Isotopic dilution methods

Isotope dilution mass spectrometry (IDMS) can be considered as a special case of the internal standard method the internal standard that is used is an isotopomer of the compound to be measured, for example a deuterated derivative. Note that an internal standard is necessary for every compound to be measured. This internal standard is as close as possible to perfection since the only property that distinguishes it from the compound to be measured is a slight mass difference, except for some phenomena that involve the labelled atoms, such as the isotopic effect. In that case, we have an absolute reference, that is the response coefficients of the compound and of the standard are identical. This method is often used to establish standard concentrations. The basic theory of this method rests on the analogy between the relative abundance of isotopes and their probability of occurrence [23]. 

The method consists of examining the spectrum of the compound that must be measured in order to select an intense characteristic peak which is used to measure the analyte. A known, exact quantity of labelled internal standard is added to the sample with an unknown concentration. When the labelled internal standard is added, the peak corresponding to the characteristic peak is moved to a different position in the spectrum, according to the number and nature of the atoms that were used in the labelling. The ratio of these two signal intensities is used to measure their relative proportion. 

Suppose that there are N atoms or molecules in the mixture that yield a peak which is characteristic of the mass rn. Suppose also that there are M atoms or molecules of the labelled compound in the substance which yield a characteristic peak for the mass m + n = o, where n corresponds to the mass displacement caused by the introduction of isotopes into the molecule. 

Principle of isotopic dilution illustrated by a molecule having two isotopes of mass m and o. 

If A represents the Avogadro number, x and y represent the quantity of natural product and the quantity of labelled product added to the sample, respectively, and E and F represent the molecular masses of the natural product and of the labelled product, respectively, then the equation is 

This is illustrated by the infrared spectra of HUP where the vOH bands 

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Data corrected for recovery using isotope dilution method. Tobacco data are reported per dry weight. 

In most alpha and mass spectrometric methods for which sample preparation is extensive and chemical recoveries can vary considerably from sample to sample, precise elemental concentrations are determined by isotope dilution methods (e.g., Faure 1977). This method is based on the determination of the isotopic composition of an element in a mixture of a known quantity of a tracer with an unknown quantity of the normal element. The tracer is a solution containing a known concentration of a particular element or elements for which isotopic composition has been changed by enrichment of one or more of its isotopes. 

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. 

For destructive measuring methods, a CRM would serve as a reference to check the recovery of a particular matrix removal procedure. This is especially important for open destructions at atmospheric pressure. Alternatively, isotope dilution methods may be used once isotopic equilibrium is established, loss of analyte does not affect the analysis result. Isotope dilution techniques are only available in a few specialised laboratories. Another type of problem is encountered in pressurised methods oxidising the matrix in a closed vessel or bomb. Due to the large amounts of gas (CO2, NO, SO2) evolving from samples with a high organic matrix content, an excessive pressure build-up occurs that prohibits the use... 

The isotope dilution method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultratrace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decay during the period of measurement, all of the labelled atoms are measured. 

A mass-isotope dilution method for determining the gamma isomer of benzene hexachloride, in which gamma-hexadeuterobenzene hexachloride is used as a tracer molecule and the dilution is determined by use of infrared spectrophotometry, has been developed by Trenner et al. (52). Impurities have no effect on the accuracy of this method. 

Study of chemical pathways in method development. Isotope dilution methods. Radioimmunoassay very important in biochemistry and medicine. Neutron activation analysis used for trace elements in geo-chemistry, semiconductor technology, pollution studies and forensic science. Relative precision of counting 1% if 104 counts are recorded. Assessment of pollution by radionuclides. 

Recently an isotope dilution method has been reported O for assaying neomycin sulphate. However, it is first necessary to prepare 14C-labelled neomycin sulphate. This is accomplished by adding l4C-labelled glucose to a small-scale fermentation of 5. nad-iat. l4C-labelled neomycin can then be extracted by solvent-extraction or by ion-exchange chromatography. 

Quantification is usually achieved by a standard addition method, use of labeled internal standards, and/or external calibration curves. In order to allow for matrix interferences the most reliable method for a correct quantitation of the analytes is the isotope dilution method, which takes into account intrinsic matrix responses, using a deuterated internal standard or carbon-13-labeled internal standard with the same chemistry as the pesticide being analyzed (i.e., d-5 atrazine for atrazine analysis). Quality analytical parameters are usually achieved by participation in interlaboratory exercises and/or the analysis of certified reference materials [21]. 

In normal RIA-procedures the labelled drug or metabolite not only serves as the tracer for recovery but also for RIA quantification. However, the isotope dilution method categorically makes a clear separation of the drug and its metabolites. Consequently, a non-specific antiserum is employed to actually quantify the total amount of both unlabelled and labelled substance present. 

Identification of possible intermediates in the cyclization was attempted by adapting the isotope dilution method which had been... 

Barker SA, Littlefield-Chabaud MA, David C. 2001. Distribution of the hallucinogens N,N-dimethyltryptamine and 5-methoxy-N,N-dimethyltryptamine in rat brain following intraperitoneal injection application of a new solid-phase extraction LC-APCI-MS-MS-isotope dilution method. J Chromatogr B Biomed Appl 751 37. 

Thus, accurate determination of the specific activities SAa and SAa immediately provides the value of the unknown Ax. Note Whenever Axstatistical variation in the measurements of quantities that are nearly identical. Should this prove to be true for your measurements, use a lesser amount of isotopic probe, so that Ax A. 

The sensitivity and specificity conferred by making isotopically labeled derivatives of drugs in vitro can be used clinically (K18, K19), as can isotope dilution methods (H2, 07). The great imposition upon time, skills, and resources that such techniques demand, limits their application to selected research studies. They nevertheless provide a basis against which other more practicable methods can be assessed. 

Bechtold WE, Sabourin PJ, Henderson RF. 1988. Reverse isotope-dilution method for determining benzene and metabolites in tissues. J Anal Toxicol 12(4) 176-179. 

Stable isotope dilution method for diagnosis of medium chain acyl-CoA dehydrogenase deficiency. 

Note that, during mass spectrometric analyses, the isotopic composition (230rh/232-ph) is measured independently from (2 U/2 2 ph) and thus the errors of these two ratios are not correlated, even though they have the same denominator in 2 h. (22 U/22 Th) is obtained by isotope dilution method while (2 h/2 2jjj is measured directly by mass spectrometers. 

The isotope dilution method consists of mixing a natural sample with an artificial spike and measuring the isotopic ratios of the mixture using mass spectrometry, providing very precise quantitative determination of the concentrations of elements in trace quantities. A spike is a solution that contains a known concentration of the element, artificially enriched in one of its minor isotopes. For example, natural Rb samples have 27.84% and 72.16% Rb (Fig. 11.lA). Rb spikes are made by artificially enrich the minor isotope Rb. And a solution with 90% Rb and 10% Rb (Fig. 11.IB) is a Rb spike. Of course, a solution with 99.99% Rb and 0.01% Rb is a better Rb spike. When the known quantities (mass) of the sample and spike are mixed, the resulting isotopic compositions can be used to calculate the concentration of the element in the sample. 

In many cases, it is more convenient to describe the isotope dilution method using isotopic ratios rather than abundances alone. / j in Eq. (11.1) can be expressed as a function of isotopic ratios in the natural sample and the spike. ... 

Glycerol kinase activity can be measured directly or indirectly however, these assays are not available as a clinical test and are done exclusively on a research basis. The direct method is as has been described previously [4, 6] and is used in the isotope dilution method indicated above. The amount of protein and the incubation time vary between cell types and it is important to be within the linear range of the assay for the given cell type. The indirect methods involve incorporation of 14C from glycerol into macromolecules and its subsequent oxidation to 14C02 [7,11]. 

LA-ICP-MS is suitable for the direct analysis of materials such as metals, semiconductors, ceramics and insulators at trace and ultratrace levels (detection limits 1 ng g ) without sample preparation. The MS detection mode makes it possible isotope analysis and also isotope dilution methods using... 

In the interim between the initial development and application of Methods 624 and 625, the Effluent Guidelines Division, under contract, developed isotopic dilution methods for purgeable volatiles and semivolatile priority pollutants (11). The primary difference between Methods 624 and 625 and their 1624 and 1625 counterparts is that stable, isotopically labeled analogs of the compounds of interest are added to the sample prior to extraction, and quantitative determination is made by using the isotope ratio values for the compounds determined relative to their labeled analogs. In addition, Method 1625 is designed as a capillary column method and uses a 30-m X 0.25-mm silicone-bonded-phase fused silica column (J W DB-5). Method 1624 uses the same column materials as Method 624. Methods 1624 and 1625 are presented as acceptable alternatives to Methods 624 and 625. 

The hydrolytic behavior of both NaX and NaY as measured at 10 2ilf. NaCl by isotopic dilution methods has also been measured as a function of the zeolite content. The results, along with standard deviations from the mean, are summarized in Table II the figure in parenthesis indicates the... 

In applying the isotope dilution method to the analysis of HMX/RDX mixts, an accurately weighed amt of carbon-14 labeled HMX of known specific activity (counts per min per g) is added to a known wt of the mixt. The mixt is made homogeneous by dissolving it in acetone. A small amt of pure HMX is then isolated from the mixt by fractional recrystallization from acetone. The specific activity of the pure HMX sample, both before and after dilution, is determined by either dissolving or suspending a known wt in a scintillator soln or gel and counting with a commercial liq scintillation counter. 

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