Nuclear magnetic resonance sample amounts

Compared with chiroptical methods and nuclear magnetic resonance spectroscopy (NMR), only chiral chromatography by direct and indirect methods is suitable for the accurate determination of enantiomeric impurities of less than 1% and for quantitative stereochemical analyses of small sample amounts (for example, in vivo studies of the metabolic pathway or pharmacokinetic effects of chiral pharmaceuticals.)... 

Consideration must be given to the quantity of sample needed for the minimum detection ]imits of the instrumental technique used. A number of techniques have been ranked in order of increasing amounts of material needed as follows mass spectroscopy (1 - 10 yg), chemical spot tests (1 - 100 yg), infrared and ultraviolet spectroscopy (10 - 200 yg), melting point (0.1 -1 mg), elemental analysis (0.5 - 5 mg), boiling point (1 - 10 mg), functional group analysis (1 - 20 mg), and nuclear magnetic resonance spectroscopy (1-25 mg). 

Nowadays, 3 P-NMR (nuclear magnetic resonance) is sometimes proposed as a better alternative, because this technique requires only a minimum amount of sample pretreatment (12-14). In our opinion, its high investment cost and need for highly qualified operators will cause P-NMR to remain a research technique rather than a quality-control method. However, being an absolute technique, P-NMR seems ideally suited to certify the composition of standard solutions whose quality largely determines the accuracy of both TLC and HPLC results (15,16). 

Forensic scientists make use of both these techniques because they are very accurate but they only require tiny amounts of sample - often only small amounts of sample are found at crime scenes. Other techniques utilised are nuclear magnetic resonance spectroscopy and ultraviolet/visible spectroscopy... 

FTMS also has the potential of becoming an important tool for determining molecular structure. Traditionally, mass spectrometry has been rather limited in its ability to determine the structure of an unknown compound unambiguously. Additional structural methods, such as nuclear magnetic resonance or crystallography, are commonly used in conjunction with mass spectrometry to elucidate the identity of a molecule. However, when the amount of sample is severely limited or when the sample is a component in a complex mixture, mass spectrometry is often one of the few analytical techniques that can be used. 

J. A. Jackson, also of this laboratory, has made room temperature nuclear magnetic resonance studies of the Knight shift of cadmium in slowly cooled CeCd, 45 alloys with different compositions and different histories. All CeCd 4 5 samples tested showed a major peak at almost the same position and shifted from that of metallic cadmium. One sample showed only this peak, while others clearly showed satellite peaks either at larger or at smaller shift. Possibly some samples had small amounts of both satellite peaks, and there was apparently some further difference in the shapes of satellite peaks and of the major peak these latter observations are tenuous, however, since they were near the resolution limit of the apparatus. The differences apparently do not correlate simply with composition however, they may correlate with differences in microphase structures. 

In 2002, through the use of nuclear magnetic resonance (NMR) and mass spectrometry/mass spectrometry (MS/MS) analysis, the planar and partial stereostructure of a SAAF from the egg-conditioning medium of C. intestinalis w s elucidated to be a previously uncharacterized sulfated steroid 3,4,7,26-tetrahydroxycholestane-3,26-disulfate (30).73 Its structure was deduced from only 4p.g (6 nmol) of sample. Thus, SAAF may represent the smallest amount of sample used in the structure elucidation of novel nonpeptidic or nonoligosaccharide natural products.74... 

Thermal analysis is capable of providing accurate information on the phase transition temperatures, degradation temperatures, heat capacity, and enthalpy of transition of polymers using comparatively simple DTA, DSC, and TG instruments. The measurement time is short compared with other techniques, such as viscoelastic measurement and nuclear magnetic resonance spectroscopy. Moreover, any kind of material, e.g., powders, flakes, films, fibers, and liquids, may be used. The required amount of sample is small, normally in the range of several milligrams. 

The solid fat content of an oil is a measure (in percent) of the amount of solid fat present in the oil at any one temperature. It is measured by means of wide-line nuclear magnetic resonance (NMR) spectrometry after a standard tempering procedure for the samples. 

The dilatometric method is time-consuming and subject to the bias introduced by the convention described. More recently pulsed Nuclear Magnetic Resonance (pNMR) has been used to measure the relative amounts of liquid and solid fat in a sample, based upon the difference in rates of relaxation of protons in the two phases after the sample has been pulsed (AOCS Method Cd 16-81). With proper calibration this gives a direct determination of the percentage of solid fat, and the results are termed sohd fat content (SFC). The analysis takes less time than dilato-metry, but the equipment is more expensive. 

Solid-state nuclear magnetic resonance spectra of Phosphoria shales (37) indicate the residence of higher amounts of aliphatic carbon in the thermally immature samples from southwestern Montana compared to thermally mature samples from Idaho. Similarly, gas chromatographic analyses and the ratio of hydrocarbon to carbon (4 ) indicate thermal immaturity and the potential for hydrocarbon generation in the southwestern Montana oil shales in contrast to the depleted hydrocarbon-producing potential of kerogenic shales in other areas. 

Identification or structural elucidation of an unknown compound is one of the most challenging tasks that can be undertaken by an analytical chemist. Where the analyst has milligram or more amounts of the unknown, MS is often used in conjunction with other techniques such as nuclear magnetic resonance (NMR) and IR spectroscopy. However, when there are only limited quantities of sample, the sensitivity of MS makes this the technique of choice for assembling structural information and, where no definitive conclusion can be reached on the mass spectral data alone, serves to limit the search to a particular class of chemicals or set of isomers. 

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