Post on 17-May-2015
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INTRODUCTION TO SPECTROSCOPY
HISTORY
• THE BEAUTIFUL PHENOMENON OF “RAINBOW” WAS THE FIRST DISPERSED SPECTRUM.• 1665 - NEWTON TOOK THE FIRST & MOST IMPORTANT STEP TOWARDS THE DEVELOPMENT OF SPECTROSCOPY.• 1752 - THOMAS MELVILL GAVE THE FIRST DESCRIPTION OF LABORATORY EMISSION SPECTRUM. • 1802 - THOMAS YOUNG SHOWED THAT THE RANGE OF WAVELENGTH IN VISIBLE SPECTRUM EXTENDS FROM 424-675 NM.• FRAUNHOFFER RULED THE FIRST GLASS TRANSMISSION GRATING.• 1848 - FOUCAULT’S WORK INDICATED A RELATION BETWEEN EMISSION & ABSORPTION SPECTRA.
• 1859 - G.R. KIRCHOFF STATED THAT “RATIO OF EMISSIVE POWER TO THE ABSORPTIVITY FOR THERMAL RADIATION IS CONSTANT FOR SAME WAVELENGTH & TEMPERATURE”.• G.R. KIRCHOFF & R.BUNSEN EMERGED AS THE “FATHER OF
MODERN SPECTROSCOPY”. • NEW DEVELOPMENTS SUCH AS DRY GELATIN
PHOTOGRAPHIC PLATE, INTERFEROMETER,BOLOMETER ETC. CAME IN THE TWENTIETH CENTURY.• INFRARED,MICROWAVE,SUBMILLIMETER,RADIO-
FREQUENCY,U.V.,X-RAY,GAMMA –RAY REGIONS CAME INTO EXISTENCE WITH THE HELP OF SPETROSCOPY.• SPECTROSCOPY PLAYED A GREAT ROLE IN THE FORMULA- TION OF QUANTUM MECHANICS & RELATIVISTIC THEORY IN THE TWENTIETH CENTURY.
IT IS DEFINED AS THE STUDY OF THE INTERACTION OF MATTER & ELECTROMAGNETIC RADIATION.
SINCE,WE ALL ARE FAMILIAR WITH “MATTER” AND THE “ELECTROMAGNETIC RADIATION”. SO,WITHOUT WASTING MUCH TIME,
…. REVIEW OF SOME BASICS• c = n x l• Angular resolution: q = 1.22 l / D radians 206,265” in a radian• E = h n• F = L / 4 p d2
• Important constants : G = 6.67 x 10-8 (c.g.s) c = 3 x 1010 cm/sec, k = 1.38 x 10-16
h = 6.626 x 10-27
mH ~ mproton = 1.67 x 10-24 grams me = 0.91 x 10-27 grams eV = 1.602 x 10-12 erg Luminosity of Sun = 4 x 1033 erg/sec Mass of the Sun = 2 x 1033 grams
THE PHYSICS OF EM RADIATION
• Light: , l n - l n = c = 2.998 x 1010 cm/s (in vacuum) - E = h n Photon energy (erg) 1 erg sec-1 = 10-7 Watt h = 6.626 x 10-27 (c.g.s) 1 eV = 1.602 x 10-12 erg - p = E / c = h / l Photon momentum - l = h / p = h / m v de Broglie wavelength Planck Function: B(T)• Emission, absorption, continua• Wave no. : Reciprocal of wavelength (in cm)
•SPECTROSCOPY : STUDY OF INTERACTION OF MATTER AND ELECTROMAGNETIC RADIATION.
• SPECTROMETRY : AN ANALYTICAL TECHNIQUE IN WHICH EMISSION (OF PARTICLE/RADIATION) IS DISPERSED ACCORDING TO SOME PROPERTY OF THE EMISSION AND THE AMOUNT OF DISPERSION IS MEASURED. EG. MASS SPECTROMETRY.
• SPECTROPHOTOMETRY : A QUANTIFIABLE STUDY OF ELECTROMAGNETIC SPECTRA.
• SPECTROGRAPHY : ANOTHER NAME FOR SPECTROSCOPY.
TYPES OF SPECTROSCOPY• Electromagnetic Waves: Emission, absorption Visual, near-IR., FIR, Radio, UV/X-ray, gamma-ray - Solids, liquids, gasses, plasmas - Emission, absorption - Spectral line, molecular bands, continua: - Thermal (~LTE, blackbody, grey-body): - Non-thermal (masers, synchrotron, …) - Electronic, vibrational, rotational transitions. - Effects of B (Zeeman), E ( Stark), motion (Doppler), pressure (collisions), natural life-time (line widths) - Radiative Transfer (optical depth)Other types (not covered in this course):• NMR• Raman• Phosprescence / Fluorecence• Astro-particle
CONTINUOUS SPECTRA ARISE FROM DENSE GASES OR SOLID OBJECTS WHICH RADIATE THEIR HEAT AWAY THROUGH THE PRODUCTION OF LIGHT. SUCH OBJECTS EMIT LIGHT OVER A BROAD RANGE OF WAVELENGTHS, THUS THE APPARENT SPECTRUM SEEMS SMOOTH AND CONTINUOUS. STARS EMIT LIGHT IN A PREDOMINANTLY (BUT NOT COMPLETELY!) CONTINUOUS SPECTRUM.
DISCRETE SPECTRA ARE THE OBSERVABLE RESULT OF THE PHYSICS OF ATOMS.
THERE ARE TWO TYPES OF DISCRETE SPECTRA :
• EMISSION (BRIGHT LINE SPECTRA) ,• ABSORPTION (DARK LINE SPECTRA) .
WHEN AN ATOM DROPS FROM EXCITED STATE TO THE GROUND STATE, THEY EMIT A WAVE OF LIGHT OF WAVELENGTH EQUAL TO THE ENERGY DIFFERENCE BETWEEN THOSE TWO LEVELS. THIS ENERGY CORRESPONDS TO A CERTAIN COLOUR, AND THUS WE ARE ABLE TO SEE AN “EMISSION SPECTRA”. THE CHANGE OF ENERGY IN AN ATOM GENERATES A PHOTON,WHICH IS THEN EMITTED. EG.
An excited Hydrogen atom relaxes from level 2 to level 1, yielding a photon. This results in a bright emission line.
WHEN AN ATOM MOVES FROM LOWER ENERGY LEVEL TO UPPER ENERGY LEVEL , THE WAVELENGTHS CORRESPONDING TO POSSIBLE ENERGY TRANSITIONS WITHIN THAT ATOM WILL BE ABSORBED AND THEREFORE AN OBSERVER WILL NOT SEE THEM. IN THIS WAY, A “DARK-LINE ABSORPTION SPECTRUM” IS BORN. EG.
A hydrogen atom in the ground state is excited by a photon of exactly the `right' energy needed to send it to level 2, absorbing the photon in the process. This results in a dark absorption line.
ABSORPTION SPECTROSCOPY
• DEFINITION : ABSORPTION SPECTROSCOPY REFERS TO SPECTROSCOPIC TECHNIQUES THAT MEASURE THE ABSORPTION OF RADIATION, AS A FUNCTION OF FREQUENCY OR WAVELENGTH, DUE TO ITS INTERACTION WITH A SAMPLE.
• THE INTENSITY OF THE ABSORPTION VARIES AS A FUNCTION OF FREQUENCY, AND THIS VARIATION IS THE “ABSORPTION SPECTRUM”. ABSORPTION SPECTROSCOPY IS PERFORMED ACROSS THE “ELECTROMAGNETIC SPECTRUM”.
ATOMIC ABSORPTION SPECTROSCOPY
• DEFINITION : ATOMIC ABSORPTION SPECTROSCOPY IS A TECHNIQUE USED TO DETERMINE THE CONCENTRATION OF A SPECIFIC METAL ELEMENT IN A SAMPLE.
• THE TECHNIQUE CAN BE USED TO ANALYZE THE CONCENTRATION OF OVER 70 DIFFERENT METALS IN A SOLUTION.
• PRINCIPLE : IT MAKES USE OF ABSORPTION SPECTROMETRY & IS HENCE, BASED ON “BEER-LAMBART’S LAW”.
• INSTRUMENT :
Atomic Absorption Spectrometer
ATOMIC EMISSION SPECTROSCOPY
• DEFINITION : IT IS THE QUANTITATIVE MEASUREMENT OF THE OPTICAL RADIATION FROM EXCITED ATOMS, WHEN THEY FALL TO GROUND STATE, TO DETERMINE ANALYTE CONCENTRATION.
• THIS TECHNIQUE MAKES USE OF HIGH TEMPERATURE OF FLAME TO EXCITE THE ATOMS.
• INSTRUMENT :
Inductively-coupled Plasma Atomic Emission Spectrometer
Excitation source
Excited electrons
Wavelengthselector
Detector
ATOMIC EMISSION SPECTROMETER
FLAME PHOTOMETRY
• DEFINITION : FLAME PHOTOMETRY (MORE ACCURATELY CALLED FLAME ATOMIC EMISSION SPECTROMETRY) IS A BRANCH OF ATOMIC SPECTROSCOPY IN WHICH THE SPECIES EXAMINED IN THE SPECTROMETER ARE IN THE FORM OF ATOMS. THE ATOMS UNDER INVESTIGATION ARE EXCITED BY LIGHT.
• THE TECHNIQUE CAN BE USED FOR QUALITATIVE AND QUANTITATIVE DETERMINATION OF SEVERAL CATIONS, ESPECIALLY FOR METALS THAT ARE EASILY EXCITED TO HIGHER ENERGY LEVELS AT A RELATIVELY LOW FLAME TEMPERATURE (MAINLY NA, K, RB, CS, CA, BA, CU).
• PRINCIPLE : IT MAKES USE OF A FLAME THAT EVAPORATES THE SOLVENT AND ALSO SUBLIMATES AND ATOMIZES THE METAL AND THEN EXCITES A VALENCE ELECTRON TO AN UPPER ENERGY STATE. Photograph of a flame photometer
• THE INTENSITY OF THE LIGHT EMITTED COULD BE DESCRIBED BY THE “SCHEIBE-LOMAKIN EQUATION”: I = K × C N
WHERE, C : CONCENTRATION OF ELEMENT, K : PROPORTIONALITY CONSTANT, N : N ~1 (AT LINEAR PART OF CALIBRATION CURVE) THEREFORE ,THE INTENSITY OF EMITTED LIGHT IS DIRECTLY PROPORTIONAL TO CONCENTRATION. • INSTRUMENT :
Fuel
Air
Sample
Aerosol enters flame
Readout
Lens
Discharge
Filter
Photo-detector
FLAME PHOTOMETER
U.V., I.R., VIS. SPECTROPHOTOMETRY
• U.V. SPECTROPHOTOMETRY : IT IS A BRANCH OF A.A.S/A.E.S IN WHICH ALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO U.V. REGION . IT IS USED IN QUANTIFYING PROTEIN AND DNA CONCENTRATION AS WELL AS THE RATIO OF PROTEIN TO DNA CONCENTRATION IN A SOLUTION .
• I.R. SPECTROPHOTOMETRY : IT IS ALSO A BRANCH OF A.A.S/A.E.S IN WHICH ALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO I.R. REGION. INFRARED SPECTROSCOPY OFFERS THE POSSIBILITY TO MEASURE DIFFERENT TYPES OF INTER ATOMIC BOND VIBRATIONS AT DIFFERENT FREQUENCIES .
• VIS. SPECTROPHOTOMETRY : IT IS THE THIRD BRANCH OF A.A.S/A.E.S IN WHICH ALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO VISIBLE REGION.
SPECTROPHOTOMETER
FLUORIMETRY
• DEFINITION : IT IS A TECHNIQUE IN WHICH THE AMOUNT OF SUBSTANCE IN A SAMPL CAN BE DETERMINED BY THE AMOUNT OF LIGHT EMITTED BY THE ATOMS OF THAT SUBSTANCE.
• THIS TECHNIQUE IS BASED ON THE PHENOMENON OF “FLUOROSCENCE”.
• RELATION BETWEEN FLUOROSCENCE INTENSITY & ANALYTE CONCENTRATION : F= K*(QE)*(Po)*[ 1- 10(A*B*C)]
Telescope
Focal Plane
Slit
SPECTROGRAPH
collimator
Dispersing element
camera
detector
SPECTROGRAPH OVERVIEW
• Slit & Decker: Restrict incoming light Spatial direction vs. Spectral direction• Collimator & Camera: Transfer image of slit onto detector.• Grating: Disperse light: dispersion => spectral resolution
• What determines spectral resolution & coverage? - Slit-width - Grating properties: Ngrooves , order number - Camera / collimator magnification (focal length ratio) - Detector pixel size and number of pixels.