uv -visible spectroscopy

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Transcript of uv -visible spectroscopy

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Instrumentation and application of UV-VISIBLE SPECTROSCOPY Keshav Narayan PaiMsc IIDOS in BotanyManasagangotri

ContentsHistory and introduction to spectroscopyBasic principlesThe law of absorptionUV visible spectroscopyInstrumentationApplicationConclusionReference

History of spectroscopy

Spectroscopy began withIsaac Newton'soptics experiments (16661672). Newton applied the word "spectrum" to describe therainbow of colors .During the early 1800s,Joseph von Fraunhofer made experimental advances with dispersive spectrometers that enabled spectroscopy to become a more precise and quantitative scientific technique. Since then, spectroscopy has played and continues to play a significant role inchemistry,physicsandastronomy.

SpectroscopySpectroscopy is the branch of science dealing the study of interaction of electromagnetic radiation with matter. Spectroscopy is the most powerful tool available for the study of atomic & molecular structure and is used in the analysis of a wide range of samples . it s two main two type Atomic Spectroscopy; This Spectroscopy is concerned with the interaction of electromagnetic radiation with atoms are commonly in the lowest energy state called as grown state . Molecular Spectroscopy ; This Spectroscopy deals with the interaction of electromagnetic radiation with molecule.

BASIC PRINCIPLESLight is supposed to duel characteristic, corpuscular and waveformThus a beam of light may be understood as electromagnetic waveform photons of energy propagated at 3*108 m/s i.e., speed of lightThe term electromagnetic in a precise description of the radiation in that the radiation is made up of electrical & a magnetic wave which are in phase & perpendicular to each other & to the direction of propagation.

A beam of light form a bulb consists of many randomly oriented plane polarised component being propagated in same direction.The distance along the direction of propagation for one complete cycle is known as wavelength.

THE LAWS OF ABSORPTION The absorption of light by any absorbing material is governed by two laws . Bouger-Lambert law Beers law Bouger-Lambert law This law is suggested by Picre Bouguer in 1729, its often attributed to Johann Heinrich Lambert .

This law is states that The amount of the light absorbed is proportional to the thickness of the absorbing material & is independent of the intensity of the incident light

100%50%25%12.5%b6.25% 3.125%

I Intensity of transmitted light - initial intensity of incident lightb thickness (path length) k linear absorption co-efficient The power term can be removed by converting to the log form.

ln(I/ )=-kb

ln( /I )=kb

Changing to common logarithms we get,

2.303 log /I =kb

Second law Beers law It states that, the amount of light absorbed by a material is proportional to the number of Absorbing molecules(concentration)Again it can be represented 2.303 log( /I) = kcK=absortivity constantc= concentration

K and k merge together = a

Log = /I = a b c

a = k & kb = thickness C = concentrationThis combined law states that the amount of light absorbed is proportional to the Concentration of the absorbing substance & to the thickness of the absorbing material(path length)The quantity /I it is absorbance (O.D optical density)The reverse I / is - transmittance T (the molecule has not used that energy)

=O.D of the unknown x concentration of stdO.D of the stdThe two terms are mathematically commutable i.e., one can be calculated from the otherA=log - log I = 100%Log 100 = 2=2-log IOr O.D is dirctly proportional to the concentration if path is constantSo if we know the value of O.D concentration can be calculatedConcentration of the Unknown(sample)

Terms describing UV absorptions 1.Chromophores: functional groups that give electronic transitions.2.Auxochromes: substituents with unshared pair e's like OH, NH, SH ..., when attached to chromophore they generally move the absorption max. to longer .3.Bathochromic shift: shift to longer , also called red shift.4.Hysochromic shift: shift to shorter , also called blue shift.5.Hyperchromism: increase in of a band.6.Hypochromism: decrease in of a band.

UV-VISIBLE Spectroscopy: Uv-vis spectroscopy is also known as electronic spectroscopy. In which the amount of light absorbed at each wavelength of Uv and visible regions of electromagnetic spectrum is measured. This absorption of electromagnetic radiations by the molecules leads to molecular excitation.

Electronic SpectroscopyUltraviolet (UV) and visible (VIS) spectroscopyThis is the earliest method of molecular spectroscopy.A phenomenon of interaction of molecules with ultraviolet and visible lights.Absorption of photon results in electronic transition of a molecule, and electrons are promoted from ground state to higher electronic states.

The first discovery of electromagnetic waves other than light came in 1800, whenWilliam Herscheldiscovered infrared light. He was studying the temperature of different colors by moving a thermometer through light split by a prism. The types of electromagnetic radiation are broadly classified into the following classesGamma radiationX-ray radiationUltraviolet radiationVisible radiationInfrared radiationTerahertz radiationMicrowave radiationRadio waves

Ultraviolet: 190~400nmViolet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm

VISIBLE LIGHTShorter wavelength and higher frequency than infrared rays.Electromagnetic waves we can seeLongest wavelength= red lightShortest wavelength= violet (purple) light

ULTRAVIOLET RAYSShorter wavelength and higher frequency than visible lightCarry more energy than visible light

Electronic transitionsThere are three types of electronic transition which can be considered; Transitions involving p, s, and n electrons Transitions involving charge-transfer electrons Transitions involving d and f electrons

Absorbing species containing p, s, and n electronsAbsorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy.

UV/VIS

Vacuum UV or Far UV (