Sampling golden rules

FIG. 5. The tunneling current between the tip and sample is derived from the overlap of the respective wave functions using Fermi s golden rule. (From Ref. 39.)... 

Exploration of a data set before resolution is a golden rule fully applicable to image analysis. In this context, there are two important domains of information in the data set the spectral domain and the spatial domain. Using a method for the selection of pure variables like SIMPLISMA [53], we can select the pixels with the most dissimilar spectra. As in the resolution of other types of data sets, these spectra are good initial estimates to start the constrained optimization of matrices C and ST. The spatial dimension of an image is what makes these types of measurement different from other chemical data sets, since it provides local information about the sample through pixel-to-pixel spectral variations. This local character can be exploited with chemometric tools based on local-rank analysis, like FSMW-EFA [30, 31], explained in Section 11.3. 

In many possible situations a sample has to be obtained under conditions that often necessitate the use of inferior sampling techniques, however, observance of these Golden Rules will lead to the best sampling procedure. 

Terms used in describing samples and sampling procedures are many and varied. The text below aims to describe those in common usage. It is clear that there is potential for confusion. It should therefore be a golden rule for any analytical chemist to define clearly the terms used in the context of the particular problem being addressed so that any procedure can be understood and repeated. ... 

These principles are embodied in The golden rules of sampling... 

This method of obtaining a sample is mentioned because it is possible that there may be circumstances in which there is no alternative but to use it, but this must not be taken to imply that such method will give satisfactory sampling. Every effort should be made to avoid this method and to use one, which satisfies the two golden rules. If a powdered material is in a container the container has been filled and presumably is going to be emptied. At both these times, the powder will be in motion, and a more satisfactory sampling procedure can then be used. 

With large tonnages, samples taken from conveyors can represent large quantities of material that need to be further reduced. Often, a traversing cutter is used as a primary sampler, and the extracted sample is further cut into a convenient quantity by a secondary sampling device. It must be borne in mind that the secondary sampler must also conform to the golden rules of sampling. This equipment is satisfactory for many applications but it has limitations, which restrict its use. These are ... 

The rotary sample divider or spinning riffler was first described in 1934 [20] and conforms to the golden rules of sampling. The preferred method of using this deviee is to fill a mass flow hopper in such a way that segregation does not occur. The table is then set in motion and the hopper outlet opened so that the powder falls into the collecting boxes. The use of a vibratory feeder is recommended to provide a constant flowrate... 

Fortunately, the solution to this problem was one of the early successes of quantum mechanics and is so central to modem physics that it carries its own name, Fermi s Golden Rule. The transition rate between the initial and final states of a system depends on the strength of the coupling between the initial and final states and on the number of ways the transition can happen, the density of the final states [1]. This rule only applies to scattering problems where the incident wavefunction is weakly perturbed by the presence of the sample. If the scattering is not strong we can suppose that the total waveftinction does not differ substantially from the incident wavefunction this supposition is the Bom approximation. 

To satisfy the above requirements, the following basic golden rules of sampling should be followed whenever practicable. 

The principle of the spinning riffler is shown in Fig. 1 and it embodies both golden rules of sampling. The sample is slowly conveyed by a vibratory feeder from the feed hopper onto a... 

USP 429) stress the importance of sampling and advise that a representative sample be prepared using a sample splitting technique. The golden rules of sampling suggested by Allen 1990 are ... 

The first decision to make is whether the analyte in question is actively pumped or whether it diffuses passively. There is always the caveat of any possible rostro-caudal gradients, in which case the volume removed is important and it is always prudent to measure the levels of analyte in blood plasma (or serum). For proteins with passive diffusion one must correct simultaneously for both the serum level and for an independent measure of barrier function. The quotient of CSF analyte/serum analyte divided by CSF albumin/serum albumin can be expressed as an index value, but if there are alterations in any of the barriers there may be a nonlinear relationship that is particularly characteristic of large molecular weight proteins. This relationship is thus best expressed by a nonlinear function. The golden rule must be to compare like with like, and make appropriate corrections for known differences due to inhomogeneous samples. 

Further, in atomic spectrometry we must face the serious problem that the behaviour (atomization/excitation characteristics) of the analyte in the calibration solutions should be the same as in the future unknown test solutions (samples) where the analyte of interest has to be quantified, otherwise peak displacement and changes of the peak shape may seriously bias the predictions. Fortunately, many atomic techniques analyse aqueous extracts or acid solutions of the (pretreated) samples and current working procedures match the amount of acids in the calibration solutions and treated samples, so the matrices become rather similar. Current practices in method development involve the study of potential interferents. The analyte is fixed at some average concentration (sometimes, studies are made at dilferent concentrations) and the effect of a wide number of potential interferents is tested. They include major cations, anions and other metals. Disappointingly, matching concomitants is not so straightforward and some doubts remain about the similarity of the calibration and unknown samples. The golden rule here is that standards and future samples should be treated in exactly the same manner. In this sense. 

One must follow the two Golden rules of sampling as proposed by Allen (1981) ... 

Sample Size Reduction. The sample obtained from the sampler is generally larger than the sample required for analysis. If a small portion of this sample is arbitrarily scooped for analysis, it will introduce a bias. The sample size can be reduced to the analytical size using various sample reduction techniques (1) spinning riffler, (2) chute riffler, (3) ICI method, and so on. All these methods follow the golden rules for sampling. For details on these sample reduction techniques, see AUen (1981). 

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