Abundance analysis accuracy

Lack of an abundance estimate for a trace element like boron has no effect on the accuracy of the abundance analysis for other elements but merely restricts astrophysical interpretations involving B. On the other hand, helium is an abundant elements with effects on the atmospheric structure and through this on the derived abundances of other elements. Although rarely stated explicitly, abundance analyses of cool stars are dependent on an assumption about the He/H ratio the assumption enters both into the model atmosphere and synthetic spectrum calculations. For normal stars, ignorance about the He/H ratio is mitigated by the fact that the He/H ratio is surely constrained within tight limits (Y = 0.24 to 0.26, see above). 

When elements that are present in the sample at high concentration have even minor isotopes that overlap another element present at low concentration, an isobaric interference exists that can be significant. However, when the converse is true, the interference will probably be insignificant. The most abundant isotope for a given element is always selected to provide the most sensitive analysis and, hence, the best detection Hmits. If other elements are present in the sample at measurable concentrations that have isotopes that overlap with this m/z value, they can have a detrimental effect on analysis accuracy. For example, the most abundant isotope of zinc is Zn, which would be the obvious selection for ultra-low-concentration level determinations. But a substantial quantity of nickel is present in the sample, the isotope Ni, even though it is only 0.92% abundant, will interfere with the trace analysis of zinc. This situation must be evaluated on a case-by-case basis to determine if an operational isobaric interference is present that will affect accuracy. 

The use of CIEF in combination with FTICR has been demonstrated in an analysis of the E. coli proteome (Jensen et al., 1999). For these experiments, E. coli was grown in a medium depleted of rare isotopes in order to increase the mass measurement accuracy. The high abundance isotopes are present at approximately 98.89% 12C, 99.63% 14N and 99.985% H. For peptides, the presence of rare isotopes does not significantly change the spectra but with undigested proteins, mass accuracy can be limited by the broadened distribution of ions of any given protein due to the incorporation... 

Approximately 90% to 95% of whole-cell MALDI MS profiles are representative of the ribosomal proteins abundant in rapid growth whole cells. Although the identification of proteins, from whole bacterial cells, by MALDI-TOF MS analysis is ambiguous, at best, due to the low-mass accuracy and resolving power, several researchers realized that many of the observed... 

Isotopic analysis of amino acids containing natural abundance levels of 15N was performed by derivatization, GC separation, on-line combustion and direct analysis of the combustion products by isotope-ratio MS. The N2 gas showed RSD better than 0.1%c for samples larger than 400 pmol and better than 0.5%o for samples larger than 25 pmol. After on-column injection of 2 nmol of each amino acid and delivery of 20% of the combustion products to the mass spectrometer, accuracy was 0.04%e and RSD 0.23%o19. 

A second method of determining major and minor element abundances in meteorites is X-ray fluorescence analysis. In this method, a sample of meteorite is ground into a fine powder and irradiated by a monochromatic X-ray beam. The atoms in the sample absorb some of the X-rays, causing electrons to be ejected. The ejection of an electron causes other electrons to drop to lower energy levels to fill the vacancy, and characteristic X-rays are emitted. The X-ray energies are unique to each element and permit the chemical composition to be determined. X-ray fluorescence was used to determine the compositions of meteorites in the late 1950s and 1960s. If done correctly the compositions have the same accuracy as wet chemical analyses, but some of the measurements in the literature have problems with alkali elements. 

Magnetic sector ion microprobes are becoming increasingly important in isotopic analysis of extratenestrial materials as spot size decreases and the precision and accuracy of the measurements improve. The first of the commercially available ion microprobes used in cosmochemistry were the Cameca ims 3f-7f series machines, which initially became available in the mid-1980s. These multipurpose instruments are able to measure isotopic compositions of most elements of interest in cosmochemistry and can also be used to measure trace element abundances. Their main drawback is that the relatively small mass spectrometer can only be operated at mass-resolving powers below about 9000, and at this mass resolving power, the transmission of the mass spectrometer is very low. 

Although the high sensitivity of activation analysis is perhaps its most striking advantage, there are a number of other favorable aspects as well. Activation analysis is basically a multielemental technique. Many elements in the sample will become radioactive during the irradiation and if each of these elements can be isolated chemically or instrumentally, their abundances may be determined simultaneously. Activation analysis can be a nondestructive method of analysis. Numerous tests have shown that with careful experimental manipulation, activation analysis is an accurate ( 1% accuracy) and precise ( 5% precision) method of measuring elemental concentrations. 

From an analysis such as described here, the relative abundances of interstellar molecules N(molecule)/N(H2) in various sources can be determined. Table 2 shows a representative list of abundances in two sources, TMC1 and Orion (OMC1). The relative abundance of a species with respect to H2 is more uncertain that the absolute column density because it is normally obtained by dividing the absolute column density by the 13CO value and then estimating the ratio of the 13CO to H2 column densities (Irvine et al. 1985). Thus, one should not assume better than one order of magnitude accuracy in the numbers shown in Table 2. Still, several trends can be discovered in this table ... 

Quantitative Analysis. When the relative accuracy of thin-film EDS analyses in an AEM is compared with other analytical techniques, it is instructive to consider the precision in terms of both spatial resolution and elemental abundance. Few techniques offer the same combination of high spatial resolution and elemental analysis for solids - analyses with 5% relative accuracy from areas <20nm in diameter. 

The aim of this study was to further explore the potential and limitations of using stable iron isotopes as tracers and EI-MS in absorption studies. Procedures were developed for preparing iron acetylacetonate from both blood and fecal samples for mass spectrometric analysis. The precision and accuracy of ion abundance measurements were evaluated. In vivo use of stable iron isotope tracers was tested with a human study in which 54pe and 57pe were given orally and absorption was estimated with the fecal monitoring and hemoglobin incorporation methods. 

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