Roadside "Preliminary Breath Test" Devices - Fuel Cell Technology
Many police agencies utilize a hand-held device to screen potential or probable DUI offenders for estimated alcohol concentration prior to making a formal arrest for DUI. These devices are often used in conjunction with the psycho-physical tests (referred to a Field Sobriety Tests). These devices are often referred to as "roadside breath testers" or "field breathalyzers" or by other similar terms. One of the leading manufacturers of this type device is Intoximeters, Inc. located in St. Louis, Mo. The current production model is the Alco-Senor IV, although the earlier model Alco-Sensor III is still widely used. Both devices, as well as the previous models, use fuel cell technology to determine alcohol in a suspect's exhaled breath. A study of fuel cell technology is therefore needed to understand the general reliability and common pitfalls of the use of Alco-Sensor devices. Fuel cells, as employed in breath testing devices such as the Alco-Sensor preliminary breath test devices and the various ignition interlock devices, work by oxidizing the target molecule. For ethanol, that means stripping away the two hydrogen molecules attached to the carbon CH2 group, and replacing those two atoms with a single oxygen which has a double bond to replace the two single bond hydrogen atoms. The two removed hydrogen atoms are combined with oxygen to form water. The ethanol is converted to acetic acid (commonly found in vinegar). The chemical transformation from ethanol to acetic acid is: Ethanol + Oxygen -> Acetic Acid + Water CH3 CH2 OH + O2 -> CH3 CO OH + H2O The byproduct of the oxidation is the release of electrons, forming a current stream of free electrons which is measured by the device. The underlying theory is that the measured current is proportional to the amount of ethanol that is present in the exhaled breath sample. Operation: In its simplest form, the alcohol fuel cell consists of a porous, chemically inert layer coated on both sides with finely divided platinum (called platinum black). The manufacturer impregnates the porous layer with an acidic electrolyte solution, and applies platinum wire electrical connections to the platinum black surfaces. The manufacturer mounts the entire assembly in a plastic case, which also includes a gas inlet that allows a breath sample to be introduced. Various manufacturers employ numerous proprietary processes in their construction. Process: The reaction that takes place in an alcohol fuel cell is alcohol oxidation. In this chemical reaction a fixed number of electrons are freed per molecule of alcohol. The oxidation occurs on the upper surface of the fuel cell. The freed H+ ions migrate to the lower surface of the cell, where they combine with atmospheric oxygen to form water, consuming one electron per H+ ion in the process. Thus, the upper surface has an excess of electrons, and the lower surface has a corresponding deficiency of electrons. If the upper and lower surfaces are connected electrically, a current flows through this external circuit to neutralize the charge. This current is a direct indication of the amount of alcohol consumed by the fuel cell. By appropriate signal processing, breath alcohol concentrations are displayed directly as grams per cent. Application to Breath Alcohol Measurement: Since the commercial introduction of fuel cell instruments for breath alcohol detection in the mid-1970s, manufacturers improved them continuously. Some of the early problems that limited their use have been eliminated. However, one continuing problem area is the inability of fuel cells to distinguish ethanol from the other types of alcohols. The fuel cell type alcohol detection devices such as the Alco-Sensor III and Alco-Sensor IV are generally accurate and reliable enough to establish "probable cause" to reasonably suspect ethanol consumption, but at present, due to the inherent inability to distinguish interferents, not reliable enough for evidential testing.