In order to determine an optimum dosage regimen for a drug and to determine its mode of metabolism, methods for analysis of the drug and its metabolites in blood, urine and tissues have to be developed. Analysis of drugs in biological fluids and tissues by GC is quite common although GC-MS (see Ch. 9) has replaced many GC methods which are reliant on less selective types of detector.
A typical application of GC to the determination of a drug in plasma is in the determination of the anti-epileptic drug valproic acid9 after solid phase extraction (see Ch. 15) by GC with flame ionisation detection. In this procedure, caprylic acid, which is isomeric with valproic acid, was used as an internal standard. The limit of detection for the drug was 1 /ig/ml of plasma. The trace shown in Figure 11.25 indicates the more extensive interference from background peaks extracted from the biological matrix which occurs in bioanalysis compared to the quality control of bulk materials.
An example of the use of GC with nitrogen selective detection is in the quantification of bupivacaine in plasma.10 Bupivacaine contains two nitrogen atoms in its structure which makes it a good candidate for this type of analysis. The limits of detection which can be achieved with a nitrogen selective detector for this compound are much better than methods based on flame ionisation detection, which are much less selective.
Plasma blank jAdLx-^
Valproic acid \
oJoj. Jl J
Caprylic acid y
(A) The GC trace of an extract of blank plasma obtained from a patient.
(B) The GC trace of an extract of plasma obtained from the same patient after treatment with valproic acid (peak 1) to which caprylic acid (peak 2) has been added as an internal standard.
1. Indicate the order of elution of the following compounds from an OV-1 column.
1 Testosterone propionate
3 Testosterone heptanoate
5 Methyltestosterone 9'E'l'SVZ SJ3MSUV
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9. M. Krogh, K. Johansen, F. T0nneson and K.E. Rasmussen J. Chromatogr. 673, 299-305 (1995). 10. L.J. Leskotand J. Ericson. J. Chromatogr. 182, 226-231 (1980).
Basic Gas Chromatography. H.M. McNair and J.M. Miller. Wiley Interscience, Chichester (1997). Capillary Gas Chromatography. D.W. Grant. Wiley Interscience, Chichester (1996). Headspace Analysis and Related Methods in Gas Chromatography. B.V. Ioffe, A.G. Vitenberg and I.A. Manatov. Wiley Interscience, Chichester (1984).
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