Sampling procedures

The purpose of analysis is to determine the quality or composition of a material and for the analytical results obtained to have any validity or meaning it is essential that adequate sampling procedures be adopted. Sampling is the process of extracting from a large quantity of material a small portion which is truly representative of the composition of the whole material. 

Sampling methods fall into three main groups  

methods in which portions of the material are selected based upon statistical probabilities. 

Procedure (1) is normally impracticable, as the majority of methods employed are destructive, and in any case the amount of material to be examined is frequently excessive. Even for a sample of manageable size the analysis would be very time-consuming, it would require large quantities of reagents, and would monopolise instruments for long periods. 

Sampling according to (2) is totally unscientific and can lead to decisions being taken on inadequate information. In this case, as the taking of samples is entirely casual, any true form of analytical control or supervision is impossible. 

---

The pressure measurment and fluid sampling procedures can be repealed at multiple depths in the reservoir. 

The copolymer composition equation relates the r s to either the ratio [Eq. (7.15)] or the mole fraction [Eq. (7.18)] of the monomers in the feedstock and repeat units in the copolymer. To use this equation to evaluate rj and V2, the composition of a copolymer resulting from a feedstock of known composition must be measured. The composition of the feedstock itself must be known also, but we assume this poses no problems. The copolymer specimen must be obtained by proper sampling procedures, and purified of extraneous materials. Remember that monomers, initiators, and possibly solvents are involved in these reactions also, even though we have been focusing attention on the copolymer alone. The proportions of the two kinds of repeat unit in the copolymer is then determined by either chemical or physical methods. Elemental analysis has been the chemical method most widely used, although analysis for functional groups is also employed. 

The quantity of sample required comprises two parts the volume and the statistical sample size. The sample volume is selected to permit completion of all required analytical procedures. The sample size is the necessary number of samples taken from a stream to characterize the lot. Sound statistical practices are not always feasible either physically or economically in industry because of cost or accessibiUty. In most sampling procedures, samples are taken at different levels and locations to form a composite sample. If some prior estimate of the population mean, and population standard deviation. O, are known or may be estimated, then the difference between that mean and the mean, x, in a sample of n items is given by the following ... 

The need for skill and experience on the part of sample designers and persoimel cannot be overemphasized in chemical plant sampling. Safety precautions are of the utmost importance. Necessary steps must be taken to document the hazards involved in an operation and to ensure that the staff are weU-trained, informed, protected, and capable. Except for bulk powder sampling, most chemical plant sampling is hazardous and difficult and must be designed with care. The following discussions are based on the assumptions that most of these decisions have been made and a satisfactory sampling procedure has been planned. 

Whenever possible a powder should be sampled when in motion and the whole of the stream of powder should be taken for many short increments of time in preference to part of the stream being taken for the whole of the time. Observance of these rules coupled with an understanding of the manner in which segregation takes place leads to the best sampling procedure. Care and skill in abstracting samples is needed and cannot be overemphasized. 

Samplers and Sample Procedure for Hazardous Waste Streams, EPA-600/2-80-018, U.S. EPA, Washington, D.C. 

Estimations based on statistics can be made for total accuracy, precision, and reproducibility of results related to the sampling procedure being applied. Statistical error is expressed in terms of variance. Total samphng error is the sum of error variance from each step of the process. However, discussions herein will take into consideration only step (I)—mechanical extraction of samples. Mechanical-extracdion accuracy is dependent on design reflecding mechanical and statistical factors in carrying out efficient and practical collection of representative samples S from a bulk quantity B,... 

Regardless of the method of evaluating the solids mixture, the sampling procedure is vital. Often a sampling thief, or other special device, is used to remove samples from the mixture without excessive disturbance of the batch. If an easier method of sampling is obvious and will bring less contamination to the batch, it should be used. 

Plant-performance analysts should be involved in reviewing the entire sampling procedure. The procedures for review are ... 

Sampling spills. Wear proper personal protective equipment (PPE) Follow proper sampling procedures Use safe sampling design CCPS G-22 CCPS G-23 CCPS G-29 Lovelace 1979... 

Develop and implement sampling procedures tailored to the need of the batch being run... 

In performing a Monte Carlo sampling procedure we let the dice decide, again and again, how to proceed with the search process. In general, a Monte Carlo search consists of two steps (1) generating a new trial conformation and (2) deciding whether the new confonnation will be accepted or rejected. 

Chemical methods involve removing a portion of the reacting system, quenching of the reaction, inhibition of the reaction that occurs within the sample, and direct determination of concentration using standard analytical techniques—a spectroscopic metliod. These methods provide absolute values of the concentration of the various species that are present in the reaction mixture. However, it is difficult to automate chemical mediods, as the sampling procedure does not provide a continuous record of tlie reaction progress. They are also not applicable to very fast reaction techniques. 

In the simple sampling procedure of generating chain conformations all successfully generated walks have equal probabihty. Walks are grown purely stochastically. Each time an attempted new bond hits a site which is already occupied, one has to start at the very beginning. Otherwise different conformations would have different probabihties and this would introduce an effective attraction among the monomers [54]. With this method, each conformation is taken randomly out of the q q — 1) possible random paths which do not include direct back-folding. However, the total number of SAW on a lattice is known [26] to be ... 

The rehability of these analytical methods may be questionable when chemical shift differences of derivatives are of the same magnitude as variations encountered from solvent, concentration, and temperature influences. Reported fluorine chemical shift ranges for tnfluoroacetylated alcohols (1 ppm), p-fluorobenzoylated sterols (1 ppm), and p-fluorobenzoylated ammo acids (0.5 ppm) are quite narrow, and correct interpretation of the fluonne NMR spectra of these denvatized mixmres requires strict adherence to standardized sampling procedure and NMR parameters. 

The sampling procedure may involve a number of stages prior to the analysis of the material. The sampling stages are outlined in Fig. 5.1. 

It should be borne in mind that although it is possible to generalise on sampling procedures, all industries have their own established methods for obtaining a record of the quantity and/or quality of their products. The sampling procedures for tobacco leaves will obviously differ from those used for bales of cotton or for coal. But although the types of samples differ considerably the actual analytical methods used later are of general application. 

Water and steam sampling procedures should follow the American Society for Testing and Materials method, ASTM D-1066 British... 

Sampling pots, see Sampling coils Sampling procedures for water and steam 599... 

Sampling Procedure. The sample extraction technique previously described was used in this work (8). However, a heated palladium thimble was also used to separate hydrogen and hydrocarbon fractions of the samples. No exchange occurred during the separation process in agreement with observations of other workers (30). In experiments using NH3 as a minor additive, the ammonia fraction was separated by means of appropriate cold traps. 

---