Methamphetamine Analysis and False Positives:
There are various types of false positives which are specific to the analysis of alleged methamphetamine samples and two of these are listed below.
These different types of false positives may arise when an instrument called a GCMS is used to analyze a sample of blood to see if any drugs of abuse are present.
The first type of methamphetamine false positive can arise when a person is using a product such as a "Vick's Inhaler" or a generic version of this product as a nasal/sinus decongestant when they have a head cold or allergies. The GCMS will tell the analyst that methamphetamine is present-- but it can't distinguish which type of methamphetamine is present.
It is a little-known fact that there are two versions (called isomers in chemistry parlance) of methamphetamine and one is legal to own, sell and purchase while the other is illegal.
The interesting thing from a chemistry perspective is that these types of decongestant products actually do contain an isomer of methamphetamine which is the mirror image of the chemical that people use as an illegal stimulant. This methamphetamine isomer is legal to purchase and possess because it does not give a person a "high" in the same way that the illegal isomer does. The legal form of methamphetamine is related to the illegal form of meth in the same way that your right hand is related to your left hand. At the molecular level, these molecules are mirror images of one another and they cannot be superimposed upon one another-- they are different and they work differently in the body because, although they are both a type of methamphetamine, they interact with the proteins in the body differently. One acts as a decongestant product (the legal form) and the other isomer (the illegal form) acts as a stimulant to the body. To continue with the "handedness" example/analogy above, just as a handshake between two people both using their right hands feels different than a handshake between two people when one uses their right hand and the other person uses their left hand, this same sort of issue occurs in the sites in the body where these two types of methamphetamine bind. One isomer "fits" in such a way that the body uses it as a decongestant while the other isomer "fits" in other sites where the body uses it as a stimulant-- their "handshake" with the body is different.
The problem with this type of false positive issue in methamphetamine analysis is that a GCMS instrument cannot tell which isomer is present unless special methods are employed; it can be rare to find these special methods used. So, if methamphetamine is found in a person's blood sample, it is usually assumed by law enforcement officers and prosecutors to be the illegal form rather than the legal form. This is not necessarily true and it is not correct to make that assumption. It may be that a person has just been using the legal form to subdue the symptoms of a head cold rather than the person using the illegal form to get "high".
The second type of false positive for methamphetamine arises in a slightly different way when a GCMS instrument is used. It turns out that a tiny amount of methamphetamine can be produced by decomposition of the sample when pseudo ephedrine (sometimes sold under the brand name of "Sudafed") is analyzed by GCMS. This methamphetamine can be detected by the GCMS instrument (even though it might not have been present in the original sample, just created by the analysis technique, itself) and, due to this false positive, the person can mistakenly be charged with being under the influence of a controlled substance (because the methamphetamine from the decomposition of pseudo ephedrine in the GCMS was found). This type of false positive is not well-known in some areas of the country, so it is useful to have an independent chemistry expert review case data to see if this decomposition reaction has occurred on the GCMS and generated the false positive for methamphetamine.