Single-calibrant quantification of drugs in plasma and whole blood by liquid chromatography–chemiluminescence nitrogen detection

Poor availability of drug reference standards may severely complicate clinical and forensic toxicology investigations. To overcome this problem, a new approach is introduced for drug analysis without primary reference standards. Liquid chromatography–chemiluminescence nitrogen detection (LC–CLND) was employed as the analytical technique, based on the detector's equimolar response to nitrogen and using caffeine as single secondary standard. Liquid–liquid extraction recoveries for 33 basic lipophilic drugs were first established by LC–CLND in blood specimens spiked with the respective reference substances. The mean recovery by butyl chloride–isopropyl alcohol extraction for plasma and whole blood was 90 ± 18 and 84 ± 20%, respectively. The validity of the generic extraction recovery-corrected single-calibrant LC–CLND was then verified with proficiency test samples, including 20 different analyses. The mean accuracy was 24 and 17% for the plasma and the whole blood samples, respectively, and the maximum error was 31% for both specimens. All 20 analyses results by LC–CLND fell within the confidence range of the reference concentrations. LC–CLND proved to be an easy-to-use and robust technique, allowing analysis of 1000 injections of biological extracts without a need for major maintenance operations.