Page 179 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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10 After ignition aspirate water continuously. Using a 10 cm measuring cylinder, check that the
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uptake rate of the nebulizer is within the range quoted by the manufacturer. Adjust if necessary.
11 Zero the absorbance reading.

12 Set the instrument integration time (or the read out update) for 2 s.

13 Aspirate the 0.2 µg ml standard. This should give an absorbance reading of about 0.2.
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14 Effect of hollow-cathode lamp current. Vary the hollow-cathode lamp current in steps of 1 mA, ±3
mA either side of the manufacturer's recommended value, ensuring that the maximum allowable lamp
current is not exceeded. Take five replicate absorbance readings at each lamp current setting.
Remember to zero the instrument at each setting whilst aspirating a blank and adjust the gain if
necessary before taking the five replicate readings. Calculate the average absorbance and relative
standard deviation (RSD) for each setting and plot a graph of absorbance against lamp current. The
RSD is defined as standard deviation (s) of the absorbances divided by the mean absorbance (x). [RSD
= (s/x) x 100]. Select the lamp current which gives the highest absorbance with an acceptable signal-to-
noise ratio (indicated by the RSD).

15 Effect of burner head height. Raise the burner head to the highest position which will not obscure
the path of the light beam. Zero the absorbance read out and obtain five replicate absorbance readings
for the 0.2 µg ml magnesium standard. Lower the burner head in 2 mm stages and obtain five replicate
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absorbance readings for the 0.2 µg ml magnesium standard at each height. Calculate the mean
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absorbance and RSD for each setting and plot a graph of absorbance against burner head height. Select
the burner head height which gives the highest absorbance with an acceptable signal to noise ratio
(indicated by the RSD).

16 Effect of varying the fuel/air ratio. Vary the fuel/air ratio by increasing the fuel flow rate in steps
until a strongly yellow (fuel rich) flame is obtained. Zero the absorbance read out and obtain five
replicate absorbance readings for the 0.2 µg ml magnesium standard. Reduce the fuel flow rate in
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stages and obtain five replicate absorbance readings for the 0.2 µg ml magnesium standard at each
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setting. Calculate the mean absorbance and RSD for each setting and plot a graph of absorbance against
fuel/air ratio. Select the fuel/air ratio which gives the highest absorbance with an acceptable signal-to-
noise ratio (indicated by the RSD).