Spectrophotometry

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Lecture 17

• Spectrophotometry

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Emission
source
sample
Absorption
Fluorescence
source
sample
Secondary emission
3
Monochromator (filter, wavelength selector)
Light Source
Detector
Sample
Spectrometer
Data Processing
4
Monochromator (filter, wavelength selector)
Light Source
Detector
Sample
Spectrometer
Data Processing
5
Light striking a sample can be 1. reflected
2. transmitted 3. absorbed 4. scattered
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Absorbing plate
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I0
IN
lt1
Transmittance
I0
I1
I2
We almost never use transmittance!
I3
I4
I5
T
This is a curve!
concentration
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Absorbing plate
NC?Volume
Volume 1 ? dx
dP
??
N C ? dx
N
? P
Incident light
Emergent light
P
P0
P1
dP
l
0
dx
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Absorbing plate
absorbance
Incident light
Emergent light
P0
P1
Sensitivity is the same for any power (P)
l
0
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Bugert, Lambert and Beer
Beers law
A
Straight line!
concentration
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Least-squares curve fitting.
The points (1,2) and (6,5) do not fall exactly on
the solid line, but they are too close to the
line to show their deviations. The Gaussian curve
drawn over the point (3,3) is a schematic
indication of the fact that each value of y is
normally distributed about the straight line.
That is, the most probable value of y will fall
on the line, but there is a finite probability of
measuring y some distance from the line.
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(No Transcript)
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ykxb straight line equation
k Slope ?y / ?x
b - blank!
Let us subtract blank y-b Y kx
Y1kx1 Y2kx2
One standard
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• Procedure
• Measure blank.
• Measure standard.
• Measure unknown.
• Subtract blank from standard and from unknown.
• Calculate concentration of unknown

If you have several (N) standards, do it several
(N) times