A function which relates the measured value \(\hat{C}_{\text{a}}\) to the instrument @R05189@, \(X\), with the value of all interferants, \(C_{i}\), remaining constant. This function is expressed by the following regression of the calibration results. \[\hat{C}_{\text{a}} = f\left(X\right)\] The analytical function is taken as equal to the inverse of the calibration function.