Collecting a representative sample

To collect a representative sample forms a vital aspect of analytical chemistry, because the samples subjected to analysis are assumed to be perfectly homogeneous and truly representative. Thus, sampling may be considered as the most critical aspect of analysis. In other words, the accuracy and significance of measurements may be solely limited by the sampling process. Unless and until the sampling process is performed properly, it may give rise to a possible weak link in the interpretation of the analytical results. For instance, the improper... 

Before a size analysis can be carried out, it is necessary to collect a representative sample of the solids, and then to reduce this to the quantity which is required for the chosen method of analysis. Again, the work of Allen gives information on how this is best carried out. Samples will generally need to be taken from the bulk of the powder, whether this is in a static heap, in the form of an airborne dust, in a flowing or falling stream, or on a conveyor belt, and in each case the precautions which need to be taken to obtain a representative sample are different. 

Collect a representative sample per the single sampling plan (general level B) and inspect. Record the observation in the attached format. Following is the sampling plan. 

Quality assurance is what we do to get the right answer for our purpose. We begin by writing use objectives, from which specifications for data quality can be derived. Specifications could include requirements for sampling, accuracy, precision, specificity, detection limit, standards, and blank values. For any meaningful analysis, we must first collect a representative sample. In an analysis of standards, the method must produce a result that is acceptably close to the known value. A method blank contains all components except analyte, and it is taken through all steps of the analytical procedure. We subtract the response of the method blank from the response of... 

Groundwater refuses to submit to a regimented written procedure, and the sampling of it may be more of an art than science. The ability to bring groundwater to the surface and collect a representative sample depends on the experience, intuition, and individual sampling technique of the sampler. 

Collecting a representative sample of snow for precipitation chemistry measurements poses special problems. Most electronic sensors on precipitation chemistry samplers do not detect snow, particularly light, dry snow, as efficiently as rain. Light, dry snow may also fall into and then be blown out of an open container or funnel. Snow may stick to sampler parts and later be blown into the sample container. Ice may coat sampler parts and prevent proper operation. Heavy snow may... 

A study by Gibs et al. (1990) found that monitoring field water-quality parameters for stability is not a reliable indicator of when to collect a representative sample for purgeable organic compounds (POCs), such as chlorinated alkanes, alkenes and aromatics. For these types of compounds, the researchers suggest direct monitoring of POC concentrations. 

With the auger type sampler (Figure 1.10) a slot inside the proeess stream is rotated to capture a eross-seetion of the process stream that is then delivered by gravity into a sample container. This type of device does not collect a representative sample unless the stream is homogeneous and has the added disadvantage that it obstructs flow. 

Milk and milk products are important components of the diet in many countries. Milk is one of the few foods produced over large areas and collected on a daily basis. Its composition is almost identical all over the world, and it is easy to collect a representative sample that can be analysed in liquid or dried form. 

The sample system is responsible for collecting a representative sample of the process and delivering it to the analyzer for analysis. Obviously, the reliability of the sample system directly affects the reliability of the overall composition analysis system. The transport delay associated with the sample system contributes directly to the overall deadtime associated with an on-line composition measurement. This difference in sampling deadtime can have a drastic effect on the performance of a control loop. Table 15.2 summarizes the dynamic characteristics and repeatability of typical control valve systems and several different types of sensors. 

Will the chosen sampling method and equipment collect a representative sample ... 

Collecting a representative sample is one major demand. Wall effects of the sampler probably cause concentration gradients close to walls, in turn causing some ions or colloids to be adsorbed preferentially to the sampler wall (Batley 1989). Further, species adsorption, contamination or alteration of biological or chemical equilibria have been observed. Oxidation by air shifts redox-systems. The sampler material can also alter species, even if made of PTFE, and seals may also cause contamination. Each contamination must be considered as a new, non-native species which may alter the native species equilibria (Batley and Gardner 1977). 

Collecting a representative sample is a major challenge. Fvmdamental to the design is the ability to collect a fully representative sample and to do this from sampling streams which have pressures of up to 3500 psig. The sampling must be carried out onto the traps at atmospheric pressure, so it is necessary to avoid any liquefaction in the traps. A series of speed loops is used to present the sample to a selection system, which introduces the relevant samples to the analyzer for measurement. The requirements for the sampling system are set out in Table 4. 

The sampling of natiual gas has been discussed and studied for many years. Serious testing on the proper sampling methods has been done in a number of locations in the recent past. From these tests, it has been determined that the samphng procedures must be carefully prepared and followed. For a person to collect a representative sample of natural gas, the proceditres learned in spot sampling operations must be followed. 

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