Grab samples

Several grab samples combined to form a single sample. 

Few populations, however, meet the conditions for a true binomial distribution. Real populations normally contain more than two types of particles, with the analyte present at several levels of concentration. Nevertheless, many well-mixed populations, in which the population s composition is homogeneous on the scale at which we sample, approximate binomial sampling statistics. Under these conditions the following relationship between the mass of a randomly collected grab sample, m, and the percent relative standard deviation for sampling, R, is often valid. ... 

Samples from municipal wastewater treatment plants and samples of industrial discharges often are collected as 24-h composites. Samples are obtained using an automatic sampler that periodically removes individual grab samples. The volume of each sample increment and the frequency of sampling may be constant or may vary in response to changes in flow rate. 

Duration of Sample. When measuring a substance having an 8 h averaging time, a single 8 h sample or several consecutive samples adding up to 8 h is best (see Eig. 1). Short period grab samples are the least satisfactory. Eor a STEL, the method should be able to collect enough material to provide adequate sensitivity. 

Molecular Weight EPA Method 3 is used to determine carbon dioxide and oxygen concentrations and dry molecular weight of the stack-gas stream. Depending on the intended use of the data, these values can be obtained with an integrated sample (see Fig. 25-28) or a grab sample (see Fig. 25-29). In addition, the instrumental analyzer... 

With the grab sampling technique, a samphng probe is placed at the center of the stack, and a sample is drawn direcfly into an Orsat analyzer or a Fyrite-type combustion-gas analyzer. The sample is then analyzed for carbon dioxide and oxygen content. With these data, the diy molecular weight of the gas stream can then be calculated. 

FIG. 25-29 Grab -sample setup for moleciilar-weigbt determination. 

A samphng probe is placed at any location in the stack, and a grab sample is collected in an evacuated flask. This flask contains a solution of siilfiiric acid and hydrogen peroxide, which reacts with the NO. The volume and moisture content of the exhaust-gas stream must be determined for calculation of the total mass-emission rate. The sample is sent to a laboratoiy, where the concentration of nitrogen oxides, except nitrons oxide, is determined colorimetrically. 

Each grab sample is obtained fairly rapidly (15 to 30 s), and four grab samples constitute one rim a total of 12 grab samples is required for a complete series of three rims. An interval of 15 min between grab samples is required. The range of this method has been determined to be 2 to 400 mg of NO (as NO9) per dry standard cubic meter (without dilution). Figure 25-34 shows a schematic of the sampling apparatus for an NO source test. 

Grab samples can be collected from periodic discharges such as from the spray booth where the fresh water is replenished only periodically. 

Grab sample Any material that can be analyzed. Samples do not vary more than +X% from final concentration. 

Grab sample A sample of air collected over a short time period in the workspace. 

The basis for assessment of river quality is valid data, but the question of data validity is secondary to the fundamental question What is the purpose of the river quality assessment activity What questions are being asked In short the means are determined by the ends. If, for example, the intent of an assessment program is to ascertain compliance with law, then a fixed interval "grab sampling program could be established at various river system sites selected on the basis of established guidelines which are consistent with the... 

Mean of 23 weekly grab samples collected during months of July, August and September, mean flow of 184 m s at Salem. DEQ values have been diumally normalized. Source (8)... 

Areal Variation. One objective of the sampling comparison studies was to determine the variation in TCDD concentration over a small area to estimate the error associated with a grab sample concentration. Accordingly, a one-square-yard area was selected adjacent to a previously sprayed road. The center of the test area was about 6 feet from a sprayed road shoulder. This one-square-yard area was divided into nine one-square-foot areas. Using clean spoons and knives, a single scoop was collected from the center of each one-square-foot area down to a depth of 2 inches. The data are presented in Figure 1. 

Benthic invertebrate communities are taxonomically and trophically complex, and their abundance and species composition in a water body often vary seasonally and among years. Sediment-dwelling invertebrates can be readily sampled but considerable effort is often required to remove benthic organisms from grab samples of sediment, to determine their taxonomic composition, and to obtain sufficient sample mass of a target taxon for analysis. Sampling would not substantially affect target populations. 

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