Determination of the elemental composition

The determination of the elemental composition of a petroleum cut is of prime importance because it provides a quick means of finding out the quality of a given cut or determining the efficiency of a refining process. In fact, the quality of a cut generally increases with the H/C ratio and in all cases, with a decrease in hetero-element (nitrogen, sulfur, and metals) content. 

AES is a useful element-specific technique for quantitative determination of the elemental composition of a surface. Although some chemical information is available in principle, the technique is used largely for elemental analysis. Electron beam damage can decompose organic adsorbates and cause damage, particularly on insulating surfaces. In some cases, the beam can reduce metal oxides. 

Applications Applications of IC extend beyond the measurement of anions and cations that initially contributed to the success of the technique. Polar organic and inorganic species can also be measured. Ion chromatography can profitably be used for the analysis of ionic degradation products. For example, IC permits determination of the elemental composition of additives in polymers from the products of pyrolysis or oxidative thermal degradation. The lower detection limit for additives in polymers are 0.1% by PyGC... 

Determination of the Elemental Composition of Ions on the Basis of Isotopic Peaks... 

Determination of the elemental composition should be started with the M + 2 peak. Chlorine, bromine, sulfur, and silicone are easily detected due to characteristic signal multiplicity for each of these elements. There is a simple mle to check the presence of the main A + 2 elements. If the intensity of the M + 2 peak constitutes less than 3% of the intensity of the M peak, the compound does not contain chlorine, bromine, sulfur, or silicon atoms. This rule is valid for the fragment ions as well, while its applicability is confirmed by the data summarized in Table 5.4. 

Mass spectrometry has gradually become an irreplaceable method for the investigation of compound structures. It is also applied to the determination of the elemental composition of inorganic samples (ICP/MS). When combined with chromatography for the separation of compounds, MS can be used to study mixtures of molecular species. The on-line coupling of these techniques, GC/MS, constitutes one of the best methods of analysis for mixtures and samples containing trace amounts of analyte. 

At one sampling point near the Technical University of Kosice (Slovakia) (see Fig. 7-13) sedimented airborne particulate matter was sampled by the above described BER-GERHOFF method (see also Section 7.2.1.1) over a period of two and a half years. Tab. 7-5 shows the analytical methods applied for the determination of the elemental composition of the dust samples. 

Tab. 7-5. Analytical methods applied for the determination of the elemental composition of the dust samples...

Fourier transform ion-cyclotron (FT-ICR-MS) provides the highest mass resolution and accuracy, and enables the determination of the elemental compositions of metabolites, which facilitates annotation procedures for unknown compounds (95). Direct infusion analysis of plant extract without a previous separation and/or derivatization can be achieved however, its use is very restricted due to the equipment cost, the difficulties in hardware handling, and the extremely large amount of data generated. Takahashi et al. applied this technique to elucidate the effects of the overexpression of the YK1 gene in stress-tolerant GM rice (96). More than 850 metabolites could be determined, and the metabolomics fingerprint in callus, leaf, and panicle was significantly different from one another. 

In addition to a library computer search for matching spectra, the usual procedure for interpretation of a mass spectrum involves (1) identification of the molecular ion peak and, if possible, determination of the elemental composition of the molecule based on the observed mass number (m/z) for the molecular ion, (2) establishment of the fragmentation patterns of the molecular ion, and finally (3) reduction of the number of possible structures for the compound, based on the interrelationships of the observed fragmentation patterns. The absence of meta-stable ions mass spectra obtained by GC-MS analysis makes (2) very difficult to achieve. 

Microspectrometry is an indispensable technique in criminalistic analyses, being a combination of optical microscopy and spectrometry. Microscopy creates, records and interprets magnified images, whereas spectrometry uses emission, absorption and reflection of radiant energy by matter to determine its structure, properties and composition. On the basis of the type of energy applied, microspectrometry can be divided into IR, visual and ultraviolet (UV-vis), and Raman microspectrometry. This group also includes X-ray microspectrometry, in which an electron microscope takes the place of an optical microscope. Infrared and Raman microspectrometry enable determination and comparison of the chemical composition of studied samples UV-vis microspectrometry serves to compare the colour of samples in an objective way that is independent of the observer and X-ray microspectrometry allows determination of the elemental composition. 

Reitseme and Allphin [59] were the first workers to develop a method for determining the absolute nitrogen content in a sample and a method for determining the nitrogen-to-carbon ratio. Their method can be used to determine the nitrogen content both in involatile compounds and in separated fractions. They mainly considered the determination of the elemental composition of compounds subjected to GC separation. 

An important method for the determination of the elemental composition of surfaces is based on the electron mUroprobe. In this technique. X-ray emissioti from the elements on the surface of a sample is stimulated by a narrowly focused beam of electrons. The resulting X-ray emission is detected and analysed with either a wavelength or an energy-dispersive spectrometer. This method is discussed in detail in Section 21F-1. 

Fig. 3.21. (a) Spectrum of a stored waveform inverse Fourier-transform (SWIFT) isolated ion at nominal mass 453 m/z. This molecule is present in dissolved organic matter at the Experimental Nutrient Removal wetland outflow, (b) Spectrum of this ion and resulting products after fragmentation by sustained off-resonance irradiation collision-induced dissociation. Formulae in parentheses represent compositions of lost fragments. From these fragments, unambiguous determination of the elemental composition of the precursor ion at 453 m/z is possible. 

Holtta P, Rosenberg RJ (1986) Determination of the Elemental Composition of Copper and Bronze Objects by Neutron Activation Analysis, in Abstracts of papers accepted at the 7th Internal. Conf, Modem Trends in Activation Analysis, p. 781, Copenhagen, RISO Laboratory... 

After the molecular weight has been deduced, the elemental composition of the molecular ion can be determined. The isotope distribution of peaks of the molecular ion may provide a thorough preliminary evaluation of the quantity and type of metal atoms. This analysis is based on the fact that most metal atoms have specific isotope distributions. The next step towards the determination of the elemental composition is to measure accurate mass(es) of the molecular ion(s). These measurements require a mass analyzer with appropriate mass resolution. Sector instruments, TOF mass spectrometers, and ICR spectrometers are the most suitable for accurate mass measurement. 

Heard, P.J., Feeney, K.A.,Allen, G.C., Shewry, PR. (2001) Determination of the elemental composition of mature wheat grain using a modified secondary ion mass spectrometer (SIMS). Plant ., 30(2), 237-245. 

Analytical electron microscopy (AEM) permits the determination of the elemental composition of a solid catalyst at the microscopic level by energy-dispersive detection of the electron-induced X-ray emission (EDX). Energy dispersive spectroscopy (EDS) is sensitive for elements with atomic numbers Z > 11. For lighter elements (Z < 11), electron energy loss spectroscopy (EELS) is applied. An example is shown in Figure 7 (bottom), which exhibits the elemental composition by EDX of two individual Pt/Rh particles on a carbon film. This analysis clearly demonstrates the heterogeneous composition of the alloy particles. 

Depth Profiling of Surfaces. Depth profiling involves the determination of the elemental composition of a surface as it is being etched away (sputtered) by a beam of argon ions. Either XPS or Auger spectroscopy can be used for elemental detection, although the lat-... 

The on-line combination of GC-MS and AED is not (yet) commercially available [25-27]. In most cases, a parallel configuration is applied. The AED allows selective detection of any element except helium, with detection limits in the pg/s range for most elements, which is almost independent of the structure of the analyte. As such, AED allows the determination of the elemental composition of an unknown compound. The power of GC-AED in combination with GC-MS is readily demonstrated in Figure 6. By combined use of GC-MS and GC-AED, the presence of elements such as sulfur, phosphorus, chlorine, and... 

Determination of the elemental composition of these films can now be considered, as well as consideration about their chemistries. Such analyses inevitably require that the sample be placed in a vacuum and be exposed to electron or photon radiation. On the basis of the above microscopic experiments, it is to be expected that the oxide will undergo some degree of decomposition. If volatile components such as water are the only ones removed in this process, then one... 

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