Here’s a bit more information for those of you interested in the ongoing investigation into how the Colorado Springs Police Department’s crime lab wound up inflating the blood alcohol tests in about 167 DUI cases last year.

First off, a tip of the hat to local lawyer Tim Bussey for thinking like a reporter and filing a Colorado Open Records Act request.

While the police department did not give Bussey everything he wanted, they did deliver a Dec. 9 memo from Crime Lab Supervisor Ian Fitch, who wrote the following:

“A root cause analysis investigation, which included input from Colorado Bureau of Investigation (CBI) Agent Tim McKibben, strongly suggested that the high blood alcohol content values reported in the previously affected cases resulted from a discrepancy between the concentration of internal standard, n-Propanol, added to the standard curve components (standards) and purchased controls (controls), and that added to the unknown blood samples being tested. Evidence suggests that in batches affected by the nonconformance in question, less n-Propanol was added to the unknown blood samples than to the standards/controls. This would explain the inaccurate high ethanol values reported, since the amount is calculated relative to the n-Propanol.

Got that?

Well, chemistry was never my strong suit in high school, so I reached out to Erik Brown, an independent forensic chemist in Arizona Monday for a bit more elaboration.

It helped that Brown had been following my stories on the flawed blood alcohol results.

Mistakes like the ones reported by Colorado Springs police are relatively rare, Brown said.

He gave kudos to the investigators and the lab for admitting the errors and seeking out the cause.

Here’s then is a bit of a primer that he provided in a phone call and an e-mail.

First off, blood alcohol or ethanol can be measured when converts to a vapor with a gas chromatograph. 

However, there are a lot of factors that a chemist can’t control in the efficiency with which the vapor is extracted for analysis.

For example, a higher temperature will result in more ethanol in the vapor and thus a higher blood alcohol count

Likewise, a higher degree of air pressure will have the opposite effect and result in a lower blood alcohol count.

Thus forensic chemists will use an internal standard, a chemical with known qualities that is similar to ethanol.

Ethanol has a two-carbon structure. n-Proponal has three-carbon structure. Brown likened it to ethanol’s “big brother.”

To create the internal standard, lab worker mix a carefully measure amount of n-Proponal in a quantity of pure water.

The exact concentration used is not as important as making sure that the concentrations are identical and constant.

Identical amounts of the standard are then added to:

The unknown blood sample (that taken from the subject or defendant.)

Quality assurance samples (the purchased controls)

Calibrator samples (the standard curve component)

The calibrator samples are measured at different concentrations and plotted.

This creates a curve against which the unknown (subject) blood sample can be measured.

The problem that the memo describes is a smaller amount of the internal standard was added to the blood sample being tested than was added to the calibrator or curve sample.

Thus, the blood alcohol levels will skew higher than they actually are.

A tip of the fedora here to Brown who has an interesting blog on forensic chemistry, which can be found at: http://www.forensicsguy.com/benchnotes/

You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.