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