Remarks concerning optimal isotope dilution work

REMARKS CONCERNING OPTIMAL ISOTOPE DILUTION WORK 

A careful examination of Figs. 1 -8 leads to some interesting points with respect to conditions needed in order to reach smallest possible uncertainties (in other words smallest possible magnification factors). 

The smallest magnification factor possible is 1. Consequently the uncertainty of an IDMS assay is at best equal to the uncertainty e of the measurement of an isotope ration (Rb) added up to the uncertainty of the spike. 

To attain smallest uncertainty on a quantitative determination by IDMS, a proper choice of sample to spike ratio has to be made. This implies the estimation a priori of the unknown sample amount. This estimation is not critical when the proper spike (Ry) is available. For instance, (see Fig. 4) for an unknown amount of an element with isotope ratio Rx = 1/99 a precision VF is obtained with Ry 10 for 0.09 q 90. (Note if the estimated q value is experimentally found outside these limits, then the assay can be done in two steps, using the first determination in order to obtain a first estimate. It is obvious however to look for this first approximation by another less precise method.) 

The lack of a suitable spike (fly) can be more prohibitive for good measurements than the measurement imprecision inherent to the techniques. Fig. 8 shows that for Rx = 1 a precision of e V5 is only obtainable for 0.022 q 1.5 with Ry = 10 or 10 i.e. the spike should be highly enriched in one of the two dilution isotopes. However, a spike with Ry = 0.1 or Ry = 10 would yield the same result if a q value of 0.6 0.1 is used. 

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REMARKS CONCERNING OPTIMAL ISOTOPE DILUTION WORK... 

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