Direct isotope dilution analysis

This is the basic equation of direct isotope dilution analysis. The unknown amount x of material A is given in terms of the amount y of added labeled material A and the two measured specific activities Si and S2. 

Direct isotope dilution analysis with a radioactive label... 

In the method of activation analysis, radioactivity is induced in the sample to be analyzed. In the method of direct isotope dilution analysis (DIDA), a radioactive form of the component of interest is added to the sample. The component is then exhaustively purified without regard to quantitative recovery and a fraction of the pure component isolated. The amount and activity of the isolated component are measured and the quantity present in the original sample is calculated using that information. 

Direct isotope dilution analysis is applied if an amount of an analyte cannot be separated quantitatively for analytical determination. A known amount of a radioactive isotope of the element of interest is added to the sample containing the analyte. Then a portion of the analyte is isolated in high purity from the sample. This separation step need not be quantitative. The mass and activity of the isolated portion are measured and used to calculate the amount of analyte in the original sample. There are several varieties known of this radiochemical method, e.g., reverse isotopic dilution. 

Direct isotope dilution analysis. This method is employed to determine the mass (weight) of an ordinary inactive compound (or an ion) in a solution, being efficacious when quantitative separation of the compound from the mixture is difficult but partial separation of the... 

For example, direct isotope dilution analysis is used for mercury inventory in industry (Cowley et al. 1966 Enomoto et al. 1975). The mass X of mercury in electrolytic chlorine cells is determined using Hg, Hg, or a mixture of them. An aliquot of known mass Wo and activity Ao and hence known specific activity So is taken from a stock of mercury labeled with the tracer, added to each electrolytic cell, and left to mix. After homogenization, a sample of mercury is taken from each cell. The samples are shaken with 15-20% HCl to decompose amalgam. The mass W and activity A of the samples are measured to yield to S and, accordingly, X. The accuracy is better than 1%. 

Substoichiometric isotope dilution analysis. It is often inconvenient to determine the mass of the isolated compound in the above direct isotope dilution analysis, as is the case for a large number of samples. Substoichiometric isotope dilution analysis is an excellent solution to this problem (Ruzicka and Stary 1968 Stary and Ruzicka 1976). In this method, the same amount of the compound is separated from each of the standard and mixed solutions using a reagent in an amount stoichiometrically less than that of the compound in the solutions. The activity of the portion separated from the standard solution (Aq) and that from the mixed solution (A) are measured. Since the mass of the portions counted are the same, the ratio of their specific activities SqIS equals the ratio of their activities Aq/A. 

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 ... 

The basic idea of isotope dilution analysis is to measure the changes in specific activity of a substance upon incorporation into a system containing an unknown amount of that substance. There are several types of IDA. We begin by considering direct IDA. 

The second general category of radiochemical analysis involves adding a radioactive substance to the sample, manipulating the sample by chemical or physical means, measuring the radioactivity, and ultimately calculating the amount of the component of interest. This category includes direct and inverse isotope dilution analysis, radiochemical titrations, and radiorelease methods of analysis. 

QUANTITATIVE TECHNIQUES Isotope dilution analysis Direct dilution analysis Reverse isotope dilution analysis Derivative analysis Double isotope dilution analysis Saturation analysis Radioenzymatic assays... 

In non-nuclear teehniques, isotopes of the same element generally eaimot be distinguished, while in nuclear techniques, specific isotopes are measured instead of elements. Therefore, direct quantitative information on the associated elements ean be obtained since poly-isotopic elements have eonstant isotope ratios. Further, isotopes of a given element may be diseriminated therefore, analytieal information may be obtained by using elements enrlehed In respeet to a partieular stable isotope or labelled with a radioisotope, e.g. in isotope dilution analysis. In addition to analytical information, isotope studies may also yield kinetie and mechanistic information. 

A unique capability of ICP-MS is its ability to measure the concentration of specific analyte isotopes. This characteristic of performing isotopic ratio measurements provides a powerful alternative method called isotope dilution analysis, comparable to gravimetric determinations, which do not require a direct relationship to a calibration standard. The ICP-MS technique allows both single and multielement isotope dilution measurements to be performed on a single sample aliquot. 

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

In the past decade, eight inherited disorders have been linked to specific enzyme defects in the isoprenoid/cholesterol biosynthetic pathway after the finding of abnormally increased levels of intermediate metabolites in tissues and/or body fluids of patients (Table 5.1.1) [7, 9, 10]. Two of these disorders are due to a defect of the enzyme mevalonate kinase, and in principle affect the synthesis of all isoprenoids (Fig. 5.1.1) [5]. The hallmark of these two disorders is the accumulation of mevalonic acid in body fluids and tissues, which can be detected by organic acid analysis, or preferably, by stable-isotope dilution gas chromatography (GC)-mass spectrometry (GC-MS) [2]. Confirmative diagnostic possibilities include direct measurement of mevalonate kinase activities in white blood cells or primary skin fibroblasts [3] from patients, and/or molecular analysis of the MVK gene [8]. 

For the isotope dilution, mass spectrometry method samples are injected directly into the gas sample injection port of the mass spectrometer [27]. These techniques do not allow concurrent analysis of TMA and TMA N-oxide in the sample. TMA N-oxide is quantified indirectly by measuring the increase in TMA after chemical reduction. 

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