Does Positive for other substance trigger GC/MS for all?
Does Positive for other substance trigger GC/MS for all?
Person takes amphetamine legally (i.e., can provide a legal script). Person smokes but at the moment has a theoretical cutoff of THC at 20 ng/mL. Person passes for substances except amphetamine. Sample is sent to the lab for confirmation (which we will assume can detect to 15 ng/mL THC).
Does person now fail for THC? I have run GC/MS myself and cant imagine why they would not. The peak is going to show up regardless unless there is some kind of different prep method (doubt it). Unless, the labs just ignore the peak for THC because they were not checking for it in the first place (i.e., passes the coloirmetric test for THC).
Thoughts? Feelings?
Does Positive for other substance trigger GC/MS for all?
Quote:
Person smokes but at the moment has a theoretical cutoff of THC at 20 ng/mL. Person passes for substances except amphetamine. Sample is sent to the lab for confirmation (which we will assume can detect to 15 ng/mL THC).
Does person now fail for THC? I have run GC/MS myself and cant imagine why they would not
Heres why it would not:
A 50 ng Immunoassay is equivalent to the 15 ng GC/MS. The cutoff numbers are different because the Immunoassay recognizes all 31 metabolite concentrations.
As you may know, THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations, with the most prevalent form being the THC-COOH metabolite concentration. In order to test negative on the I/A, the whole composite of the 31 metab concentrations must register below 50 ng.
The GC/MS cutoff is 15 ng because the machine recognizes only one metabolite concentration: the THC-COOH concentration. In order to test negative, that one metabolite concentration must register below 15 ng.
Therefore if a given sample tests negative on the I/A, that same sample will also test negative on the GC/MS.
But once the sample passes the I/A, it will never see the GC/MS. It would be a huge waste of money for both the client an the lab; and the GC/MS is far from being cheap. Therefore, the GC/MS is only used to confirm positives for only the drugs that tested positive on the I/A.
Does Positive for other substance trigger GC/MS for all?
Quote:
Originally Posted by Burnt Toast
Heres why it would not:
A 50 ng Immunoassay is equivalent to the 15 ng GC/MS. The cutoff numbers are different because the Immunoassay recognizes all 31 metabolite concentrations.
As you may know, THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations, with the most prevalent form being the THC-COOH metabolite concentration. In order to test negative on the I/A, the whole composite of the 31 metab concentrations must register below 50 ng.
The GC/MS cutoff is 15 ng because the machine recognizes only one metabolite concentration: the THC-COOH concentration. In order to test negative, that one metabolite concentration must register below 15 ng.
Therefore if a given sample tests negative on the I/A, that same sample will also test negative on the GC/MS.
But once the sample passes the I/A, it will never see the GC/MS. It would be a huge waste of money for both the client an the lab; and the GC/MS is far from being cheap. Therefore, the GC/MS is only used to confirm positives for only the drugs that tested positive on the I/A.
Hmm. Well a GC works by vaporizing the sample you inject into it so I dont understand why it would not detect all of the metabolites by their respective weights. Now if they were all different masses they may choose to only look at one particular peak which for all I know could be that one you mentioend with the -COOH group. That would make sense I suppose. But still, if they run a sample looking for a peak at m/z 135 for amphetamine-whatever and lets say this THC peak is at m/z 300 then its going to show up regardless. I guess what I am saying is you cant run a GCMS for one sample unless you are running a really narrow calibration range that only covers a particular M/W range..... But from a business prospective it would save time and money to just have one general calibration that covered all the shit they are looking for.
Oh and just some info. I set up a study where GC/MS runs cost $80 - 120 each for water analysis (of ground water) +$10 for filtration. So I dont think GCMS is as expensive as many of you think it is (but maybe after the cost of the analyst it is or something). Anyways......Thanks for the response I do appreciate it I guess what I need to know is how they quantitate each substance.
Does Positive for other substance trigger GC/MS for all?
