Isotope-dilution mass spectrometry internal standards

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Isotope Dilution Mass Spectrometry (IDMS) A quantitative mass spectrometry technique in which an isotopically enriched compound is used as an internal standard. See Chapter 14 for a more detailed explanation. 

A single measurement of a calibration sample can give the concentration of the test solution by a simple ratio. This is often done in techniques where a calibration internal standard can be measured simultaneously (within one spectrum or chromatogram) with the analyte and the system is sufficiently well behaved for the proportionality to be maintained. Examples are in quantitative nuclear magnetic resonance with an internal proton standard added to the test solution, or in isotope dilution mass spectrometry where an isotope standard gives the reference signal. For instrument responses As and /sample for internal standard and sample, respectively, and if the concentration of the internal standard is Cjs, then... 

Calibration for the head-space gas chromatography method is based on calibration curves with individual amines in distilled water. Isopropylamine is the internal standard [28]. For the isotope dilution mass spectrometry method [2H9]-TMA is used as the internal standard [27]. 

Table 2 Comparison of Cholesterol Reference Method Laborato- tute, NIST National Institute of Standards and Technology (US ry Network (CRMLN) traceability model to the International Or- NMI), IDMS isotope dilution mass spectrometry, ACL accredited ganization for Standardization (ISO) model. BIPM International calibration laboratory, AK Abell-Kendall Bureau of Weights and Measures, NMI National Metrology Insti-...

There are two main types of internal standards. The first ones are stable isotope labeled (SIL) internal standards. They are compounds in which several atoms in the analytes are replaced by their respective stable isotopes, such as deuterium (2H, D or d), 13C, 15N, or 170. Labeling with the first three isotopes are most common, particularly labeling with deuterium (due to less difficulty in synthesis and therefore less expensive). For examples, raloxifene-d4-6-glucuronide was used as the internal standard for the determination of raloxifene-6-glucuronide [5] and 1, 2, 3, 4-13C4 estrone (PCJEl) was used as the internal standard for estrone (El) [6], The usage of stable isotope labeled internal standards in quantitative LC-MS or GC-MS analysis is often termed as isotope dilution mass spectrometry (IDMS) [7],... 

Targeted analysis refers to metabolome analysis that targets one, or a few metabolites, and typically uses an internal standard for quantitation. The most common method is isotope dilution mass spectrometry (IDMS) [34], which relies on the use of stable isotope internal standards to enable the absolute quantitation of metabolites. This method has proven highly effective and has been successfully used in numerous studies. 

Dehennin, L., Reiffsteck, A., and Scholler, R. (1980). Simple methods for the synthesis of twenty different, highly enriched deuterium labelled steroids, suitable as internal standards for isotope dilution mass spectrometry. Biomed. Mass Spectrom. 7, 493-499. 

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]. 

Quantitation by mass spectrometry or radiometric counting requires reference to known standard material. This reference can be internal to the sample, wherein the reference material is added to the sample at an appropriate stage of processing, or it can be external when the response of the analyte in the sample is compared to the response measured for the reference material. A commonly employed method, isotope dilution mass spectrometry, is to add a known amount of an isotopically altered tracer (sometimes called a spike) to the sample. 

Some analytes are difficult to standardize owing to the complexity of developing a primary standard, or analyte inhomogeneity, an International Standard is used to provide comparability. Such standards are based on the principle of transferability. A definitive method, e.g., isotope-dilution mass spectrometry (MS), is used to obtain the best possible estimate of the accurate concentration of the analyte. This value would be transferred to a reference method, which is usually a very carefully documented analytical procedure of known and impeccable performance, e.g., Abel-Kendal assay for cholesterol, and standards are compared in the procedure before being used in the filed. 

The greatest accuracy attainable by MS is achieved by Isotope Dilution Mass Spectrometry using an exact signal matching method. Like other SIM techniques, an isotopically labeled internal standard is added gravimetrically to the analyte, prior to extraction from its matrix. The quantity of internal standard is then adjusted, over a number of iterations, such that the signal obtained exactly matches that of... 

Surrogate Internal Standard (SIS), often just referred to as Internal Standard (IS) A compound of known chemical purity that is added, in a known amount, to the sample for analysis before any extraction and work-up procedures used to monitor and correct for analyte losses and variations in instrumental sensitivity with mass spec-Iromelric detection isotope-labeled versions of the analyte (isotopologs) are the best (this procedure is referred to as isotope dilution mass spectrometry, IDMS) but isomers or analogs can be used, see Table 2.1. 

Fig. 2. Trideuterated methoxypyrazines, used as internal standards in quantitative wine analysis by isotope-dilution mass spectrometry, and the corresponding chloropyrazines used in their synthesis...

The method described here makes use of stable isotope dilution mass spectrometry (IDMS) for quantitative analysis of niacin. Isotopically labelled versions of both nicotinic acid and niacinamide are commercially available at a reasonable cost. The use of an isotopically labelled internal standard has distinct advantages in quantitative analysis, as it can correct for analyte losses and makes possible high levels of accuracy and precision (Fassett and Paulson 1989). Sample digestion and clean-up is based on a previously published LC-UV method (LaCroix et al. 1999, 2002 LaCroix and Wolf 2001). This chapter expands on a previous report of LC-IDMS analysis of niacin (Goldschmidt and Wolf 2007), with material from that report used with permission of the publisher. 

Stable isotope dilution mass spectrometry (IDMS) IDMS is a technique (method) based on the addition to an analytical sample of a known amount of a stable isotopically labelled analogue of a desired analyte as an internal standard. The ratio of the amount of added isotopic analogue and the naturally occurring compound, as measured by mass spectrometry, provides highly accurate values. Definitive accuracy of the IDMS technique requires full equilibration between endogenous natural amounts of the compound... 

Stable isotope dilution mass-spectrometry (MSID) is the most accurate technique for determining lanthanide abundances in geochemical materials.. The superior quality of the method may be attributed principally to the inherent sensitivity of mass-spectrometers, and to the use of the ideal internal standard, namely, an artificially enriched isotope of each element to be determined. The utilization of isotopic internal standards virtually eliminates such potential analytical problems as quantitative recovery and instrument calibration. The sensitivity of the mass spectrometer is such that the lower limit of measurable abundance is usually controlled by the purity of the reagents used in preparing the sample for analysis. 

Mass spectrometry is increasingly used in vitamin Be analysis. Hachey et al. (155) described analysis of Be vitamers in biological samples by isotope dilution mass spectrometry. Deuterated forms of the different Be vitamers were added in the early stages of the sample preparation procedure these deuterated vitamers were used as internal standards to compensate for analytical losses during the isolation and derivatization steps. The Bg vitamers in the homogenized tissue sample were separated by cation exchange HPLC as described previously (76). Acetylation was chosen as derivatization procedure for GLC. However, prior to... 

Acronyms BHT = butylated hydroxytoluene FL = fluorescence detection HILIC = hydrophilic-interaction LC IDMS = isotope-dilution mass spectrometry IS = internal standard MSPD = matrix solid-phase dispersion QqQ = triple quadrupole SRM = standard reference material TCA = trichloroacetic acid TEA = trifluoroacetic acid UPLC = ultraperformance liquid chromatography. 

Clarke, D.B., Lloyd, A.S., Botting, N.P., Oldfield, M.F., Needs, P.W., and Wiseman, H., Measurement of intact sulfate and glucuronide phytoestrogen conjugates in human urine using isotope dilution liquid chromatography-tandem mass spectrometry with [13C(3)]isoflavone internal standards, Anal. Biochem., 309, 158, 2002. 

Spark source (SSMS) and thermal emission (TEMS) mass spectrometry are used to determine ppb to ppm quantities of elements in energy sources such as coal, fuel oil, and gasoline. Toxic metals—cadmium, mercury, lead, and zinc— may be determined by SSMS with an estimated precision of 5%, and metals which ionize thermally may be determined by TEMS with an estimated precision of 1% using the isotope dilution technique. An environmental study of the trace element balance from a coal-fired steam plant was done by SSMS using isotope dilution to determine the toxic metals and a general scan technique for 15 other elements using chemically determined iron as an internal standard. In addition, isotope dilution procedures for the analysis of lead in gasoline and uranium in coal and fly ash by TEMS are presented. 

The ideal internal standard is the same element as the analyte because it has similar mass, ionization energy, and chemical properties. Therefore, isotope dilution based calibration provides high accuracy as long as isotope equilibration is attained and the measured isotopes are free of spectral overlaps [192,193]. Standards do not need to be matrix-matched. Quadrupole-based ICP-MS instruments can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope ratio precision can be obtained by using simultaneous MS detection, such as a multicollector-based instrument or perhaps time-of-flight MS. In comparison to thermal ionization mass spectrometry, ICP-MS provides much higher sample throughput and simpler, faster sample preparation. 

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