Supercritical fluid extraction-immunoassay for the rapid screening of cocaine in hair

Abstract

Supercritical fluid extraction-radioimmunoassay (SFE-RIA) was evaluated as a rapid screening tool for the detection of cocaine residues in human hair. SFE was performed using carbon dioxide modified with triethylamine (TEA) and water, with off-line collection of extracted cocaine in methanol. Extracts were analyzed for the presence of cocaine using a commercially available solid-phase RIA kit. In order to develop a suitable RIA calibration method for reliable measurement of cocaine in SF extracts, calibration data was compared for methanolic standards both with and without the presence of the SF modifier. Methanol had only a minor impact on immunoassay performance, producing an 11% decrease in maximum binding counts. In contrast, the presence of TEA/H2O profoundly degraded assay performance, producing a 60% suppression in assay counts. To preserve RIA sensitivity, SF extracts were evaporated under nitrogen to remove the modifier and reconstituted in methanol for RIA analysis. Incorporation of the evaporation step permitted the use of modifier-free cocaine calibrators in methanol. Calibration data was at to a four-parameter logistic model. SFE-RIA analysis of a series of drug-free hair samples established an RIA cut-off value for distinguishing between a negative and presumptive positive cocaine sample at an SF extract concentration of 1.2 ng/mL, or a hair concentration of 0.07 ng/mg. The robustness of the SFE-RIA method was demonstrated by the analysis of a variety of hair samples from both drug users and non-users. The quantitative SFE-RIA findings correlated well with the values obtained by an acid incubation/GC-MS method. © 1996 John Wiley & Sons, Inc.