At Forensic Fluids Laboratories, we can identify and quantitate over 130 different compounds. For scientists looking for more information about oral fluid drug testing, Forensic Fluids provides the following summary of the detection process, advantages and associated instrumentation. Below, you'll also find references to various publications regarding tandem mass spectrometry, ELISA, LC/GC, and more.
Forensic Fluids Laboratories uses FDA approved ELISA (Enzyme Linked ImmunoSorbent Assay) plates for all of our drug screens. These plates are coated with polyclonal (rabbit) antibodies, making them less specific (that is, more likely to detect across a range of species) than monoclonal (mouse) antibodies.
This screening process tells you whether a sample is negative or positive for a certain drug or group of drugs (e.g., Opiates and Methamphetamines are groups of drugs, whereas PCP is a single drug) but does not give you a quantitative value or number.
The immunoassay will also cross react with other chemicals/drugs that are structurally similar to each other. For example, Ephedrine and other cold pills cross-react with a Methamphetamine or Amphetamine screen, causing a false positive. False positives may also occur due to reactions with other medications as well.
For these reasons, all of our screens are followed by confirmation by LC/MS/MS (liquid chromatography tandem mass spectrometry), for positive identification and quantitation.
Forensic Fluids Laboratories uses the most sensitive instruments available. The LC/MS/MS can detect lower levels than a single MS, and is also more accurate at identifying drugs.
The scientific advantage of tandem mass spectrometry (MS/MS) is seen by the way it identifies peaks. Daughter, or fragment ions in Multiple Reaction Monitoring (MRM) analysis are inextricably sourced from one parent each. This makes this method more sensitive and accurate than single mass spectrometry. MRM, with tandem MS, provides exceptionally clean fragment ion chromatograms for quantification. By selectively detecting a product ion of interest from a specified precursor ion, the signal-to-noise ratio is optimized, thus lowering limits of detection (LOD). The MRM analysis mode sets MS1 to pass one parent at a time to the collision cell. This single parent ion is dissociated to fragment ions and these are measured by MS2.
The advantage of liquid chromatography (LC) is that it gives the laboratory the ability to analyze for many more drugs than gas chromatography (GC). Some compounds are not very amenable to GC, such as polar analytes, metabolites, and involatiles. Drugs do not need to be derivatized with liquid chromatography like they do with gas chromatography. Getting rid of this step decreases sample preparation time in some cases by more than one hour. Sample prep for liquid chromatography may be as easy as: pipet the saliva sample, add internal standard, centrifuge, and analyze.
Data collected for any daughters is exclusive to the parent from which it was sourced. Co-eluting parents may be monitored which produce the exact same fragment ions with no confounding of the data.
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