Breath-Alcohol Simulators: Scientific Basis and Actual Performance

Commercial breath-alcohol simulators were studied to determine their suitability as calibrating devices and for providing control spocimens in breath-alcohol analysis. The best-fit exponential equation for the air/water partition of ethanol was found to by y = 0.0145e 0.06583x, where x = temperature ~ and y = Ka/wxl03. Temperature, volume, pressure, and alcohol depletion of simulators and their effluents were measured. The consequences of temperature tluctustlons upon effluent-alcohol concentration were determined, as was the number of times a simulator can be used with various breath-alcohol analyzers before reaching 1% alcohol depletion. I n t r o d u c t i o n The use of controlled-temperature equilibration of alcohol 1 between water and air with breath-alcohol simulators is very widespread in breath-alcohol analysis. As the name implies, simulators were originally developed as training aids to provide surrogate specimens for breath-alcohol analysts. However, they are now widely employed to produce control specimens for checking the proper functioning of quantitative evidential breath-alcohol analyzers and as calibrating devices for such analyzers. Despite these critical applications of simulators, published information concerning the theoretical basis for their use and actual performance data are scarce. These matters, which are of great significance in laboratory use and field testing of breath-alcohol analyzers and in research on breath-alcohol analysis, were therefore studied in this laboratory. T h e o r e t i c a l B a s i s of E thano l Par t i t ion B e t w e e n A i r and W a t e r The evolution of alcohol vapor from dilute solutions of alcohol under equilibrium conditions is governed by Henry's Law (1), from which it follows that, at definite temperature and independent of pressure, Concentration of solute in the liquid phase = Constant (I) Concentration of solute in the gaseous phase for equilibrium systems of volatile substances in aqueous solution. If a dilute aqueous solution of alcohol is brought to equilibrium with air, the partial pressure of the alcohol in the vapor phase is a function of the alcohol concentration of the liquid phase and of the system temperature. Hence, at constant temperature, the equilibrium alcohol concentration of the vapor phase is controlled by the alcohol concentration of the liquid phase; and at a constant alcohol concentration in 1The unmodified term alcohol in this article refers to ethanol. the liquid phase, the equilibrium alcohol concentration of the vapor phase is governed by the temperature of the system. The vapor pressure of a volatile aqueous solute is proportional to the absolute temperature of the system. Thermodynamic consideration for such systems indicate that the logarithms of the air/water Ostwald partition coefficients for alcohol should vary linearly with the reciprocal of the absolute (~ temperature. For short temperature ranges of interest for simulators and equilibrators, a direct linear relation also holds for temperatures expressed in ~ Experimental air/water partition data for alcohol (or the basis for such data) are contained in seven leading studies published between 1911 and 1974 (2-8). Least-squares, best-fit regression analysis of those data (N = 23) yields the following exponential equation governing the equilibrium partition of alcohol between water and air, over the range of temperature interest for equilibrators and simulators: y = 0.04145eO.06sa3x