Basic Training 4th Annual ICP Conference

3:9 Sampling and sub-sampling

Sub-sampling

This is the process of removing a sample aliquot for preparation and measurement from an individual sample or the aggregate sample submitted for analysis. Obtaining a representative sample is the goal and homogeneity is the primary concern, not only for the original sample collection, but the sub-sampling as well. The smaller the sample aliquot, the greater the risk of achieving sub-sampling errors that will significantly influence the accuracy of the analysis. Sub- sampling may involve an attempt to homogenize the sample by grinding, sieving, blending, or mixing the original sample. Contamination is of particular concern when the sample is handled, ground, sieved, etc. Therefore, trace analysts should be versed on the contaminants resulting from the use of various ball mills and grinders. The use of Nylon sieves will eliminate contamination risks for all metals and non-metals, except carbon.

3:10 Sampling and sub-sampling

Determination of Sampling and Sub-Sampling Errors

Relative error is defined as the standard deviation divided by the mean (sd / X). The relationship between the sampling error and the analytical method (preparation and measurement) is shown in the following expression:

Expression 3.1 (sd / X) 2 total error

= (sd / X) 2 sampling

+ (sd / X) 2 analytical

Example: The total relative error (sd / X) total error has been determined to be 0.23 (23% relative) on a solid catalyst sample submitted in pellet-form for total Ni analysis.

§ How much of this error is the analytical error and how much is the sampling error? § A CRM is not available for determining (sd / X) analytical § Calculate the (sd / X) sampling using (sd /X) analytical = 0.07 § Is there any point in reducing the analytical error further?