Page 180 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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17 Adjusting the nebulizer. Measure the sample uptake rate using a 10 ml measuring cylinder and
stop-watch. Report the result in ml min Compare this with your previous reading. Note that the sample
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uptake rate and flame conditions are interdependent and that adjusting one will require re-optimization
of the other. Therefore, do not adjust the sample uptake rate once the fuel/air ratio has been optimized.

18 Effect of monochromator slit width. For a range of monochromator slit width settings between 0.2
and 2.0 nm, zero the absorbance readout and obtain five replicate absorbance readings for the 0.2 µg
ml magnesium standard. Calculate the mean absorbance and RSD for each setting and plot a graph of
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absorbance against monochromator slit width. Select the monochromator slit width setting which gives
the highest absorbance with an acceptable signal-to-noise ratio (indicated by the RSD).

19 The instrument should now be optimized with regard to the above parameters.
20 Determination of magnesium in synthetic human urine. Using the calibration standards already
prepared, obtain five replicate readings for each magnesium standard and calculate the mean
absorbance and RSD for each standard. Using a statistical software package to construct a calibration
graph which includes RSD error bars. Report the linear regression equation and the squared product
moment correlation coefficient for the line of best fit. Identify the linear working range. Using a blank
magnesium 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 *
bl
lod
+ 3s )].
bl
21 You are provided with a sample of synthetic urine. Dilute it 200x and aspirate. Obtain five replicate
absorbance readings. Calculate the mean absorbance and RSD. Ensure that the absorbance reading lies
in the linear response range of your calibration graph. If not, dilute appropriately. Use the regression
equation to determine the concentration of magnesium in the sample.

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

B.3.5 Discussion.

Include answers to the following questions:

1 Explain the effect of adjusting each of the instrumental parameters on the quality of the analytical
signal.