Uv Spectroscopy Flashcards
Which molecules absorb uv/ visible light
Conjugated pi systems
Longer conjugation= longer wavelength absorption
What happens when radiation passes through a sample
Some of the radiation is absorbed by the smoke
Some radiation is reflected/ scattered
Some radiation passes straight through
How to minimise the effects of reflection and scattering
Taking the intensity of light (I0) passing through a blank solution and then With a sample solution
Measure the difference
Transmittance (amount of radiation which passed through the sample) equation
T= I/Io
Intensity of unknown sample
Intensity of blank
What’s does the transmittance depend on
Path length of the cell
Concentration of the absorbing substance
Nature of the substance
Effects of Intensity with path length
I decreases exponentially with an increase in path length
I decreases exponentially with an increase in concentration in a fixed path length
Relationship between absorbance A and path length
Absorbance is directly proportional to the concentration of absorbing substance and path length
Beer- Lambert law
A= acl
a= absorption coefficient
C=1, l=1
Molar absorption coefficient
L= cm, c=M
a= molar absorption coefficient (epsilon)
Represents the absorption of a 1 Moldm-3 solution in a cell of path length 1 cm
Specific absorbance
L=cm, c= %m/v
Specific absorbance= A(1%, 1cm)
Represents the absorbance of a 1% m/v solution in a cell of path length 1cm
luminescence
emission of light by molecules without heat
fluorescence
excited molecules rid themselves of the excess electromagnetic energy by emitting the absorbed energy as light of a lower wavelength
chemi-luminescence
some product molecules are left in an excited state
light emitted when these molecules return to the ground state is known as chemi-luminescence
absorption
excitation to higher electronic and vibrational states
vibrational relaxation
through collisions with surrounding molecules, the molecule returns to the v=0 state within the higher electronic state
emission
the molecule releases its energy as a photon and returns to the ground state
emits fluorescence
stokes shift
emitted light is always of lower energy
because some energy is lost to vibrational relaxation in the excited state
because the molecule can return to an excited vibrational state giving a spectrum of emitted light
Difference in energy between light absorbed and light emitted when it returns to its ground state
Kasha’s rule
emission of spectrum is independent of the excitation wavelength
because fluorescence always occurs from the vibrational ground state due to rapid vibrational relaxation
reasons for using fluorescence
more selective than absorption
a substance will absorb radiation at once wavelength and emit at another
not all substance fluoresce
more sensitive
relationship between chemical structure and fluorescence
rigid structures cannot twist/turn
prevents loss of energy
fluorescent
also large number of delocalised electrons
factors affecting the intensity of fluorescence
how much light is absorbed by the sample (more energy absorbed=more energy emitted)
the efficiency with which absorbed light is converted to fluorescence
the experimental setup used to measure the emission
equation relating fluorescence and intensity of light
F is directly proportional to (incident intensity- transmitted intensity)
quantum efficiency
ratio of number of molecules which fluoresce to number that actually absorb energy
no. of quanta emitted/ no. of quanta absorbed
primary inner filter effect
at high concentration, excitation light is mainly absorbed near the entrance face of the cell
creates an uneven distribution of fluorescence
