5.2. Measurement procedure

Brief summary: The main steps of a measurement/analysis procedure are presented on the example of pesticide measurement are presented: Sample preparation (in this case: homogenization, extraction(s), extract purification), instrument calibration, actual analysis. It is stressed that the measurement procedure must correspond to measurand definition.

Measurement procedure 
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This scheme of a chemical analysis procedure is very general. In specific cases there can be deviations from this scheme (more steps or less steps). In particular, sampling is not introduced here as a step of chemical analysis procedure. This holds if samples are brought to the laboratory for analysis and the laboratory itself does not do sampling (see section 5.3 for more details).

It is worth stressing the importance of sample preparation as a step in analytical procedure. The majority of analytical procedures need that the sample is converted into a solution which contains as large as possible share of the analyte from the sample (ideally all of it) and as little as possible of the other components of the sample matrix. In analytical chemistry sample matrix is the term for describing jointly all sample components except the analyte(s). The matrix components often act as interfering compounds, which can artificially increase or decrease the result. Therefore it is important to minimize their content in the solution obtained from the sample. If the interfering compounds cannot be fully eliminated and the interference cannot be corrected (which is quite usual in chemical analysis) then their effect has to be taken into account in measurement uncertainty estimation.

Sample preparation is often the most work-intensive part of chemical analysis and in most cases it is also the part, which has the largest uncertainty contribution. Sample preparation usually involves either of the two approaches:

1. Essentially destroying the sample matrix so that a solution containing the analyte(s) and few matrix components is obtained. This is often done by digestion with acids or fusing with alkalies or salts. This approach is suitable for determining elements.
2. Separating the analyte(s) from the sample matrix so that a solution containing the analyte(s) is obtained where the amount of matrix components is as small as possible. This is usually done by a set of extractions. This approach is suitable for organic analytes.

Obviously the choice of sample preparation procedure depends on whether the measurand corresponds to the total analyte content in the sample or some part of it, e.g. the bioavailable analyte content (see the text in section 5.1). In the case of e.g. total phosphorus content determination in soil the analyst can make the choice of the sample preparation procedure. All procedures that lead to determination of the total phosphorus content (often involving complete destruction of the matrix) are suitable. In the case of determining of e.g. bioavailable phosphorus content in soil the sample preparation procedure must mimic the way the plants get phosphorus from soil. So, sample preparation involves leaching at predefined conditions. Such sample preparation procedures are often standardized and whenever the results are meant to be mutually comparable they must be obtained with the same procedure. Thus, in this latter case the sample preparation procedure becomes part of the measurand definition.