Sample purity effects

The two types of overload have quite different effects on the separation process, i.e., the resolution that is obtained from the column and the shape of the resulting peaks. Both sampling techniques can be very effective, but need to be carefully controlled and the procedure well understood if sample purity is to be maintained. 

A major source of error in most measurements is the presence of impurities in the sample. The effect of an impurity depends upon its amount in the sample and upon the difference between its density and the density of the principal constituent. Even when the sample purity is provided quantitatively, the impurities often are not identified individually. Nevertheless, a report of sample purity reduces the estimated uncertainty because it can be taken as evidence that the investigator has considered sample purity. The most ubiquitous impurity in liquids is water, and, because its density differs significantly from those of hydrocarbons, it is a common source of error. Exclusion of water requires that the sample be protected from the atmosphere during transfer, and that special precautions be taken to remove the sample from containers. 

The above-reviewed pharmacological results show some slight discrepancies specially with ciguatoxin. These discrepancies may be explained partly by the variability of the sample purity and partly by the presence in the extracts of secondary toxins. The complex description of the effects observed experimentally can explain the polymorphism of the clinical features. These pharmacological data can help the physician to improvise an appropriate treatment in ciguatera fish poisoning. 

Sample purity The matter of sample purity is somewhat more complex than might be thought at first. The effect of an impurity on the diffraction pattern depends on its mole fraction in the sample vapor and on its scattering power relative to that of the material of interest. (The structure-sensitive scattering from a molecule is approximately proportional to n//Z,Z//r//.) However, the effect of an impurity on the desired structure determination need not bear much relation to its effect on the diffraction pattern. If the impurity has a distance distribution sufficiently different from that of the substance of interest, the peaks of the D(r) curve will be resolved accordingly, the parameter values of the two molecules will be essentially uncorrelated and the desired results unaffected. On the other hand, small amounts of an impurity with a distance spectrum similar to the substance of interest can seriously disturb the results. It is perhaps worth noting that the... 

Hydrogenation was carried out in n-hexane. Kinetic lag was observed. Sample purity was 97.1%. The entry above is the arithmetic mean of two sets of experimental results, each of 9 determinations. Kinetic effects and impurities had a relatively small effect on the error limits because kinetic lag was slight and the major contaminants were isomers of the designated branched octene having a similar AhydH. ... 

The effects of concentration and sample purity on the redox potentials of the Fen/Fein couple in heme have also been studied usually the midpoint potentials are much higher in concentrated solutions than in more dilute ones [Adler (1)]. Reduction of FemCl36 TPP with Cr11 salts leads to a 96% percent transfer of the radioactivity towards the Crm product which confirms the inner-sphere electron transfer across an Fe—Cl—Cr-bridge [Cohen (37)]. 

In either case, extremely small amounts of impurities and crystalline defects can play an important role in determining both the magnitude of the conductivity and the activation energy. The elucidation of such effects is often diflScult, and careful control of sample purity and form as well as experiments on samples in various stages of purification are required. 

Similar conditions also effected cleavage of the interflavanyl bond in the fisetinidol-(4a-+8)-catechin permethylaryl ether (68) to afford tetra-0-methylcatechin (75) (21%), the l,3-diarylpropan-2-ol (78) (12%), and tri-O-methylfisetinidol (81) (12%). Such a rupture of the interflavanyl bond in the permethylaryl ether (68) introduced an important dimension to these cleavages in relation to the chemistry of the 5-deoxyoligo-flavanoids where the additional chromatographic steps involved with derivatization are often prerequisites for sample purity. The liberation of the chain-terminating flavan-3-ol unit (5) or (75), irrespective of... 

With the advent of FAB, derivatization was usually not required to obtain a molecular ion, (M -I- H] or [M — H) from an involatile thermally labile compound. However, the overall information yield in the FAB spectrum of the native compound frequently fell short of what was required for structure identification. Structurally significant fragment ions were often absent from the FAB spectrum, and low abundance fragments from the sample could be difficult to differentiate from the collection of ions originating from the liquid matrix. In addition, overall sensitivity varied widely with compound structure and sample purity. In these circumstances, derivatization has proved to be highly effective method for tailoring the response of a sample compound in order to maximize the yield of structural information. 

Thin-layer chromatography (TEC) is a simple separation technique that is usually applied in the pharmaceutical industry, organic chemistry, and environmental laboratories for fast, basic analyses of sample purity, detection of the reaction products, or verification of the presence or absence of some compounds in the mixtures. TLC methodology and equipment are simple and cost-effective, and such separations may also be performed outside the laboratory, for example, wherever the sample has been taken from the source localized hundreds of miles from the nearest facility. One of the weakest points of TLC separation is visualization of the obtained chromatogram. 

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