Gas Chromatograph

Forensics for Writers: It’s Written in the Blood

I’ve discussed blood spatter, the external signals that blood at a crime scene sends, earlier in the series. Today we are going to look at what is in the blood, and very briefly cover three large topics. While I would happily cover toxicology, serology, and DNA tests in at least three posts, I am trying to wrap up this series and not bore you all with the minutiae. I know I find it fascinating, but then, this is what I want to do. The field investigator collects the samples that will tell lab analysts their story – there is very little that can be done in the field with these three topics, and no conclusive tests. The stories written inside the blood, the flesh, the stomach and the brain can only truly be read in the lab, so to the lab we will go.

I don’t think I have ever read a mystery that was, realistically, about an analyst. Unlike the TV shows, and the Jefferson Bass books I’ve read and (mostly) enjoyed, while an analyst may go into the field, they don’t carry guns and they don’t do the detective’s work. They look at the inanimate, the inert, and they make it talk. Which is why they play an important role in solving real-life mysteries, and why mystery writers want to include them, even if they sometimes make that participation in solving a mystery rather implausible. Reality is that the analyst in a lab may be miles and miles from the crime scene, may never even know where it was, and may not initially know the name or face of the victim. They will in the long run, though. Melendez-Diaz vs. Massachusetts set the precedent that the person who analyzed the evidence must appear in court to present that analysis.

So what is the lab boffin reading in the samples they receive? Serology, the study of serum and other bodily fluids, is the division that handles blood typing, among other things. While this may seem old-fashioned in the face of DNA tests, blood typing is fast, relatively simple, and able to exclude some suspects almost immediately. Because typing could actually be done in the field, it is an interesting idea to hand to a writer who wants to put some constraints on their investigator and his abilities to utilize a lab. Stuck in the wilderness? Marooned on a distant planet? If you mix a known sample from, say, Suspect A, with a suspected sample from the crime scene, and the blood agglutinates (read: curdles, clots, and goes lumpy) then you know it wasn’t him. Blood type A has anitbodies for Blood type B, and ne’er the two shall mix. Same reason you cannot just randomly transfuse blood from one person into another one without first typing the blood. Good way to kill someone fast, that. Which makes for a gruesome if somewhat obvious murder weapon.

Toxicology is the analysis of foreign substances in the blood. Blood carries both nutrients and wastes, which means that testing the blood can tell if a poison is present. Not always – there are substances that don’t remain in the blood, but we’ll start here since there is a specific substance which is the most tested-for in a toxicology lab. Alcohol in the blood has a relationship to the level of alcohol in the brain – and to the level of alcohol in the air you exhale, as the alcohol diffuses out of the capillaries in the lungs into the air there. That constant equation is what allows the ‘breathalyzer’ to be used and accurate for field sobriety tests (the device used is no longer the actual breathalyzer, but the name has stuck, and I suspect will stay with us as long as people are blowing into funny tubes to prove they aren’t drunk ossifer, even when we are scattered to the stars). The lab backs up the breathalyzer result by analyzing the blood alcohol levels. Which brings me to something I learned in class. I knew that in the US, the BAC for legal drunkenness is .08 %, but we learned that in other countries, it is much lower. In France, it was 0.05, but may have been lowered to 0.02 which was already the level in Sweden. Finland and Australia (among others) have their level set at 0.05.

Moving away from the most common intoxicant, we come to the more interesting (IMO) part of the toxicologist’s job. To take a sample of body fluids, stomach content, or a piece of an organ, and to accurately discover the presence of a drug or poison in them. With the gas chromatograph-mass spectrometer, this is a much more plausible task, but still, we are not talking about a lot of whatever-it-is. The toxicologist may have an idea of what the offending substance is from descriptions of the symptoms and pathology of the victim, but they might not. Screening out all the ‘nopes’ can take a good bit of time and patience. You aren’t talking about the amount that is/was in the initial dose, which may have ranged from microgram to milligram in amount, since the body will have absorbed or metabolized some if not all of it. There are some drugs, like the infamous succinylcholine, which metabolizes completely to succinic acid. Cases like those would be impossible to solve if there was not already a suspicion of what was used, and equally important, a knowledge of what to look for – in this case, in the brain. Another example is heroin, which immediately metabolizes to morphine, a known reaction, and even then, since it bonds with carbohydrates, morphine can only be found in the urine in a relatively small percentage of the dose taken.

I mentioned the GC-Mass Spec, and as it is a favorite of lab jargon in TV shows, I thought I’d describe what it is and what it does in more detail. It is actually two linked instruments, the gas chromatograph, and a mass spectrometer. The gas chromatograph operates on the principle that some molecules will travel faster than others through a specific substrate/gas medium. The sample is injected in very small amounts – when I was doing this in the lab, we were injecting a microliter (for those who aren’t familiar – that’s less than a water droplet, which is usually closer to 2-3 microliters), and told that very little of that would actually travel through the coil after vaporization – heated instantly, and the vapor passed through a very thin tube – little larger than a human hair – which is coated with a specific stationary phase. Some molecules will pass through without stopping, others will cling to the stationary phase briefly before passing through. In this way, when the sample reaches the spectrometer, they are separated into discrete parts. They are bombarded with high-energy electrons, which breaks them up even more, and then they pass by a detector which records the spectrum emission. This analysis of their mass is then read against a comparison library of known samples, and the unknown can be determined with a very high degree of accuracy.

Gas Chromatograph
The interior of a gas chromatograph oven, and the coil. Photo by Cedar Sanderson

And I’m all out of room for DNA in this post. I mean, I could get into it, but it’s a fascinating topic so if you all want to hear about it? I’ll talk it over next week. Let me know in the comments, but be aware that because of heightened public awareness of DNA through media and entertainment, it is considered the gold standard of physical evidence.

GC in use
Injecting a sample into the GC carefully. The very fine needle can bend and the sample will go astray. Photo by Cedar Sanderson


  1. all this goes in the long term “writing stuff” file for reference…but then I write this sort of stuff, and I only know the street cop side.

  2. Back in my research days I not only routinely used a quadrupole mass spectrometer, I assembled and tested a time-of-flight mass spec. If anyone is interested in the details of how that works I can expound 🙂 Most fun are the tribal legends concerning the interpretation of odd data that shows up. I used the quadrupole in an ultrahigh vacuum chamber to diagnose leaks, contamination, etc. One story involved the “cracking pattern” of your common cockroach, which, contrary to their PR, *cannot* survive in a vacuum. They can, however, outgas for MONTHS.

    * “cracking pattern* refers to how large complex molecules, especially hydrocarbons, break when exposed to high voltage. Cockroach guts contain many large complex molecules, it turns out….

  3. I was right last week..Sux is a detection pita.

    Good overview for all this. As to the BAC stuff, it is a law not science question mostly. Other than time the TV shows are not horrible on this aspect

  4. BAC for legal drunkeness varies by state. Like most other laws, that is ignored on cop shows.

    1. And it has changed. At some point – I am going to say the 1940s – it was 1.5, then it was lowered in the 60s to 1.0 before reaching the current level. Not all states have adopted the 0.08 but it is more common.

      1. And it is age dependent. When I got my license in the oughts it was .02 iirc for those under 21. IIRC the DOT funding cudgel is partly to blame for the commonality of .08

  5. OT, but anyone who uses the ATH-MGC Comments greasemonkey script, I had it cut out temporarily until I added the HTTPS versions of the site links to the active sites list, and made sure they ended with a *.

    Is there an updated version beyond 1.3.1?

      1. I just went into the options and added the urls. I didn’t remember who created it or where the latest version lives.

        A lot of sites are moving to forced https for security reasons.

  6. Continue with DNA, by all means, please! Can’t stop without discussing the gold standard. Which it will continue to be – until if/when we meet aliens that are not DNA-based. Which leads to some interesting speculation about what would be the forensics of the future.

    Hum. Another note in the story idea files. (Along with another note for some rather intensive research – I’m completely aware that these are the view from 40,000 feet, here…)

    1. Heh. Yeah. The idea of how an alien DNA or (in my case) lycanthropic DNA works. I just handwave with a “I don’t understand it” mentality

  7. The “problem” is people like me. In _College_ Chem, I destroyed the 3rd(?) law of thermo. *Twice.* First time, I *lost* mass in the classic Zn+HCl= water + H2. Second time, in a “half life demo,” the samples got *more* radioactive. And, *NO,* I will never _knowingly_ enter a Nuclear Power plant. 🙂

    1. This is what I’m studying right now, I’ll be getting a BS in Forensic Science and Investigation later this year. It has also been a long term interest of mine.

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