Materials and Methods

BACtrack® Instruments

The instruments selected for this study were the BACtrack® Pro, C8, and C6 (BACtrack® Breathalyzers, San Francisco CA, USA). One of each instrument was purchased new from an internet retailer. The same instruments were used for the entirety of this study. The analyzers connect via Bluetooth® to an app installed on the user’s smartphone where measurements can be displayed and recorded. Two of the instruments, the C6 and C8, can also be used in a standalone mode where a smartphone app is not required. When used in standalone mode, results display on a small screen. The dimensions of the instruments are: 6.98 x 4.44 x 1.60, 6.35 x 5.58 x 1.67, and 5.61 x 4.76 x 1.67 cm, for the Pro, C8 and C6 respectively.
The instruments utilize an electrochemical fuel cell to measure vaporous ethanol. As the user blows into the instrument, ethanol is introduced to the instrument’s fuel cell via a miniature solenoid pump (BACtrack, 2019a). Ethanol on the user’s breath is oxidized with the platinum black coated fuel cell, producing electrons (Wigmore & Langille, 2009). The electrical response generated is proportional to the ethanol concentration on the user’s breath (Alan W. Jones, 2019b).
Before analysis can take place, one must abstain from drinking alcohol for a period of at least 15-minutes to allow for any residual mouth alcohol to dissipate (Anstie, 1867; Bogen, 1927; Caddy, Sobell, & Sobell, 1978; Rod G. Gullberg, 1992; Spector, 1971). The instruments do not have a mechanism to monitor for mouth alcohol, such as those used in more advanced infrared breath alcohol analyzers (Alan Wayne Jones & Andersson, 2008; Pon, Dagenais, & Macalpine, 2002; Wigmore & Leslie, 2001). To start the testing process, the user powers-on the instrument and follows onscreen instructions to blow into the instrument. The user must provide a breath at a flow rate of 12-14 L/m for approximately 5 seconds, resulting in a total volume of breath provided of approximately 1.2L (email communication, BACtrack Breathalyzers). The result of the breath alcohol analysis is shown onscreen to the third decimal in g/210L. There is no exhalation profile as seen in more sophisticated infrared breath alcohol analyzers (Alan Wayne Jones & Andersson, 2008; Pon et al., 2002; Wigmore & Leslie, 2001).
After analysis, the app will display a predicted “time-to-sober” (BrAC of 0.000), based on an ethanol elimination rate of 0.015 g/210L/hr. The instruments and companion app are programmed to calculate “time-to-sober” using zero-order kinetics, even when results are below 0.020 g/210L when zero-order kinetics cannot be assumed (Alan W. Jones, 2019a). Instruments are programmed for a single breath sample to be obtained for analysis and cannot be programmed for a duplicate test sequence. Results can be saved in the smartphone app but cannot be exported to a spreadsheet for further analysis.
The cost of the instruments was $79, $99, and $99 for the C6, C8, and Pro respectively. The instruments cannot be calibrated by the end-user but must be sent back to the manufacturer every 6-12 months for recalibration. The results of breath tests can be remotely monitored by approved third parties (BACtrack, 2019b).

Breath Simulators

Reference vapors were produced using two breath simulators, RepCo Model 3402C (RepCo Marketing, Co. Raleigh, NC, USA), in tandem as described by Dubowski (Dubowski, 1979). Breath simulators were driven by human breath.

Aqueous Reference Standards

Aqueous reference standards were volumetrically prepared to produce reference vapors at specified concentrations when heated to 34℃ in breath simulators in accordance with Henry’s Law (Henry, 1832). 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 at 34℃ (R. G. Gullberg, 2005; Harger R.N., Raney B.B., Bridwell E.G., 1950). Potential interfering substances were prepared to produce vapor concentrations of acetone, isopropanol, and methanol at 0.5, 0.1, 0.1 mg/L respectively. Water-air partition coefficients for potential interfering substances at 34℃ were obtained from the literature (Cowan, McCutcheon, & Weathermon, 1990).

Method

Ten measurements were taken with each instrument at each ethanol reference vapor concentration, as well as ten measurements of each potentially interfering substance concentration. Measurements were alternated with the Pro, C8, and C6 at approximately 2-minute intervals. The mean, percent coefficient of variation (%CV), percent bias, R-squared, and ordinary least squares regression was calculated. Apparent ethanol response to potential interfering substances was recorded.
An evaluation of the potential sources of uncertainty was evaluated using an Ishikawa diagram (Ishikawa, 1989, pp. 229–232) seen in Figure 1 . Standard uncertainties were combined using the root-sum-squares approach where,\(\mathrm{u}_{\mathrm{\text{combined}}}\mathrm{=}\sqrt{{{\mathrm{u}_{\mathrm{\text{reference}}}^{\mathrm{2}}\mathrm{+u}}_{\mathrm{\text{precision}}}^{\mathrm{2}}\mathrm{+u}}_{\mathrm{\text{bias}}}^{\mathrm{2}}}\), as detailed in the literature (Anghel, 2008; Archer, De Vos, & Visser, 2007; Brockley-Drinkman & Barkholtz, 2019; Rod G. Gullberg, 2006; Hwang, Beltran, Rogers, Barlow, & Razatos, 2016; Hwang, Rogers, Beltran, Razatos, & Avery, 2016; Philipp et al., 2010; Souza et al., 2006). Bias was incorporated as a component in the estimate of measurement uncertainty, expanding the coverage interval (Magnusson & Ellison, 2008; Phillips, Eberhardt, & Parry, 1997). The combined standard uncertainty was expanded to the 95% coverage interval (\(U=2u_{\text{combined}}\)). Figure 2 shows the individual uncertainty component’s percent contribution to the combined standard uncertainty.