Sequential extraction of amphetamines, opiates, and 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid from a limited volume of urine using a monolithic silica spin column coupled with gas chromatography-mass spectrometry

A method for sequential extraction of amphetamines (AMPs), opiates (OPIs), and 11-nor-Delta(9)-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) from a limited volume of urine was developed. The method used two different spin columns packed with octadecyl (C-18) and mixed-mode C-18-strong anion exchange (SAX)-bonded monolithic silica for extraction before analysis by gas chromatography-mass spectrometry (GC-MS). The urine (0.5 ml), which was hydrolyzed with 10 M NaOH solution (0.1 ml), and 0.3 M Sorensen's glycine buffer (pH 13, 0.1 ml) were poured into the preactivated C-18-SAX spin column and then centrifuged at 10,000 rpm for 2 min to load the sample solution. AMPs and THC-COOH, which adsorbed to the column, were eluted with methanol containing 2% HCOOH. For the second extraction, the remaining solution, which was hydrolyzed by acid, and 0.2 M Sorensen's glycine buffer (pH 12, 0.1 ml) were poured into the other preactivated C-18 spin column. The adsorbed OPIs were subsequently eluted with methanol containing 2% NH4OH. The eluates were combined and the solvent evaporated under nitrogen before the target drugs in the residue were successively converted to O-trimethylsilyl-N-trifluoroacetyl derivatives by N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA)-trimethylchlorosilane (TMCS) and N-methyl-bis(trifluoroacetamide) (MBTFA) followed analysis by GC-MS. Linearity from 10 to 1,000 ng/ml was observed for all the tested drugs using an internal standard method. The correlation coefficients of the calibration curves were greater than 0.990. The coefficients for intraday and interday variations at 50, 200, and 800 ng/ml in urine were between 0.7 and 11.1 %. The proposed method was applied to forensic and clinical poisoning cases and is considered suitable for routine analysis of drugs in biological materials.