The Myth of the Untraceable Poison by DP Lyle
One of the most common questions I get from writers is: Is there a poison that can’t be found in a corpse? The answer is No. And Yes.
Much depends on the state of the corpse when it is found. If severely decayed or completely skeletonized, the ME and the forensic toxicologist have their hands tied. Mostly. There are some toxins, such as the heavy metals (Mercury, Lead, and Arsenic are common ones), that can be found in bones and hair. But most toxins can’t be found in corpses that are severely decayed or simply bones.
In a more or less intact body, your villain can still get away with the murder by poison. That is, until your clever sleuth figures out that something is amiss and solves the crime.
The first thing your murderer must consider is how to make the poisoning look like something else. An example would be an elderly person with heart and lung disease who dies in his sleep. In this case, the person's private physician would sign the death certificate as a natural cardiac death and the almost always the ME will accept it. Why? Because there is an old adage in medicine that says: Common things occur commonly. Most people who die in this situation do indeed die from natural causes so searching for something more sinister would be neither logical nor practical. If the ME accepted the private physician’s cause of death, no autopsy would be done and no toxicological examinations would be undertaken. An overdose of Morphine or digitalis or arsenic or anything else would go undetected.
Unless someone asked questions. Maybe a high-dollar inheritance or insurance policy is in play. If an inheritance, one family member could suspect another and ask questions. In the case of a large insurance policy, the insurance company would look under every stone before paying off the policy. Or your sleuth could have some reason to suspect that things are not as they seem. In any of these situations, the ME might be moved to open a file and investigate.
But if your killer is cleaver, he might be able to keep the ME completely out of the picture or at least give him an easy answer for the cause of death. If no murder is suspected, he'll take the path of least resistance, which is also the cheapest route. Remember, he must live with and justify his budget annually. If he is wasteful he'll be looking for a job. So, give him a cheap and easy out. Your sleuth will then have to battle the ME to get the case re-opened.
The second thing a clever poisoner can do is to use a poison that is not readily detectable and will slip through most drug screens. Toxicology testing follows a two-tiered approach. Screening Tests, which are easier, faster, and cheaper, are used to identify common classes of drugs such as narcotics or amphetamines. This only tells the ME and toxicologist that some type of narcotic or amphetamine is present, but not which one. Determining which one requires more sophisticated, time-consuming, and expensive Confirmatory Testing. And if the screening tests are normal, no further testing is warranted and the ME would not spend the time and money to go further down that road.
Drug screens typically test for alcohol, narcotics, sedatives, marijuana, cocaine, amphetamines, and aspirin. Some screen for a few other classes. Once a member of a class is identified, then confirmatory testing will determine exactly which member of the class is present and in what amount. For example, if narcotic is found in the screen, further testing might show that the actual narcotic present is morphine. Or an amphetamine might be further analyzed and this might show that methamphetamine is the culprit.
Your poisoner could use a poison that would not be found in the typical screen. Things such as arsenic, selenium, and most plants (oleander, deadly nightshade, etc.) do not show up on the typical tox screen and when the screen came back negative, the ME might not go further. Why would he spend the time and money without a good reason? This is where your sleuth steps in to shake things up.
But, if a poison is suspected and if the funds and interest to pursue it are present, anything can be found in an intact corpse. Using gas chromatography in conjunction with either mass spectrometry (GS/MS) or infrared spectroscopy (GC/IR) will give a chemical fingerprint for any molecule. And since each molecule has its own structure and thus its own fingerprint, every compound can be distinguished from every other one.
To write a good mystery that will keep the reader guessing to the end, you must plot the nearly perfect murder. This way when your sleuth cracks the case, he or she will be a true hero. If poisoning is your killer’s chosen weapon, then use the above principles to make your plot as clever and convoluted as possible. Have your killer mask the death as natural or use some poison that is not readily detectable in screening tests and then your sleuth must be very clever to solve the case.
There are several sources for you to search out poisons and to discover how they act and how they are identified. Google, of course, and try plugging into your state poison control center. My books, Forensic For Dummies, Murder and Mayhem, and Forensics and Fiction cover a number of poisons. I also recommend Howdunnit: Book of Poisons by Serita Stevens and Anne Bannon fro Writers’ Digest Books. It is a great resource for poisons of all types.
D. P. Lyle, MD