Exhaled air can be categorized into essentially three types of samples: tidal breath air, reserve breath air, and alveolar breath air. Tidal breath air is air exhaled in the course of normal breathing. It is the most shallow of the three types. Reserve breath air is exhaled when the body is exerted. It is produced through deeper breathing than tidal breath air, but great volumes of air are both inhaled and exhaled with little residence in the lung. Alveolar breath air is deep lung air. Since breath testing instruments are intended to measure indirectly the concentration of alcohol in the blood, it is essential for accuracy that the breath sample captured by the instrument for analysis be representative of the air in the alveoli of the lung, because it is in the alveoli that the 2100:1 equilibrium ratio between alcohol in the breath and alcohol in the blood occurs.
Infrared breath measuring instruments operate on the principle that each chemical compound has unique infrared energy absorption characteristics. Ethyl alcohol absorbs energy in the 3.42 micron region of the infrared spectrum. The amount of alcohol contained in a sample can be calculated by observing energy loss when a known energy is applied to the sample. In the infrared devices, infrared energy is projected through a breath sample. A photo-detector identifies a decrease in wave amplitude caused by the absorption of energy by the alcohol. The amount of energy absorbed is equal to the breath alcohol concentration. The greater the alcohol concentration, the lower the wave amplitude. A computer on the instrument determines the breath alcohol content based upon the amount of energy loss, and then applies the 2100:1 conversion ratio to provide a digital readout of the suspect's blood alcohol content.
Because infrared instruments are based upon infrared absorption spectra, which are chemically unique, they cannot be influenced by compound such as acetone, which may have some chemical characteristics in common with ethyl alcohol. In fact, some infrared instruments also provide data on the concentrations of other compounds contained in the breath sample as well as that of alcohol.
When infrared instruments are not used, law enforcement generally uses wet chemical instruments, which operate on the basis of color changes produced through the chemical reaction of ethyl alcohol with chromate salts. These devices obtain a measured volume of alveolar breath and pass that sample through a known volume and concentration of a solution of chromate salt and acid. Chromate salt is yellow. As it reacts with the alcohol in the breath sample, it is chemically altered, resulting in a lighter color. The higher the alcohol concentration, the greater the color change.
Preliminary Breath Testing Instruments
PBT instruments are portable instruments for the purpose of BAC screening as part of the pre-arrest field testing. The suspect driver blows for several seconds through a plastic or glass tube, and the PBT provides an instantaneous determination of blood alcohol content.
Disposable chemical PBTs are glass or plastic tubes containing a measured amount of the chemical, which is reactive with alcohol. As the suspect exhales through the tube, alcohol contained in the breath reacts with the chemical contained within. The greater the breath alcohol content, the greater the chemical reaction observed.
Non-Invasive or Passive Alcohol Sensors
Passive alcohol sensors (PAS) are instruments that detect the presence of alcohol in normally expelled breath. They require no cooperation from the driver. During the roadside interview of the driver and examination of documents, the officer places the PAS within six inches of the driver's mouth. It contains a small fan which samples the ambient air for examination. An electro-chemical mechanism analyzes the air for the presence of alcohol. Some instruments are concealed within a flashlight and can be used as a passive or active detector. NHTSA studies indicate these devices are effective during sobriety checkpoints when the decision whether or not to continue breath testing must be made quickly.