1. Lecture 17
Spectrophotometry

2. Emission Absorption Fluorescence source sample source sample Secondary

3. Spectrometer Monochromator Detector (filter, wavelength selector)
Light Source
Detector
Sample
Spectrometer
Data Processing

4. Spectrometer Monochromator Detector Light Source (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

6. Absorbing plate

7. I0 IN
<1
Transmittance = I0 I1 I2 I3
We almost never use transmittance! I4 I5 T
This is a curve!
concentration

8. dP=  dP P P0 P1 Absorbing plate N=CVolume Volume= 1  dx N = C  dx
Incident light
Emergent light P P0 P1 dP l dx

9. Sensitivity is the same for any
Absorbing plate
absorbance
Incident light
Emergent light P0 P1
Sensitivity is the same for any
power (P) l

10. Beer’s law
Bugert, Lambert and Beer A
Straight line!
concentration

11. 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.

13. k = Slope = y / x y=kx+b straight line equation b - blank! Y1=kx1
Let us subtract blank:
y-b = Y = kx
Y1=kx1
Y2=kx2
One standard

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