Physical properties mass spectra

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

Oxidation products from tetrahydropapaveroline with potassium ferricyanide were reinvestigated by Mak and Brossi 21), who confirmed structure 28 for the initial product. Air oxidation of 29 gave after acetylation a product with the same physical properties reported by Harley-Mason, but its mass spectrum showed a parent ion at m e 900 and molecular composition C48H40N2O16, suggesting it to be a dehydro dimer of 12. The H-NMR spectrum of the dimer showed that the double bonds at the 5,6 positions were intact. The point of dimerization was determined by comparing the multiplicities of carbons in the C-NMR spectra of the dimer and dibenzopyrrocoline models. For monomeric compounds the peak ascribed to C-12 in compound 11 was a triplet and a doublet in 12, but a singlet in the dimer. Thus, the products from oxidation of 28 had structures 31 and 32 rather than 29 and 30, respectively. 

Perchlorotriptycene, mass spectrum, 33 20-21 iV-Perchloryl compounds, 19 49-52 infrared spectra, 19 49-52 PerehloiyJ fluoride, 18 371-385 chemical properties, 18 380-384 dipole moment, 18 374 eigenvalues for, 18 377 frequency values, 18 374 ionization data for, 18 376 molecular structure of, 18 373-377 physical properties of, 18 377-380 preparation and reaction, 27 177-178 preparation of, 5 66-68 properties of, 5 68... 

It may be determined whether known or new peptide alkaloids are present and whether or not the laborious isolation of pure constituents is warranted. The mass spectrum is particularly useful for purposes of identification because the physical properties are sometimes indefinite and often similar and nearly identical Iif numbers in several solvent systems are becoming very common. On the other hand, mass spectra are specific and only rarely require further chemical studies (adouetine-Y and frangufoline). 

In problems of structure elucidation an NMR spectrum may provide useful, even vital data, but it is seldon the sole piece of information available. A knowledge of the source of the compound or its method of synthesis is frequently the single most important fact. In addition, the interpretation of the NMR spectrum is carried out with concurrent knowledge of other physical properties, such as elemental analysis from combustion or mass spectral studies, the molecular weight, and the presence or absence of structural features, as indicated by infrared or ultraviolet spectra or by chemical tests. Obviously, the procedure used for analyzing the NMR spectrum is highly dependent on such ancillary knowledge. 

Because the mass of the molecular ion equals the molecular weight of a compound, a mass spectrum can be used to distinguish between compounds that have similar physical properties but different molecular weights, as illustrated in Sample Problem 13.1. 

First, wc purify the compound and determine its physical properties melting point, boiling point, density, refractive index, and solubility in various solvents. In the laboratory today, we would measure various spectra of the compound (Chap. 13), in particular the infrared spectrum and the nmr spectrum indeed, because of the wealth of information to be gotten in this way, spectroscopic examination might well be the first order of business after purification. From the mass spectrum we would get a very accurate molecular weight. 

Now, a single mass spectrum amounts to dozens of physical properties, since it ow s the relative abundarTces of dozens of different fragments. If we measure the mass spectrum of an unknown compound and find it to be identical with the spectrum of a previously reported compound of known structure, then we can conclude that - almost beyond the shadow of a doubt the two compounds are identical. 

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