A reader who says he’s a former forensic chemist emails:

Just in case you didn’t already know:

When someone blows into a breathalyzer, the breath sample is “split.” Half goes to the instant-read dohickey that gives the officer an on-the-spot BAC. The other half goes to a silica gel “trap.” The silica gel holds onto the alcohol and moisture from the breath. This silica gel sample is separated and documented as evidence, so that the accused can have a separate lab run an analysis to determine what the accused’s BAC was. In theory, it would prove a defendant’s innocence, or cement the case against him.

In practice, these silica gel samples were +/- 0.02 from the recorded value. So, if someone was arrested with a BAC of 0.08, the true value could be 0.06 - 0.10%.

During the time I tested these, I never heard of a calibration procedure for the portable breathalyzers.

What I learned from my time in forensic chemistry is that I will never drink and drive, since blind, stupid luck could determine if I’m arrested for a DUI.

Also, as Lawrence Taylor will tell you, the Supreme Court has ruled that police have no obligation to provide you with those separate breath or blood samples for independent testing. In nearly every other facet of criminal law, the defense is given access to forensic evidence for independent testing.

This entry was posted on Tuesday, February 21st, 2006 at 6:49 pm by Radley Balko and is filed under Alcohol, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.