Sample preparation, generally measurement process

In general, the total variation of a measured result y is composed of several variation components y, yi,..., ym- In the course of the analytical process, all the steps of the analytical procedure (e.g., sampling, sample preparation, separation, and measurement) and of single operations (e.g.,... 

The compound to be analysed, the analyte, is generally contained in a liquid or solid matrix it is rarely found in a form that allows direct measurement. Interfering species that may lead to unwanted interactions, particularly during trace analysis in the presence of abundant matrix components, have to be eliminated. As a result, analysts have long acknowledged the need for efficient and reproducible sample preparation methods. The pre-treatment process has to take into account the analyte, matrix and measurement technique chosen. This situation has led to a number of specific sample pre-treatment protocols that describe sample treatment from sampling all the way to recording of the results (Fig. 20.1). 

As far as experimental protocol is concerned, this will correspond to the mode of operation chosen. The protocol is essentially the analysis recipe and is generally a process controlled by predefined standards. This procedure involves standardisation at each stage of analysis from sample preparation to experimental measurement. 

Before a new analytical method or sample preparation technique is to be implemented, it must be validated. The various figures of merit need to be determined during the validation process. Random and systematic errors are measured in terms of precision and bias. The detection limit is established for each analyte. The accuracy and precision are determined at the concentration range where the method is to be used. The linear dynamic range is established and the calibration sensitivity is measured. In general, method validation provides a comprehensive picture of the merits of a new method and provides a basis for comparison with existing methods. 

Once a representative sample has been obtained from the object of interest, the next step is to prepare the sample for analysis. Since sample preparation depends upon both the analyte (e. g. iron in water, polycyclic aromatic compounds in benzene, etc.) and the instrumentation used to perform the spectroscopic measurement (e. g. UV—VIS or IR absorption, luminescence, Raman, HPLC-fluorescence, and GC-MS, etc.), details of the preparation process will vary from analysis to analysis. Many general procedures have been developed over the years for the preparation of various types of samples prior to analysis. Most of these procedures can be classified based upon the types of samples that are to be analyzed, either sohds or liquids. Within each of these categories exist several subcategories based upon the type of analyte to be measured. 

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