Presentation1

UV SPECTROSCOPY

Shruti VermaM.Sc. 3rd sem

(INSTRUMENTATION)

INTRODUCTION A spectrophotometer is an instrument for measuring the transmittance or absorbance of a sample as a function of the wavelength of electromagnetic radiation.

The key components of spectrophotometer are-

Light Source

Wavelength selectors

Sample Cuvet

Detectors

Display Unit

SOURCE

• It must be stable

• It must be of sufficient intensity for the transmitted energy to be detected at the end of the optional path.

•It must be supply continuous radiation over the entire wavelength region in which it is used.

Tungsten Lamp• Use in the region 350-2000nm.• These measure most effectively in the visible region 320-1100nm. • Instruments that only use tungsten lamp as a light source will only measure in the visible region.

Hydrogen Discharge Lamp• Consist of two electrode contain in hydrogen filled silica envelop.•Gives continuous spectrum in region 180-380nm.• Above 380nm emission is not continuous.

Deuterium Lamp• Measure in UV region 190-370nm.• Operate at high temperature. Normal glass housing can not used for casing. Instead, a fused quartz, UV glass, or magnesium fluoride is used.• Lamp life is approximately 1000hrs.

Xenon Discharge Lamp• Use in region 200-800nm.• Life time is approximately 500-1500hrs.•It consist of fused quartz or other heat resistant glass arc tube, with a tungsten metal electrode at each end.

Mercury Arc• Wavelength range is 200-600nm.• It is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light.• Mercury lamps are more energy efficient with luminous efficacies of 35-65 lumens/watt.

WAVELENGTH SELECTORS

Filters

Monochromators

FILTERS• It permit certain bands of wavelength (bandwidth of ~50nm)to pass through.• The simplest kind of filter is absorption filters, the most common of this type of filters is colored glass filters.

DISADVANTAGE- They are not very good wavelength selectors because they allow the passage of broad bandwidth and they absorb a significant fraction of the desired radiation.

MONOCHROMATOR The monochromator is used to disperse the radiation according to the wavelength. The essential elements of a monochromator are an entrence slit, a dispersing element and an exit slit. The entrence slit sharply defines the incoming beam of hetrochromatic radiation. The dispersing element disperses the hetrochromatic radiation into its component wavelengths. The position of despersion device is always adjust by rotating motor.

The exit slit allows the nominal wavelength together with a band of wavelengths together with a band of wavelengths on either side. The dispersing element may be Prism of Grating.

Prism

• Dispersion by prism depends on refraction of light which is wavelength dependent.• violet color with higher energy (shorter wavelength) are diffracted or bent least.•As a result, the polychromatic light is dispersed into its individual color.

Grating

• Polychromatic radiation from the entrance slit is collimated (made into beam of parallel rays) by a concave mirrors.• These rays fall on a reflection grating, whereupon different wavelengths are reflected at different angles.• The orientation of the reflection grating directs only one narrow band wavelengths to the exit slit of the monochromator.• Rotation of grating allows different wavelengths to pass through the exit slit.

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SAMPLE CELLS• For Visible and UV spectroscopy, a liquid sample is usually contained in a cell called a cuvette.• Glass is suitable for visible but not for UV spectroscopy because it absorbs UV radiation. Quartz can be used in UV as well as in visible spectroscopy.

DETECTORS

• The detectors are devices that convert radiant energy into electrical signal.• A Detector should be sensitive, and has a fast response over a considerable range of wavelengths.• In addition, the electrical signal produced by the detector must be directly proportional to the transmitted intensity (linear response).

Barrier Layer Cell• It is also known as photo voltai cell.• It constitutes- 1- A steel support plate. 2- Layer of metallic selenium, which is few hundredths of a millimeter in thickness. 3- A thin transparent electrically conductive layer, applied by cathodic sputtering.

Photodiode• Photo diodes operates on principle of p-n junction diode.

• Its responsivity is 250-500mA W-1

across the visible spectrum.

Photomultiplier Tube• It is a very sensitive device in which electrons

emitted from the photosensitive cathode strike a second surface called dynode which is positive with respect to the original cathode.

• Electrons are thus accelerated and can knock out more than one electrons from the dynode.

• If the above process is repeated several times, so more than 106 electrons are finally collected for each photon striking the first cathode.

photochathode

anode

high voltage

voltage divider network

dynodeslight

electronse-

SINGLE-BEAM SYSTEM

• Single beam instruments are less expansive.• High energy throughput due to non-splitting of source beam results in high sensitivity of detection.

DOUBLE-BEAM SYSTEM

• Little or no lamp warm up time is required. This not only improves throughput of results but also conserves lamp life.• Modern improvements in optics permit high level of automation and offer the same or even better level of detection.

Absorbance & Transmittance displayPower indicator light

Sample holder

Wavelength selector

Absorbance & Transmittance controlPower switch Zero control

1) Power on

2) Select wavelength

3) 0% Transmittance adjustment(Calibration)

4) Blank (Reference cell) is inserted into cell holder

5) 100% Transmittance adjustment

6) Sample cell is placed in the cell compartment

7) Readout absorbance

8) Power off

Working procedure-

ExamplesIsoprene - acyclic butadiene =

217 nm one alkyl subs.

+ 5 nm

222 nmExperimental value

220 nm

Allylidenecyclohexane- acyclic butadiene =

217 nmone exocyclic C=C +

5 nm2 alkyl subs.

+10 nm

232 nmExperimental value

237 nm

1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene heteroannular diene = 214 nm

3 alkyl subs. (3 x 5) +15 nm

1 exo C=C + 5 nm234 nm

Experimental value 235 nm

APPLICATIONS

• Detection of impurities.• Structure elucidation of organic compound.•Quantitative analysis.• Qualitative analysis.• Dissociation constants of acids and bases.• Chemical kinetics.• Quantitative analysis of pharmaceutical substances.• Molecular weight determination.• As HPLC detector. • Examination of polynuclear hydrocarbons.

REFERENCES www.molecularinfo.com/MTM/UV.pdf

Instrumental Methods of Chemical Analysis by Gurdeep R. Chatwal and Sham K. Anand

Spectroscopy of Organic Compounds by P.S.KALSI

http://www.youtube.com/watch?v=pxC6F7bK8CU&feature=player_detailpage

THANKS