Development of a rapid, in‐situ analysis method using sheath‐flow probe electrospray ionisation‐mass spectrometry for the direct identification of cocaine metabolites in dried blood spots

RATIONALE: Small amounts of biofluid samples are frequently found at crime scenes; however, existing gold standard methods such as LC–MS frequently require destructive extraction of the sample before a time‐consuming analysis which puts strain on forensic analysis providers and can preclude further sample analysis. This study presents the application of sheath‐flow probe electrospray ionization‐mass spectrometry (sfPESI–MS) to the direct analysis of drug metabolites in dried blood spots (DBS) as a high throughput, minimally destructive alternative. METHODS: A rapid direct analysis method using a sfPESI ionisation source coupled to an Orbitrap Exactive mass spectrometer was applied to detect cocaine metabolites (benzoylecgonine, BZE, cocaethylene, CE, and ecgonine methyl ester, EME) from DBS. An optimisation study exploring the use of different chemical modifiers (formic acid and sodium acetate) in the sfPESI probe extraction solvent was conducted to enhance the sensitivity and reproducibility of the sfPESI–MS method. RESULTS: Optimisation of the extraction solvent significantly enhanced the sensitivity and reproducibility of the sfPESI–MS method. A quantitative response over a five‐point calibration range 0.5 to 10 μg/ml was obtained for BZE (R(2) = 0.9979) and CE (R(2) = 0.9948). Limits of detection (LOD) of 1.31, 0.29 and 0.15 μg/ml were achieved for EME, BZE and CE, respectively, from 48 h aged DBSs with % RSD (relative standard deviation) across the calibration range ranging between 19%–28% for [BZE + H](+), 13%–21% for [CE + H](+) and 12%–29% for [EME + H](+). CONCLUSIONS: A rapid (< 20 s) quantitative method for the direct analysis of cocaine metabolites from DBS which requires no prior sample preparation was developed. Although the LOD achieved for BZE (LOD: 0.29 μg/ml) was above the UK threshold limit of exposure for drug driving (0.05 μg/ml), the method may be suitable for use in identifying overdose in forensic analysis.