Isotope dilution analysis principles

An excellent possibility for quantifying analytical data in inorganic mass spectrometry is isotope dilution analysis (IDA) based on more precise isotope ratio measurements. IDA uses highly enriched isotope spikes of analytes of known concentration for calibration and is the method of choice if a high accuracy for element concentrations is required. The principles and applications of this method will be discussed below. 

The principle of isotope dilution analysis is surprisingly simple. It relies on the intentional alteration of the isotope abundance of an endogenous element in a given sample by the addition of a known amount of an enriched isotope of the same element (spike). Therefore, the element to be analysed must have, at least, two stable isotopes that can be measured free of spectral interferences in a mass spectrometer. This principle is illustrated in Figure 1.11 for an element containing two different isotopes, a and b. As can be observed, the a isotope is the most abundant one in the sample whereas the spike is isotopically enriched in the b isotope. It is clear that the abundances of the two isotopes... 

Figure 1.11 Illustration of the principle of isotope dilution analysis for an element containing two different isotopes (a and b).

The principle of the isotope dilution analysis (IDA) is described in Section 6.4. Due to its advantages as a definitive and accurate analytical method for the determination of element concentration via isotope ratio measurements, IDA is being increasingly applied in mass spectrometry, especially in ICP-MS and LA-ICP-MS as one of the most frequently used techniques. For example, the isotope dilution technique is employed in species analysis in biological systems, " e.g., for the determination of mercury species in tuna material,or in aquatic systems. Further applications of the isotope dilution technique are the determination of selenomethionine in human blood serum by capillary HPLC-ICP (ORC) MS ° or sulfur speciation in gas oil, diesel or heating fuel by LA-ICP-MS. Evans and co-workers have reported on the high accuracy analysis of sulfur in diesel fuel by IDA. ICP-SFMS has been employed for Si species analysis in biological or clinical samples and... 

The principle of isotope dilution analysis (IDA) is illustrated in Fig. 17.4. The sample contains an unknown number of atoms or molecules, and it may also contain an unknown number N of labelled atoms or molecules of the same kind. Known numbers N and N are added. These atoms or molecules (subscript 1) must not be identical with the atoms or molecules x, but they must exhibit the same behaviour under the given conditions. After mixing to obtain homogeneous distribution, any fraction is taken and the numbers N2 and W2 are determined in this fraction. 

The preceding method is known as reversed isotope dilution analysis when the compound to be measured is already radioactive. The principle remains the same the activity of the subject compound (measured from a fraction), is carried out before and after dilution with the same compound, non-labelled. The calculations are identical. This analysis is used for the determination of the isotopic carrier in a solution of a radionuclide using one of its stable isotopes. 

Isotope dilution analysis require the determination of either the chemical yield in the separation process or of the specific activity. This can be avoided by applying the substoichiometric principle, which may also increase the sensitivity of the analytical method. 

When the substoichiometric principle is applied to isotope dilution analysis, the relationship becomes... 

In DIDA, a radioactive form of the component of interest is added to the sample and the quantity of the inactive form initially present is determined. In some instances, one may wish to determine the amount of a radioactive substance in the sample. A method similar in principle to DIDA can then be used wherein a quantity of an inactive form of the component of interest is added to the sample, the sample is purified without regard to quantitative recovery, and the amount of the recovered component and its activity are measured. From this information, the quantity of the radioactive substance initially present in the sample is calculated. This method is referred to as inverse isotope dilution analysis (IIDA). 

The principle of the substoichiometric analysis is as follows To the element of interest (M ), one adds a known amount of its labeled radioisotope with the specific activity S = A/M, where A is the radioactivity and M the amount of carrier hence the specific activity of the mixture becomes S = A/(M -1- M). Knowing the change in specific activity from S to S, the element of interest can be sinqily determined. This is the same as the principle of isotope dilution, but in practice the accurate determination of S and S is very tedious. This is one important reason why isotope dilution analysis is not very popular in trace analysis. However, in substoichiometric analysis, equal amounts of the element (m) are isolated substoichiometrically from the radioisotope solution and the mixed solution, and subsequently the radioactivities of the separated portions (a and a ) are measured, then the amount of the element of interest can be calculated according to the equation ... 

See also Activation Anaiysis Neutron Activation. Atomic Emission Spectrometry Principies and Instrumentation. Bleaches and Sterilants. Chiroptical Analysis. Chromatography Principles. Conductimetry and Oscillometry. Coulometry. Fire Assay. Food and Nutritional Analysis Overview. Gas Chromatography Principles. Gravimetry. Indicators Redox. Infrared Spectroscopy Overview. Ion Exchange Oven/iew. Isotope Dilution Analysis. Lipids Fatty Acids. Liquid Chromatography Size-Exclusion. Radiochemical... 

After separation from excess reagent (by liquid-liquid distribution, chromatography, precipitation, etc.), the mass or concentration of this product is determined from activity measurement. The determination is based on a radioactive substance chemically different from the analyte substance (in contrast to isotope dilution analysis) therefore, the chemical reaction is of prime importance. By variation of this key reaction, the principle can be adapted to various procedures. The superiority of radio-reagent methods over classical separation techniques arises from the use of an inactive carrier and the high sensitivity of the activity measurements, which are not subject to interference by the carrier or other substances. 

See also Atomic Absorption Spectrometry Interferences and Background Correction. Atomic Emission Spectrometry Principles and Instrumentation Interferences and Background Correction Flame Photometry Inductively Coupled Plasma Microwave-Induced Plasma. Atomic Mass Spectrometry Inductively Coupled Plasma Laser Microprobe. Countercurrent Chromatography Solvent Extraction with a Helical Column. Derivatization of Analytes. Elemental Speciation Overview Practicalities and Instrumentation. Extraction Solvent Extraction Principles Solvent Extraction Multistage Countercurrent Distribution Microwave-Assisted Solvent Extraction Pressurized Fluid Extraction Solid-Phase Extraction Solid-Phase Microextraction. Gas Chromatography Ovenriew. Isotope Dilution Analysis. Liquid Chromatography Ovenriew. 

When the separation is not strictly stoichiometric, a cahhration curve determined from a series of known samples is used. Radioimmunoassay is based on this principle of suhstoichiometric isotope dilution analysis. 

The use of radionuclide techniques in analytical chemistry was first reported in 1913 by G. Hevesy and F. Paneth in a study of the solubility of lead sulfide in water, using the natural lead isotope " Pb as indicator [67], Isotope dilution analysis was introduced by O. Hahn in 1923 [68J, using Pa to determine the yield of Pa. The development of radioreagent methods followed, and further development of radioanalytical chemistry has established a range of analytical methods and techniques ll]-[4], [61], [65], [87], [93], [95], [97]. These include the use of artificial radionuclides and labeled compounds, the principles of nuclear activation [4]-[10], [66] (- Activation Analysis), and absorption and scattering of radiation [11], [12]. The most important procedures are shown in Table 1. 

The principle of isotope dilution analysis (IDA) [31], [47], [90]. [97], [98] involves measurement of the change in isotopic ratio when portions of a radiolabeled and nonlabeled form of the same chemical species are mixed. To perform a radioisotope IDA. an aliquot of a radioactive spike substance of known specific activity o,=A,/ni/ is... 

Isotope Dilution Analysis.—The principle of isotope dilution analysis (i.d.a.) is the measurement of the change in specific activity of a radioactive tracer when mixed with the test sample. The concentration of the determined component of the mixture is calculated using the well-known equation ... 

The principle of isotope dilution analysis consists of synthesizing the flavor compounds to be quantified (native compounds) so as to label them with stable isotopes (for example, deuterium or carbon-13) and adding them to the food matrix before sample workup. 

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]. 

The isotope dilution technique makes possible the analysis of mixts in which complete separation of the components is very difficult or impossible, since it is only necessary to isolate a small amt (enough for radioassay) of each component in the pure form. The method is based on the principle that the change in initial specific activity (counts per minute per unit wt) of the tracer will be proportional to the amt of inactive form of the same compd in the mixt that is, the change in counting rate of the original radioactive material is a function of the amt of dilution by the chemically equivalent inactive material... 

Combination of isotope dilution with the principle of substoichiometric analysis offers the possibility of avoiding determination either of the chemical yield of the separation procedure or of the specific activity in the isolated fraction. Two identical aliquots of the radiotracer solution are taken, both containing the tracer with mass... 

Two radioanalytical methods described in chapter 17 are applied preferentially in the life sciences, activation analysis and isotope dilution, the latter mainly in combination with the substoichiometric principle. 

Isotope dilution in combination with the substoichiometric principle is applied in various ways. The most important examples are radioimmunoassay for protein analysis and DNA analysis. In radioimmunoassay, radionuclides are used as tracers and immunochemical reactions for isolation. Radioimmunoassay was first described in 1959 by Yalow and Berson, and since then has found very broad application in clinical medicine, in particular for the measurement of serum proteins, hormones, enzymes, viruses, bacterial antigens, drugs and other substances in blood, other body fluids and tissues. Only one drop of blood is needed, and the analysis can be per-fonned automatically. Today more than 10 immunoassays are made annually in the United States. The most important advantages of the method are the high sensitivity and the high specificity. In favourable cases quantities down to 10 g can... 

The isotopic dilution radioimmunoassay is based on the principle of saturation analysis, which is outlined in Figure 1. A test compound (in this case insulin, represented by X) is quantified by its ability to progressively saturate a suitable reagent (in this case an antibody specific for insulin, represented by Y). In the first step, a known quantity of radioactively labeled tracer insulin (X ) is added to the sample. 

Chapter 8 provides the reader with the basic principles of isotope dilution mass spectrometry used for elemental analysis and also discusses more advanced features of this calibration approach, such as its use in direct solid sample analysis and in elemental speciation work, wherein not the total amount but that of various chemical species of a target element need to be determined. 

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