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UV VISIBLE SPECTROSCOPY - INSTRUMENTATION By Pradnya Mardolkar 1 st year M. Pharm Quality Assurance Department

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Essential components of spectrophotometer are :- Source of Electro magnetic radiation Monochromator Sample compartment Detector/ transducer Recorder/ display

Block diagram of a spectrometer:

Block diagram of a spectrometer


LIGHT SOURCES Should be able to meet the following conditions: Provide sufficient radiant energy over required wavelength region. Provide stable output power during the course of measurements. Supply continuous radiation over entire wavelength used Should be free from any fluctuation

UV Light sources:

UV Light sources HYDROGEN DISCHARGE LAMP H₂ stored under pressure. Electric discharge through lamp excite hydrogen molecules, emitting radiations. High pressure cause collisions between H₂ molecules & results in pressure broadening. Stable, robust & widely used.


DEUTERIUM DISCHARGE LAMP Deuterium used in place of hydrogen. 3 -5 X increase in emission intensity. Expensive than hydrogen lamps.


XENON DISCHARGE LAMP Contain two tungsten electrodes separated by 8mm. Lamp filled with xenon gas stored under pressure ( 10 – 30 atm ) Low voltage application produce intense arc between electrodes & produce UV light. High intensity of light. Limitation:- uv radiation released by lamp causes generation of ozone by ionisation of the oxygen molecule.


Hg (vapor form) held under high pressure. Electric discharge excites atoms. MERCURY ARC LAMP


VISIBLE LIGHT SOURCES TUNGSTEN FILAMENT LAM P Tungsten filament in glass envelope. Useful over range of 350 to 2000nm. Lamp life limited due to- evaporation of tungsten which darkens the inside of the lamp reducing incident light.


TUNGSTEN HALOGEN LAMP Contain small quantity of iodine inside quartz envelope which houses tungsten filament. Iodine reacts with gaseous tungsten to form tungsten iodide. Molecules strike the filament, tungsten is re-deposited. Quartz required for high operating temperature of the lamp.


CARBON ARC LAMP Used when more intense source of visible light is required.


MONOCHROMATORS AND FILTERS Filters Device which allows878 radiation of required wavelength to pass through Wholly or partially absorb unwanted radiation.

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Absorption filters Interference filters


DIFFERENCE Absorption filters Work by selective absorption Made of glass Contain chemicals(dyes) that absorb all radiations except that desired to pass. Color absorbed is complement of the color of filter Interference filters Narrower bandwidth of about 15nm Consist of two parallel glass plates silvered internally Separated by thin film of dielectric material ( e.g.- cryolite ). Interference of light waves eliminate undesired radiation


Monochromators Converts polychromatic beam of light into a monochromatic beam Can isolate a selected narrow band of wavelength anywhere within a comparatively wide spectral range. Consists of:- Entrance slit Collimator 1 Prism or grating Collimator 2 Exit slit

Types of Monochromators:

Types of Monochromators Prism monochromators : Non linear dispersion- shorter wavelength dispersed to greater degree than the longer ones. Made of glass, quartz or fused silica. Glass- visible. Quartz, fused silica- ultraviolet region.

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White light, through prism disperses into polychromatic light(rainbow.) Rotation of prism to pass required wavelength through exit slit. Effective wavelength depends on- dispersive power of wavelength material optical angle of the prism.

Types of mounting:

Types of mounting CORNU TYPE Optical angle 60° Emerging light fall on exit slit on rotation. LITTROW TYPE Optical angle 30° One surface is aluminized. Light passes through prism & emerges on same side as that of light source.

Diffraction gratings :

Diffraction gratings Large number of parallel lines (grooves), ruled on highly polished surface such as alumina. Done with a suitable shaped diamond tool. For UV/ Visible region about 15000- 30000 lines/square inch are drawn. Incident light diffracted over range of angles. (grooves act as scattering centre)

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Construction of master grating is tedious, time consuming & expensive. Identical grooves, exactly parallel, equally spaced over length of gratings (3-10cm). Replica gratings prepared by coating original with epoxy. Made reflective by coating surface with aluminium, gold or platinum.

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Law of diffraction nλ= d (sini±sinθ)

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ECHELLETE GRATING Grooved to have broad faces for reflection and narrow unused faces. Efficient diffraction of radiation due to geometry CONCAVE GRATING Grating formed on concave surface disperses radiation and focuses it on exit slit. Eliminates need of auxillary collimating/ focusing mirrors. Cost reduction Reduction in no. of optical surface increase energy throughout monochromator .

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Holographic grating Pair of identical lasers used on glass surface coated with photoresist . Interference fringes from two beams sensitize photoresist ,dissolves leaving a grooved surface. Coated with aluminium . Greater perfection with respect to shape and dimension. Give spectra free from stray radiation & ghosts.


SAMPLE CONTAINERS Cells or cuvettes used to hold the sample. Shape- rectangular or cylindrical. thickness- 1 cm Cells --- transparent in the wavelength region being measured. REGION TYPE OF MATERIAL USED FOR CELLS OR CUVETTES VISIBLE GLASS OR PLASTIC UV REGION QUARTZ OR FUSED SILICA

Radiation Transducers/ detectors :

Radiation Transducers/ detectors Properties of radiation transducers High sensitivity High signal to noise ratio Constant response over a considerable range of wavelength Exhibit fast response time Zero output signal in the absence of illumination

Types of radiation transducers:

Types of radiation transducers Two types :- Photon transducers/ photoelectric or quantum detectors- respond to photons Heat transducers- responds to heat Types of photon transducers - Photovoltaic cells Phototubes Photomultiplier tubes Photoconductivity transducers Silicon photodiodes Charge transfer transducers

Barrier layer or photovoltaic cell:

Barrier layer or photovoltaic cell

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Advantages Requires no power supply. Disadvantages Lack of sensitivity.


Photoemissive tubes

Advantages: :

Advantages: More sensitive than BLC- used in UV & Visible region Output current can be amplified- used in measuring low intensity radiations.


PHOTOMULTIPLIER TUBE Successive phototubes built into one envelope. Photo cathode, series of electrodes, each at more positive potential than the one before it, an anode.


Disadvantages Measures low power radiations as intense light damages photo electric surface. Sensitivity limited by its dark current emission. Advantages More sensitive then phototube in UV and visible region Ideal for measuring weak light intensity Fast response time


RECORDERS/ READ OUT SYSTEMS Signal from the detector is proportional to the intensity of light incident on the detector. After amplification it is displayed as percentage transmittance (%T) or as absorbance. Common systems employed for displaying %T or absorbance are: Moving coil meter Digital display Strip chart recorder.

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REFERENCES Skoog , Holler, Crouch; INSTRUMENTAL ANALYSIS , Indian Edition; Pg no: 203-213;392-396 Willard, Meritt , Dean,Settle ; INSTRUMENTAL METHODS OF ANALYSIS , seventh edition; Pg no: 118-148 A.H Beckett,J.B . Stenlake ; PRACTICAL PHARMACEUTICAL CHEMISTRY , fourth edition- part two; Pg no: 264-270;272-274 G.R. Chatwal , S.K. Anand ; INSTRUMENTAL METHODS OF CHEMICAL ANALYSIS ; Pg no: 2.167-2.172 www.google.com

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