.e., [M
Na]
).
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