Purity, samples

The nOe difference spectrum is highly demanding, since even the slightest variation in the spectra recorded with and without preirradiations will show up as artifacts in the difference spectrum (Fig. 4.8). The errors can be random, due to phase instability caused by temperature effects on the Rf circuits, variations in spinner speed, etc. The problem of phase instability is reduced in the latest generation of instruments with digital 

Avoid having moving metal objects near the magnet when carrying out nOe difference experiments, to prevent random variations in frequency. A small line-broadening ( 2 Hz) can also be applied to the spectra before or after subtraction, to reduce subtraction artifacts. 

What precautions are normally taken during sample preparation for the nOe experiment  

The nOe experiment is one of the most powerful and widely exploited methods for structure determination. nOe difference (NOED) or the two-dimensional experiment, NOESY, is used extensively for stereochemical assignments. It provides an indirect way to extract information about internuclear distances. The other use of nOe is in signal intensification in certain NMR experiments, such as the broad-band decoupled C-NMR experiment. 

The positive nOe observed in small molecules in nonviscous solution is mainly due to double-quantum relaxation, whereas the negative nOe observed for macromolecules in viscous solution is due to the predominance of the zero-quantum W) cross-relaxation pathway. 

---

Other analytical techniques ate also available for the determination of maleic anhydride sample purity. For example, maleic anhydride content can be determined by reacting it with a known excess of aniline [62-53-3] in an alcohol mixture (170). The solution is then titrated with an acid to determine the amount of unconsumed aniline. This number is then used to calculate the amount of maleic anhydride reacted and thus its concentration. Another method of a similar type has also been reported (171). 

Favorable polymer physical properties include water solubiUty and sample purity. Environmental conditions to consider in evaluating biodegradabihty are temperature, pH, moisture, oxygen, nutrients, suitable microbial population (fungal, algae, bacterial), concentration, and test duration. 

Since conductivity is highly sensitive to sample purity and temperature, group demarcations can be given only to within an order of magnitude. It should always be considered that conductivity under practical conditions... 

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. 

C and 5 N values were determined by mass spectrometry (MM 903, VG Isogas), equipped with a CN elemental analyzer (Roboprep CN, Europa Scientific). As control for sample purity, C N ratios from the elemental analyser were compared with C N ratios as calculated from the amino acid profile of the same sample. 

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 thin-layer thickness calculated in this manner may well have some serious errors associated with it due to sample purity, errors in the weighing out of the solution, the diffusion of species near the thin layer into it within the timescale of the experiment, etc. 

Sample purity is documented with SEM, TEM, and electron microprobe elemental analysis. Raman and UV-vis-near-IR spectra are also useful techniques that can be used to examine the quality of CNTs at the different stages of the purification procedure. 

Compound impurities can lead to biased results when plate readers are used for detection since the total absorbance at a particular wavelength is employed for the determinations. The yt/PLC system employed in these determinations can easily compensate for this problem since only the value for the peak area due to the compound (at the corresponding retention time) is considered for the calculations. For example, in the case of a nifedipine sample, purity was determined to be... 

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. 

Possible determinations from DSC or DTA measurements include (1) heat of transition, (2) heat of reaction, (3) sample purity, (4) phase diagram, (5) specific heat, (6) sample identification, (7) percentage incorporation of a substance, (8) reaction rate, (9) rate of crystallization or melting, (10) solvent retention, and (11) activation energy. Thus, thermo-calorimetric analysis can be a useful tool in describing the chemical and physical relationship of a polymer with respect to temperature. 

As many physical properties of the actinide metals depend significantly on the sample purity, refining of the metals is mandatory. The choice of the refining methods is determined by the chemical reactivity of the actinide metal in the presence of the constituents of air, by high temperature reactions with crucible materials, by the specific radioactivity and the availability of the actinide elements. 

In TGA, a sensitive balance is used to follow the weight change of a polymer as a function of time or temperature. In making both TGA and thermocalorimetric measurements, the same heating rate and flow of gas should be employed to give the most comparable thermograms. TGA and DTA can be used to determine (1) the sample purity, (2) the material identity,... 

Purification. a-Methylstyrene was purified by distillation through a 6-foot, Heli-grid-packed, modified Podbielniak column under a reduced pressure of dry helium. Purity of the distillate was established by vapor phase chromatographic analysis, using flame ionization detection and an appropriate column. Sample purity generally ranged from 99.90 to 99.98%. 

Sensitivity to Heat and Electric Charge The Ignition (or Explosion) Temperature Test measures thermal stability, which is discussed under Chemical Properties . The test is run with the sample either confined or unconfined (see Vol 1, XVI). Test results are quite sensitive to conditions and to sample purity, and as a result, ignition values for TNT reported in the literature differ by as much as 200° C (Ref 156 see discussion in Vol 4, D583-L ff). Careful studies using different procedures with confined samples have given ignition temps of 275° (Ref 156),... 

---