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Line Wing Overlap

[Cell definitions and equations] [Student assignment handout]

This worksheet has only one simple purpose: to demonstrate graphically how the Lorentzian line wings of an intense matrix line can cause a background spectral interference even if the analyte line is many line-widths away from the interfering matrix line. (Because this simulation deals with spectral effects far from the line center of the interfering line, it does not include the effects of Doppler broadening or of instrumental broadening due to non-zero spectral bandpass). Note: The effect of continuum background emission is commonly compensated by using a wavelength modulation system or photodiode array detector with appropriate software.

Download links: LineWingOverlap.wkz;

Wingz player application and basic set of simulation modules, for windows PCs or Macintosh

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Inputs:

lamMax = wavelength of analyte band, nm
awidth = width of analyte band, nm
(intensity of analyte band is fixed at 1)
position = wavelength of interfering band, nm
width = width of interfering band, nm
intensity = intensity of interfering band

Graphs:
analyte band shape = 1/(1+((lamMax-wavelength)*2/awidth)^2)
interfering band shape = intensity/(1+((position-wavelength)*2/width)^2)

The red line on the graphs shows sum of analyte and interfering bands; the
blue line shows the interfering band alone for comparison.pre>

Student Assignment

This particular worksheet is set up for the example of the analysis of Mg at 285.21 nm in the presence of variable amounts of Na, which has a line at 285.28 nm. The Na line at 285.28 nm is not the usual analytical line for Na analysis; it is much weaker than other Na lines, but Na is often present in such large  concentrations in biological and environmental samples that it's quite possible  that this line may be much stronger that the lines of trace components.

The plot at the bottom of the screen shows both the analyte line (on the
left) and the interfering line (on the right). The smaller plot on the top right shows only the region around the analyte line. You can change the numbers in boldface in the table on the left; except that the intensity of the analyte line is fixed at 1.0. Set both line widths to 0.002 nm (a reasonable "average" value).

1. Start with the intensity of the interfering line (Na) equal to 1 and increase by decades to 10, 100, 1000... At first there is no interference; the analyte line intensity stays at 1.0. But note how the background shifts up and tilts as the Na intensity increases.

2. Change the width of the interfering line and note the effect.

3. Set the width of the interfering line back to 0.002 nm. Determine the largest interfering line intensity that will produce a relative error of no more than 10% in the intensity of the analyte line. Can you suggest a measurement procedure that would provide an effective background correction for this interference and improve the tolerance for interfering intensity? (You may print out the worksheet to get a clearer picture).

4. There is a much stronger sodium line at 590 nm. Assuming it has a width of 0.004 nm and an intensity 100X greater that the 285.28 nm line, would that line cause a significant interference?


(c) 1992, 2000, Prof. Tom O'Haver , Professor Emeritus,
The University of Maryland at College Park.
Comments, suggestions and questions should be directed to
Prof. O'Haver at toh@umd.edu.
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