Certification of drugs of abuse in a human serum standard reference material: SRM 1959

A new standard reference material (SRM) for drugs of abuse in human serum (SRM 1959) has been developed. This SRM is intended to be used as a control material for laboratories performing analysis of drugs of abuse in blood to evaluate the accuracy of their methods. SRM 1959 is a frozen human serum material fortified with seven compounds for which analyses are performed to determine evidence of illegal drug use: benzoylecgonine (BZE), methadone (METH), methamphetamine (MAMP), morphine (MOR), nordiazepam (NOR), phencyclidine (PCP), and 11-nor-Δ⁹-tetrahydrocannabinol-9-carboxylic acid (THC-9-COOH). Two independent methods involving isotope dilution (ID)-gas chromatography/mass spectrometry (GC/MS) and ID-liquid chromatography/mass spectrometry (LC/MS) were used for the value assignment. For THC-9-COOH, an additional measurement using LC/tandem mass spectrometry (LC/MS/MS) was also included. All methods used isotopically labeled compounds as internal standards and solid-phase extractions to isolate the analytes from the serum. The GC/MS methods used different clean-up procedures from those used for the LC/MS-based methods. Repeatability with within-set coefficients of variation (CVs) ranged from 0.5% to 4.3% for the GC/MS methods and from 0.2% to 1.2% for the LC/MS-based methods. Intermediate precision with between-set CVs for all the methods ranged from 0.1% to 1.1%. Agreement between the GC/MS and LC/MS methods ranged from 0.8% to 8.8%. The results from the methods were combined to obtain the certified concentrations and their expanded uncertainties.