I NTERNAL Q UALITY C ONTROL M EASURES

7.4.2 I NTERNAL Q UALITY C ONTROL M EASURES

IQC measures 97 must be included in the analytical batches to enable the decision whether the batches satisfy the preset quality criteria and that a set of the results can

be accepted.

1. Reagent blank eliminates false positives by contamination in the extraction process, instruments, or chemicals used. Moreover, a matrix blank has to be analyzed to detect interferences of sample matrix. If a matrix blank is not available, the use of a simulated homemade matrix is allowed.

2. Certified Reference Materials (CRMs) or homemade reference materials (checked for stability and homogeneity), prepared by spiking a blank sample matrix with 5% –10% of the target pesticides, can be used to study the variation between batches of samples. This reference must be analyzed everyday by applying the analytical method, providing additional informa- tion about instrument performance (instrument sensitivity, column perform- ance, etc.). The variation in the data obtained from the analysis of the quality control sample is normally monitored on a quality control chart. However, CRMs are expensive and, although several materials have been produced for the analysis of OCs, most of them show limitations, such as a limited number of certified pesticides, wide uncertainty ranges, concentra- tions well above the current values of interest, or a physical state not matching routine samples (e.g., freeze-dried materials).

3. A calibration curve must be carried out for every sample batch. A minimum of three standard concentrations, prepared in solvent or in extract of blank matrix, has to be used for each pesticide. The first of them has to be equal or preferably lower than the MRL allowed for each pesticide in the target matrix, but must be still higher than or equal to the limit of quantification. The fit (linear, quadratic, etc.) of the calibration function must be inspected to ensure that it is satisfactory. Individual points of the calibration curve

approached).

198 Analysis of Pesticides in Food and Environmental Samples

TABLE 7.5 Acceptable Recovery Ranges of Quantitative Methods

Mass Fraction (mg=kg) Range of Acceptable Recovery (%)

4. Recoveries measured from spiked matrix blanks must be used to check the extraction efficiency in each sample batch, by adding all analytes to a matrix blank at a concentration level about 30% above the LOQ. This level of addition may be varied to have information over a range of

concentrations. The Council Directive 96=23=EC 98 has established accept- able recovery ranges in function of the amount of the analyte(s) added to the

blank matrix (Table 7.5).

5. Replicated samples provide a less formal means of checking for drift than quality control samples. The results obtained from the analysis of these samples must be comparable, taking into account the uncertainty of the method. They are located in the batch every certain number of samples (e.g., each 10 samples) and their presence is known by the analyst.

6. Blind samples are replicated samples placed in the analytical batch without known by the analyst. They are complementary to replicated samples, providing information about the analyst’s proficiency.