INVESTIGATING LIGHT-MATTER INTERACTIONS USING A MANUAL SPECTROMETER - A NEW CHM151 LAB
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Transcript of INVESTIGATING LIGHT-MATTER INTERACTIONS USING A MANUAL SPECTROMETER - A NEW CHM151 LAB
INVESTIGATING LIGHT-MATTER INTERACTIONS USING A MANUAL
SPECTROMETER - A NEW CHM151 LAB
Developed by E. Kwan, H. Ohorodnyk, I. Miller, A. Orozco, and A. Dhirani with assistance from F. Bures and J. Jackiewicz (electronics)
as well as J. Ford and F. Shaw (machining).Department of Chemistry, University of Toronto.
Introduction
Beer’s Law Intensity of a beam of light decreases exponentially with the number of absorbing particles in the beam:
I/I0 = 10-A
where I is the final intensity, I0 is the initial intensity, and A is the absorbance. A = c b, where is the extinction coefficient, c is the concentration of the solution, and b is the path length.
Goals of Project Students explore the validity, limitations, and applications of Beer’s Law Students are introduced to spectroscopy in a hands-on way
The Spectrometer
Double Convex
Lens
Lamp
Sample Holder
Light Detector and I-V Converter
Multimeter
Diffraction pattern
Photodiode
Diffraction Grating
Light intensity measured by photodiode and multimeter Detector position on rail determines wavelength detected
Rails
Samples
Filter ground up leaves soaked in
methanol
Extracted chlorophyll
(1) Various filters
(2) Copper (II) Sulfate
Geraniums
(3) Chlorophyll
0 1 2 3 4 5 6 7 8 9 10
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Chi^2 = 2.49934R^2 = 0.99753
y = 0.28 + 2.75*e-x/1.31
Sig
na
l (V
)
Number of Blue Filters
Blue Filters: Background Removal
Red light intensity measured against number of filters Data fit to y = y0 + y1 e-x/t, an exponential decay y0 is the background (stray light)
Blue Filters: Absorbance Plot
0 1 2 3 4 5 6
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
y = 0.31x + 0.02
R2=0.99611Std. Dev.=0.17359
Ab
sorb
an
ce
Number of Filters
Absorbance calculated as –log[(I-y0)/(I0-y0)] Absorbance increases linearly with numbers of filters Slope is 0.31 (represents absorbance per blue filter) Signal:noise ratio gets much worse
Green Filters: Absorbance Plot
0 1 2 3 4 5 6 7 8-0.5
0.0
0.5
1.0
1.5
2.0
y = 0.15x + 0.01
R2=0.99934Std. Dev.=0.05805
Ab
sorb
an
ce
Number of Green Filters
Absorbance increases linearly but with a different slope, 0.15. Data measured at same red wavelength
Blue and Green Filters: Absorbance
0 1 2 3 4-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
y = 0.45x + 0.02
R2=0.99615Std. Dev.=0.20074
Ab
sorb
an
ce
Number of Filter Pairs
Slope is now 0.45, statistically the same as the sum of 0.31 + 0.15 => slopes add when filters combined So absorbance is additive
Copper Sulfate Absorbance
0.00 0.02 0.04 0.06 0.08 0.10 0.12-0.25
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
y = 10.6x + 0.01
R2=0.99713Std. Dev.=015755
Absorb
an
ce
Concentration (M)
Absorbance increases linearly with concentration Graph can now be used to determine concentration of a solution by measuring its absorbance
Green light transmitted Red light absorbed and re-emitted (sideways also) Some blue light converted to red light -> fluorescence (covered slit with blue filter and observed red light out)
Spectroscopy of Chlorophyll
Green Diffraction Maximum
Green Reflection
Chlorophyll appears red!Light Shield
Transmitted Light
Chlorophyll viewed from the sideChlorophyll viewed in
normal light
Special thanks to P.E. Trudeau and Y. Suganuma