A representative sample is one that truly reflects the composition of the material to be analyzed within the context of a defined analytical problem.
Sample Storage:
Due to varying periods of time that may elapse between sample collection and analysis, storage conditions must be such as to avoid undesirable losses, contamination or other changes that could affect the results of the analysis.
Sample Pre-treatment:
Preliminary treatment of a sample is sometimes necessary before it is in a suitable form for analysis by the chosen technique and method. This may involve a separation or concentration of the analytes or the removal of matrix components that would otherwise interfere with the analysis.
Sample Preparation:
Samples generally need to be brought into a form suitable for measurements to be made under controlled conditions. This may involve dissolution, grinding, fabricating into a specific size and shape, pelletizing or mounting in a sample holder.
Representative Sample:
The importance of obtaining a representative sample for analysis cannot be overemphasized. Without it, results may be meaningless or even grossly misleading. Sampling is particularly crucial where a heterogeneous material is to be analyzed. It is vital that the aims of the analysis are understood and an appropriate sampling procedure adopted. In some situations, a sampling plan or strategy may need to be devised so as to optimize the value of the analytical information collected. This is necessary particularly where environmental samples of soil, water or the atmosphere are to be collected or a complex industrial process is to be monitored. Legal requirements may also determine a sampling strategy, particularly in the food and drug industries. A small sample taken for analysis is described as a laboratory sample. Where duplicate analyses or several different analyses are required, the laboratory sample will be divided into sub-samples which should have identical compositions.
Homogeneous materials (e.g., single or mixed solvents or solutions and most gases) generally present no particular sampling problem as the composition of any small laboratory sample taken from a larger volume will be representative of the bulk solution. Heterogeneous materials have to be homogenized prior to obtaining a laboratory sample if an average or bulk composition is required. Conversely, where analyte levels in different parts of the material are to be measured, they may need to be physically separated before laboratory samples are taken. This is known as selective sampling. Typical examples of heterogeneous 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 sediments or suspended particulate matter may each be of importance;
● materials stored in bulk, such as grain, edible oils, or industrial organic chemicals, 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 particular 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 randomly selected from different parts of the shipment.
(ii) The pieces are crushed, ground to coarse granules and thoroughly mixed.
(iii) A repeated coning and quartering process, 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).
The distribution of toxic heavy metals or organic compounds in a land redevelopment site presents a different problem. Here, to economize on the number of analyses, a grid is superimposed on the site dividing it up into approximately one- to five-metre squares. From each of these, samples of soil will be taken at several specified depths. A three-dimensional representation of the distribution of each analyte over the whole site can then be produced, and any localized high concentrations, or hot spots, can be investigated by taking further, more closelyspaced samples. Individual samples may need to be ground, coned and quartered as part of the sampling strategy.
Repeated sampling over a period of time is a common requirement. Examples include the continuous monitoring of a process stream in a manufacturing plant and the frequent sampling of patients’ body fluids for changes in the levels of drugs, metabolites, sugars or enzymes, etc., during hospital treatment. Studies of seasonal variations in the levels of pesticide, herbicide and fertilizer residues in soils and surface waters, or the continuous monitoring of drinking water supplies are two further examples.
Having obtained a representative sample, it must be labeled and stored under appropriate conditions. Sample identification through proper labeling, increasingly done by using bar codes and optical readers under computer control, is an essential feature of sample handling.
Sample Storage:
Samples often have to be collected from places remote from the analytical laboratory and several days or weeks may elapse before they are received by the laboratory and analyzed. Furthermore, the workload of many laboratories is such that incoming samples are stored for a period of time prior to analysis. In both instances, sample containers and storage conditions (e.g., temperature, humidity, light levels and exposure to the atmosphere) must be controlled such that no significant changes occur that could affect the validity of the analytical data. The following effects during storage should be considered:
● increases in temperature leading to the loss of volatile analytes, thermal or biological degradation, or increased chemical reactivity;
● decreases in temperature that lead to the formation of deposits or the precipitation of analytes with low solubilities;
● changes in humidity that affect the moisture content of hygroscopic solids and liquids or induce hydrolysis reactions;
● UV radiation, particularly from direct sunlight, that induces photochemical reactions, photodecomposition or polymerization;
● air-induced oxidation;
● physical separation of the sample into layers of different density or changes in crystallinity.
In addition, containers may leak or allow contaminants to enter.
A particular problem associated with samples having very low (trace and ultra-trace) levels of analytes in solution is the possibility of losses by adsorption onto the walls of the container or contamination by substances being leached from the container by the sample solvent. Trace metals may be depleted by adsorption or ion-exchange processes if stored in glass containers, whilst sodium, potassium, boron and silicates can be leached from the glass into the sample solution. Plastic containers should always be used for such samples.
Conversely, sample solutions containing organic solvents and other organic liquids should be stored in glass containers because the base plastic or additives such as plasticizers and antioxidants may be leached from the walls of plastic containers.
Sample Pre-treatment:
Samples arriving in an analytical laboratory come in a very wide assortment of sizes, conditions and physical forms and can contain analytes from major constituents down to ultra-trace levels. They can have a variable moisture content and the matrix components of samples submitted for determinations of the same analyte(s) may also vary widely. A preliminary, or pre-treatment, is often used to condition them in readiness for the application of a specific method of analysis or to pre-concentrate (enrich) analytes present at very low levels. Examples of pretreatments are:
● drying at 100°C to 120°C to eliminate the effect of a variable moisture content;
● weighing before and after drying enables the water content to be calculated or it can be established by thermogravimetric analysis (Topic G1);
● separating the analytes into groups with common characteristics by distillation, filtration, centrifugation, solvent or solid phase extraction (Topic D1);
● removing or reducing the level of matrix components that are known to cause interference with measurements of the analytes;
● concentrating the analytes if they are below the concentration range of the analytical method to be used by evaporation, distillation, co-precipitation, ion exchange, solvent or solid phase extraction or electrolysis.
Sample clean-up in relation to matrix interference and to protect specialized analytical equipment such as chromatographic columns and detection systems from high levels of matrix components is widely practised using solid phase extraction (SPE) cartridges (Topic D1). Substances such as lipids, fats, proteins, pigments, polymeric and tarry substances are particularly detrimental.
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This is very useful to Sampling handing people.....
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