Welcome viewers to yet another video on analytical instrumentation
In the current video we will be going through the basics of fluorescence and
phosphorescence
so before starting with the basics
let us understand some terms.
We all know that the electrons present in the
atom have a specific spin associated with them.
these electrons pair themselves
with electrons having spins opposite to theirs. as a result the net spin of
the pair of electrons is zero. When the electrons are paired with
electrons having opposite spins they are said to be in a singlet state
however certain times arise when both the paired electrons have same spin
associated with them such a state is called a triplet state. Looking at the
energy level diagram we can try to understand how phosphorescence and
fluorescence are produced
the energy level diagram helps one understand the
production of fluorescence or phosphorescence
As seen in the diagram
there are different energy states in the ground state of molecules the molecular
energies are constant and possess minimum value.
When external radiation
or EMR fall on the molecule it absorbs the energy from external EMR. Due
to this absorption an electronic transition takes place and molecules
transition to higher electronic states, like second electronic singlet state or
first electronic singlet state as depicted in the diagram. It should be
noted that electronic transitions will occur only for specific molecules and
only when bombarded with specific wavelengths of EMR. This is because only
radiation of a specific wavelength and energy is absorbed by characteristic
structure. As seen in the diagram, lambda 1 refers to the wavelength of light
absorbed by the molecules to transition to the first electronic singlet state.
The excited singlet State remains for certain time, its order being 10 to the
power minus 8 to 10 to the power minus 4 seconds after the short time interval
the electrons return back to the Ground singlet State in this transition energy
is released in the form of emissions this phenomenon is called fluorescence
this transition involves less energy than the original absorption so emitted
wavelength is greater than the absorption wavelength that is lambda 3
is greater than lambda 1 one can perform both qualitative and quantitative
analysis of a sample by studying and measuring its fluorescence sometimes
electrons instead of transitioning from first electronic singlet state to ground
state go to first excited triplet state they
stay there for some time before transitioning to the ground singlet
state, this transition is accompanied by emissions being released. this phenomena is
called phosphorescence even phosphorescence helps one perform
analysis of an analyte. fluorescence is immediate emissions of absorbed
radiations whereas phosphorescence is delayed release of absorbed EMR now let
us try to understand the principle of fluorescence measurement again we can
refer to the diagram shown here. As seen the detector is placed
perpendicular to the light path of emitted source, the analyte is placed in
the cuvette these instruments are employed with multiple filters or
monochromators. one filter is placed between light source and analyte. this
filter helps to achieve a good monochromatic beam of a particular
wavelength that will be absorbed by the analyte the second filter is placed
between the analyte and the detector. this filters out the phosphorence radiation
of the stray light and only fluorescence is measured