Dilution analysis

Although isotope-dilution analysis can be very accurate, a number of precautions need to be taken. Some of these are obvious ones that any analytical procedure demands. For example, analyte preparation for both spiked and unspiked sample must be as nearly identical as possible the spike also must be intimately mixed with the sample before analysis so there is no differential effect on the subsequent isotope ration measurements. The last requirement sometimes requires special chemical treatment to ensure that the spike element and the sample element are in the same chemical state before analysis. However, once procedures have been set in place, the highly sensitive isotope-dilution analysis gives excellent precision and accuracy for the estimation of several elements at the same time or just one element. 

It is not necessary that there be two isotopes in both the sample and the spike. One isotope in the sample needs to be measured, but the spike can have one isotope of the same element that has been produced artificially. The latter is often a long-lived radioisotope. For example, and are radioactive and all occur naturally. The radioactive isotope does not occur naturally but is made artificially by irradiation of Th with neutrons. Since it is commercially available, this last isotope is often used as a spike for isotope-dilution analysis of natural uranium materials by comparison with the most abundant isotope ( U). 

For the purposes of this illustration, suppose that the level of of a trace element (X) is to be determined in a sample and that the element has two suitable isotopes (A,B). 

Where N is the total number of atoms of element X In the sample and Ab (A), Ab (B) are the percentage abundances of the two isotopes. 

From this measured ratio, the proportional abundances are 

In the majority of the studies on the composition of food aromas, AEDA is used for the determination of the relative odour potency of the compounds detected by GC-O (reviewed in [1]). The odour potency is proportional to the odour activity value (OAV) of the compound in air. The OAV is defined as the ratio of the concentration of a compound to its odour threshold [3]. 

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A more detailed study of the nitration of quinolinium (l) in 80-05 % sulphuric acid at 25 °C, using isotopic dilution analysis, has shown that 3-) 5-) 6-, 7- and 8-nitroquinoline are formed (table 10.3). Combining these results with the kinetic ones, and assuming that no 2- and 4-nitration occurs, gives the partial rate factors listed in table 10.4. Isoquinolinium is 14 times more reactive than quinolinium. The strong deactivation of the 3-position is in accord with an estimated partial rate factor of io for hydrogen isotope exchange at the 3-position in the pyridinium ion. It has been estimated that the reactivity of this ion is at least 10 less than that of the quinolinium ion. Based on this estimate, the partial rate factor for 3-nitration of the pyridinium ion would be less than 5 x io . 

Isotopic dilution analysis is widely used to determine the amounts of trace elements in a wide range of samples. The technique involves the addition to any sample of a known quantity (a spike) of an isotope of the element to be analyzed. By measuring isotope ratios in the sample before and after addition of the spike, the amount of the trace element can be determined with high accuracy. The method is described more fully in Figure 48.13. 

A major example of isotope-dilution analysis lies in the procedure itself, which does not require any quantitative isolation of the elements being investigated. The relation between the abundance of the element under investigation and the spike is such that, once the spike has been intimately mixed with the sample, any losses of sample have no effect on the result (Figure 48.14). 

Oxidation of [2- C]acetone produced the following yields of products identified by isotope dilution analysis... 

Very little hydrogen peroxide was found as a product (by isotope dilution analysis), and it was ruled out as an intermediate by addition of 0-labelled H2O2 before reaction. 

Separation and detection methods Ion chromatography is routinely used for the isolation of bromate. Diverse detection methods are mentioned in the literature, including isotope dilution analysis (Creed and Brockhoff 1999), conductivity measurement (Jackson et al. 1998), fluorimetric determination (Gahr et al. 1998), ICP-MS (Seubert and Nowak 1998) and spectrophotometry (Achilii and Romele 1999),... 

Creed JT, and Brockhoff CA (1999) Isotope dilution analysis of bromate in drinking water matrixes by ion chromatography with inductively coupled plasma mass spectrometric detection. Anal Chem 71 722-726. 

The main advantages of plasma-source mass spectrometry (PS-MS) over other analytical techniques, such as PS-AES and ETAAS, are the possibilities of quantitative isotope determination and isotope dilution analysis the rapid spectral scanning capability of the mass spectrometer and semiquantitative determinations to within a factor of two or three. Several labelling methods are used for the quantification of analytes present in complex mixtures. In these methods, the sample is spiked... 

Fractional separation of tin compounds used as stabilisers in PVC was based on substoichiometric isotope dilution analysis [446]. The tin compounds were isolated by extraction and complexed with salicylideneamino-2-thiophenol, followed by controlled addition of y-irradiated tributyl tin oxide and measurement of the y-activity. PVC containing a nominal 0.63% (Bu Sn analysed 0.614 0.016% in nine determinations. 

The photochemical interconversion of cis- and tra/w-5,6-diphenyl-bicyclo[3.1.0]hexanones has been extensively studied by Zimmerman, Hancock, and Licke.<77) Using isotope dilution analysis, these workers obtained very accurate quantum yields for the various photochemical processes involved in this reaction ... 

Elderfield and Greaves [629] have described a method for the mass spectromet-ric isotope dilution analysis of rare earth elements in seawater. In this method, the rare earth elements are concentrated from seawater by coprecipitation with ferric hydroxide and separated from other elements and into groups for analysis by anion exchange [630-635] using mixed solvents. Results for synthetic mixtures and standards show that the method is accurate and precise to 1% and blanks are low (e.g., 1() 12 moles La and 10 14 moles Eu). The method has been applied to the determination of nine rare earth elements in a variety of oceanographic samples. Results for North Atlantic Ocean water below the mixed layer are (in 10 12 mol/kg) 13.0 La, 16.8 Ce, 12.8 Nd, 2.67 Sm, 0.644 Eu, 3.41 Gd, 4.78 Dy, 407 Er, and 3.55 Yb, with enrichment of rare earth elements in deep ocean water by a factor of 2 for the light rare earth elements, and a factor of 1.3 for the heavy rare earth elements. 

The tributyltin-117m chloride was purified by extraction into benzene. This isotope of tin, which has a half-life of 14 days, was used in the isotope dilution analysis of environmental organotin compounds. 

A mixture is being assayed by radioisotope dilution analysis. 10 mg of the labelled analyte (0.51 pCi mg-1) was added. 1.5 mg of the pure analyte was separated and its specific activity measured and found to be 0.042 pCi mg1. What was the amount of analyte in the original sample ... 

Aroma compounds, 11 517 Aroma extract dilution analysis (AEDA), 11 519-520... 

Webster, R. K. (1960). Mass spectrometric isotope dilution analysis. In Methods in... 

Stable-isotope dilution analysis is an analytical technique in which a known quantity of a stable-labelled isotope is added to a sample prior to extraction, in order to quantitate a particular compound. The ratio of the naturally abundant and the stable-labelled isotope is a measure of the naturally abundant compound and can be determined only by gas chromatography-mass spectrometry since the naturally abundant and the stable-labelled isotope cannot be completely separated gas chromatographically. 

Isotope dilution analysis is the definitive method for the quantitative analysis of many compounds but is usually only carried out in specialist laboratories. 

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. 

Blank I, Grosch W, Evaluation of potent odorants in dill seed and dill herb Anethum graveolens L.) by aroma extract dilution analysis,Sci 56 63-67, 1991. 

Giusti, P., Schaumloffel, D., Encinar, J. R., and Szpunar, J., Interfacing reversed-phase nanoHPLC with ICP-MS and on-line isotope dilution analysis for the accurate quantification of selenium-containing peptides in protein tryptic digests. Journal of Analytical Atomic Spectrometry 20(10), 1101-1107, 2005. 

Diagnosis of medium chain acyl-CoA dehydrogenase deficiency by stable isotope dilution analysis of urinary acylglycines retrospective and prospective studies, and comparison of its accuracy to acylcamitine identification by FAB/mass spectrometry. 

Nishimura, O. Identification of the characteristic odorants in fresh rhizomes of ginger (Zingiber officinale Roscoe) using aroma extract dilution analysis and modified multidimensional gas chromatography-mass spectroscopy. J Agr Food Chem 1995 43(11) 2941-2945. 

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