Stable isotope measurement mass spectrometry

Varelis P, Smythe GA, Hodgson D, Lazarus L. Measurement of urinary free cortisol by stable isotope dilution mass spectrometry using a new cortisol derivative. J Chromatogr B Biomed Appl 1994 660 151-9. 

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. 

Flegal and Stokes [59] have described a sample processing technique necessary for avoiding lead contamination of seawater samples prior to lead stable isotope measurements by thermal ionisation mass spectrometry. Levels down to 0.02 ng/kg were determined. 

TI is a very precise and accurate method in stable isotope ratio measurements and quantification of inorganic elements, for example, by isotope dilution mass spectrometry [8]. Because TI is a continuous ion source, it could be coupled to any analyzer that is suitable for such sources. However, because the strength of TI lies in the quantitative precision and accuracy, sector analyzers are preferred to ensure maximum quality. 

While competitive methods to determine KIE s are free from errors due to differences in reaction conditions (impurities, temperature, pH, etc.) they do require access to equipment that allows high precision measurements of isotope ratios. The selection of an appropriate analytical technique depends on the type of the isotope and its location in the molecule. For studies with stable isotopes the most commonly used technique (and usually the most appropriate) is isotope ratio mass spectrometry (IRMS). 

The Faraday cup was widely used in the beginning of mass spectrometry but all the characteristics of this detector mean that it is now generally used in the measurement of highly precise ratios of specific ion species as in isotopic ratio mass spectrometry (IRMS) or in accelerator mass spectrometry (AMS). To obtain a highly accurate ratio in such relative abundance measurements, the intensities of the two stable beams of specific ions are measured simultaneously with two Faraday cups. 

Methods of Isotopic Measurement. Two methods of stable isotope measurement are applicable 1) Mass Spectrometry, and 2) Neutron Activation Analysis. The former is described elsewhere in this volume and so we will provide a brief description of the latter method. We have given more detailed discussions elsewhere (, 9 ). 

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

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