Page 194 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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2 Calculate the weight of a 1 µg g solution of enriched Cd that you would need to add to the
106
-1
natural Cd to give a Cd: 111 Cd ratio of approximately 1, using Eqn. B.3. Accurately weigh as close as
106
possible to this mass of the enriched Cd solution into the polypropylene bottle.
106
3 Make up to 100 g with distilled, de-ionized water.
4 This is the 'optimization solution'.

B.7.5.2 Dwell Time

The dwell time is the time spent acquiring data at each of the channels which make up a peak in the
mass spectrum. The length of time is measured in fractions of a millisecond, and will ultimately affect
the frequency with which data is acquired at each mass. This will have a bearing on the final precision
of the isotope ratio because of the influence of various sources of 'noise' on the analytical signal.

To optimize the dwell time, set up a data acquisition procedure using the dwell times shown in Table
B.4, aspirate the 'optimization solution' and acquire count rate data for the 106 Cd and 111 Cd isotopes. The
method for setting the data acquisition parameters will vary between instruments, but a similar
procedure should be possible for all makes of instrument. Record the data in Table B.4, calculate the
mean and RSD for the Cd: Cd ratio for each dwell time and hence determine the best precision.
111
106
B.7.5.3 Points Per Peak

The number of points, or channels, which are chosen for each isotopic peak will also affect the
frequency of data acquisition, and hence the final precision of the isotope ratio measurement. Three
points per peak is a typical value, but this too could be optimized.
Table B.4 Dwell times and precision.
Dwell Mean RSD
time (ms) Replicates 106 Cd: 111 Cd ratio SD (%)

0.05 10
0.1 10

0.5 10
1 10
5 10

10 10
50 10