An individual suspected of operating a motor vehicle while intoxicated is required to submit to evidential breath testing administered by law enforcement officers. Evidential breath testing devices produce printouts that are admissible as evidence in a court of law.

Evidential breath testing instruments are tested to very high standards before being approved by the Department of Transportation. Widely relied upon by law enforcement, evidential breath testing can also be used in the workplace for pre-employment testing, random testing, post-accident testing and reasonable suspicion employee testing.

IR Evidential Breath Testing

Infrared spectrometry (IR) continues to be the most widely accepted form of evidential breath testing for illegal blood alcohol levels. There is more case law surrounding the use of IR instruments than any other non-invasive type of alcohol testing analysis. The Intoxilyzer® is by far the most commonly used evidential breath testing instrument for collecting evidence in DUI cases.

How IR Evidential Breath Testing Works

The basic premise of IR technology in evidential breath testing is that all things will absorb electromagnetic radiation in a unique and consistent manner. Molecular chemistry tells us that all substances, including the alcohol molecular structure, have a unique and consistent quality. Furthermore, the bond between one atom and another establishes that substance’s sensitivity to various wavelengths of electromagnetic radiation and specifically, to infrared light energy. Since no two substances have the same molecular structure, it is possible to analyze or detect a substance’s presence due to the manner in which that substance will absorb the various wavelengths of the infrared spectrum. This absorption or “sensitivity” is caused by the resonating of the molecular bonds when exposed to the infrared energy. The IR energy is absorbed by these resonating bonds and is depleted. Therefore, it is possible to measure the amount of energy that is used, due to the unique and consistent manner in which it occurs. Since these bonds will resonate to different degrees at different wavelengths of IR light, a “fingerprint” of that substance’s absorption or sensitivity to those wavelengths is created. This fingerprint is most commonly expressed in percent transmittance, which depicts the loss of IR light able to pass through the molecule.

When IR light of a particular frequency passing through a chamber with no alcohol present strikes a detector, a certain voltage level is created. This can be called X. As an alcohol sample is introduced into the chamber, some of the IR light is absorbed or attenuated. As the alcohol level in chamber increases, the amount of light able to pass through the chamber and strike the detector decreases. At the end of the sample, a very different amount of light is striking the detector creating a different level of voltage. This new level of voltage can be called Y. If the amount of IR light passing through the chamber with no alcohol present can be determined—X, and the amount of light passing through the chamber with alcohol present—Y—can be determined, the difference between the two will represent the concentration of alcohol in the breath sample. The more alcohol present, the greater the absorption.

Additional Resources: Alcohol Testing