Page 148 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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Figure 5.10
The relative response for equimolar concentration of elements of different mass,
corrected for abundance. This reflects the variable transmission of ions through
the ion lenses and quadrupole.
where C = mass bias correction factor, R = true isotopic ratio for the isotope pair and R =observed
o
t
isotope ratio for the isotope pair.
For the best results, such a correction should be applied to each individual isotopic pair that is to be
ratioed, although this is not always possible in practice since a large number of isotopic standards
would be needed to cover every eventuality.
Also important is the effect of detector dead time. When ions are detected using a pulse counting (PC)
detector, the resultant electronic pulses are approximately 10 ns long. During and after each pulse there
is a period of time during which the detector is effectively 'dead' (i.e. it cannot detect any ions). The
dead time is made up of the time for each pulse and recovery time for the detector and associated
electronics. Typical dead times vary between 20 and 100 ns. If dead time is not taken into account there
will be an apparent reduction in the number of pulses at high count rates, which would cause an
inaccuracy in the measurement of isotope ratios when abundances differ markedly. However, a
correction can be applied as follows:
