Stability of Cocaine Compounds in Biological Fluids During Post-Analytical Sample Storage

The objective of this study is to evaluate in vitro stability of cocaine compounds, cocaine (COC), benzoylecgonine (BE), ecgonine methyl ester (EME) and benzoylecgonine ethyl ester (EBE), in blood and urine, during post-analysis custody. Stability was evaluated by measuring percent recovery. Parameters evaluated were time of custody (1 year), storage temperature (−20°C and 4°C), influence of preservative (only for blood samples) and pH (only for urine samples). The impact of the temperature is very important in blood samples. At −20°C all compounds demonstrated to be stable, with recoveries higher than 80% after 1 year. In contrast, degradation was observed in the concentration for all four compounds when the samples were maintained at 4°C. In these same conditions, the influence of the preservative was also noticeable and a higher stability was found in samples preserved with NaF. COC and EBE had similar profiles, and both compounds disappeared after 30 days in samples without NaF and after 150 days in samples with NaF added. EME disappeared after 185 days and after 215 days in samples without and with preservative, respectively. BE recoveries, after 365 days of storage, were 68.5% (in samples with NaF) and 3.7% (in samples without NaF). In urine samples, the four compounds were stable in all the studied conditions except when samples were at pH 8 and stored at 4°C where the compounds disappeared (COC and EBE after 75 days of storage and EME after 15 days). The exception was BE, with a recovery of 23% after 1 year of storage. Of the temperatures evaluated, −20°C seems to be optimal for storage to maintain the stability of cocaine and metabolites in biological samples. This can be further enhanced by maintaining a pH of 4 in urine samples and adding a NaF preservative to blood.