Every instrument used in chemical analysis can be characterised by a specific calibration function – an equation relating the instrument output signal (S) to the analyte concentration (C). This response function may be linear, logarithmic, exponential, or any other appropriate mathematical form. The exact form of this response function depends on the system being measured and the measurement process itself.
Illustration of the instrument calibration function, S = f(C). Note that the concentration of analyte detected within the instrument may not necessarily be the same as the concentration in the sample, but will be related to it.
While the calibration function may be known theoretically, various factors (such as the specific analyte being measured, interference effects caused by other components of the sample matrix, or random experimental errors) require that we calibrate each instrument for the specific analyte and measurement conditions to be used in a particular experiment.