Scientific Defenses That May Be Available In Arizona DUI Cases

Challenging the reliability of a chemical test result is usually essential to effectively defending a DUI case.  In order to accomplish this task a thorough understanding is needed of the types of chemical testing used in Arizona and the scientific principals behind those tests. Here are the three types of chemical tests used by Arizona law enforcement:

  1. Breath Testing
  2. Blood Testing
  3. Urine Testing

Blood and breath testing are by far the most common.  Moreover, there is a trend in Arizona law enforcement moving towards blood as the primary testing method.  However, some police agencies such as the Department of Public Safety appear to be staying primarily with breath testing.  Urine testing is utilized mostly in cases where it is suspected that a person is under the influence of drugs.  Urine testing is seldom used in our state for blood alcohol testing.  Below is a summary the potential defenses for cases involving both breath and blood alcohol testing. 

BREATH TESTING DEFENSES

In order to know what defenses may be available to an attorney, an understanding of the breath alcohol testing process is required.  Breath testing is based on the scientific principal of Henry's law (also referred to as Henry's coefficient).  Henry's law was conceived by the English scientist William Henry. The principal provides in a closed container over time some of the molecules in a liquid will travel into the gas above the liquid. The amount of molecules that travel into the gas will be directly proportional to the number of molecules in the liquid.  Henry's law assumes a constant temperature and a closed system.  Consequently, all the factors that may influence Henry's law may also effect the results of a breath test.

The instrument used to apply Henry's law to breath testing is an Intoxilyzer.  In Arizona, most agencies now use the Intoxilyzer 8000.  The instrument is manufactured by a company named CMI.  The instrument is supposed to take a sample of the subject's lung air and use the principals of Henry's law to estimate a person's blood alcohol concentration.

Her are some of the challenges to evidentiary breath testing:

  • Range of Accuracy
  • Temperature Changes
  • Lack of a Deprivation Period
  • Radio Frequency Interference
  • Failure Keep Calibration Records
  • Partition Ratios
  • Lack of Warranty
  • Forced Agreement of Tests
  • Breathing Patterns
  • Mouth Alcohol
  • Hermatocrit Levels
  • Calibration Errors
  • Source Code Disclosure

 BLOOD TESTING DEFENSES

Similar to breath testing, blood testing also relies on the principals of Henry's law.  However, a different instrument is used to test the blood sample.  Most forensic laboratories use a headspace gaschromatograph.  Headspace refers to the space in a vial above the sample where the gas portion is located. Headspace analysis is the analysis of what is present in that gas. In its simplest terms, gas chromatography attempts to separate and identify what is in that head space gas. 

Some of the attacks that can be made on this blood testing process are:

  • Margin of Accuracy
  • Improper Tube Inversion
  • Chain of Custody
  • Contamination of Sample
  • Proper Site Cleaning
  • Lab Testing Errors
  • Expired Materials
  • Serum Samples
  • Failure to Follow Manufacturer's Instructions

While blood testing can be an accurate and precise measure of a person's blood alcohol concentration, it is not perfect.  Even under ideal conditions, there will still be a range of accuracy regarding test results.  However, conditions are not always ideal.  When basic scientific protocols are not followed the reliability of the test comes into question.

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