Kinetics of ethanol degradation in forensic blood samples

To determine ethanol in human post-mortem blood samples is problematic, largely due to the inappropriate and variable methods of preserving and storing, which can cause decomposition and loss of alcohol concentration. In this study, four crucial parameters of sample conservation were studied: temperature (T), percentage of air chamber in container (%CA), ethanol concentration in blood and post-mortem time. Blood samples from post-mortem cases were stored under different conditions (ethanol levels were known in all cases); factorial design variables: (%CA) 0, 5, 20, 35, 65%; storage temperature: 25, 4 and −10 °C; in a total of 15 experiments. No preserving agent was used in samples. Quantification of ethanol in blood was carried out by gas chromatography with head-space FID detector. Initial ethanol concentration ranged from 0.50 to 4.30 g/L. The kinetics of degradation observed was pseudo-first-order. The parameter that characterised the kinetics of ethanol degradation (k0) ranged from (4 × 10−4 and 5.0 × 10−1 day−1), depending on storage conditions. A strong dependence between ethanol degradation and the content of the air chamber was observed and this dependence was found to be stronger than that between degradation and temperature; there was an experimental relation between (k0) and (%CA). Activation energy for different conditions, i.e. 0, 5, 20, 35 and 65 (%CA), were calculated and contour plots were made. A mathematical equation relating air chamber, temperature and ethanol concentration at a certain time was determined. This equation allowed estimation of initial concentrations of ethanol with minimal error. A good correlation between experimental data and data calculated with the equation was obtained (r2 = 0.9998). The best storage conditions were: 0% CA and storage at −10 °C, obtaining an ethanol degradation of 0.01% after 15 days. However, 33% of ethanol degradation was obtained with 35% CA at 25 °C after 15 days. This equation is useful in forensic cases in which original concentration of ethanol has to be estimated under different sample storage conditions.