Logic of Conventional Correction Models

The need for a formal mass bias correction model arises when a specific isotope amount ratio is used to calibrate the amount ratio of another pair of isotopes, either of the same or of a different element, as for example when N( Sr)/N( Sr) is used to caUbrate N( Sr)/N( Sr) or when is used to caUbrate 

As with TIMS, the core strength of MC-ICP-MS is its ability to register ion signals simultaneously. However, the traditional calibration approaches use sequential analysis of the measurand and the calibrant as a result of their temporally different introduction into the source. Hence a paradox of the choice in contrast to sequential measurand- alibrator-measurand bracketing calibration, the calibrator can be admixed with the sample and both isotope amount ratios measured simultaneously. This, however, can only be done when the calibrator is a ratio of isotopes other than the measurand, which, in turn, requires a sound model for the mass bias transfer. 

Mass bias in MC-ICP-MS varies with both time and nuclide mass. Since calibration is typically performed sequentially in a calibrator-measurand-cahbrator bracketing mode, the effect of time has to be separated from that of mass in the mass bias correction model. Hence the logic of creating a mass bias discrimination model is to express the calibration factor, Ki/j, as a product of two functions one that varies with time only and the other that varies with the nuclide masses only [16, 17, 19]. For example, 

Qearly, there are many ways according to which one can express the caUbration factor Ki/j as functions of time and mass. Equation (5.4), for example, is commonly known as the linear correction law, Eq. (5.5) as Russell s law, and Eq. (5.6) as the exponential law. To add flexibility to the fitting function in the mass domain, it has been proposed that a discrimination exponent, n, be introduced into the mass bias correction factor [19]  

When using any of these models, a calibrator with known isotope amount ratio is measured and from the difference between this known value and the corresponding measurement result, the value off(t) is obtained. This value then permits calculation of the isotope amount ratio correction factor (Kqj) for the measurand. 

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