Linearity
Definition
The range of analyte concentrations over which an analytical method produces instrument responses that are proportional to concentration (the calibration curve is linear); characterised by a coefficient of determination r² typically required to be at least 0.999 for forensic drug quantification; results outside the linear range must be diluted and re-analysed.
- Definition
- Range where instrument response is proportional to concentration
- Typical requirement
- r-squared of at least 0.999 for drug quantification
- Out-of-range action
- Dilute and re-analyze
Common questions
Why does a low r-squared value matter for a case result?+
A poorly fitting calibration curve means the relationship between instrument response and concentration is unreliable across the tested range, so any concentration read from it carries unquantified extra error.
What is done if a sample's response falls above the linear range's upper limit?+
The sample is diluted so its response falls back within the validated linear range, then re-analyzed, and the dilution factor is applied to the reported concentration.
Related terms
- Certified Reference Material (CRM)
- A sample with one or more analyte concentrations established by an accredited body and traceable to a national metrology institute. NIST SRM...
- 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 17034
- The international standard specifying general requirements for the competence of reference material producers; an ISO 17034-accredited producer has demonstrated the technical competence...
- Limit of Detection (LOD)
- The lowest concentration of analyte that produces a signal reliably distinguishable from the instrument noise, conventionally three times the standard deviation of...
- Limit of Quantification (LOQ)
- The smallest concentration that can be quantified with acceptable precision and accuracy, typically corresponding to a signal-to-noise ratio of 10:1 or an...
- 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...
- Metrological Traceability
- A property of a measurement result whereby the result can be related to a reference (ultimately the SI system of units) through...
- NIST SRM (Standard Reference Material)
- A reference material issued by the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, whose property values are established...
- System Suitability Test
- A pre-run check in which a standard mixture is injected into the analytical instrument and key parameters (retention times, peak widths, signal-to-noise...