Irrespective of the state of materials to be sampled, the level of heterogeneity, the size, nature and volume of the sample and configuration of the site , the interest of the analyst is to generate gross sample from which laboratory size sample is obtained for analysis.
Ideally, gross sample is a miniature replica of the bulk of materials to be analysed. It corresponds to the “whole lot” both in chemical composition and in particle size distribution. A certain portion of the whole must be removed through any of the sampling methods earlier discussed. The sample may be grab or composite depending on the judgement of the analyst. The competence and expertise required for obtaining the gross sample vary and depend on the situation at hand. This range from sampling homogeneous situation of liquid and gases, to sampling particulate solids, to sampling of metal and alloys.
The size of the gross sample needs not to be larger than necessary. The size is determined by the following factors
i. the uncertainty that can be tolerated between the composition of the samples as a whole;
ii. the degree of heterogeneity of materials being sampled; and
iii. the level of particle size at which heterogeneity begins.
Laboratory size sample
This is the ultimate sample on which analysis is carried out from non homogeneous
material. The gross sample may weigh several hundreds of kg or more. A laboratory
size that is almost one over thousands or less is obtained. Diminutions in particles
size is essential as the weight of the sample is decreased to ensure that the sample composition continue to be representative of the original materials.
The sample obtained upon arrival at the laboratory received further treatment before it is eventually analysed. The integrity of sample to maintain chain-of-custody
procedure must be ensured.
Coning and Quartering method of sample selection
It is a method sampling selection which aim at reducing the samples without creating a systematic bias. The technique involves pouring the sample so that it takes on a conical shape, and then flattening it out into a cake. The cake is then divided into quarters and two quarters which face opposite one another are discarded, whilst the other two are combined and constitute the reduced sample. The same process is continued until a reasonable amount of material is obtained. Analyses are made with respect to the sample obtained.
The chain of custody procedure is the following:
1 Sample label (including bar code label)
2 Sample seals
3 Fields log book
4 Chain of custody books
5 Sample analysis request sheet
6 Sample delivery to the laboratory
7 Receipt and logging of sample
8 Assignment of sample for analysis
The chain of custody procedure helps in identifying the source of contamination (if any) and help the analyst in planning.