Forensic Gas Analysis?

[Redshift]

If he had convulsed and drowned and then the gas analysis had shown 21% O2 then further investigation would make sense. From what I read everything, from his behavior beforehand, to what happened later was consistent with oxygen toxicity.

If a pedestrian arrives at a crossing and it’s red light for pedestrians, but trips, falls and is hit by a car, does the police try to find some kind of drug that could have affected the person’s balance or motor control?

 

[Storker]

As lab equipment goes, that is near chump change today

It is chump change. Heck, If I could get a decent GC for 12k USD, I'd be the happiest guy on campus that month.

it doesn’t sound like a one-pass full spectrum analysis. What I don’t really understand is do gas chromatographs report on all gasses in a sample or do they have to specifically look for them one-by-one?

A standard issue GC is a pretty dumb instrument. All it does is to separate the components in a mixture of gases or a mixture of volatile components. You put the sample in on one side, the individual components come out of the other side, one after the other. Some substances behave very differently and are easy to separate, some behave very similarly, and you need an expensive hi-res instrument to even have a chance of separating them. After that, they're detected on a device (often it's a type of device that's called "FID") that basically says "there's something here".

So, you've got a list of how much of "something" that came out at which time. If you have an idea what this "something" might be, you run standard samples and check if the time of the unknown and the time of the standard match. Usually, that's enough, because you have a pretty decent idea of what's in the mix. If you don't have the time for that "something" in your library, you only know that "there's something here".

I am also a little fuzzy on the differences between gas chromatographs and mass spectrometers. Can a mass spec do the same job with gas mixtures?

A mass spectrometer can't work on mixtures. It works on single substances, essentially by smashing the molecules into pieces of varying size and detecting those pieces. Different substances end up in different sized pieces, so you can find out what that substance was. Since the GS just separates but can't really identify, while the MS identifies but can't separate, they're a great team. Hooking up an MS after a GC is a pretty popular setup for analysts.


And just so that it's said: Analytic chemistry is a bit like detective work. You have to know what to look for, or at least have a decent hunch as to what kind of stuff you're looking for. Otherwise, you're in for a pretty big job...

 

[Jax]

Perhaps another way to ask Akimbo's question - under what circumstance would one call for a broader gas analysis?

In an episode of Sea Hunt, the Bad Guy wanted to kill Bridges, so he fed exhaust fumes into the compressor intake.

One could imagine the physical characteristics of CO poisoning would lead to an analysis, but what would cause a quest for other gasses?

 

[Dr. Lecter]

Perhaps another way to ask Akimbo's question - under what circumstance would one call for a broader gas analysis?

In an episode of Sea Hunt, the Bad Guy wanted to kill Bridges, so he fed exhaust fumes into the compressor intake.

One could imagine the physical characteristics of CO poisoning would lead to an analysis, but what would cause a quest for other gasses?

A delusional belief that each and every police force and ME has a magic box at their lab just waiting to take a scuba tank gas sample and spit out a Fisher-Price level explanation of exactly what's in there a few seconds later?

 

[boulderjohn]

It seems to me that any hint of a contaminated tank would call for a full inspection. I suspect that a percentage of unexplained scuba deaths might be related to CO contamination that was not checked out. It is pretty rare, however, for such a possibility to exist. Usually the issues are more obvious.

 

[hroark2112]

If he hadn't done his own fills at his own home, there might be a tiny iota of a shred of a reason to contemplate analyzing his tank beyond what they did.

 

[Akimbo]

Severe CO poisoning has some pretty obvious symptoms like cherry-red lips and the skin under fingernails. Unfortunately people are near death by that point and you aren't going to notice it underwater. CO is absorbed by Hemoglobin more readily than Oxygen (highly simplified).

I understand that the "redness" gets even worse after a person expires, but I have no idea how long it persists after the body starts to decay. I would "guess" a fairly common blood test would show it, which would lead to testing cylinders.

---------- Post added January 31st, 2015 at 05:17 PM ----------

If he hadn't done his own fills at his own home, there might be a tiny iota of a shred of a reason to contemplate analyzing his tank beyond what they did.

This discussion is beyond any specific accident. It is more about forensic conventions, analysis methods, and what might trigger further investigation.

---------- Post added January 31st, 2015 at 05:19 PM ----------

A delusional belief that each and every police force and ME has a magic box at their lab just waiting to take a scuba tank gas sample and spit out a Fisher-Price level explanation of exactly what's in there a few seconds later?

That can't be true, they do it on TV all the time. Scubaboard really needs a sarcastic icon.

 

[Abdullah]

I managed an environmental lab and we operated both GC and GCMS, as well as ICP, ICPMS, and FTIR analyzing environmental contaminants regulated by the USEPA. The fundamental problem you'd have with testing for "anything under the sun" using GC and GCMS is the columns are fairly specific for categories of compounds. The separation in the column, which is required for differentiating the compounds at the detector, is achieved through a difference in "reactivity" between the compound and the solid phase coating on the inside of the column. The solid phase coating is engineered to be fairly specific and extremely reproducible.

Mass spectrometry (MS) is just another detector - a very low detection limit detector, but still just a detector. The GC column separates the compounds so that the detector can analyze the compounds one at a time. For example, when analyzing for dioxin and furans, which are pretty similar compounds, the column separates out the individual isomers on a very reproducible timeline (elution time), whereupon the detector blows them into very reproducible bits, which are measured very accurately by the mass spectrometer. The software is calibrated to consider the elution time and the atomic weights of the fragments when determining the compound. The quantity of fragments is directly proportional to the concentration.

Next the analysis time isn't going to be quick. The columns can be pretty long (coiled fused silica tubes dozens of meters long) and the flow is metered though a standardized sequence of steps involving temperature changes that improve separation efficiency. Add QC samples and the process time becomes significant for many compounds. To find anything that could be a hazard, you'd need a lot of instruments or a lot of column changes on the instruments you have. The column change is another QC driven process with standardized protocols that don't favor haste. Then there is the preparatory analysis to concentrate potential contaminants for the various instrument tests (wet chemistry)...

Combine the specificity of the columns, changing columns to test for anything under the sun, wet chemistry, and analysis time itself - and determining that there is nothing but air in the tank becomes a significant endeavor (assuming that "air" has a clean standard that everyone can agree on). The analyses can be done, but the instrument and analyst time would be pretty expensive. You'd need to be able to justify that cost with some sort of objective evidence that anything other than "standard" air was hazardous.

In saturation diving, there is quite a bit of evidence that certain compounds are hazardous at saturation pressures, and the OSHA duty of care requirements would force a company to have a process to ensure that these compounds were below an established threshold or permissible exposure level for that operating depth. The litigable liability for the company makes the instrumentation and analysis a worthwhile investment. Having a dedicated instrument with a column engineered for known hazards would be the way to go. Saturation diving is really expensive for a lot of reasons, and this would be another contributor to those costs.

For a recreational diving mishap I don't see how this analysis could be justified without some reasonable expectation that a hazardous compound is present. You would really need to know what to look for to price this in an affordable range. So, unless the family or a benevolent MegaBall winner wanted to fund the analyses, I don't see how this could be at all feasible for a local law enforcement accident investigation. Especially if the goal is to eliminate "any possible" (gianaamari proof) suspicion of foul play.

 

[boulderjohn]

That can’t be true, they do it on TV all the time.

In TV land, DNA analysis comes back from the lab in less than 5 minutes. In the rest of the world it takes months.

When I worked as the curriculum director for an online education company, we made most of our courses, but we purchased some as well. It was my job to review potential purchases for quality. We looked at a high school level course in forensic science, written by someone who worked in the field. The first lesson was a contrast between what you see on TV and what happens in the real world. Anyone who thinks what they see related to real forensic science on a TV show is at all like the real world is very far off the mark.

 

[Storker]

In TV land, DNA analysis comes back from the lab in less than 5 minutes. In the rest of the world it takes months.

Obligatory XKCD: