Types of Sampling

Probability (or random) sampling allows a statistical evaluation to be applied to the data. It is used when a representative sample is required. There are three approaches which give rise to the three types of random sample described in Section 3.2.3. 

Simple random sampling involves taking increments from the bulk material in such a way that any portion of the bulk has an equal probability of being sampled. This type of sampling is often used when little information is available about the material that is being sampled. It is also commonly used when 

This is used when a representative sample cannot be collected or is not appropriate. It is the correct sampling approach to use to produce a selective sample (see Section 3.2.2). There are three main non-probability sampling strategies. 

Judgement sampling involves using knowledge about the material to be sampled, and the reason for sampling, to select specific samples. 

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Sample size is 100 ml and distillation conditions are specified according to the type of sample. Temperature and volume of condensate are taken simultaneously and the test results are calculated and reported as boiling temperature as a function of the volume recovered as shown in Table 2.1. 

CT was applied to various types of samples, such as a) concrete, b) asphalt c) wood and trees d) soil samples e) ceramic materials f) works of art. 

The following experiments may he used to illustrate the application of titrimetry to quantitative, qtmlitative, or characterization problems. Experiments are grouped into four categories based on the type of reaction (acid-base, complexation, redox, and precipitation). A brief description is included with each experiment providing details such as the type of sample analyzed, the method for locating end points, or the analysis of data. Additional experiments emphasizing potentiometric electrodes are found in Chapter 11. 

Atomic emission is used for the analysis of the same types of samples that may be analyzed by atomic absorption. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomization and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization. 

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

The following experiments may he used to illustrate the application of chromatography and electrophoresis to a number of different types of samples. Experiments are grouped by the type of technique, and each is briefly annotated. 

The first set of experiments describes the application of gas chromatography. These experiments encompass a variety of different types of samples, columns, and detectors. Most experiments maybe easily modified to use available equipment and detectors. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Sample Integrity. In order to be able to rely on the results of measurements, it is necessary to be sure that the sample as analy2ed is the same as it was when collected, and that it is properly identified in the field, in the laboratory, and in the report. Transit times and temperatures should be within the limits allowed for the type of sample and analysis. A series of documents which estabhsh a chain of custody should exist so that it is possible to be sure that the right result goes with the right sample. 

Samples logged into the system are assigned a unique number which is often physically attached to the sample using a bar-code label. The testing protocol is contained in the LIMS and is based on the point in the process from which the sample was taken. This information and the identification of the type of sample enable the LIMS to schedule the testing of the material. 

Some forms of agarose are specifically designed to work with large (mol wt >500,000) molecules (27,28). The types of samples for which the agarose ief system are utilized are larger plasma proteins such as immunoglobulins, tissues, and tumors. 

In addition to polymeric support media, capillaries and flowing buffers have been used as support media for electrophoresis. Although these are not used as frequendy, there are definite advantages for certain types of samples and appHcations. 

Sample Statistics Many types of sample statistics will be defined. Two very special types are the sample mean, designated as X, and the sample standard deviation, designated as s. These are, by definition, random variables. Parameters like [L and O are not random variables they are fixed constants. 

Sample Containers More saiTiple containers will be required for a complex test than are typically used for normal operation. The number and type of sample containers must be gathered in advance, recognizing the number of measurements that 1 be required. The sample containers should be tagged for the sample location, type, and conditions. 

Type of sample Number of samples Urban Rural ... 

Fig. 13-1. Schematic diagram of various types of sampling systems.

Static sampling systems are defined as those that do not have an active air-moving component, such as the pump, to pull a sample to the collection medium. This type of sampling system has been used for over 100 years. Examples include the lead peroxide candle used to detect the presence of SO2 in the atmosphere and the dust-fall bucket and trays or slides coated with a viscous material used to detect particulate matter. This type of system suffers from inability to quantify the amount of pollutant present over a short period of time, i.e., less than 1 week. The potentially desirable characteristics of a static sampling system have led to further developments in this type of technology to provide quantitative information on pollutant concentrations over a fked period of time. Static sampling systems have been developed for use in the occupational environment and are also used to measure the exposure levels in the general community, e.g., radon gas in residences. 

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