I INTRODUCTION TO SPECTROSCOPY
HISTORY THE BEAUTIFUL PHENOMENON OF “RAINBOW” WAS THE FIRST DISPERSED SPECTRUM NEWTON TOOK THE FIRST & MOST IMPORTANT STEP TOWARDS THE DEVELOPMENT OF SPECTROSCOPY THOMAS MELVILL GAVE THE FIRST DESCRIPTION OF LABORATORY EMISSION SPECTRUM THOMAS YOUNG SHOWED THAT THE RANGE OF WAVELENGTH IN VISIBLE SPECTRUM EXTENDS FROM NM. FRAUNHOFFER RULED THE FIRST GLASS TRANSMISSION GRATING FOUCAULT’S WORK INDICATED A RELATION BETWEEN EMISSION & ABSORPTION SPECTRA.
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 = xx Angular resolution: = 1.22 / D radians 206,265” in a radian E = h F = L / 4 d2d2 Important constants : G = 6.67 x (c.g.s) c = 3 x cm/sec, k = 1.38 x h = x mH mH ~ m proton = 1.67 x grams me me = 0.91 x grams eV = x erg Luminosity of Sun = 4 x erg/sec Mass of the Sun = 2 x grams
THE PHYSICS OF EM RADIATION THE PHYSICS OF EM RADIATION Light: - = c = x cm/s (in vacuum) - E = h Photon energy (erg) 1 erg sec -1 = Watt h = x (c.g.s) 1 eV = x erg - p = E / c = h / Photon momentum - = 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 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 Wavelength selector 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. P hotograph 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: N grooves, order number - Camera / collimator magnification (focal length ratio) - Detector pixel size and number of pixels.