Even with the most advanced technology and the best-trained technicians, test results from a Crime Lab won't prove guilt beyond a reasonable doubt.
If you took television programs like CIS & NCIS seriously, you may as well believe lab personnel are wizards who cast magical spells in their lairs, bending and shaping time, matter, and space.
Cops found a trace of something? Well, look no further. The sorcerers in lab coats and safety goggles determine what happened from that sample – who did it, when, and why – in record time. It's so iron-clad that the perp promptly gives up all hope of getting away with the crime.
There are no mystical powers and no all-knowing oracles in a lab. Lab technicians do not possess superpowers.
Most television crime labs are fairy tales. The only mystery of labs is that most people aren't aware of what they do, or their work seems too complicated to understand.
How to Demystify Crime Labs
One great way to demystify crime labs is to visit one. In September, I toured the Texas DPS Crime Laboratory in Weslaco, Texas. The Weslaco lab tests multiple kinds of evidence, including DNA, fingerprints, ballistics, and blood alcohol.
The featured image above the headline shows a black cubed box called a Gas Chromatograph (GC), which is used to analyze blood samples in DWI cases. Round trays with slots for many vials, perhaps a hundred, sit atop the GC. Some of the vials contain blood samples from actual people, while others have known standards and blanks for quality control. The technician prepares the samples, then crimps metal caps with rubber centers onto the vials to seal in the gas. The vials are placed in trays. The samples are heated, causing volatile substances like ethanol to come out of the liquid and into the air within the vial. You are already familiar with this concept as evaporation.
A device called an autosampler is shown as the vertical tower above the cube-shaped GC. The autosampler uses a needle to extract a sample of gas from each vial and inject it into the GC.
That leads us to how the GC works. This diagram illustrates the GC process from left to right. The needle from the autosampler removes a headspace gas sample from the vial and injects it into a port on the GC. Please see my explanation here if you want to know how the GC works when introducing the gas sample . You will notice a graph on the right with peaks called a chromatogram .
The chromatogram graphs the evaporated volatile substances in the headspace gas, including Ethanol, the alcohol we drink. Ethanol is represented by the first labeled, prominent peak. The value of 1.965 represents the number of minutes from when the injection of gas occurred to when the gas exited the GC. That specific time helps identify it as Ethanol instead of another substance. The time is called retention time because that's how long the GC retains that substance.
A peak with a later retention time, 2.987, is labeled as "n-Propanol." That substance is used in GCs as a "standard" for blood alcohol testing. A precise amount of that standard is added to a DWI suspect's blood sample. The GC operator knows the retention time of n-Propanol and Ethanol from calibration.
During calibration, standard reference materials , or SRM, from the National Institute of Standards and Technology (NIST) are used to identify the proper retention times and amounts. N-propanol and Ethanol SRMs are used to make sure the GC can identify them correctly. SRMs are extraordinarily precise and pure substances, giving the calibrating technician a reliable reference for measurement.
The GC operator looks for a peak on the graph at the designated retention time. If no peak appears at the expected time, that is a massive red flag. A test result like that is categorically unreliable.
When the sample leaves the GC, the flame ionization detector (FID) shown in the previous diagram burns what comes out of the GC. The amount of space on the graph under the triangular area represents how much of the substance was detected when it was burned. The greater the area, the more substance was present in the sample and the higher the peak.
Labs and Lab Personnel Are Just Doing a Job, Not Magic
Each test could result in testimony from people who work at the Weslaco lab. Technicians with various titles spend most of their time doing work in the lab rather than testifying. When they ultimately testify, they are frequently the only "experts" in the case.
In court, a lab technician may be called to interpret the test results as an expert witness. The witnesses typically explain what they do to get the result in the case. They walk through the basics of what happens with the samples, the testing, and how they reach conclusions. They should explain the underlying scientific principles in play during the testing.
“Scientific” Numbers, Like .08
Contrary to popular wisdom, the results don't speak for themselves. Even seemingly plain and uncontested findings don't tell the full story. For example, a .08 blood-alcohol content (BAC) result may have a legal meaning but doesn't end the legal inquiry or challenge and can mean far different things, depending upon the exact time the sample was obtained. The test result may be identical, but the factual implications of the result for each situation can differ widely.
I encourage everyone to visit the DPS open house opportunities near you. They typically are offered in the early fall, and you can see lab technology on display and talk to people who work there.
Caption: When I visited the Weslaco lab, I wasn't able to go into the testing room for blood alcohol, but I was taken to another room with a new GC, practically straight out of the box. A couple of high-ranking staff members were kind enough to have a conversation with me and a fellow lawyer. You can see the type of vial rack is rectangular, rather than round in the earlier graphic. Autosamplers come in many forms, but they do the same things described in this blog.
When you or your loved one needs help with a case involving a crime lab, you'll choose a lawyer who will visit the lab, too. It’s a win-win.