Inductively coupled plasma mass uncertainty

Early investigations on Mg isotope variations have been limited by an uncertainty of 1 to 2%o. Catanzaro and Murphy (1966) for instance concluded that terrestrial Mg isotope variations are restricted to a few %o. The introduction of multicoUector-inductively coupled-plasma mass spectrometry (MC-ICP-MS) increased the precision by one order of magnitude and has initiated a new search of natural isotope variations (Galy et al. 2001, 2002). These authors obtained an overall 4%c variation in 5 Mg. 

F. Laborda, J. Medrano and J. R. Castillo, Estimation of the quantification uncertainty from flow injection and liquid chromatography transient signals in inductively coupled plasma mass spectrometry, Spectrochim. Acta, Part B, 59(6), 2004, 857-870. 

If the reference materials are pure substances and can be specified on the microscopic level, then they represent the unit amount of substance. Because there are no absolute pure substances the representation is in all cases an approximation. The degree of approximation is given by the accuracy of the contents of the main component. In case of pure elements, e.g. metals Fe, Cu, Zn the determination of the main component by coulometry is limited by an uncertainty of 0.01%. The determination of all impurities needs completeness and requires a great deal of analytical equipment. However, a combination of inductively coupled plasma-mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS) and isotope dilution mass spectrometry (ID-MS) covering all elements of the periodic table allows a decrease of total uncertainty to 0.0032% (Cu, see Fig. 8). 

The table below lists some common spectral interferences that are encountered in inductively coupled plasma mass spectrometry (ICP-MS), as well as the resolution that is necessary to analyze them.1 The resolution is presented as a dimensionless ratio. As an example, the relative molecular mass (RMM) of the polyatomic ion 15N160+would be 15.000108 + 15.994915 = 30.995023. This would interfere with 31P at a mass of 30.973762. The required resolution would be RMM/8RMM, or 30.973762/0.021261 = 1457. One should bear in mind that as resolution increases, the sensitivity decreases with subsequent effects on the price of the instrument. Note that small differences exist in the published exact masses of isotopes, but for the calculation of the required resolution, these differences are trivial. Moreover, recent instrumentation has provided rapid, high-resolution mass spectra with an uncertainty of less than 0.01%. 

Wolff-Briche, C.S.J., Harrington, C.F., Catterick, T, and Fairman, B., Orhodox uncertainty budgeting for high accuracy measurements by isotope dilution inductively coupled plasma mass spectrometry, Anal. Chim. Acta, 437, 1 (2001). 

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