Preparation

Sample preparation refers to the process of selecting a representative piece from a larger amount of material and preparing it to be studied or analysed. The small amount ideally should represent the larger amount of the material. Sample preparation or sampling has significance in analytical chemistry since it initiates all kinds of analysis and underlies all the subsequent work in the same, without which anything would be meaningless in analytical chemistry. For example, a sample of the atmosphere can be prepared to analyse the seasonal and societal trends. The Antarctic Ozone hole and the use of fluorocarbons were linked by using sampling. This preparation study material shall answer all your questions.

Designing a Sample Preparation 

Your research topic may be spatial dimensions, like a mineral deposit, or a dynamically changing system, like a river. In both circumstances, the sampling strategy’s effectiveness is determined by how precisely a much larger system is reflected in the microcosm of the laboratory sample. 

Sample preparation must reflect the goals of the analysis. A material scientist, for example, who wants to characterise the surface chemistry of metal is more likely to pick a freshly exposed surface generated by cleaving the sample under vacuum rather than a surface previously exposed to the environment. 

From Where to Sample? 

A sampling error can occur when the sample is not identical to the composition of the larger material. If the target population is homogenous, then collecting a sample would not be a problem. The problem arises when the target population is heterogeneous, as in most cases. A drug provided as an oral suspension may have a higher concentration of its active components near the bottom of the container due to settling. 

When a clinical sample, such as blood or urine, is taken, its composition might change. The following methods of collecting a sample may be useful in cases of the heterogeneous target population- 

• Random Sampling- Random sampling is the easiest way to satisfy an unbiased collection of samples. Despite its apparent simplicity, obtaining a genuinely random sample is challenging. Haphazard sampling, which involves collecting samples without a strategy, is not random and may reveal an analyst’s inadvertent biases.

• Judgemental Sampling- The opposite of random sampling is selective or judgmental sampling, in which we utilise previous knowledge about the target population to influence our sample selection. Although judgmental sampling is more biassed than random sampling, it needs fewer samples.

• Systematic Sampling- The extremes of bias and sample size required to characterise the target population are represented by random and judgmental sampling. Systematic sampling falls somewhere in the middle of these two extremes. We sample the target population at regular intervals in space or time in systematic sampling.

• Systematic-Judgemental Sampling- Combinations of the three major sampling procedures are also feasible. Systematic–judgmental sampling is one such combination, in which we use previous information about a system to drive a systematic sampling plan. When monitoring garbage leaking from a landfill, for example, we anticipate the plume to go in the same direction as the flow of groundwater—this allows us to target our samples, saving money and time.

• Stratified Sampling- Individual strata are frequently more homogenous than the target population, which is a benefit of stratified sampling. The total sample variance acquired by stratified sampling is always at least as excellent, if not better, than that obtained by ordinary random sampling.

• Convenience Sampling- In convenience sampling, sample locations are chosen based on criteria other than minimising sampling error and variation. In a study of rural groundwater quality, for example, we can drill wells at random locations or use existing wells, which is typically the favoured option.

Sample Preparation of Liquids 

The majority of solution samples do not require any further preparation before analysis. This is true for natural water and wastewater samples. Solution samples with extremely complex matrices, such as blood and milk, may require further processing to isolate analytes from interference.

Sample Preparation of Gases 

There is usually no requirement for sample preservation or processing after obtaining a gross sample of urban air. When a gas sample is collected using a solid sorbent, a filter, or cryogenic refrigeration, its chemical composition is typically stable. When a solid sorbent is used, gaseous components are liberated for analysis by thermal desorption or extraction with a suitable solvent. If the sorbent is only selective for one analyte, the rise in sorbent mass may be used to calculate the quantity of analyte in the sample.

Sample Preparation of Solids 

In contrast to gases and liquids, which normally require little sample preparation, a solid sample usually necessitates considerable processing before the examination. This is due to two factors. First, the standard deviation for sampling is determined by the number of particles in the sample rather than the total mass of the particles. The gross sample for a heterogeneous substance, including big particles, may be too large to analyse.

For example, to extract an acceptable sample from a Ni-bearing ore with an average particle size of 5 mm, a sample weighing one tonne may be required. By reducing the average particle size of the sample, we may collect the same number of particles with a lower, more manageable bulk. Second, many analytical procedures need the presence of the analyte in the solution.

Conclusion 

When we use an analytical method of solving an issue, precision is an important factor that needs to be considered. One reason that your precision may suffer is related to sampling. You may have chosen the wrong sample, or chosen the right sample of the wrong size. Either way, the sample fails to represent the larger material, and the analysis suffers. Indeterminate sample preparation may render the analysis useless. Therefore, sample preparation holds a significant place in analytical chemistry, which cannot be ignored. Refer to the study material notes for preparation for clarification.