Calibration

Abstract

• To describe the linear calibration model and how to estimate uncertainties in the calibration parameters and test concentrations determined from the model. • To show how to perform calibration calculations using Excel. • To calculate parameters and uncertainties in the standard addition method. • To calculate detection limits from measurements of blanks and uncertainties of the calibration model.… Calibration is at the heart of chemical analysis, and is the process by which the response of an instrument (in metrology called ‘‘indication of the measuring instrument’’) is related to the value of the measurand, in chemistry often the concentration of the analyte. Without proper calibration of instruments measurement results are not traceable, and not even correct. Scales in supermarkets are periodically calibrated to ensure they indicate the correct mass. Petrol pumps and gas and electric meters all must be calibrated and recalibrated at appropriate times. A typical example in analytical chemistry is the calibration of a GC (gas chromatography) analysis. The heights of GC peaks are measured as a function of the concentration of the analyte in a series of standard solutions (‘‘calibration solutions’’) and a linear equation fitted to the data. Before the advent of computers, a graph would be plotted by hand and used for calibration and subsequent measurement. Having drawn the best straight line through the points, the unknown test solution would be measured and the peak height read across to the calibration line then down on to the x-axis to give the concentration (figure 5.1). Nowadays, the regression equation is computed from the calibration data and then inverted to give the concentration of the test solution. Although the graph is no longer necessary to determine the parameters of the calibration equation, it is good practice to plot the graph as a rapid visual check for outliers or curvature. Because we can choose what values the calibration concentrations will take, the concentration is the independent variable, with the instrumental output being the dependent variable (because the output of the instrument depends on the concentration).