Analytical procedure applied

In summary, official German analytical methods for pesticide residues are always validated in several laboratories. These inter-laboratory studies avoid the acceptance of methods which cannot readily be reproduced in further laboratories and they do improve the ruggedness of analytical procedures applied. The recently introduced calibration with standards in matrix improves the trueness of the reported recovery data. Other aspects of validation (sample processing, analyte stability, extraction efficiency) are not considered. 

The use of sophisticated instrumental systems such as high-resolution GC-MS does not guarantee satisfactory quantitation of the hundreds of chemicals sometimes present in SPMDs without some fractionation of sample residues. Thus, the complexity of target residues, as well as interferences from the matrix sampled can be determinants in the cleanup and separation procedures needed for satisfactory analyses. The following discussion presents the salient features of the typical processing and analytical procedures applied to SPMD samples. 

The analytical procedures applied at Level 2 may be extensions of the Level 1 procedures. In most cases, however, Information developed at Level 1 will provide background for selection and utilization of more sophisticated sampling and analysis techniques. Because Level 2 analyses must positively identify the materials in sources which have already been found to cause adverse environmental effects, these analyses are the most critical of all three levels. It is equally important, however, that the analyses be conducted in an information-effective manner. This is because increasing specificity and accuracy result in cost escalations which are, at best, exponential rather than proportional. Due to the multiplicity of analytical techniques required and the potential for unnecessarily high expenditures, the analyses must be conducted with a full awareness of the information requirements of the environmental assessment program. 

The objective of improvement schemes is to study and validate each step of different analytical procedures applied by different laboratories in a collaborative manner. Such programmes usually involve groups of 20-50 laboratories. In the best case, each critical step of the procedure should be evaluated in an adapted exercise. The individual steps may be studied with a series of different materials in a stepwise manner. In principle the strategy consists of starting from the simplest matrix, e.g. pure solutions and/or mixtures of compounds in solution, for testing the performance of the detector. The analysis of more complex matrices (e.g. raw extract, purified extract) enables the separation and/or clean-up steps to be tested, whereas solid samples are used to test the entire procedure. Spiked samples can be analysed to evaluate the extraction procedure, within the limits of this evaluation (as commented in Section 2.3.1). Such an approach is actually similar to the steps that should be followed when developing and validating a new method in a laboratory. 

Error is defined as the difference between the expected (true) value and the value obtained as a result of the determination. Thus, error can be calculated as a measured quantity value minus a reference value [3]. Measurement error is the consequence of the accuracy (as trueness and precision) of the analytical procedure applied for obtaining the measured quantity ... 

MS/MS was used by Gosetti et al. [287] to determine Se in dietary supplements. Infante et al. [291] reviewed the analytical procedures applying mass spectrometry to the speciation of Se in food products. 

The analytical accuracy of methods can only be discussed with regard to the complete analytical procedure applied. Therefore, it is necessary to optimize sample preparation procedures and trace-matrix separations specifically to the requirements of the analytical results in terms of accuracy, power of detection, precision, cost and the number of elements and increasingly of the species to be determined. However, the intrinsic sensitivity to matrix interferences of the different methods of determination remains important. 

An important precondition for the successful determination of carbon isotope ratios is the prevention of isotopic shifts as a result of the analytical procedures applied. Therefore, five recovery experiments were performed in order to detect changes of the carbon isotope ratios during sample preparation and measurement. The compounds selected for these experiments are known riverine contaminants and comprise hexachlorobutadiene, several musk fragrances, phthalates and other plasticizers, a flame retardant and a pesticide. All recovery samples were spiked with concentrations between approx. 800 ng/L and 1500 ng/L for each compound representing a common abundance level in river systems. 

Isotopic shifts or higher variations of the isotope ratios as a result of the analytical procedures applied were observed only for selected contaminants. Therefore, for accurate compound specific carbon isotope analyses of riverine contaminants it is recommended to conduct supplementary recovery experiments of the individual substances. 

As a first requirement, the sample analyzed should represent as closely as possible the lipid composition of the whole matrix from which it was taken furthermore, sample preparation should be carried out in such an environment as to minimize any changes in lipid properties prior to analysis. In food analysis, proper sampling of the lipid fraction requires knowledge of the physical structure and location of the major lipids in the sample, and the choice of the most adequate procedure depends on the t)q)e of food being analyzed, the nature of the lipid fraction, as well as the analytical procedure applied for the extraction. Foods consisting almost entirely of lipids, such as vegetable oils, often require little, if any, sample preparation prior to analysis. Qn the other hand, for more complex foods, such as meat or milk, extraction and purification of the lipid fraction is necessary prior to analysis. Official methods have been developed, which recommend the sample preparation and extraction procedures to be followed for a specific t)q5e of food. Solvent extraction methods are usually used, to separate lipids from water-soluble food components, prior to chromatographic analysis these are described in the sections that follow. A number of steps are usually required, prior to the solvent xtraction of lipids from a matrix ... 

The analytical procedures applied in the study were fluoromet-ric determination of chlorophyll pigments / /, absorption spectrophotometry for the analysis of carotenoids /2/. Hydroperoxides were determined by the iodometric titrational or spectro-photometric procedures /3,4/. 

Analytical procedures applied to diagnosis and retrospective verification of exposure to OP include (Worek et al., 2005) (i) biochemical determination of ChE activity (ii) identification of imbound OP (iii) identification of decomposition products (iv) fluoride-induced reactivation of inhibited ChE, followed by analysis of the inhibitor and (v) analysis of phosphyl-protein-adducts after tryptic digestion of the protein. The last procedure is regarded to be the most specific and sensitive, but it has the drawback of being strongly dependent on the analysis of butyrylcholinesterase (BChE), the most abim-dant plasma serine esterase with a half-life of about 16 days. 

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