sample preparation
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What is sample preparation in analytical chemistry?
In analytical chemistry, sample preparation involves taking a representative piece of material and getting it ready for analysis. After a sample is collected, it undergoes pretreatment, either chemically or physically, at the sampling site. Once the samples arrive at the lab, they might be combined with other samples. The process involves converting the laboratory sample into a test sample, from which a test portion is selected for analysis.
If the test portion is a particulate solid, it might need to be turned into a solution. If the substance being measured is present in low concentrations, or if interfering substances are present, it may be necessary to isolate or concentrate the substance through separation and purification steps.
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Why is sample preparation important before analyzing a substance?
Sample preparation is a critical step in analytical chemistry because it readies a representative piece of material for analysis. The process ensures that the portion being analyzed accurately reflects the whole.
Sample preparation involves converting the laboratory sample into a test sample and selecting a test portion for analysis. This may involve dissolving a particulate solid into a solution or isolating and concentrating the analyte (the species being determined) through separation and purification steps to eliminate interfering substances. Additives can also be used to mask interference, or the analyte can be chemically converted to facilitate measurement. Proper sample preparation is essential for accurate analytical results.
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What are some common steps in the sample preparation process?
Sample preparation in analytical chemistry involves readying a representative piece of material for analysis. Common steps include:
- Sampling Extracting a representative piece of material.
- Preparing the test sample Reducing and processing the laboratory sample to a smaller, representative subsample with a finer particle size.
- Isolation Separating the analyte from the sample matrix. Techniques include thermal evolution, solvent extraction, and ion exchange.
- Preconcentration Isolating or concentrating the analyte by separation and purification steps because the analyte might be present at a low concentration.
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How does filtration help in preparing a sample for analysis?
Filtration is a crucial step in preparing samples for analysis because it removes interfering substances, such as particles, that could affect the accuracy of analytical results. Filtration separates particles of different sizes using a porous material. The mixture is poured through a filter, trapping solids while allowing the liquid to pass. This process is vital for isolating the analyte from the sample matrix. For instance, filtration is used to remove precipitates after selective precipitation. Depending on the application, filters can range from simple filter paper to sophisticated devices like baghouses for air pollution control.
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What is the purpose of dissolving or diluting a sample during preparation?
In analytical chemistry, dissolving or diluting a sample during preparation serves multiple crucial purposes. It is often necessary to prepare an acidic aqueous solution from the test portion.
Reasons for dissolving or diluting a sample:
* To remove or mask interferences, and sometimes to isolate the intended analyte from its sample matrix and dissolution medium.
* To remove interferences from an analyte prior to an assay. An interference is a substance, other than the assayed material, that can be measured by the chosen analytical method or that can prevent the assayed material from being measured, which can cause erroneous analytical results.
* In many modern analytical procedures, once the test portion has been dissolved, it is diluted to some fixed volume and measured.
* To prepare the sample for techniques such as isotope dilution, a radiochemical method of analysis for measuring the mass and quantity of an element in a substance when precise quantitative analysis by conventional means is difficult.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How can improper sample preparation affect the results of an analysis?
In analytical chemistry, sample preparation readies a representative piece of material for analysis. Improper techniques can introduce errors that compromise accuracy.
Contamination from the atmosphere is one factor. For example, a clean metal piece left in the air for an hour will pick up a carbon contamination layer.
If the test portion is a particulate solid, it may be necessary to convert it to a solution. If the analyte is at a low concentration, or if interfering substances are present, it may be necessary to isolate or concentrate the analyte through separation and purification steps. Additives might be needed to mask interference, or the analyte must be chemically converted to facilitate measurement.
The quantitative transfer of a precipitate to a filter is a manipulative art that becomes more critical at higher analyte concentrations.
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sample preparation, in analytical chemistry, the processes in which a representative piece of material is extracted from a larger amount and readied for analysis. Sampling and sample preparation have a unique meaning and special importance when applied to the field of analytical chemistry. Analytical chemistry in all its diverse forms can be looked upon as a multistep endeavour with the measurement phase but one link near the end of a chain of operations. That chain begins with sampling, an essential process that underlies all subsequent work and imparts relevance to what would otherwise be a meaningless exercise.
Sampling is critically relevant everywhere that analytical chemistry has a role to play. Ambient sampling of the atmosphere is used to provide analytical data on seasonal or other trends that can be correlated with natural or societal processes. For example, the extent of the Antarctic ozone hole and its relation to fluorocarbon use were confirmed by this means. Near ground level, monitoring sites provide data for air-quality assessment, for the design of pollution-control strategies, and for regulatory enforcement. Groundwater-monitoring wells are used to sample aquifers in order to ensure water quality. Rivers and streams are sampled to track pollution from industry, agriculture, sewers, and cities. The ocean is sampled to study the carbon cycle budget for Earth, and seafloor hydrothermal vents are sampled to obtain clues about geochemistry deep in Earth’s crust.
Analytical chemistry that studies other worlds follows upon careful sampling. The Apollo astronauts who explored the Moon were trained in geological sampling. Various robotic probes have sampled Mars and Halley’s Comet for automated onboard analyses. The European Space Agency’s Huygens probe sampled the atmosphere and surface of Saturn’s moon Titan in 2005.
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Back on Earth, manufactured products are sampled to ensure consumer safety; foods are sampled to assay nutrients and to monitor pesticide residues and other potentially harmful contaminants. Sampling methods are also used in connection with forensic analyses, chemical analyses in customs work, and industrial processes.
Following close upon sampling is sample preparation, the entire process whereby the sample is readied for measurement. The sample that arrives at the laboratory is commonly called the laboratory sample. This is then converted by a set of operations to the test sample, from which an analyst selects a test portion for an analytical determination. If the test portion is a particulate solid, it may be necessary to convert it to a solution. If the analyte (i.e., the species being determined) is present at low concentration, or if interfering substances are present, it may be necessary to isolate or concentrate the analyte by one or more separation and purification steps. In some cases additives are required to mask interference, or the analyte must be chemically converted to another form to facilitate its measurement.
Sampling
Theory
The sampling plan is the strategy employed to represent the distribution of one or multiple analytes in the object of study. The object of study may encompass objects with only spatial dimensions, such as a mineral deposit, or it may be a dynamically changing system, such as a river, which has a temporal component. In both cases the success of the sampling plan depends upon how accurately a much larger system is represented in the microcosm of the laboratory sample.
Materials vary widely in the degree of large- and small-scale uniformity that they exhibit. It is most useful to speak of the heterogeneity of a material as a scalar function that approaches perfect homogeneity in its limit. It is also essential to speak in terms of a given analyte or suite of analytes, since some components in a material may be much more heterogeneously distributed than others.
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The most comprehensive sampling theory was formulated by French chemist Pierre Gy in the second half of the 20th century. Gy defined two types of material heterogeneity: constitution heterogeneity, which is the intrinsic heterogeneity of the material’s components, and distribution heterogeneity, which is the heterogeneity that derives from the spatial mixing of the components. While this dichotomy can be usefully applied to many material types, it is best described and understood in reference to particulate solid mixtures. For example, if one considers a mixture of silt and sand to be sampled for the presence of calcium, the variation of that analyte among the silt and sand particles represents two forms of its constitution heterogeneity. The degree of uniformity in the spatial arrangement of silt and sand particles then determines the distribution heterogeneity of calcium. Appropriate grinding of such a mixture to reduce the average particle size may diminish the constitution heterogeneity, and the correct blending of such a mixture may lower its distribution heterogeneity.
Gy developed another concept that involves the likelihood that all a material’s constituents have a high and equal probability of being included in the sample. Many commonly employed sampling practices are seriously flawed in that some constituents have a zero probability of being sampled. “Grab sampling,” in which one movement of a sampling device is used to select the sample, most often falls into this category, which is called nonprobabilistic sampling. Such methods can never satisfactorily represent highly heterogeneous material. In contrast, probabilistic sampling methods are techniques in which all constituents of the material have some probability of being included. However, it is only in a correctly designed sampling plan that probabilistic sampling achieves true representation.