Page 176 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

Page 162

Find the characteristic concentration (i.e. the concentration corresponding to an absorbance of 0.0044)
for nickel (in iron solution) using this instrumentation by (a) interpolation of the graph; (b) calculation
from the slope.

If a 1% solution of steel gave an absorbance of 0.36, what would be the concentration of nickel in the
solution, and hence the steel sample as a w/w percentage?

(Answers: 0.08 mg , 6.5 mg , 0.065%)
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2 The procedure for the method of standard additions was described in Section 1.4.1. The following
data were obtained for the determination of copper in a contaminated stream by inductively coupled
plasma atomic emission using the 324.754 nm copper line.

Analysis solution
Stream water + Distilled water + 1 µg cm Cu standard Net signal
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3
3
3
(cm ) (cm ) (cm ) (cps)
5 5 0 440
5 4 1 1540

5 3 2 2640
5 1 4 4800

Plot the emission signal (y-axis) against the added copper content of the solution (in µg; x-axis),
extrapolate the graph and, from the negative x-intercept, calculate the concentration of copper in the
original stream water sample.

(Answer: 0.08 µg cm )
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3 The calculation of the standard deviation of a series of readings can be used both to give an estimate
of the precision and to calculate the limit of detection (see Section 1.4.2). The following data were
obtained for cadmium at m/z 111 when spraying a standard solution of 0.1 ng ml cadmium, using an
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inductively coupled plasma mass spectrometer when using a 60 s scan over the range m/z 5-255.
Sample No. 1 2 3 4 5 6 7 8 9 10 11
Signal (CPS) 510 430 480 570 550 510 440 490 550 590 590

Calculate the mean count rate.

Calculate the standard deviation for the series of readings.

Calculate the detection limit as the concentration which would apparently give a reading equal to three
times the standard deviation.

(Answers: 519 cps; 56 cps; 0.022 ng ml )
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