Evaluation of analytical calibration based on least-squares linear regression for instrumental techniques: A tutorial review

• Compilation of analytical calibration information from validation guidelines.
• Report of very common mistakes in the analytical calibration process.
• Compilation of methods for assessing linearity of calibration curves.
• The correlation and determination coefficients are misleading for testing linearity.
• Deviation from back-calculated concentration is very useful for testing linearity.

The analytical calibration of an instrumental method is very important, being considered as a key point in method validation. There are different validation guidelines; showing that analytical calibration process variety prevails in terms of nomenclature, methodology employed and acceptance criteria. Very common mistakes in the analytical calibration process are the use of correlation and/or determination coefficients as a test for linearity, the negligence in the heteroscedasticity of the experimental data and selection of appropriate weighting factor, misunderstanding about the regression through the origin and using zero-point calibration. Once the calibration function is established, their linearity can be confirmed by using different procedures such as graphical plots, statistical significance tests and numerical parameters. In particular, deviation from back-calculated concentrations expressed in the form of percentage of relative error (%RE) can be considered very useful for unambiguous linearity evaluation. Some case studies were included to explain the linearity assessment from a practical viewpoint.