Chemical Evidence involves the results gathered from the collection and testing of blood, breath, and urine samples to determine a suspect’s blood-alcohol concentration (BAC). In 2000, urine tests in California were discontinued except when neither breath nor blood tests are available.

Chemical Evidence is often the cornerstone for a DUl / DWl / Drunk Driving prosecution. A sample of the driver’s breath, blood, or urine taken shortly after the act of driving supplements the arresting officer’s observation. When an arresting officer in California requests that a DUI / DWI / Drunk Driving suspect submit to and complete a chemical test, the suspect has the option to comply or refuse. This “option” is called “implied consent” or “informed consent”. Implied consent laws exist because the courts have upheld the belief that if a person opted to drive on the state’s roadways, and thereby obtained the benefit of this public expenditure of money, then the person also has implicitly agreed to submit to giving a breath, blood, or urine sample to the police so that analysis for alcohol (and drugs) can be performed. If the suspect refuses to submit to and complete a chemical test, 3 consequences could result:

His/her license may be suspended/revoke for:

  • 1 year, if there are no prior DUI / DWl/ Drunk Driving convictions within 10 years.
  • 2 years, if there is a separate prior DUI / DWI / Drunk Driving conviction within 10 years.
  • 3 years, if there are 2 or more separate prior DUI/ DWI/ Drunk Driving convictions with 10 years.

His/her sentence may be increased because of sentence enhancements statutes

At trial, the jury may assume that the defendant refused to submit and complete a chemical test because the defendant knew the results would reflect that he/she was intoxicated.

If a suspect chooses to submit and complete a chemical test, the identity and integrity of the sample shall be maintained from the time of collection through analysis, reporting, and adjudication. Procedures for collection of these samples must comply with the following the rules and regulations established in the California Code of Regulations under Title 17, Division 1, Chapter 2, Subchapter 1, Group 8. Among other guidelines, Title 17 dictates that samples of blood or urine shall be tested only by a laboratory approved and licensed by the California Department of Health Service (CA DHS) and in accordance with regulations established by the DHS.

To learn more about the 3 different types of chemical tests that help build a case’s Chemical Evidence, please click below:


Breath Tests

A breath test is a practical yet indirect way for the police to estimate a suspect’s Blood-Alcohol Concentration (BAC). The amount of alcohol in a properly collected breath sample is governed by the amount of alcohol in the blood of the suspect’s lungs so it is vital that all residual mouth alcohol is eliminated before a test is conducted.

Currently, California uses desktop models to collect a suspect’s breath sample, with the most commonly used being the Intoxilyzer 5000. It utilizes the infrared spectrophotometric method of analysis, which is based on the fact that different molecules absorb light energy at different frequencies. In the case of alcohol, absorption of light takes place in the band range from 3.380 to 3.398 microns. The Intoxilyzer 5000 shoots a beam of infrared light through a captured breath sample and measures how much of it is absorbed in the above mentioned band range; the more light energy absorbed, the higher the BAC.

Police officers typically attend a breath testing school and are taught only what the designers of the instrument feel is necessary for the officers to know. In fact, many states have constructed elaborate certification systems that are a highly sought-after training by police officers. NOTE: Police officers have no constitutional duty to preserve breath samples, and there is no requirement for a second test sample as there is with blood or urine tests.

Some of the key reasons why the police use breath tests include:

  • results are obtainable in minutes as opposed to hours for laboratory reports
  • results are obtainable in easy-to-read digital display and printed readout
  • shortens amount of custody time
  • low operator skill requirements
  • low cost
  • little invasiveness of the human body.

Some of the shortcomings of breath tests include:

  • breath samples are difficult to preserve
  • must rely upon cooperation of the suspect
  • require a waiting period of 15-20 minutes before administering to eliminate residue mouth alcohol
  • interference from foreign sources, i.e. radio frequency interference (RFI)
  • interference by other objects/odors/contaminants in mouth.

Blood Tests

Blood tests measure the % of alcohol in the DUI / DWI / Drunk Driving suspect’s blood at the time of testing. Blood samples are usually drawn from the arm at a location as close to the traffic stop as possible and are deposited in a clean, dry container that is closed with an inert stopper. An anticoagulant and a preservative, such as sodium fluoride or mercuric chloride, are mixed with the blood sample in order to avoid putrefaction from beginning in just a few hours. The quantity of the specimen required, as well as the type and quantity of the preservative used, would be dependent upon the method of analysis employed by the laboratory’. Blood samples shall be retained for a minimum period of one year from date of collection, and if during the retention period, a sample for BAC analysis is requested by the defendant for analysis and a sufficient sample remains, the laboratory shall provide a portion of the sample to the defendant. Blood samples need to be protected from exposure to light and heat.

The most commonly used laboratory test for alcohol in a blood sample is called the Dichromate Oxidation Method. The Dichromate Oxidation Method involves using ready-made ampoules consisting of a mix of chromium dichromate and sulfuric acid. If anything containing alcohol is introduced into this solution, a residue of chromic sulfate will form due to oxidation. Either the unconsumed amount of dichromate or the chromic sulfate formed is then measured which gives the percentage of alcohol in the sample expressed in percent weight per volume, which is exactly the same formula for estimating BAC.


Urine Tests

Urine normally contains about 1.3 times as much alcohol as blood, but attempts to relate this to BAC depend on a number of bladder conditions. If alcohol is consumed with a full bladder, a urine test could produce a false negative by inaccurately underestimating the suspect’s BAC If the person consumed alcohol with an empty bladder,the test would produce a false positive by inaccurately overestimating the suspect’s BAC. As a result of all these factors, the only approved urine sample for BAC analysis is a sample collected no sooner than 20 minutes after first completely voiding the bladder.

Urine samples are deposited in a clean, dry container that is closed with an inert stopper. A preservative is mixed with the urine sample in order to avoid putrefaction from beginning in just a few hours. The quantity of the specimen required, as well as the type and quantity of the preservative used, would be dependent upon the method of analysis employed by the laboratory. Urine samples shall be retained for a minimum period of one year from date of collection. If during the retention period, a sample for BAC analysis is requested by the defendant for analysis and a sufficient sample remains, the laboratory shall provide a portion of the sample to the defendant. Urine samples need to be protected from exposure to light and heat.

Related Links

California Code of Regulations
The CCR is available at the offices of County Clerks or County Law Libraries and lOO,state depository libraries.

CMI, Inc. Home Page
CMI, Inc. is the manufacturer of the Intoxilyzer line of breath alcohol testers.