Breath
Breath has become the biological specimen of choice for measuring alcohol concentration in association with the enforcement and prosecution of DUI. The preference for breath-alcohol (BrAc) measurement has resulted from several considerations, which include:
• noninvasive nature of sample collection
• rapid analysis and result
• minimal operator training
• sampling and analysis allowed in non-laboratory environment
• acceptable metrological properties of current instrumentation that demonstrate fitness-for-purpose
• minimal cost
• legislative mandates and parliamentary or high court and/or supreme court endorsement.
These factors have combined to establish breath alcohol analysis as the most common forensic analysis employed in drink-driving enforcement. As breath-alcohol analysis is typically per formed in a police station or other nonlaboratory environment by a trained user (but who is not a chemist or forensic scientist) it must be performed with the minimum possible input or influence on variability in the test result from the subject or operator.
At a very minimum, a sound forensic breath test protocol which is to be used for prosecution purposes should include:
1. Trained and qualified breath-test instrument operators
2. Validated instrumentation which is re-calibrated on a routine basis
3. At least 15 minutes pre-exhalation observation
4. Internal instrument standards verifying calibration
5. Duplicate breath samples with appropriate agreement (i.e. 10% of the mean)
6. External control standards (wet bath or gas)
7. Blank tests between all analyses
8. Printout of all results
9. Error detection systems throughout the test procedure
If the subject has recently consumed alcohol and there has not been time for this to dissipate from the mouth, an erroneously high BrAc value will be obtained. This presence of residual alcohol is often referred to as mouth alcohol. A period of at least 15 minutes should be allowed to elapse between the last drink the subject is known to have consumed and the breath alcohol test to ensure any mouth alcohol has dissipated. In addition, all instruments should be evaluated periodically according to an established quality assurance procedure to ensure optimal analytical performance, including accuracy, precision, linearity and critical-system evaluations (e.g. acetone detection, radiofrequency interference detection, mouth alcohol detection, etc.).
