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lecture 8; sampling by Dr. Salma Amir

population, sampling, importance of sampling

salmaamirfcw
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lecture 8; sampling by Dr. Salma Amir

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  1. Lecture No. 08Course title: Fundamental Analytical ChemistryTopic: Sampling Course instructor: Dr. Salma Amir GFCW Peshawar

  2. Population, census, sample and sampling • Population: A population is the pool of material from which a statistical sample is drawn for a study. A population is the entire group that you want to draw conclusions about. • Census: A census is an official survey of the population that is carried out in order to  obtaindetailsof material found in population.

  3. Sample : Sample is defined as a smaller set of data that a researcher chooses or selects from a larger population by using a pre-defined selection method. Or A portion of material selected from a larger quantity of material. Sample consist of two parts 1. Analyte 2. Matrix 1. Analyte: The constituent of interest in sample or the component measured is known as analyte 2.Matrix: all the other constituents in a samlple except analyte are called matrix

  4. Sampling: Sampling is the process of extracting a small portion from a large quantity of material. The small portion is truly representative of the composition of the whole material. Or The process by which a representative fraction is acquired is termed sampling. In sampling, a sample population is reduced in size to an amount of homogeneous material that can be conveniently handled in the laboratory and whose composition is representative of the population.

  5. Steps in obtaining representative sample • Lot: A lot is the total material from which the sample is taken. e.g., it may be truck full of reagent, a lake full of water. Frequently, lots are composed of sample units, such as individual boxes in the truck. • Bulk material: A bulk sample (also called the gross sample) is taken from the lot for analysis. The bulk sample is usually chosen to be the representative of the lot. • Laboratory sample: From the bulk material, a small laboratory sample is formed that must have exact same composition as the bulk sample. e.g., this might be done by grinding the entire solid bulk sample to a fine powder, mixing thoroughly, and keeping one bottle of powder for testing. • Test portions: Test portions (aliquots) of laboratory sample are then used for individual analysis.

  6. Lot Bulk sample Laboratory sample Test portions

  7. Importance of sampling 1. Economical problems: When whole material is analyzed, it will be highly expensive. 2. Time: It is also an important factor that double the importance of sampling. For the analysis of whole bulk, too much time is required which is not affordable at some time. 3. Handling: Due to large size of bulk material, the handling is also a problem. So, sampling should be done to reduce the size of bulk.

  8. 4. Destructive process: Some analysis are destructive, so if all the material is analyzed, the it would be the waste of useful material, which is sometimes unwanted because some materials are very rare and precious. 5. Accuracy: Every analyst want to be highly accurate. With large size of material, it is difficult to maintain the accuracy.

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