Aqueous Reference Standards

Aqueous reference standards were volumetrically prepared to produce reference vapors at specified concentrations when used in a breath simulator heated to 34°C, in accordance with Henry's Law \cite{Henry_William1803-tu}. Class A glassware, American Chemical Society/United States Pharmacopeia grade chemicals, and purified water were used. Ethanol reference standards were prepared to produce vapor concentrations of 0.020, 0.040, 0.080, 0.160, 0.250 g/210L using Harger’s water/air partition coefficient of 2539:1 at 34°C \cite{HargerR.N.RaneyB.B.BridwellE.G.1950}. Reference vapors of potentially interfering substances were prepared to produce vapor concentrations of acetone, isopropanol, and methanol at 0.5, 0.1, 0.1 mg/L respectively \cite{analyzers}. Water/air partition coefficients for potential interfering substances at 34°C were obtained from the literature \cite{Cowan1990-xl}.

Methods

Ten measurements were taken with each instrument at each ethanol reference vapor concentration, as well as ten measurements of each potentially interfering substance concentration \cite{2013}. Measurements were alternated with the Pro, C8, and C6 at approximately 2-minute intervals. The mean, standard deviation (SD), bias, R2 and ordinary least squares regression (OLSR) were calculated. Apparent ethanol response to potential interfering substances was recorded. An evaluation of the potential sources of uncertainty was evaluated using an Ishikawa diagram \cite{Ishikawa1990-em} as seen in figure \ref{657792}.