Let us first investigate what is meant by sample preparation. Sample preparation in analytical chemistry encompasses processes during which a representative sample is extracted from a larger amount of material and readied for analysis. Sampling and sample preparation have particular importance in analytical chemistry. Samples may be solid, gas, liquid or solutions, biological matter or cells, or mixtures. Errors in any preparation stage can lead to inaccuracies, which are compounded in the analytical process.
Correct sampling for analytical chemistry is a vitally important link in the preparation chain. All subsequent steps in the process rely on accurate sampling. It is vital to get the right sample and ensure it is representative of what you want to test. You may need to get a sample from the edge of the subject, and in some cases, you may wish to get a sample from the middle. You may even want to take a composite sample of many mixed samples to give an average sample. Obtaining a sample requires thought and preparation to ensure that it is possible to get the correct analysis.
Sampling and Sample Preparation in Analytical Chemistry
The method of sampling and the storage of the sample is also critical. Spotlessly cleaning sampling equipment and sample storage containers are essential to prevent contamination. The materials used to collect and store samples are also necessary to prevent the sample from reacting with the storage container. For example, keeping an acidic sample in an iron container would create iron salts by the reaction. Accurate labeling of samples is also crucial to prevent mix-ups.
Samples are sometimes analyzed as they are, which makes preparation reasonably simple. A small amount of sample is measured and analyzed. Care must be taken to use measuring containers of suitable materials that are scrupulously clean. Occasionally, a solid, liquid, or gas sample component will need to be extracted from the mixture before analysis. In analytical chemistry, sample preparation refers to how a sample is treated before it is analyzed. Preparation is an essential step in some analytical techniques because the sample will not respond to the analyte in its natural form. Analytical results can be distorted by interfering species. Knowing your chemical reaction may be critical in the preparation of samples. To get an accurate result, it may be necessary to precipitate or extract potential contaminants that interfere with the analytical reaction.
If analytical chemistry is used to monitor discharges from manufacturing or treatment plants, it may be carried out automatically online. This requires the design of automatic sampling equipment, which will take the sample and then prepare the sample for analysis. This preparation could be as simple as filtration before adding reagents but may be more complex.
One key aspect of sample preparation is cleanliness. It is critical that all sampling, storage, and handling equipment is spotlessly clean to avoid contamination of samples. Laboratory glassware cleaning equipment is an industry in itself. Companies like Cole-Palmer, Camlab, Fisher Scientific, and many others have extensive cleaning sampling product lines.
Sample preparation from initial sampling through sample handling to final analysis preparation is critical. The accuracy of steps in the preparation process will influence the final analytical result, and any inaccuracies can be multiplied by further steps. Some analytical methods like chromatography analysis cannot take place without a significant amount of preparation to get the sample into a usable form.
What we have highlighted above is an example of random sampling, which is often applied to a grid design as shown above. Random sampling strategies can be applied to any target population (i.e. evaluating a solid sample or measuring water quality parameters within a creek). However, random sampling can at times be expensive and not necessarily cost effective, as you often need a greater number of samples to ensure that your samples represent the target population. If you know something about the target population, other sampling methods may be possible or appropriate. Here are a few examples.
Cluster Sampling- is a sampling technique where the population is divided into groups or clusters and random samples are selected from the cluster for analysis. The main objective of cluster sampling is to reduce costs by increasing sampling efficiency.
Q22. Another factor to consider in a sampling plan is the sample holding time. Can you think of ways in which sample holding time may impact the concentration of different species such as nitrate, pH, and Ca2+ in the sample?
When sampling, you need to make sure the sample is not too small, so that the composition is not substantially different from the target population. You also want to ensure that you collect an appropriate number of samples for analysis.
Q24. It is known from analyses conducted in 2008 that the % relative sampling error for water hardness by EDTA titration is 0.8%. How many samples should you collect to limit the relative standard deviation for sampling to 1.0% within the 95% confidence level? Is this a feasible task? (May help by referring to the Analytical Chemistry 2.0 by David Harvey section 7.2: How Many Samples to Collect, =452)
Q25. Based on your results from the prior question, if the cost of collecting a sample is $25 and the cost of analyzing a sample is $50 what budget should you allocate for the project and what sampling strategy would be most effective for the given number of samples?
Q26. Using the map from the beginning of this module that has the area around Machias, where the confluence of the Machias, Middle, and East Machias Rivers empty into the Machias Bay. Design your sampling plan. Think about random, systematic, clustering, etc. sample strategies. Will you take grab samples or pool samples together?
Equipment and sampling procedures are typically specific to the specific water quality parameter one wishes to determine. However, there are two fundamental tasks that are common to any water analysis: the preparation of the sampling containers and the sample collection.
The variability associated with testing wheat for deoxynivalenol (DON) was measured using a 0.454 kg sample, Romer mill, 25 g comminuted subsample, and the Romer Fluoroquant analytical method. The total variability was partitioned into sampling, sample preparation, and analytical variability components. Each variance component was a function of the DON concentration and equations were developed to predict each variance component using regression techniques. The effect of sample size, subsample size, and number of aliquots on reducing the variability of the DON test procedure was also determined. For the test procedure, the coefficient of variation (CV) associated with testing wheat at 5 ppm was 13.4%. The CVs associated with sampling, sample preparation, and analysis were 6.3, 10.0, and 6.3%, respectively. For the sample variation, a 0.454 kg sample was used; for the sample preparation variation, a Romer mill and a 25 g subsample were used; for the analytical variation, the Romer Fluoroquant method was used. The CVs associated with testing wheat are relatively small compared to the CV associated with testing other commodities for other mycotoxins, such as aflatoxin in peanuts. Even when the small sample size of 0.454 kg was used, the sampling variation was not the largest source of error as found in other mycotoxin test procedures.
The adequacy and condition of the sample or specimen received for examination are of primary importance. If samples are improperly collected and mishandled or are not representative of the sampled lot, the laboratory results will be meaningless. Because interpretations about a large consignment of food are based on a relatively small sample of the lot, established sampling procedures must be applied uniformly. A representative sample is essential when pathogens or toxins are sparsely distributed within the food or when disposal of a food shipment depends on the demonstrated bacterial content in relation to a legal standard.
The number of units that comprise a representative sample from a designated lot of a food product must be statistically significant. The composition and nature of each lot affects the homogeneity and uniformity of the total sample mass. The proper statistical sampling procedure, according to whether the food is solid, semisolid, viscous, or liquid, must be determined by the collector at the time of sampling by using the Investigations Operation Manual (5). Sampling and sample plans are discussed in detail in ref. 6.
Whenever possible, submit samples to the laboratory in the original unopened containers. If products are in bulk or in containers too large for submission to the laboratory, transfer representative portions to sterile containers under aseptic conditions. There can be no compromise in the use of sterile sampling equipment and the use of aseptic technique. Sterilize one-piece stainless steel spoons, forceps, spatulas, and scissors in an autoclave or dry-heat oven. Use of a propane torch or dipping the instrument in alcohol and igniting is dangerous and may be inadequate for sterilizing equipment.
Place containers in a freezer long enough to chill them thoroughly. Keep frozen samples solidly frozen at all times. Cool refrigerated samples, except shellfish and shell stock, in ice at 0-4C and transport them in a sample chest with suitable refrigerant capable of maintaining the sample at 0-4C until arrival at the laboratory. Do not freeze refrigerated products. Unless otherwise specified, refrigerated samples should not be analyzed more than 36 h after collection. Special conditions apply to the collection and storage of shucked, unfrozen shellfish and shell stock (1). Pack samples of shucked shellfish immediately in crushed ice (no temperature specified) until analyzed; keep shell stock above freezing but below 10C. Examine refrigerated shellfish and shell stock within 6 h of collection but in no case more than 24 h after collection. Further details on sample handling and shipment may be found in the Investigations Operation Manual (5) and the Laboratory Procedures Manual (3). The Investigations Operation Manual (5) contains sampling plans for various microorganisms. Some of those commonly used are presented here. 2ff7e9595c
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