Testing the Accuracy of Analytical Standard Solutions Used for Quantitative Determination of Pesticide Residues

Pure analytical standards of pesticide residues may decompose or change their concentrations, especially in diluted standard solutions. The SANTE/11945/2015 guidance document on analytical quality control permits a ±10% difference between the concentrations in the new (100%) and the old standard solutions, which should be tested by comparing the average responses of the corresponding components obtained with a minimum of five replicate injections. To verify that the difference is within the permitted ±10%, the application of the two one-sided t-test (TOST) is recommended. It is shown here, with practical examples, that the two-sample t-test is not suitable for this purpose, because it is designed to prove that the old and new standard solutions have different means, not that the means of two sets are equal or within a specified limit. Furthermore, it is recommended that the standard solutions be prepared based on weighing with 0.01 mg accuracy and in duplicate, and that their average concentrations, determined with an appropriate instrumental technique, should be compared with the TOST to identify potential errors in their preparation. An automated Excel template, provided as supplemental material, simplifies the calculation