The limit of detection is the smallest concentration at which a method can say "this compound is present" with stated confidence. Below the LOD, the signal can't be separated from instrument noise and blank variation. It is often estimated as about three times the standard deviation of blank measurements, though methods differ.
In practice
A COA might list myclobutanil with LOD 0.01 µg/g and result "ND". That means the lab saw nothing at or above 0.01 µg/g, not that the sample contains zero. A result between the LOD and the LOQ shows the compound is probably present but the number isn't reliable; many labs report those as "<LOQ" or "detected". LODs vary between labs and methods, so ND at one lab and a detection at another can both be honest.
Units and formula
Same units as the result (% w/w, mg/g, µg/g, ppm, ppb). A common estimate is LOD ≈ 3 × SD of the blank, divided by the calibration slope.
Don't confuse with
- LOQ: the higher threshold where the number becomes reliable.
- Action limit: the regulatory pass/fail level, usually well above the LOD.
