Calibration Curve and Analytical Range

Quantitative methods are usually based on a comparison of the response from an analyte in a sample with the response from standards of the analyte in solution at known concentrations. In method development and validation, the calibration curve should first be determined to assess the detector response to standards over a range of concentration. These concentrations should cover the full range of analytical interest, and, although it is usually recommended practice to include a suitable blank with the calibration samples, this does not imply that it is acceptable to extrapolate into the region of the curve below the lowest calibration standard or to force the curve through the origin. 

There are recognized definitions and guidance documents related to the assessment of the analytical response for an analyte in a measurement system. These usually are expressed in terms of the relationship between the concentration of analyte present in the sample material and the associated response from a detector. The relationship between the analytical response and the analyte concentration is plotted and usually expressed as a linear regression equation. lUPAC defines the calibration curve in terms of the calibration function in analysis in the lUPAC Gold Book the calibration function in analysis is defined as follows  

The functional (not statistical) relationship for the chemical measurement process, relating the expected value of the observed (gross) signal or response variable E(y) to the analyte amount x. The corresponding graphical display for a single analyte is referred to as the calibration curve. When extended to additional variables or analytes which occur in multicomponent analysis, the curve becomes a calibration surface or hypersurface. 

Two earlier publications are cited as original source documents for this detinition  

A function which relates the measured value Ca to the instrument reading, X, with the value of all interferants, 

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