PASCO OS-8539 Educational Spectrophotometer - Manual (Page 25)

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M o d e l N o . O S - 8 5 3 7 a n d O S - 8 5 3 9 A c t i v i t y 2 : A b s o r p t i o n ( D a r k L i n e ) S p e c t r u m
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1. Darken the room. Examine the spectrum closely. Determine which of the two first order spectral patterns is
brightest.
2. Use the Light Sensor Arm on the Spectrophotometer to turn the Degree Plate until the light sensor is beyond
the last color in the brightest first order spectral pattern.
Record Data - Empty Cuvette
1. Set the GAIN select switch on top of the High Sensitivity Light Sensor to 10.
2. Start recording data.
3. Push on the threaded post under the light sensor to slowly and continuously scan the spectrum in one direc-
tion. Scan all the way through the first order spectral pattern on one side of the central ray, through the cen-
tral ray itself, and all the way through the first order spectral pattern on the other side of the central ray.
4. Stop recording data.
Record Data - Cuvette with Liquid Sample
1. Remove the cuvette and fill it three-quarters full with the liquid sample you are testing. Cap the cuvette and
replace it in front of the sensor.
2. Start recording data.
3. Push on the threaded post under the light sensor to slowly and continuously scan the spectrum in one direc-
tion. Scan all the way through the first order spectral pattern on one side of the central ray, through the cen-
tral ray itself, and all the way through the first order spectral pattern on the other side of the central ray.
4. Stop recording data.
Analyze the Data
Refer to the User’s Guide for your version of the data acquisition software for detailed information on displays.
1. Use the Graph display to compare the plot of Light Intensity versus Actual Angular Position for the first run
of data (empty cuvette) to the plot of Light Intensity versus Actual Angular Position for the second run of
data (cuvette plus liquid sample).
2. Use the built-in analysis tools of the program to find the angle of the first gap or “dark line” in the absorption
spectrum of the liquid sample. Find the angle of the corresponding gap or line in the first order on the other
side of the central ray.
3. Determine the difference between the angles and use one-half of the difference as the angle, , to determine
the wavelength, , of that gap or dark line. (If you did not calibrate the Diffraction Grating, assume d = 1666
nm.)
 = d sin 
4. Repeat the process for the other gaps (if any) in the first order spectral pattern.