Enriched IRMs

The principal advantage of conventional GC-IRMS in this context is its ease of use and lower sample size requirements, while the IRMS approach provides superior accuracy and precision, particularly at lower enrichments. IRMS data are usually expressed using the 6yX notation, given as... 

With the use of IRMs, these false assumptions can be avoided and traceable isotope amount ratios can be generated. At present, however, only for a limited number of elements is an IRM available, as can be seen from Tables 6.1 and 6.2. Natural-like IRMs (Table 6.1) can be used to correct for mass discrimination, whereas enriched IRMs (Table 6.2) are mainly used in tracer studies and isotope dilution experiments. The enriched IRMs offer sufficient quality, but the number of elements covered is too low. This has been discussed at several meetings with the summarized outcome that several enriched isotopes have been exhausted... 

Table 6.2 Enriched IRMs more details can be found on the websites of NIST [51], IRMM [52], and ERM [53],...

The authors demonstrated the importance that correct use of the MDGC-IRMS System is essential for the achievement of precise and accurate measurements. Table 10.4 reports the GC-IRMS measurements of some standard reference materials, obtained with different cut conditions. As can be seen from this table, premature cuts result in 8 C values which are significantly higher than the true values, while delayed cuts give lower 8 C values. This fact indicates that the beginning of the peak is enriched in C, while the end is depleted. 

Ordinarily electrical amplification is used to compensate for differences in isotope abundances in the gas being measured. Thus, for carbon dioxide all three Faraday collectors are used with relative signal amplification at m/z = 44, 45, and 46 of 1 91 500 (since the normal abundance ratios 12C/13C 91, and 160/180 500). The amplified signals from all three detectors are thus comparable in intensity. Because of this feature, however, IRMS should only be used on gases with isotope composition close to natural abundance. Enriched material should not be used without careful recalibration since there is no guarantee of a linear response of electric signal to ion current for widely different isotope ratios. 

The chemistries of Ru, Os, Rh, and Ir in seawater are poorly understood. Available evidence suggests that the probable oxidation state of Ru is + IV OSO4 has a large field of stability on oxidation potential vs pH diagrams and can thereby be considered as an important species in seawater. Rh and Ir are expected to exist in lower oxidation states than Ru and Os. Available evidence (Baes and Mesmer, 1976) indicates that the most important oxidation state for Rh and Ir is + III. Rhm and Irm form strong chloride complexes and should, as well, hydrolyse extensively in solution at pH 7.4-8.2. This speciation assessment is consistent with the relative enrichment of Os in seawater compared to Ir. OSO4 should be substantially less reactive towards particles than is the case for Ir(Cl) - and Ir(OH) - species. 

Final points to consider include the degree of replication and treatment of the initial, or time zero, concentrations and enrichment of both the product and source pools. The ability to replicate analyses has been greatly enhanced by the advent of automated CF-IRMS so that there is no longer a reason not to measure duplicate samples as a minimum. However, given the high analytical precision of modem mass spectrometers, the replication should be from the field by incubating replicate bottles... 

Several reviews of CSIA principles, techniques and important application areas have been published in the past years [70-72]. A current problem of CSIA is the rather poor sensitivity of the IRMS that frequently limits potential applications to highly contaminated samples [70]. Improvements of this situation are directly related to the appropriate use of enrichment techniques as discussed in detail above for MTBE and related compounds. 

Fig. 2 Schematic overview of a GC/C/IRMS for determination of values. Following headspace injection or enrichment with SPME or purge Trap the analytes were separated hy GC. After separation the target analytes were completely combusted to CO2 and H2O hy using a PT/NiO/CuO catalyst containing combustion oven. Water is removed by a Nafion membrane to prevent formation of C02H (m/z 45) during ionization. Following combustion the CO2 is ionized in the ion source of the mass spectrometer. After ionization the formed isotopologues C02 (m/z 44), C02 (m/z 45) and (m/z...

As was the case for Hinkel Reservoir, the Ti-normalized data help us distinguish weathering from coal-combustion input. When compared to the youngest reservoir sediments, patterns in the Ti-normalized data from MTR are only slightly different from those previously discussed for Hinkel Reservoir (Fig. 12). Zn and S resemble the Hinkel core data in that they are enriched in reservoir sediments relative to sods. Similarly, Pb and magnetite (IRM) are relatively enriched in soil (although there is scatter in the IRM data). However, in contrast to the Hinkel data, Ti-normalized Fe and As in MTR soil samples tend to be depleted compared to reservoir sediments. 

The Ti-normalized data emphasize the enrichment in Cu at 10.5 cm (1987), a very striking feature of the MTR data. Associated with the copper peak are corresponding major peaks in S and IRM and lesser ones in Fe, Zn, and As (Zn only shows up as a peak in Ti-normalized plots). We believe these maxima at 10.5 cm are unrelated to power plant emissions. Copper sulfate is commonly added to reservoirs to retard algal growth. The manager of the water department in the town of Spencer, Lowell Hardmann, reported to us that copper sulfate was indeed added to the reservoir, but he did not have information on the exact date. Since Cu is not an element associated with power plant emissions and we know of no other significant Cu source nearby, the most likely explanation is that the copper sulfate additions were made in about 1987 coincident with the peak. The dramatic increase in Cu/Ti for reservoir sediments at 10.5 cm compared to nearby soils (Fig. 12) and rocks (Table 7b) is consistent with this conclusion. 

Mass spectrometric approaches are also very useful for the measurement of stable isotopes in drug metabolism studies. The application of MS to the quantitative measurement of stable isotope has been limited due to the high cost and sophistication of the instruments necessary for stable isotope enrichment studies. Nonetheless, recent improvements in instrument design and performance, as well as computer software for instrument control, data acquisition, and analysis, have increased the sensitivity and reliability of stable isotopic enrichment studies. These new MS instruments, including continuous-flow isotope ratio mass spectrometry (CF-IRMS) and HPLC-chemical reaction interface mass spectrometry (HPLC-CRIMS) are increasingly less expensive, easier to operate, and accessible for mass balance/ metabolite identification studies with stable isotopes. 

Although spectroscopic methods are far less precise than IRMS for isotope ratio measurements, they can have advantages in terms of costs and ease of operation. These techniques also remove the laborious chemical preparation required for conventional IRMS. They are most commonly applied to the analysis of samples that are artificially enriched in the heavier isotopes of nitrogen, carbon, and oxygen. 

Stable isotope ratio analysis (SIRA, GC-IRMS [Gas Chromatography-Isotope Ratio Mass Spectrometry], and Site Specific Isotope Fractionation — Nuclear Magnetic Resonance [SNIF-NMR]) have proven useful in many adulteration situations. In nature, and exist at relative proportions of 1.11 98.89 [27]. The photosynthetic process selectively enriches the plant in dependent upon the type of photosynthetic process used by the plant. Plants using the Hatch-Slack pathway (e.g., com, sugar cane, millet, and lemon grass) give 8 C values the closest to the standard, i.e., 5 C values of ca. -10. The 5 C value is calculated as ... 

The variation in isotope abundance measurement in GC-C-IRMS and GC-P-IRMS is much less than in GC-MS. Therefore, molar enrichments of 0.01% can still be quantified. For this reason, IRMS techniques can replace MS techniques at low enrichment levels or can be added to the MS technique in order to extend an isotopic decay curve to enable multicompartment analysis. However, the latter application may be unnecessary since linearity of GC-C-IRMS is usually excellent. Linearity has been shown from 0.01 to 25% molar enrichment for valine... 

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