Spectroscopy Measurements – SPECORD UV VIS
Introduction:
Absorbancy (absorption) is the physical quantity servant to describe amount of
absorbed (rapt) radiation. As shows fig. 1 intensity of bundle after the passage through the
matter decrease. We can describe absorbancy by equation:
A  log
I0
I
.
x
I0
I
Fig. 1
Transmittance is the physical quantity which describe amount of bulk radiation. It is
definite by the relationship:
T
I
I0
or in percentage:
T
I
100% .
I0
Relationship between absorbancy and transmittance shows equation:
log
or exspress in percentage:
1
A
T
A  2  log T
where:
A -- the absorbancy
I 0 - intensity of incident beam
I - intensity of beam which pass through the solution
T – transmittance.
The absorption spectrum describe the relationship between amount of absorb radiation
and wavelength of incidental radiation. In the visible range it could be dark lines or band on
the constant emission spectrum and it is characteristic for every substance.
Wavelength λ – distance between two neighboring points in the same phase.
wavelenght
The most popular units express wavelength are :
1nm  10 9 m
1 m  10 6 m .
1A  10 10 m
Range of wavelength of electromagnetic radiation
Range of
radiation
Wavelenght
[ nm]
UV-C
UV
UV-B
UV-A
100-280
280-315
315-380
VIS
380-780
IR-A
IR
IR-B
780-1400
1400-3000
IR-C
300010000
Others physical quantity characteristic for electromagnetic wave are:


1
Frequency ν. Frequency’s unit is Hz  . It define number of vibration in time unit.
s
Wave number k, which describe number of wavelengths in length unit. It’s unit is Kejzer.
1K 
1
cm
Relationship between wave number and wavelength describe equation:
k
1
.
Absorption laws:
Lambert law
I  I 0 e  x
where :
A - the absorbancy
I 0 - intensity of incident beam
I - intensity of beam which pass through the thickness x
α – absorption coefficient which is characteristic for every substance
x - thickness of the layer
Hence, I of electromagnetic radiation exponentially decreasing with the increasing length of
absorber.
x
I0
I
Fig. 1
Beer`s Lambert law- refer multicomponents solutions:
I  I 0e  cx
where c is concentration of solution.
α
A
c
c
Absorption coefficient is independent of concentration of absorber. Absorption increase with
the increasing concentration..
Absorption Additivity law
Absorption value of several component is equal to sum of absorption values of each
components.
Ac  A1  A2  ...  An
Application:
NMR spectroscopy, detection of various matters
Golas:




Identification of unknown substance with the help of absorption spectrum
Determine the absorption spectrum of chosen substance
Check the relationship between absorption and concentration of the substance
Find concentration of the substance witch the help of curve A=f(c)
1
2
3

Procedure:
1.
2.
3.
4.
Turn on the device (switch nr 1 on the position 1).
Turn on the source of light (switch nr 2 on the position 1).
Put the graph paper and marker on the right place.
On the path of light put the measurements cuvettes, one with the distilled water
(reference) and another with the solution under investigation (sample). Put their smoth
surfaces to the light.
5. Choose the value of wave number k (for which we begin the measurement) with the
help of switch nr 3 (change „+” or „-„). Actual value of wave number k we read of the
scale.
6. Turn on measurement of absorption spectrum – switch nr 3 set on the position „-„.
7. During the measurement make some control- read of wave number k – stop on the
moment the measurement (switch nr 3 on position „0”).
8. Repeat the measurements for other 10 different concentrations solutions. Read and
tabulate only the extreme values of absorbance and transmittance.
Amax




Tmin
C
Make graphs Amax=f(c) and Tmin=f(c).
With the help of the SPECORD UV VIS study the same substance with unknown
concentration. Mark Amax and using the graphs Amax=f(c) find it’s concentration.
With the help of the SPECORD UV VIS study the absorption spectrum of another
substance.
With the help of the SPECORD UV VIS study the third substance and using collected
absorption spectrum define it’s composition.