Page 150 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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sample transport, ionization efficiency and ion extraction) inherent in the sample introduction system
and the ICP source. The best conditions must be determined empirically for each individual instrument.

5.7.2 Isotope Dilution Analysis

A direct result of the ability to measure isotope ratios with ICP-MS is the technique known as isotope
dilution analysis. This is done by spiking the sample to be analysed with a known concentration of an
enriched isotopic standard, and the isotope ratio is measured by mass spectrometry. The observed
isotope ratio (R ) of the two chosen isotopes can then be used in the isotope dilution equation (Eqn. 5.7)
m
to calculate the concentration of the element in the sample:

where: R = observed isotope ratio of A to B, A = atom fraction of isotope A in sample, B = atom
x
m
x
fraction of isotope B in sample, A = atom fraction of isotope A in spike, B = atom fraction of isotope B
s
s
in spike, W = weight of sample, W = weight of spike, C = concentration of element in sample and C
s
s
x
x
= concentration of element in spike, and M and M s = molar mass of element in sample and spike
x
respectively.
Equation 5.7 can be rearranged into the appropriate form to calculate C as follows:
x
The concentration of any isotope, for example isotope A, can be calculated from the relationship
C A M /M , where M is the molar mass of isotope A.
A
x
x
x
A
The best precision is obtained for isotope ratios near unity (unless the element to be determined is near
the detection limit, when the ratio of spike isotope to natural isotope should be between 3 and 10) so
that noise contributes only to the uncertainty of natural isotope measurement. Errors also become large
when the isotope ratio in the spiked sample approaches the ratio of the isotopes in the spike
(overspiking), or the ratio of the isotopes in the sample (underspiking), the two situations being
illustrated in Fig. 5.11. The accuracy and precision of the isotope dilution analysis ultimately depend on
the accuracy and precision of the isotope ratio measurement, so all the precautions that apply to isotope
ratio analysis also apply in this case.

Isotope dilution analysis is attractive because it can provide very accurate and precise results. The
analyte acts as its own de facto internal standard. For instance, if the isotopic spike is added prior to any
sample preparation,