Page 18 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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Solid-phase microextraction (SPME) is becoming a modern alternative
to SPE and LLE. SPME is a solvent-free and concentrating extraction tech-
nique especially for rather volatile analytes. It is based on the adsorption of
the analyte on a stationary phase, coating a fine rod of fused silica. The
analytes can be desorbed directly in the GC injector. Fast GC/MS procedures
73
72
for screening, e.g., for benzodiazepines, for barbiturates, clozapine, or
71
for drugs of abuse 74–82 have been published in recent years.
Extractive alkylation has been proved to be a powerful procedure for
simultaneous extraction and derivatization of acidic compounds. 65–69,83 The
acidic compounds are extracted at pH 12 as ion pairs with a phase-transfer
catalyst (tetrahexyl ammonium iodide, THA I ) into the organic phase (tol-
+ –
uene). In the organic phase, the phase-transfer catalyst could easily be sol-
vated due to its lipophilic hexyl groups, whereas poor solvation of the anionic
analytes leads to a high reactivity with the alkylation (most often methyla-
tion) reagent alkyl (methyl) iodide. Part of the phase-transfer catalyst can
also reach the organic phase as an ion pair with the iodide anion formed
during the alkylation reaction or with anions of the urine matrix. Therefore,
the remaining part had to be removed to prevent a loss of the GC column’s
separation power and to exclude interactions with analytes in the GC injec-
tion port. Several SPE sorbents and different eluents have been tested for
efficient separation of the vestige of the phase-transfer catalyst from the
analytes. A diol sorbent yielded best reproducibility and recovery under the
described conditions. Further advantages of such SPE columns were easy
handling, commercial availability, and that they had not to be manually
prepared as described by Lisi et al. 84
Derivatization steps are necessary if relatively polar compounds contain-
ing, e.g., carboxylic, hydroxy, primary, or secondary amino groups are to be
determined by GC/MS, and/or if electronegative moieties (e.g., halogen
atoms) have to be introduced into the molecule for sensitive NICI detection.
The following procedures are typically used for basic compounds: acetylation
(AC), trifluoroacetylation (TFA), pentafluoropropionylation (PFP), hep-
tafluorobutyration (HFB), trimethylsilylation (TMS), or for acidic com-
pounds: methylation (ME), extractive methylation, PFP, TMS or tert-
butyldimethylsilylation. Further details on derivatization methods can be
found in References 57, 63, and the pros and cons of derivatization proce-
dures were discussed in a review of Segura et al. 85

1.2 Screening for Drugs in Blood, Serum, or Plasma
by GC/MS

GC/MS procedures have been published for blood screening, mainly of drugs
of abuse 6,86–89 because they have to be monitored or confirmed after

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