1 The Sample

In the text the word sample is used in two ways. The sample referred to in Part 1 is the portion of material submitted for chemical analysis, e.g. a 10 mL sample of olive oil removed from a one litre bottle of oil for the purpose of chemical analysis. When statistical procedures are applied to the results of analytical measurements (Part 6) the term sample refers to the segment of all the possible results, i.e. the population that is being used to calculate the statistic. Ten measurements are used to calculate the sample standard deviation (see random errors, Section 6.2); the ten results are only a portion of the infinite number of possible measurements.

Terms used in describing samples and sampling procedures are many and varied. The text below aims to describe those in common usage. It is clear that there is potential for confusion. It should therefore be a golden rule for any analytical chemist to define clearly the terms used in the context of the particular problem being addressed so that any procedure can be understood and repeated.

1.1 Sampling

Sampling Sampling schedule
Sampling plan Sampling scheme
Sampling programme Sample variability

One of the first considerations in any analytical work is what information is required from the analysis. Fundamental to this is the sample and the procedure by which it is obtained, i.e.sampling. Consideration needs to be given to whether the sample is part of a larger whole, how it should be removed, and how it should be stored and processed prior to the analytical measurement.

Samples are taken from an infinite variety of sources and the sampling procedure has to be designed accordingly. There should be a requirement to hold discussions between the customer, the analyst, the sampling officer etc. to reach agreement on what is a representative sample.

Example: 1.1

Sampling

Sampling a day's production of shampoo, already divided into several thousand individual containers, is very different from sampling water from a reservoir. Various means of removing samples are available (sampling method).

The method of sampling selected should be appropriate for the analysis. Sampling a tank of stratified liquid by dipping an empty sample bottle in from the top may be inappropriate, as this would discriminate with respect to the top layer. This method of sampling may, however, be suitable if the top layer was the object of the analysis.

With the exception of portable on-site testing kits, analytical equipment is rarely sited adjacent to the sampling point. Consideration should be given to the need for sample storage and transportation. For unstable samples it may be appropriate to stabilise the sample in some way before analysis can be carried out, e.g. this may involve chilling the sample.

Example: 1.2

Sampling programme, sampling plan, sampling schedule, sampling scheme

The use of a sampling programme is common practice in the operation of a major chemical manufacturing plant. Analysis is carried out to ensure the plant is running efficiently and that feedstock and products are within the specified limits. Samples of the feeds and production streams are, therefore, taken regularly. The sampling plan is formulated for each analyte or group of analytes and combined into a sampling schedule or sampling scheme where the sampling point, sampling times, and frequency of sampling are defined.

The following require different sampling plans:

If feedstock is taken from a well-mixed supply tank for periods of days without the tank being refilled it is clearly unnecessary to analyse the feedstock each hour.

If the composition of production streams is known to vary slowly on changing the operating parameters of the plant then the sampling should reflect the fact that, even if operational changes are being made, analyses will not be required at a higher frequency than the time constant of the process.

Samples are taken from designated and carefully chosen sample points to meet the needs of the process. Therefore a sample taken to ensure product quality may be taken prior to the product storage tank so, if problems are identified, the material can be diverted to an off-specification tank for reprocessing.

Sampling of materials which are at non-ambient conditions also require special considerations - this is clearly the case with compressed gases and liquids where removing the sample under different conditions can result in a change to the sample.

Generally, the sampling process used must reflect a detailed consideration of the requirements of the analysis and the material being sampled.

A sampling plan needs to be established which involves decisions as to when, where, and how the sample should be taken, and whether the process of sampling should be a one-off or whether it should be repeated and how often. Safety implications of sampling procedures, sample preservation and transportation should be considered in any sampling plan. When there is a regular requirement for analysis the sampling plan is referred to as a sampling scheme or a sampling schedule.

The term sampling programme is commonly used to describe a combination of procedures where several related sampling schemes are combined.

Example: 1.3

Sample variability

Samples taken from a water supply reservoir may vary with the time of day and the position from which they are taken. These factors would have to be considered in order to determine an overall analytical result for the reservoir.

Samples can vary with time, or with location, and the range of samples conceivably attainable is referred to as the sample variability.

1.2 Types of Sample

Aggregate sample Judgmental sample
Batch Lot
Bulk sample Random sample
Composite sample Representative sample
Gross sample...