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and benzimidazoles) in serum. Forty-seven compounds were determined
with GC-MS after SPE on Oasis HLB cartridges, whereas LC/ESI/MS was
applied for 14 thermolabile and polar pesticides, such as carbamates and
benzimidazoles. These substances were isolated with Oasis MCX cation
exchange cartridges. LC/MS was applied for clinical diagnostics in carbofuran
and aldicarb self-poisonings.

2.6.2 General Undirected Screening
Studies on the application of LC/MS as a universal screening technique may
be divided into two main groups: those performed using a single quadrupole
instrument and in-source fragmentation, and those done with an MS/MS
technique (triple quadrupole, qTOF, or ion trap). The general strategy of
each approach is different; in the case of single quadrupole MS, all substances
reaching the analyzer are fragmented and monitored without any preselec-
tion. This is in agreement with the basic concept of general screening analysis.
On the other hand, in-source fragmentation requires optimized sample pre-
treatment and thorough chromatographic separation of all compounds
present in the sample. The latter is often not feasible, therefore mass spectra
of substances may overlap, or ionization suppression may occur. On the
contrary, tandem MS gives much better sensitivity and specificity but at the
cost of preselection.

2.6.2.1 Undirected Screening Using LC/MS Procedures
The main drawback of LC/MS as an universal identification tool is high
variability in the degree of fragmentation of the examined compounds,
observed for different instruments or even for similar instruments but used
187
in different labs. As a solution, selected substances were used for tuning
the fragmentation energy. Particular applications will be discussed below.
Marquet et al. 193,194 developed a library for over 1500 compounds using
positive and negative ionization modes and two fragmentation energies. The
substances were separated in a gradient of acetonitrile and ammonium for-
mate buffer. Reconstructed mass spectra (containing quasi-molecular ion
and fragments) were used for identification. Glafenin was used as a standard
for tuning the degree of fragmentation. About 1100 reconstructed positive
and about 500 negative mass spectra were stored. In the next study by the
same French group, a complete identification procedure was presented,
which comprised sample preparation, optimized chromatographic separa-
195
tion, and ESI/LC/MS detection. The comparison of three extraction meth-
ods (Extrelut, Oasis HLB, and Oasis MCX) showed that the last procedure
was the best one. Extracted compounds were separated on a C18 column
(150 ¥ 1 mm) in a gradient of acetonitrile–ammonium formate buffer with
pH 3.0. Mass spectra were stored in positive and negative ionization modes

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