Isotope ratio analysis standards

The reference principle for on-line methods has to be different as compared to classic dual-inlet isotope ratio analysis, as it is not possible to introduce sample and reference gas in exactly the same maimer into the mass spectrometer. Reference gas pulses have to be set at different points during one mn, but also laboratory standards with knovm 8-values have to be analysed periodically and exactly in the same manner and under the same conditions as the samples under investigation (identical treatment principle ]13]). A general problem of on-line HPLC and GC methods in isotope ratio analysis is the lack of international reference materials suitable to fulfil the above mentioned requirements. This implies the necessity to establish suitable laboratory standards by EA (elemental analysis) or classic measurements. Meanwhile, also efforts in supplying suitable international organic standards have been overtaken by the IAEA (e.g. caffeine, glutamic acid ]176, 177]). This programme (benzoic acid reference materials with different 8 0-values) will also support the on-line EA and GC 8 0-meas-urement by reductive pyrolysis (carbon reduction) methods (standardisation problems are compiled in ]178]). 

Most flavourings are complex mixtures of many compounds. As IRMS makes only sense with pure analytes, a strict purification of individual substances is indispensable. Therefore GC-IRMS has been further developed and optimised to multi-compound isotope ratio analysis by its coupling IRMS to capillary (c) and multidimensional (MD) gas chromatography (see 6.2.2.2.2). This methodology demands a strict intrinsic control and standardisation [340] apart from the international standards (see Table 6.3) also secondary standards like the polyethylene foil IAEA-CH7 or the NBS22 oil are available from the IAEA in Vieima. However, as these substances are also not suitable for the direct standardisation of data from a coupled GC system for flavour isotope analysis, certificated tertiary laboratory standards for hydrogen have been developed by parallel analysis of flavour compounds by TC/EA-IRMS and MDGC-P-IRMS [210]. 

Blood collected at each time point was allowed to remain on ice for 15 min prior to centrifugation at 1800g. Plasma was transferred to 5-ml cryogenic vials and stored at -8(PC until analyzed. Plasma lipids were extracted from duplicate 2.2-g plasma aliquots for isotope ratio analysis or from 0.25-g aliquots for HPLC quantification of retinol and /3C after addition of internal standard (retinyl acetate). Plasma aliquots were de-proteinized with 1 vol of ethanol and lipids extracted with 3 vol of hexane (Optima Grade, Fisher Scientific, Rochester, NY). 

In the best of instruments, isotope ratio errors may occur as a result of inefficient counting, from mass bias, and other causes. Correction standards at approximately the same concentrations as quality control (QC) samples and unknown samples are used to correct for these sources of error. A natural uranium spike is used because low-concentration endogenous uranium in the base urine and any small amounts of contamination from environmental sources are presumed to be natural, thus will not affect the true ratio in the event of environmental contamination. Correction with an unadulterated isotope ratio is assured. Considering the nature of the urine sample and the problems that must be overcome for uranium accurate isotope ratio analysis with acceptable sample throughput, two sample preparation... 

The best precision is obtained for isotope ratios near one, however, if the element to be determined is near the detection limit when the ratio of spike isotope to natural isotope should be between 3 and 10 so that noise contributes only to the uncertainty of natural isotojje measurement. Errors also become large when the isotope ratio in the spiked sample approaches the ratio of the isotopes in the spike (overspiking), or the ratio of the isotopes in the sample (underspiking). The accuracy and precision of the isotope dilution analysis ultimately depends on the accuracy and precision of the isotope ratio measurement, so all the precautions that apply to isotope ratio analysis also apply in this case. Isotope dilution analysis is attractive because it can provide very accurate and precise results. The analyte acts as its own de facto internal standard. For instance, if the isotopic spike is added prior to any sample preparation then the spike will behave in exactly the same way as the analyte because it is chemically identical, providing it has been properly equilibrated by a series of oxi-dation/reductions. Hence, any losses from the sample can be accounted for because the analyte and spike will be equally affected. 

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 ... 

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

Both absolute quantitation and relative quantitation of species in mixtures is of interest in some circumstances. Quantitation in a 5-minute analysis can be achieved by addition of an internal standard, ideally the target microorganism grown in special media to incorporate heavy isotopes92-95 and determination of the relative peak heights of pairs of proteins from the analyte and the standard. Isotope-labeled proteins or peptides, selected to match proteins or peptides characteristic of target microorganisms, can also serve as internal standards for isotope ratio measurement. The addition of unmatched proteins or peptides is less reliable for either ESI or MALDI measurements because of unpredictable suppression in the variable mixture. 

In most natural situations, physical and chemical parameters are not defined by a unique deterministic value. Due to our limited comprehension of the natural processes and imperfect analytical procedures (notwithstanding the interaction of the measurement itself with the process investigated), measurements of concentrations, isotopic ratios and other geochemical parameters must be considered as samples taken from an infinite reservoir or population of attainable values. Defining random variables in a rigorous way would require a rather lengthy development of probability spaces and the measure theory which is beyond the scope of this book. For that purpose, the reader is referred to any of the many excellent standard textbooks on probability and statistics (e.g., Hamilton, 1964 Hoel et al., 1971 Lloyd, 1980 Papoulis, 1984 Dudewicz and Mishra, 1988). For most practical purposes, the statistical analysis of geochemical parameters will be restricted to the field of continuous random variables. 

A study was made of the effects of derivatization on the 13C analysis of amino acid enantiomers. Conventional isotope ratio MS and GC-isotope ratio MS were used. The latter method requires volatilization of the analytes, which was accomplished by introducing O-isopropyl and A-trifluoroacetyl groups, causing a change in the 13C analysis of the original analytes. It was proposed to use a set of known standards for such analyses, which are applied in geological studies68. 

Corrections for instrumentally-produced mass fractionation that preserve natural mass dependent fractionation can be approached in one of two ways a double-spike method, which allows for rigorous calculation of instrumental mass fractionation (e.g., Dodson 1963 Compston and Oversby 1969 Eugster et al. 1969 Gale 1970 Hamelin et al. 1985 Galer 1999 see section Double-spike analysis ), or an empirical adjustment, based on comparison with isotopic analysis of standards (Dixon et al. 1993 Taylor et al. 1992 1993). The empirical approach assumes that standards and samples fractionate to the same degree during isotopic analysis, requiring carefully controlled analysis conditions. Such approaches are commonly used for Pb isotope work. However, it is important to stress that the precision and accuracy of isotope ratios determined on unknown samples may be very difficult to evaluate because each filament load in a TIMS analysis is different. 

Flow-injection sample introduction has been successfully applied in the analysis of standard reference materials and in the measurement of accurate and precise isotope ratios, and, hence, isotope dilution analysis. The rapid sample throughput possible with FI should allow a four-fold increase in the sampling rate compared with conventional nebufization techniques. Also, the amount of sample consumed per analytical measurement by FI is considerably less than continuous nebufization. TTiese considerations are of particular importance for the cost-effective operation of ICP-MS. 

Samples were prepared for Cu isotope analysis on the Multicollector Inductively-Coupled Plasma Mass Spectrometer (MC-ICPMS) at University of Arizona. The Cu-rich samples were loaded and dissolved in pure HNO3 and the Cu-poor samples were loaded and dissolved in a mixture of HCI and HNO3, Chromatographic separation of the Fe and Cu ions was deemed necessary for the Cu-rich samples. The diluted solutions were injected into the MC-ICPMS using a microconcentric nebulizer. Samples were run numerous times to increase precision. The Cu isotope ratios are reported in conventional per mil notation, relative to the NIST 976 standard. Mass bias was also accounted for by bracketing methods with the NIST 976 standard. 

Fitzsimons et al. (2000) have reviewed the factors that influence the precision of SIMS stable isotope data. All sample analyses must be calibrated for instrumental mass fractionation using SIMS analyses of a standard material. Under favorable circumstances, precision can reach a few tenths of a per mill. The latest version of ion-microprobe is the Cameca-lMS-1280 type, allowing further reduction in sample and spot size and achieving precise analysis of isotope ratios at the 0. %o level (Page et al. 2007). 

A direct result of the ability to measure isotope ratios with ICP-MS is the technique known as isotope dilution analysis. This is done by spiking the sample to be analysed with a known concentration of an enriched isotopic standard, and the isotope ratio is measured by mass spectrometry. The observed isotope ratio (RJ of the two chosen isotopes can then be used in the isotope dilution equation (Eqn. 5.7) to calculate the concentration of the element in the sample ... 

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