Structure mass spectrometry

Kiselar, J.G., Chance, M.R. (2010) Future Directions of Structural Mass Spectrometry Using Hydroxyl Radical Footprinting. J. Mass. Spectrom. 45 1373-1382. 

One has seen that the number of individual components in a hydrocarbon cut increases rapidly with its boiling point. It is thereby out of the question to resolve such a cut to its individual components instead of the analysis by family given by mass spectrometry, one may prefer a distribution by type of carbon. This can be done by infrared absorption spectrometry which also has other applications in the petroleum industry. Another distribution is possible which describes a cut in tei ns of a set of structural patterns using nuclear magnetic resonance of hydrogen (or carbon) this can thus describe the average molecule in the fraction under study. 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

In contrast to IR and NMR spectroscopy, the principle of mass spectrometry (MS) is based on decomposition and reactions of organic molecules on theii way from the ion source to the detector. Consequently, structure-MS correlation is basically a matter of relating reactions to the signals in a mass spectrum. The chemical structure information contained in mass spectra is difficult to extract because of the complicated relationships between MS data and chemical structures. The aim of spectra evaluation can be either the identification of a compound or the interpretation of spectral data in order to elucidate the chemical structure [78-80],... 

According to early theoretical calculations Kloptnan and I carried out in 1971, the parent molecular ions of alkanes, such as CH4, observed in mass spectrometry, also prefer a planar hypercarbon structure. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

As we have just seen interpreting the fragmentation patterns m a mass spectrum m terms of a molecule s structural units makes mass spectrometry much more than just a tool for determining molecular weights Nevertheless even the molecular weight can provide more information than you might think... 

Section 13 22 Mass spectrometry exploits the information obtained when a molecule is ionized by electron impact and then dissociates to smaller fragments Pos itive ions are separated and detected according to their mass to charge (m/z) ratio By examining the fragments and by knowing how classes of molecules dissociate on electron impact one can deduce the structure of a compound Mass spectrometry is quite sensitive as little as 10 g of compound is sufficient for analysis... 

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS). 

The techniques described thus far cope well with samples up to 10 kDa. Molecular mass determinations on peptides can be used to identify modifications occurring after the protein has been assembled according to its DNA code (post-translation), to map a protein structure, or simply to confirm the composition of a peptide. For samples with molecular masses in excess of 10 kDa, the sensitivity of FAB is quite low, and such analyses are far from routine. Two new developments have extended the scope of mass spectrometry even further to the analysis of peptides and proteins of high mass. 

Tandem mass spectrometry (MS/MS) produces precise structural or sequence information by selective and specific induced fragmentation on samples up to several thousand Daltons. For samples of greater molecular mass than this, an enzyme digest will usually produce several peptides of molecular mass suitable for sequencing by mass spectrometry. The smaller sequences can be used to deduce the sequence of the whole protein. 

In the structure sections, labelled compounds have often been used to solve a spectroscopic problem involved in microwave (Section 4.04.1.3.2), nitrogen NMR (Section 4.04.1.3.5), IR (Section 4.04.1.3.7(i)) or mass spectrometry (Section 4.04.1.3.8). The synthesis usually involves non-radioactive compounds ( H, N) by classical methods that must be repeated several times in order to obtain good yields. 

Benzo[b]thiophene-2,3-quinone, 5-chloro-oxidation, 4, 824 Benzothiophenes, 4, 863-934 biological activity, 4, 911-913 intramolecular acylation, 4, 761 mass spectrometry, 4, 739 metabolism, 1, 242 phosphorescence, 4, 16 reactivity, 4, 741-861 spectroscopy, 4, 713-740 structure, 4, 713-740 substituents reactivity, 4, 796-839... 

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