Page 184 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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calibration graph of the pooled data (ascending and descending) which includes RSD error bars.
Identify the linear working range. Report the linear regression equation and the squared product
moment correlation coefficient for the line of best fit. Using a paired t-test determine whether there
is any significant difference between the emission data of the standards introduced in ascending
and descending order. The significant difference t is defined as t = , where is the mean
d
difference between equivalent standards in ascending and descending order, n is the number of
standards and s the standard deviation of the differences. The critical value of [t] for a confidence
d
interval of 95% is 2.78. Using a blank sodium standard, estimate the instrumental limit of detection
[where the limit of detection is defined as the sum of the mean blank signal plus three standard
deviations of the mean blank signal (X = X * + 3s )]. Obtain five replicate emission intensity
bl
lod
bl
readings of the prepared sample solution. Calculate the mean emission intensity and RSD. Ensure
that the emission reading lies in the linear response range of your calibration graph. If it does not,
dilute appropriately. Use the regression equation to determine the concentration of sodium in the
sample.

16 Shut down the instrument using the manufacturer's recommended procedure.

B.4.5 Discussion

Include answers to the following questions:

1 Explain the shape of the graph showing the effect of adding potassium to sodium. What is the
mechanism of this effect and why is potassium added to all solutions in the determination of the sodium
in the extract? Would you expect to see the same effects in atomic absorption and, if so, why?

2 Report the analytical figures of merit (i.e. limit of detection, linear range) for the sodium calibration
and compare with literature values. Discuss the implications of using a curved calibration graph.

3 Report (with RSD) the result for the concentration of sodium in the soil extract sample and compare
with the literature values for soils.

4 What are the most probable sources of error in this experiment? Does the method used offer an
acceptable level of precision and accuracy for the determination of sodium in soil extracts and what are
the reasons for your conclusions?
5 Comment on the results of the paired t-test.