Instant Notes: Analytical Chemistry

measured, they may need to be physically separated before laboratory samples

are taken. This is known as selective sampling. Typical examples of hetero-

geneous materials where selective sampling may be necessary include:

● surface waters such as streams, rivers, reservoirs and seawater, where the

concentrations of trace metals or organic compounds in solution and in sedi-

ments or suspended particulate matter may each be of importance;

● materials stored in bulk, such as grain, edible oils, or industrial organic chem-

icals, where physical segregation (stratification) or other effects may lead to

variations in chemical composition throughout the bulk;

● ores, minerals and alloys, where information about the distribution of a partic-

ular metal or compound is sought;

● laboratory, industrial or urban atmospheres where the concentrations of toxic

vapors and fumes may be localized or vary with time.

Obtaining a laboratory sample to establish an average analyte level in a highly

heterogeneous material can be a lengthy procedure. For example, sampling a

large shipment of an ore or mineral, where the economic cost needs to be

determined by a very accurate assay, is typically approached in the following

manner.

(i) Relatively large pieces are randomlyselected from different parts of the

shipment.

(ii) The pieces are crushed, ground to coarse granules and thoroughly mixed.

(iii) A repeated coning and quarteringprocess, with additional grinding to

reduce particle size, is used until a laboratory-sized sample is obtained.

This involves creating a conical heap of the material, dividing it into four

equal portions, discarding two diagonally opposite portions and forming a

new conical heap from the remaining two quarters. The process is then

repeated as necessary (Fig. 1).

A4 – Sampling and sample handling 11

2

4

13

2

4

13

2

4

13

Fig. 1. A diagrammatic representation of coning and quartering (quarters 1 and 3, or 2 and 4 are discarded each time).