Molecular formula, from high-resolution

Molecular formulae from high-resolution mass measurements... 

This hybrid is used in one form to measure highly accurate m/z values to obtain excellent elemental compositions of ions and therefore molecular formulae from molecular ions in the other form, it is used to obtain MS/MS data at high resolution. 

With the molecular weight available from the M+ peak with reasonable certainty, the next step is to determine the molecular formula. If the resolution of the instrument is sufficiently high, quite exact masses can be measured, which means that ions with mje values differing by one part in 50,000 can be distinguished. At this resolution it is possible to determine the elemental composition of each ion from its exact m/e value (see Exercise 9-44). 

If a high-resolution mass spectrum is not available, it is still possible to obtain information about the molecular formula from the low-resolution spectrum. In the mass spectrum of benzene, shown in Figure 15.5, the molecular ion appears at mlz 78. In addition, there is a smaller peak at mlz 79, called the M + 1 peak, that is 6.8% of the intensity of the Mt peak. The M + 2 peak, at mlz 80, is 0.2% of the Mt peak. The M + 1 and M + 2 peaks are caused by the presence of isotopic atoms of heavier mass in some of the molecules. Their intensities relative to the Mt peak can be used to deduce information about the formula. Let s look at how these peaks arise in more detail. 

In addition we include derivatives that are naturally occurring or have been synthesized. Finally, plant sources (genera, species, family, plant part) for the compound are given. The molecular formula and molecular weight of the alkaloid are given alongside the structure. When an accurate mass from high resolution MS exists it is noted in parentheses. The found, not the calculated, value is presented. 

High-resolution mass spectrometry involves the use of a detector that can measure tw/z values to four decimal places. This technique allows for the determination of the molecular formula of an unknown compound. In order to analyze the data obtained from high-resolution mass spectrometry, we must first review some background information. Specifically, we must discuss why atomic masses are not whole numbers (despite the fact that we have treated them as such until now). 

The mass spectra of alkynes, like those of alkenes, frequently show prominent molecular ions. Thus high-resolution measurements can reveal the molecular formula and therefore the presence of two degrees of unsaturation derived from the presence of the triple bond. 

The preceding section described a method of determining molecular formulas using data from high-resolution mass spectrometers. Another method of determining molecular formulas is to examine the relative intensities of the peaks due to the molecular ion and related ions that bear one or more... 

In general terms, the main function of the magnetic/electric-sector section of the hybrid is to be able to resolve m/z values differing by only a few parts per million. Such accuracy allows highly accurate measurement of m/z values and therefore affords excellent elemental compositions of ions if these are molecular ions, the resulting compositions are in fact molecular formulae, which is the usual MS mode. Apart from accurate mass measurement, full mass spectra can also be obtained. The high-resolution separation of ions also allows ions having only small mass differences to be carefully selected for MS/MS studies. 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

Camphor, a saturated monoketone from the Asian camphor tree, is used among other things as a moth repellent and as a constituent of embalming fluid. If camphor has M+ = 152.1201 by high-resolution mass spectrometry, what is its molecular formula How many rings does camphor have ... 

B Nicotine is a diamino compound isolated from dried tobacco leaves. Nicotine has two rings and M + = 162.1157 by high-resolution mass spectrometry. Give a molecular formula for nicotine, and calculate the number of double bonds. 

The following is a procedure recommended for elucidating the structure of complex organic molecules. It uses a combination of different NMR and other spectroscopic techniques. It assumes that the molecular formula has been deduced from elemental analysis or high-resolution mass spectrometry. Computer-based automated or interactive versions of similar approaches have also been devised for structural elucidation of complex natural products, such as SESAMI (systematic elucidation of structures by using artificial machine intelligence), but there is no substitute for the hard work, experience, and intuition of the chemist. 

Currently, LC-MS is widely used for the analysis of polar compounds, such as medicinal metabolites and bioactive peptides, since the interface has been improved and several new ionization methods have been developed. The sensitivity and reproducibility are sufficient for a daily quantitative analysis. The usefulness of the LC-MS has been demonstrated for studies on Type II pheromones using a time-of-flight MS with electrospray ionization (ESI) [180]. Each epoxydiene derived from the (Z3,Z6,Z9)-triene shows three ion series of [M+NHJ+, [M+H]+, and [M-OH]+ with high resolution and good sensitivity, indicating its molecular formula. In addition to these, characteristic fragment... 

Peripentadenia is a monotypic genus of the family Elaeocarpaceae represented by Peripentadenia mearsii (C. T. White) L. S. Smith, a tree growing in rain forests of northern Queensland. Besides unidentified minor alkaloids, the species yields one major alkaloid, peripentadenine (49) (87). Its molecular formula, C22H34N203, was established by high-resolution mass spectroscopy the presence of phenol, ketone, secondary amide, and tertiary amine functionalities was deduced from examination of spectroscopic data. 

Pseudovinblastinediol (21) (formerly named pseudovincaleukoblas-tinediol) has been isolated from C. rose us as a minor constituent (44). The high-resolution mass spectrum of 21 established a molecular formula of C44H5ftN40g. The characteristic vindoline fragments of miz 469 and 282 were shifted 58 mass units to miz 411 and 224, respectively, indicating the... 

A unique molecular formula (or fragment formula) can often be derived from a sufficiently accurate mass measurement alone (high-resolution mass spectrometry). This is possible because the nuclide masses are not integers (see Table 2.2). For example, we can distinguish at a unit mass of 28 among CO, N2, CH2N, and C2H4. 

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