Limit of detection (revision 16)
Old revision·15:53, 12 Jul 2025·ColumnOvenCoy
| Limit of detectionMethod characteristic | |
|---|---|
| Abbreviation | LOD |
| Distinguished from | Limit of quantitation (LOQ) |
| Common convention | Signal-to-noise ratio of about 3:1 |
| Topic infobox · conventions | |
The limit of detection is the lowest concentration of an analyte that can be reliably distinguished from background, though not necessarily measured with acceptable precision. The limit of quantitation, at which a number can be reported, is higher — conventionally by a factor of about three.[1]
The distinction is practical rather than pedantic. Between the two limits an analyte is detectable but not measurable, and a determination reported in that region carries an uncertainty comparable with the value itself.[1]
For peptide impurity work the relevant question is usually not what the limit is in absolute terms but whether it is below the reporting threshold. An impurity present at 0.05% is irrelevant if the method cannot see below 0.5%, and a certificate reporting "no impurities detected" is a statement about the method as much as about the material.[2]
How it is determined
[edit]Three approaches are recognised. The signal-to-noise approach compares the analyte signal with baseline noise and takes a ratio of about 3:1 for detection and 10:1 for quantitation; it is simple and is the usual choice for chromatographic methods. The standard-deviation-of-the-blank approach uses replicate blank measurements. The calibration-curve approach uses the residual standard deviation of the regression and its slope.[1]
The three do not necessarily agree, and a reported limit should state which was used. Signal-to-noise figures in particular depend on how noise was measured — over what region of the baseline, and after what smoothing — and are not comparable between laboratories without that detail.
The limit is a property of the method as applied, not of the technique. The same instrument with a longer injection, a more concentrated preparation or a more sensitive detector setting has a different limit.[3]
Reporting thresholds
[edit]In pharmaceutical practice the reporting threshold — the level above which an impurity must appear in a report — is set by guidance according to the daily dose, and is distinct from the detection limit. A method must be able to detect below the reporting threshold; a method that cannot is inadequate regardless of how good its other characteristics are.[4]
| Term | Meaning |
|---|---|
| Detection limit | Presence can be distinguished from background |
| Quantitation limit | A number can be reported with acceptable precision |
| Reporting threshold | Level above which an impurity must be listed |
| Identification threshold | Level above which an impurity must be identified |
| Qualification threshold | Level above which safety must be addressed |
Research-chemical certificates rarely state any of these, which is why "no other peaks observed" is uninformative without the method behind it. The statement may reflect a clean material or a method with a high detection limit, and nothing in the document distinguishes the two.[2]
Practical consequences
[edit]Two certificates reporting different numbers of impurities may describe identical material analysed by methods of different sensitivity. The one listing more impurities is not describing worse material; it may simply be looking harder.[1]
This inverts the naive reading. An itemised related-substances table listing six impurities at 0.05–0.2% is evidence of a sensitive method and a transparent report; a certificate stating "purity 99.9%, no other peaks" may be evidence of a method that cannot see them.
See also
- Analytical method validation
- Area percent purity
- Residual solvent
- Bacterial endotoxin test
- Certificate of analysis
References
- ^ a b c d International Council for Harmonisation, Q2(R2): Validation of Analytical Procedures (2023).
- ^ a b United States Pharmacopeia, General Chapter <1503>, Quality Attributes of Synthetic Peptide Drug Substances.
- ^ United States Pharmacopeia, General Chapter <621>, Chromatography.
- ^ International Council for Harmonisation, Q3A(R2): Impurities in New Drug Substances (2006).