Page 80 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_C02.fm Page 68 Wednesday, November 12, 2003 9:36 AM

and high flow rate (4 ml/min) allowed achieving ultrafast analysis (below 1
min) of nonpolar and polar analytes like morphine and its glucuronides and
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midazolam and its hydroxylated metabolites with LC/MS/MS. Recently,
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Naidong et al. eliminated evaporation and reconstitution steps in 96-well
SPE. This was possible due to application of straight-phase LC/MS/MS, where
the eluting solvent was compatible for injection into mobile phase containing
high percentage of organic solvent. Such an approach allowed processing a
batch of 96 samples (containing, e.g., fentanyl, omeprazole, or pseudoephe-
drine) in 1 h.
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Zweigenbaum et al. demonstrated the feasibility of rapid sample prep-
aration and fast analysis by LC/ESI/MS/MS. As model drugs, six benzodiaz-
epine derivatives were used. The drugs were extracted from plasma using 96-
well plates and separated on C18 column (15 mm ¥ 2.1 mm, particle size 3
mm) in less than 30 sec. As a result, 1152 samples could be analyzed in less
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than 12 h. Cheng et al. studied the possibility of the rapid LC/MS/MS
analysis of a mixture of various compounds. Amitriptyline, ibuprofen,
lidocaine, naproxen, and prednisolone were used as model drugs. From
theoretical considerations, a column capable of ultrafast liquid chromatog-
raphy should provide a good peak capacity at a short gradient run time and
high flow rate. The study was done on XTerra MS C18 columns 20 mm ¥
2.1 mm, 2.5 mm, 30 mm ¥ 2.1 mm, 3.5 mm, and 50 mm ¥ 2.1 mm, 5.0 mm.
The drugs were eluted in acetonitrile–acid or acetonitrile–base gradient. The
gradient duration time (5 to 100% ACN) was 0.7 to 1.7 min, and the flow
rate was 1.5 ml/min. MS/MS and UV detection was applied. In these condi-
tions the drug mixture was separated in less than 1 min. The study of elec-
trospray response at different pH values showed that one of the drugs
examined (lidocaine) did not follow the rules of electrospray theory. The
authors stated that the optimization for particular drugs should be done
checking different solvents, buffer types, and pH values since the best results
are sometimes achieved outside the expected range.

2.2.2 Detection Issues

2.2.2.1 Choice of Ionization Source
In the scientific literature, a general trend could be observed regarding the
use of electrospray ionization in toxicological analysis. Most probably, this is
caused by an overwhelming use of this source in the LC/MS of large mole-
cules. However, the choice of ESI or APCI may be of relevance for particular
substances. As a rule, the response of ESI for very polar substances is higher
than that of APCI. On the other hand, less polar compounds, including a
majority of parent drugs, need an active ionization mode for better detect-
ability. It is therefore a good practice to try both ionization sources in the

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