Grab sampling methods

In order to assess the accuracy of the present method, we compared it with two other methods. One was the Track Etch detector manufactured by the Terradex Corp. (type SF). Simultaneous measurements with our detectors and the Terradex detectors in 207 locations were made over 10 months. The correlation coefficient between radon concentrations derived from these methods was 0.875, but the mean value by the Terradex method was about twice that by our detectors. The other method used was the passive integrated detector using activated charcoal which is in a canister (Iwata, 1986). After 24 hour exposure, the amount of radon absorbed in the charcoal was measured with Nal (Tl) scintillation counter. The method was calibrated with the grab sampling method using activated charcoal in the coolant and cross-calibrated with other methods. Measurements for comparison with the bare track detector were made in 57 indoor locations. The correlation coefficient between the results by the two methods was 0.323. In the case of comparisons in five locations where frequent measurements with the charcoal method were made or where the radon concentration was approximately constant, the correlation coefficient was 0.996 and mean value by the charcoal method was higher by only 12% than that by the present method. 

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... 

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. 

An extensive benchmark stndy has been carried ont in order to identify the optimal mass redaction principle(s) [6], This was achieved by assessing and validating the almost nniversally nsed mass redaction approach, grab sampling, as opposed to a comprehensive series of more advanced techniqnes and methods, 17 in total. All techniqnes and methods were tested with regard to a fall snite of qnality parameters, some scientihcally important, others related to cost and minimizing practical operating conditions and expenses. The most important merit of any mass redaction method is the ability to deliver an unbiased split of material with the smallest possible variation in repeated rnns snbject to the best possible accnracy. These two featnres are summed up in the TOS qnantitative measnre of representativeness. 

Occasionally attempts to justify a process analyzer are made on the basis that it will reduce the number of grab samples that need to be taken from the process to be analyzed, and therefore reduce the sample load on the QC lab. This is not usually a good justification for an on-line analyzer for two reasons (1) The reduction in lab analyses rarely offsets the cost of analyzer installation and operation (2) An on-line analyzer relies on high quality lab data for calibration, validation, and long-term model maintenance, so the reference method can never be eliminated entirely. 

Ideally, an on-line analyzer will be calibrated before it is installed in the process. It may be possible to accomplish this by calibrating it off-line with process grab samples and/or synthetic samples. It may be possible to install the analyzer in a lab-scale reactor, or in a semi-works or pilot plant. It may be possible to transfer to the on-line analyzer a method developed on an off-line analyzer or on another on-line analyzer (e.g. at a different plant site). However, sometimes none of these are possible and the analyzer will need to be calibrated on-line. The challenges of on-line model development (calibration) and validation, as well as approaches to dealing with them, are discussed below. For information related to calibration transfer issues, please see Chapter 12 of this book. 

During the Western Project, a total of 3000 grab samples were assayed by using the rapid spectral method to determine 32 elements and chemical adsorption to determine gold. Spectral assay data were processed in order to determine the distribution of 24 elements. Background (Cf) values were calculated using Lognormal Statistics software. 

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