1
2304 Analytical chemistry
Continuation of sources of radiation
Recent lamps – halogen lamps
Has the same dimensions as older lamps.
Made from quartz-halogen or quartz iodine lamp. At temps above 250° C, iodine vaporizes
combining chemically with the evaporated tungsten to form tungsten halogenide (WI 2) which
diffuses (or migrates) back to the hot filament and chemically decomposes releasing the tungsten.
This permits operation of the lamp at temperatures approx. 3600K, nearly a 5 fold increase in the
spectral radiance relative to the earlier W – lamp.
Quartz envelope permits wider wavelength range eg 200 -3000 nm. Also Quartz tolerates a higher
operating temperature. This lamp has the ability to maintain 90% of its initial light output thoughout
their lifetime.
Arc discharge sources
The spectral radiance at 200nm for W-lamps is low, thus not particularly useful.
Continuous spectrum in UV region is achieved using H2 or D2 arc discharge lamps
They are comprised of a suitable gas at low pressure (0.2-0.5 torr) sealed in an envelope, operated at
low voltages (30 -100V, 0.3 -1.3A) if a preheated cathode is used. The preheated cathode produces
some ionisation of the fill gas which facilitates firing or initiation of the arc.
The short wavelength cutoff is dependent on the envelope material
Quartz 200nm
Fused silica 185 nm
Uviol glass 240 nm
The maximum radiant intensity is at 280 nm, with an upper limit at 375 nm. D 2 lamp gives X3-5 times
more intensity than H2.
We also have Hg Arc lamp, used for line source.
D2 lamp and W lamps are used in UV-visible spectral photometers with automatic switching from
one lamp to the other.
High pressure Xe discharge lamps are used in fluorescence equipment
, 2
Wavelength selection
In order to isolate from the continuous source a beam of radiation of high spectral purity of any
desired or required wavelength, filters or a monochromator is used.
Monochromator uses slits and a dispersing device to isolate the spectral band. Radiation from the
source is focused on the exit slit, collimated by lens or mirror to a parallel beam which falls onto the
dispersing element.
Filters are used in simple equipment. Filters absorb all the radiation except the required range of
frequencies. They give poor selectivity since they do not offer a beam or band pass with a narrow
range of transmission characteristics. Also the scattered radiation is relatively high. Filters give 50-
80% radiation throughput efficiency and are characterised by the nominal wavelength, maximum
percentage transmittance and effective bandwidth.
Effective bandwidth is called the bandpass.
Absorption filters
They are made from a coloured glass plate which functions by absorption of part of the incident
radiation. The common types are:
Gelatin filters – made using a suitable organic dye that is sandwiched between two glass
plates
Glass filters – made from a solution of absorbing components sandwiched between glass
plates
, 3
Tinted glass filters – solid sheet of glass that has been coloured with pigment which is either
dissolved or dispersed in the glass
Plastic filters – plastics in which colour has been introduced in the batch or during dye
treatment on a clear base.
Normally, filters are used in series. Short λ cutoff and long λ cutoff filters are popularly used as
blocking filters to suppress unwanted spectral orders from interference filters and diffraction
gratings and for the elimination of stray light. When applied to select a band, they give low
transmittance, 5-20% for peak transmission, a wide band width 20-70 nm. For narrower bandwidths,
they may give less than 1% transmittance at nominal wavelength. They have the advantage of
insensitivity to input angle.
Interference filters
Work on the principle of optical interference to provide a relatively narrow beam of radiation. They
consist of a thin film of dielectric spacer (transparent material) eg CaF 2 or MgF2 or SiO2 sandwiched
between two parallel, partially metal films (usually silver) mirrored glass plates. These plates reflect
half of the incident radiation while the other half is transmitted. Also called dichroic filters.
2304 Analytical chemistry
Continuation of sources of radiation
Recent lamps – halogen lamps
Has the same dimensions as older lamps.
Made from quartz-halogen or quartz iodine lamp. At temps above 250° C, iodine vaporizes
combining chemically with the evaporated tungsten to form tungsten halogenide (WI 2) which
diffuses (or migrates) back to the hot filament and chemically decomposes releasing the tungsten.
This permits operation of the lamp at temperatures approx. 3600K, nearly a 5 fold increase in the
spectral radiance relative to the earlier W – lamp.
Quartz envelope permits wider wavelength range eg 200 -3000 nm. Also Quartz tolerates a higher
operating temperature. This lamp has the ability to maintain 90% of its initial light output thoughout
their lifetime.
Arc discharge sources
The spectral radiance at 200nm for W-lamps is low, thus not particularly useful.
Continuous spectrum in UV region is achieved using H2 or D2 arc discharge lamps
They are comprised of a suitable gas at low pressure (0.2-0.5 torr) sealed in an envelope, operated at
low voltages (30 -100V, 0.3 -1.3A) if a preheated cathode is used. The preheated cathode produces
some ionisation of the fill gas which facilitates firing or initiation of the arc.
The short wavelength cutoff is dependent on the envelope material
Quartz 200nm
Fused silica 185 nm
Uviol glass 240 nm
The maximum radiant intensity is at 280 nm, with an upper limit at 375 nm. D 2 lamp gives X3-5 times
more intensity than H2.
We also have Hg Arc lamp, used for line source.
D2 lamp and W lamps are used in UV-visible spectral photometers with automatic switching from
one lamp to the other.
High pressure Xe discharge lamps are used in fluorescence equipment
, 2
Wavelength selection
In order to isolate from the continuous source a beam of radiation of high spectral purity of any
desired or required wavelength, filters or a monochromator is used.
Monochromator uses slits and a dispersing device to isolate the spectral band. Radiation from the
source is focused on the exit slit, collimated by lens or mirror to a parallel beam which falls onto the
dispersing element.
Filters are used in simple equipment. Filters absorb all the radiation except the required range of
frequencies. They give poor selectivity since they do not offer a beam or band pass with a narrow
range of transmission characteristics. Also the scattered radiation is relatively high. Filters give 50-
80% radiation throughput efficiency and are characterised by the nominal wavelength, maximum
percentage transmittance and effective bandwidth.
Effective bandwidth is called the bandpass.
Absorption filters
They are made from a coloured glass plate which functions by absorption of part of the incident
radiation. The common types are:
Gelatin filters – made using a suitable organic dye that is sandwiched between two glass
plates
Glass filters – made from a solution of absorbing components sandwiched between glass
plates
, 3
Tinted glass filters – solid sheet of glass that has been coloured with pigment which is either
dissolved or dispersed in the glass
Plastic filters – plastics in which colour has been introduced in the batch or during dye
treatment on a clear base.
Normally, filters are used in series. Short λ cutoff and long λ cutoff filters are popularly used as
blocking filters to suppress unwanted spectral orders from interference filters and diffraction
gratings and for the elimination of stray light. When applied to select a band, they give low
transmittance, 5-20% for peak transmission, a wide band width 20-70 nm. For narrower bandwidths,
they may give less than 1% transmittance at nominal wavelength. They have the advantage of
insensitivity to input angle.
Interference filters
Work on the principle of optical interference to provide a relatively narrow beam of radiation. They
consist of a thin film of dielectric spacer (transparent material) eg CaF 2 or MgF2 or SiO2 sandwiched
between two parallel, partially metal films (usually silver) mirrored glass plates. These plates reflect
half of the incident radiation while the other half is transmitted. Also called dichroic filters.
