Limit of Detection (LOD)
Definition
The smallest amount a method can reliably distinguish from background, without being able to say how much is present. A result below it is reported as not detected rather than as zero.
In more detail
Every measurement sits on a noisy baseline. The limit of detection is the point at which a signal can be reliably told apart from that noise, conventionally where it stands around three times the noise above it.
A substance present below that point produces no distinguishable signal, which is why a result there is reported as not detected rather than as zero. The two are different claims: not detected states the limit of what was looked for, and a report that says it without naming the limit has not said very much.
Terms used in this definition
Other entries this definition refers to.
Where this term comes up
Entries whose definition mentions Limit of Detection (LOD).
What this means for research sourcing
Purity and identity verification are central to research reproducibility. Every Volta compound ships with >99% HPLC-verified purity and a certificate of analysis.
Related Terms in Quality
Limit of Quantitation (LOQ)
The smallest amount a method can measure with acceptable accuracy and precision. It is higher than the limit of detection, and a figure reported below it carries more uncertainty than the digits suggest.
Reporting Limit
The lowest value a laboratory will put on a certificate, set at or above the limit of quantitation. Anything smaller is reported against that limit rather than as a number nobody can defend.
Chromatogram
The detector trace a chromatographic run produces: signal against time, with each component appearing as a peak. The purity figure on a certificate is arithmetic performed on the areas of those peaks.
Retention Time
How long a component takes to travel through the column and reach the detector. It is characteristic of a compound under fixed conditions, which is why identity is often confirmed by a sample and a reference standard eluting at the same time.
Co-elution
Two components leaving the column together and arriving as one peak. It is the main way a purity figure can be honestly measured and still be too high, and it is why an orthogonal method is worth running.
Peak Integration
Deciding where a peak starts and stops so its area can be calculated. Where the baseline is drawn changes the number, which is why integration parameters belong in the method rather than in the analyst's judgement on the day.
Reversed-Phase HPLC (RP-HPLC)
The chromatographic mode almost all peptide purity work uses: a non-polar stationary phase with a polar moving phase, separating components by hydrophobicity. The gradient usually runs water and acetonitrile with an acidic modifier.
HPLC-UV
High-performance liquid chromatography with an ultraviolet detector, typically reading at 214 nm where the peptide bond absorbs. It reports how much of the absorbing material is the target peak, which is a purity measurement and not an identity one.