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.