Sample Collection, Transport and Storage

Based on the physical form of the specimen viz. gases, liquids or solids the sampling procedures can be broadly separated into three groups. It is clear that different sampling techniques will be required for each group. The sampling method may also vary, depending on the level of the components of interest within each sample group. The major types that the analyst might meet will be described and reference given to other applications that may be appropriate.

Gas Samples

There are broadly two types of gas sample: one, where the sample contains a limited number of major components (e.g. flue gasses containing mainly nitrogen. oxygen and carbon dioxide with, perhaps, small quantities of carbon monoxide, two, where the sample contains trace concentrations of vapors (e.g. the essential oils in the atmosphere around a flower or hydrocarbon vapors in the vicinity of a gasoline pump). These two examples must be sampled by very different procedures. The motives behind these analyses are also quite different. The first is to control the combustion in a furnace efficiently whereas the determination of the hydrocarbons in the vicinity of a fuel pump would for pollution analysis.

Sampling for the Major Components of a Permanent Gas Mixture

Gases expand and contract with temperature and pressure and, thus, the sample must be taken under known and conditions (e.g. if the sample is in a flue duct it is not likely to be at ambient temperature or pressure). Ideally, the sample vessel should be thermostatted and taken at atmospheric pressure. If the detector has a known or calibrated response to each component, however, as sample expansion or contraction will occur identically for all components (assuming they behave as permanent gases) the relative proportions of each component will not change and so there will be no need to control or measure the temperature and pressure.

Figure 1. Sampling a Flue Duct