Deducing Elemental Compositions

In summary, if n A + 2 elements are present, the (A + 1) peak [and possibly other (A + 2n + 1) peaks] will contain isotopic contributions from the (A — 1) peak [and possibly other (A — 2n — 1) peaks]. In calculating the number of carbon atoms by using [(A + 1)]/[A], one should correct for such isotopic interferences. Alternatively, use [(A + 2n -I- 1)]/[A - - 2n)] corresponding to the peaks of highest mass, since the (A — 1) peak cannot make an isotopic contribution to these. 

Erroneously low values for an element. Occasionally a significant proportion of the peak A intensity will be due to the isotopic contribution of an A + 1 element in the (A — 1) peak, or an A -f 2 element in the (A — 2) peak. For example, the spectrum of butylbenzene shows the group of peaks  

C7Hg = 47.3%, and their ratio is 7.7%, as expected for seven carbons. 

Above we derived peak intensities in the methyl bromide spectrum from knowledge of abundances of particular ion compositions however, for an unknown spectrum we face the opposite problem. The suggestions that follow should increase your efficiency in arriving at all reasonable solutions to such a problem. 

Highest intensity peaks. Within these limits, the accuracy of elemental composition calculations obviously should improve with increasing peak intensity. 

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Changing the magnetic or electric fields that effect separation causes ions of different m/z values to reach the collector. On-line computer systems produce sets of mass and abundance values directly by measuring the field and ion current corresponding to each peak. These systems can record complete mass spectra several times per second, which is especially valuable for GC/MS (see below). However, the scan rate affects the accuracy of ion-abundance measurement, which is important for deducing elemental compositions from isotope ratios check the performance of your instrument with known samples to be sure you have not sacrificed this accuracy unnecessarily. 

In attempting to deduce elemental compositions in this group of peaks, start with the most intense peak. The presence of one or more A -I- 2 elements should be immediately obvious. Although the [m/z 132]/[m/z 130] value of 0.98 strongly suggests Brj, this is ruled out by the higher mass ratios. Note that the ratios of the series [129]/[131]/[133]/[135] are very similar to those of the series [130]/[132]/[134]/[136], and that these correspond closely with the isotopic cluster for CI3 in Tables 2.3 and A.2. The relative abundances at m/z 131,... 

Characterisation of the antibiotic monordene (also referred to as radicicol) with the elemental composition Cig/Zz/OgCl isolated from Monosporium bonorden gave the macrolide structure 1. The relative configuration of the H atoms on the two conjugated double bonds (6,7-cis, %,9-trans-) could be deduced from the 60 MHz H NMR spectrum The relative configuration of the C atoms 2-5, which encompass the oxirane ring as a partial structure, has yet to be established. 

The degradation of 2,6-xylenol (2,6-dimethylphenol) by bacteria produces a metabolite with elemental composition C8///0O2 as determined by high-resolution mass spectrometry Which carbon skeleton and which relative configuration are deducible from the NMR experiments 44, all obtained from one 1.5 mg sample ... 

The strong emphasis placed on concentration dependences in Chapters 2-5 was there for a reason. The algebraic form of the rate law reveals, in a straightforward manner, the elemental composition of the transition state—the atoms present and the net ionic charge, if any. This information is available for each of the elementary reactions that can become a rate-controlling step under the conditions studied. From the form of the rate law, one can deduce the number of steps in the scheme. In most cases, further information can be obtained about the pattern in which parallel and sequential steps are arranged. 

Interpretation of molecular spectra involves four basic steps. First, major skeletal and functional group components of the molecule are identified, either from assumptions about the compound origin or from features of the spectra. Second, non-localized molecular properties such as the molecular weight, elemental composition, and chromatographic behavior are considered. These global constraints can be used to eliminate unlikely functional groups, deduce the presence of groups and skeletal units which have no distinctive features in the spectra, and detect multiple occurrences of... 

Other approaches have been taken for on-line analysis of individual aerosol particles as well. Laser spark spectroscopy (33) vaporizes individual particles in the breakdown plasma created by a pulsed laser. Atomic emission spectra can then be used to deduce the elemental composition of the particle that was vaporized. The timing of the laser pulse is critical because the particle must be caught in the focal volume of the pulsed laser, so a second laser is used to detect the particle and trigger the pulsed laser. To date the technique has been applied to large particles, that is, coal particles on the order of 60 to 70 xm in diameter in combustion studies. The use of inductively coupled plasma would eliminate the complex triggering and might allow on-line analysis of smaller particles spectroscopically. 

In contrast to a chemical property which can be measured, a molecular descriptor is computed from the molecular structure. Contained in the structural information are the atoms making up the molecule and their spatial arrangement. From the coordinates of the atoms, the geometric attributes (i.e., the size and shape of the molecule) can be deduced. A straightforward example is the molecular mass, which is computed by adding up the masses of the individual atoms making up the molecule and indicated in the elemental composition. The result is accurate since the atomic masses are independent of the chemical bonds with which they are involved. However, the molecular mass reflects few of the geometrical and chemical attributes of a compound and M is therefore a poor predictor for most properties. 

This information can be used to deduce die elemental composition of a compound. For example, the oxidation of 1,2-diazacyclohexane was carried out in cyclohexane. A product was isolated and was found to have a molecular ion of m jc = 84. At this point the experimenter realized that both the expected product and the reaction solvent have a molecular weight of 84. 

From the elemental composition data it could be deduced that the nonvolatiles consist of low hydrogen containing species with a fair amount of oxygen. Since most of the experimental work was done on a small sample under atmospheric conditions, the higher sulfur content of the fractions could be due to the incorporation of S02, perhaps in its oxidized form as sulfates. In a large scale process such incorporation could be minimized. 

The methods listed yield the concentrations either of water-soluble ions measured in terms of certain oxidized states or of elements. For example, materials appearing as sulfate and nitrate may include lower oxidation states, but the methods basically do not distinguish among them. The metal elements found are either oxides or soluble salts. The actual composition of the material is indeterminant, but workers have deduced the composition suspected to be present by a material balance, combined with knowledge of the origins of the particulate material. 

Sentences are often found meaning explicitly or implicitly 5 ppm accuracy is sufficient to deduce the elemental composition . This is absolutely not true and is at the origin of many exaggerations. This value comes from a rule of the American Chemical Society that states For most new compounds, HRMS data accurate within 5 ppm or combustion elemental analysis accurate within 0.4% should be reported to support the molecular formula assignment [11]. Thus, this rule does not tell that 5ppm accuracy can be used to deduce an elemental composition, but that it can be used in support of a proposed formula, but not as a proof of that formula. 

One can deduce much information concerning the elemental composition from the masses of the neutrals or of the low-mass fragments. 

That the core is not solely an Fe-Ni alloy, but contains —5-10% of a light mass element alloy, is about the extent of the compositional guidance that comes from geophysics. Less direct information on the makeup of the Earth is provided by studies of meteorites and samples of the silicate Earth. It is from these investigations that we develop models for the composition of the bulk Earth and primitive mantle (or the silicate Earth) and from these deduce the composition of the core. 

Almost all the characterizations performed by us until now are ensemble characterizations (i.e. probing many nanostructures simultaneously). HRTEM and HRS EM do probe the structure (and elemental composition) of individual nanostructures, but they do not correlate this structure with a specific property. STM and STS measurements are real single-object measurements that reveal the size, shape, and surface atomic structure, as well as the electronic density of states (deduced the I-V characteristics). The STM/STS measurements offer a way to correlate the electronic properties of SiNWs with the nanostructure size. 

The most serious problem in the study of humic substances is the lack of reproducibility of analytical results. One would expect soil humates to vary with soil type, aquatic humates to vary with water sources, and coal humates to vary with coal rank. But even within one well-defined source, the elemental composition will vary between samples, depending on extraction and fractionation procedures. There are cases in which the same authors have used the same source and the same extraction procedure and have obtained significantly different elemental analyses. Before any meaningful structural conclusions can be deduced from elemental analysis, a rational definition of humic substances will have to be established (MacCarthy, 1976 Malcolm and MacCarthy, 1979). 

When molecules are synthesized by multi-step sequences or when new compounds are extracted from natural sources, their structure and purity must be elucidated. For this, a quantitative elemental analysis must be performed. This particular type of analysis allows us to find the percent elemental composition, in the pure state, of the molecule under study. The measurement of a single element, indeed two (C and H are the most frequent) will verify the accuracy of the molecular formula proposed for a molecule not as yet fully defined but for which a structure has been deduced from spectral studies. Elsewhere, purify of a compound for which the composition and the molecular weight are known, can be determined by comparison of the experimental results obtained from a sample with the theoretical ones (Figure 18.1). 

Armed with the empirical knowledge that each element in the periodic table has a characteristic spectmm, and that heating materials to a sufficiently high temperature dismpts all interatomic interactions, Bunsen and Kirchoff invented the spectroscope, an instrument that atomizes substances in a flame and then records their emission spectmm. Using this instmment, the elemental composition of several compounds and minerals were deduced by measuring the wavelength of radiation that they emit. In addition, this new science led to the discovery of elements, notably caesium and mbidium. 

Elemental compositions may also be deduced in certain cases by comparing a spectrum of an unknown substance with those of model compounds. As the examples at... 

One of the most important pieces of information required to elucidate the molecular structure of an unknown organic compound is its molecular mass, which provides a window within which the elemental composition and the final structure of the compound must fit. Therefore, the first essential step to identifying a compound is to measure its molecular mass by determining the m/z value of the molecular ion. Molecular mass measurements can be performed at either low or high resolution. A low-resolution measmement provides information about the nominal mass of the analyte, and its elemental composition can be also determined for low-molecular-weight compounds from the isotopic pattern. From a high-resolution mass spectrum, the accurate molecular mass can be determined, from which it is also feasible to deduce the elemental composition. Chemists who work with synthetic compounds and natural products rely heavily on the exact mass measurement data for structmal assignment. This value is acceptable in lieu of the combustion or other elemental analysis data. An acceptable value of the measured mass should be within 5 ppm of the accmate mass [1]. As shown below, the mass measurement error is reported either in parts per million (ppm) or in millimass units (mmu). 

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