Methodology investigation of expirograms for enabling contact free breath alcohol analysis

The present techniques for breath alcohol determination have usability limitations concerning practical use and the time and effort required for the test person. The rationale of the physiological assumptions in a recently demonstrated technique for breath analysis without a mouthpiece is investigated in this paper. Expirograms quantifying ethanol, carbon dioxide (CO₂) and water (H₂O) from 30 test subjects were analysed, with respect to the influence of individual variations in end-expiratory CO₂ and H₂O concentrations, and possible benefits from simultaneous measurement of CO₂ or H₂O. Both healthy subjects and patients suffering from pulmonary diseases performed breath tests with small and maximum volume expiration. The breath tests were recorded basically with a standard evidential instrument using infrared absorption spectroscopy, and equipped with a mouthpiece. Average concentrations were significantly higher for the maximum than for the small expirations. For the maximum expirations, the healthy subjects had a significantly higher end-expired PCO₂ of 4.4 ± 0.5 kPa (mean ± standard deviation) than the patients (3.9 ± 0.7 kPa). The corresponding values for H₂O were 39 ± 1 and 38 ± 1 mg l⁻¹. The results indicate that the CO₂ variability is consistent with the requirements of accuracy for alcohol ignition interlocks. In addition, CO₂ as tracer gas is preferable to H₂O due to its low concentration in ambient air. In instruments for evidential purposes H₂O may be required as tracer gas for increased accuracy. Furthermore, the study provides support for early determination of breath alcohol concentration, indicating that determination after 2 s will introduce an additional random error of 0.02 mg l⁻¹ or less.