Sampling operations

If the standard deviation of the lot caimot be estimated, a sampling program of greater sample size is required to generate an estimate of the standard deviation for future sampling operations. In some cases, sample size can be increased and sampling costs reduced by the use of automatic samplers. These offer a substantial reduction in labor costs but an increase in capital costs. 

By far the largest proportion of gas sampling operations in industry is carried out for environmental reasons and the sampling methods employed have been thoroughly researched and are well documented (5—12). The preparation, precautions and equipment requirements involved in the sampling of air pollution sources are appHcable to most other gaseous environments (see Airpollution control methods). 

The errors arising in sampling, particularly in the case of heterogeneous solids, may be the most important source of uncertainty in the subsequent analysis of the material. If we represent the standard deviation of the sampling operation (the sampling error) by ss and the standard deviation of the analytical procedures (the analytical error) by sA, then the overall standard deviation sT (the total error) is given by... 

There are two basic kinds of centering and scaling. Data can be treated variable by variable, or they can be treated sample by sample. For example, if we are dealing with a system of absorbance spectra measured on samples each containing two components, a variable by variable operation would deal with one component at a time, or one wavelength at a time while a sample by sample operation would deal with one spectrum or one sample at a time. 

Figure 3.1 Schematic of sampling and analytical operations. Note the lower A of the sampling operations continues with the upper A of the analytical operations [1]. Reproduced by permission of the International Union of Pure and Applied Chemistry, from Horwitz, W Pure Appl. Chem., 62, 1193-1208 (1990). 

Functions to validate on automated dissolution systems may include bath operation, balance operation, media dispensing operations, media removal, sampling operations, media replacement, thermistor operation, robot operation, sample timing, sequence, and dilution. 

In general, zero-headspace procedures are employed when the concentrations of volatiles in the soil are relatively low, and solvent extraction methods are used for more polluted soils. Irrespective of which procedure is used, quantitation of volatiles in soil is subject to serious errors if sufficient care is not taken with the sampling operation. Although direct purge-and-trap methods are frequently advocated for the determination of volatiles in samples collected by zero-headspace procedures, there are certain problems associated with this technique. Caution is advised since the procedure really collects only that fraction of the volatile that exists in a free form within the soil pore spaces or is at least in a facile equilibrium with this fraction. 

Field sampling operations are often costly in time and manpower Those collecting samples should be aware of the possibility of bias and contamination ... 

The Facility Closure Plan includes two sampling and analysis plans. The first, Sampling and Analysis Plan for HWMU Closure (Appendix 2 in the Plan), includes requirements and procedures for conducting field sampling operations and investigations of soils and structures associated with the MDB and the HWMUs, as well as data quality objectives and field sampling protocols that could be used to verify decontamination (U.S. Army, 2000a). 

It is essential to avoid changes in water content of the material during sampling operations. Errors from this source are particularly significant in the analysis of materials having low water or having the tendency to readily absorb moisture, such as absolute alcohol. 

Experimental effort has been devoted in parallel to evaluate the sampling operation as well as the analytic procedure. 

The sampling operation involves collection of an aerosol sample that is representative of the particle size distribution and concentration of the sampled atmosphere. The efficiency of particle transport and collection operations are dependent on the particle size, sampling velocity, the geometry of the sampling apparatus and the properties of the collection medium. In the present work, a 37 mm diameter membrane filter (0.3 ym pore size) is the primary collection medium under evaluation. The filter is housed in a standard filter cassette and effects of filter-holder inlet geometry are also being investigated. 

Cover sheet (includes general information concerning sample, operator, equipment, analytical method)... 

The generous support of the U.S. Air Force Air Weather Service and the National Center for Atmospheric Research Aircraft Facility for aircraft sampling support is gratefully acknowledged. The author is especially indebted to S. E. Poet who carried out the laboratory measurements and assisted in the sampling operations that provided the data in Table I. 

---