limit of quantitation
/ LIM-it of kwon-tih-TAY-shun /
Reading a bathroom scale, there is a difference between sensing that you are standing on it and being able to read off an honest number of kilograms. A scale that twitches between 60 and 70 each time you step on tells you something is there, but not a number you would trust. The limit of quantitation is where a chemical measurement crosses from 'I can detect it' to 'I can put a believable number on it'.
The limit of quantitation is the lowest concentration at which the analyte can be measured with acceptable accuracy and precision, not merely detected. A common rule places it where the signal is about ten times the standard deviation of the blank noise — comfortably above the limit of detection. Below the limit of quantitation you may report 'detected but below the limit of quantitation', but you should not quote a precise figure.
It matters because regulatory limits and decisions often hinge on a number, and that number is only meaningful at or above the limit of quantitation. The honest caveat is that the exact value depends on how strict you set 'acceptable precision' — typically a relative standard deviation around ten percent — so two labs can list slightly different limits of quantitation for the same method.
A pesticide method has a limit of detection of 0.5 µg/kg and a limit of quantitation of 1.5 µg/kg; a residue measured at 0.8 µg/kg is reported as detected but not quantifiable, while one at 4 µg/kg is reported as a firm number.
The lowest level at which a residue can be quoted as a trustworthy number.
The limit of quantitation is always higher than the limit of detection for the same method; the band between them is where you can honestly say something is present but cannot give a reliable amount.