Detection limit

The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the exact threshold (level of decision) used to decide when a signal significantly emerges above the continuously fluctuating background noise remains arbitrary and is a matter of policy and often of debate among scientists, statisticians and regulators depending on the stakes in different fields. In analytical chemistry, the detection limit, lower limit of detection, also termed LOD for limit of detection or analytical sensitivity (not to be confused with statistical sensitivity), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) with a stated confidence level (generally 99%). The detection limit is estimated from the mean of the blank, the standard deviation of the blank, the slope (analytical sensitivity) of the calibration plot and a defined confidence factor (e.g. 3.2 being the most accepted value for this arbitrary value). Another consideration that affects the detection limit is the adequacy and the accuracy of the model used to predict concentration from the raw analytical signal. As a typical example, from a calibration plot following a linear equation taken here as the simplest possible model: where, corresponds to the signal measured (e.g. voltage, luminescence, energy, etc.), "b" the value in which the straight line cuts the ordinates axis, "a" the sensitivity of the system (i.e., the slope of the line, or the function relating the measured signal to the quantity to be determined) and "x" the value of the quantity (e.g. temperature, concentration, pH, etc.) to be determined from the signal , the LOD for "x" is calculated as the "x" value in which equals to the average value of blanks "y" plus "t" times its standard deviation "s" (or, if zero, the standard deviation corresponding to the lowest value measured) where "t" is the chosen confidence value (e.