Sampling differences

Sample Collection Solids are usually heterogeneous, and samples must be collected carefully if they are to be representative of the target population. As noted earlier, solids come in a variety of forms, each of which is sampled differently. 

Flavor Description. TypicaHy, a sensory analyst determines if two samples differ, and attempts to explain their differences so that changes can be made. The Arthur D. Litde flavor profile (FP), quantitative descriptive analysis (QDA), and spectmm method are three of the most popular methods designed to answer these and more compHcated questions (30—33). AH three methods involve the training of people in the nominal scaling of the flavor quaHties present in the food being studied, but they differ in their method for quantitation. 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Combining Sepharose CL 2B and CL 4B singnificantly increases resolution in the low dp range of broad distributed samples. As an example, wild-type potato starch and two fractions of this sample differing in their branching... 

Populations are very large collections of values. In practice, experimental pharmacology deals with samples (much smaller collections) from a population. The statistical tools used to deal with samples differ somewhat from those used to deal with populations. When an experimental sample is obtained, the investigator often wants to know about two features of the sample central tendency and variability. The central tendency refers to the most representative estimate of the value, while the variability defines the confidence that the estimate is a true reflection of that value. Central tendency estimates can be the median (value that divides the sample into two equal halves) or the... 

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. 

These samples differed slightly in the degree of hydrophobe branching. Although the difference in hydrophobe branching is small, it may be worthy of note that the more branched surfactant provided the lower IFT value. 

Spectra of s.o. samples differed markedly from those of a.p. samples and were unaffected by a subsequent evacuation up to 673 K (Fig. 4, a). Spectra consisted of a composite envelope of heavily overlapping bands at 980-1070 cm-, with two weak bands at 874 and 894 cm-. Irrespective of the preparation method, the integrated area (cm- ) of the composite band at 980-1070 cm- was proportional to the V-content up to 3 atoms nm-2. An analysis of spectra by the curve-fitting procedure showed the presence of several V=0 modes. The relative intensity of the various peaks contributing to the composite band depended only on the V-content and did not depend on the method used for preparing the catalysts. Samples with V > 3 atoms nm-2 R-spectra features similar to those of pure V2O5 (spectrum 8 in Fig. 4, a). 

The two techniques differ in that HDC employs a nonporous stationary phase. Separation is affected as a result of particles of different size sampling different velocities in the interstitial spaces. Size exclusion chromatography is accomplished by superimposing a steric selection mechanism which results from the use of a porous bed. The pore sizes may vary over a wide range and the separation occurs as a result of essentially the same processes present in the gel permeation chromatography of macromolecules. 

If your experiment is short, you don t need to worry about field drift. Modem magnets are quite stable and can be used for at least a few minutes without drifting too far. The disadvantage with this approach is that shimming is normally performed on the deuterium signal and you will need to shim your sample differently if there is no deuterium in the sample. 

Whereas several techniques may thus be used to study a certain characteristic of a polymer sample, for instance IR and Raman spectroscopy and X-ray diffraction as well as NMR may be used to determine or infer the crystallinity level of a sample, different techniques work differently and therefore usually do not measure the same. What this means is that crystallinity levels obtained from the same sample may differ when a different technique is applied, see, for example, ref. [23] and chapter 7 and references therein. However, these differences do not necessarily imply one technique being better than another. In fact these differences may contain useful information on the sample (see, for example, ref. [25]). 

If the Fixman potential is not added to the Hamiltonian, the system is sampled differently and therefore correction terms must be added to (4.27). First, the weight Ze r in (4.24) must be reintroduced. Second, the Lagrange multiplier is different since the Fixman potential I /(2/3) In is not used. Considering (4.20) for A, it is possible to show that the Lagrange multiplier AF simply needs to be replaced by... 

Depending on the toxicological or criminalistic problems to be solved and/or the availability of the sample, different matrices must be analyzed by using GC-MS or LC-MS techniques. 

The second study used a similar method to induce specific mood in odour donors (Ackerl, Atzmueller and Grammer 2002). In this case, subjects watched a horror movie or a neutral movie. From three presented odours, female raters were asked to choose the sample different from the other two and say which one smells like fear . The different sample was picked significantly more often, but not judged correctly as fearful odour . However, fearful odour was rated as less pleasant and more intense. 

Transect sampling for nonpoint sources is done the same way and with the same purpose as it is in point source sampling. Figure 7.7 shows a transect across an area to be sampled. Different markers indicate that different sampling will be accomplished at these sites. These would also be sites for depth sampling if it is called for. 

Consider the analysis of a blood sample for alcohol content (imagine that a police officer suspects a motorist to be intoxicated). The problem here is not sampling different locations within a system, but rather a time factor. The blood must be sampled within a particular time frame in order to demonstrate intoxication at the time the motorist was stopped. 

---