Orthogonality (Analytical)
Definition
The requirement that two methods used in a drug identification measure genuinely different physical or chemical properties. Orthogonality ensures that the probability of a false positive in both methods simultaneously is the product of the individual false-positive probabilities, not merely the higher of the two. Two GC runs with the same stationary phase are not orthogonal; GC-MS and FTIR are orthogonal.
- Requirement
- Two methods measure genuinely different properties
- Benefit
- Combined false-positive rate is a product, not a maximum
- Non-orthogonal example
- Two GC runs on the same stationary phase
- Orthogonal example
- GC-MS paired with FTIR
Common questions
Why does SWGDRUG require orthogonal rather than merely repeated testing for identification?+
Repeating the same technique, such as two GC runs on identical columns, can produce the same false result twice if an interferent happens to co-elute at the same retention time in both runs. An orthogonal method measures a different physical property, such as infrared absorption instead of chromatographic retention, so an interferent that fools one technique is unlikely to also fool the other.
How does orthogonality reduce the overall false-positive risk in a drug identification scheme?+
When two methods are truly independent, the probability that both simultaneously return the same wrong identification is approximately the product of each method's individual false-positive rate, which is far lower than either method's rate alone, giving the combined result much stronger evidentiary weight.
Related terms
- Category a Method
- An analytical method that provides the highest level of discriminating power in drug identification, generating structural or molecular fingerprint information. Category A...
- Category B Method
- An analytical method with intermediate discriminating power, measuring a characteristic but less structurally specific property. Includes UV-Vis spectroscopy, TLC, GC without MS,...
- ENFSI Drugs Working Group (DWG)
- European Network of Forensic Science Institutes Drugs Working Group. Publishes best-practice manuals for drug analysis aligned with SWGDRUG tiering principles, operates an...
- Internal Standard (IS)
- A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known...
- ISO/IEC 17025:2017
- The international standard specifying general requirements for the competence of testing and calibration laboratories. Covers management (document control, non-conforming work, audits) and...
- Measurement Uncertainty (MU)
- A quantified parameter associated with a result that characterises the dispersion of values reasonably attributable to the measurand. Estimated by the GUM...
- Multiple Reaction Monitoring (MRM)
- An LC-MS/MS acquisition mode in which the first quadrupole (Q1) selects a precursor ion at a specific m/z, the second (Q2) fragments...
- NDPS Small Quantity / Commercial Quantity
- Quantity thresholds defined in the schedules to India's NDPS Act 1985 that determine the severity of criminal charge and applicable mandatory minimum...
- SWGDRUG
- Scientific Working Group for the Analysis of Seized Drugs, established in 1997 by the US DEA and the Office of National Drug...