Chem 314: Instrumentation Analysis

OPTICAL METHODS OF ANALYSIS

A) WAVE CHARACTERISTICS:Amplitude, A length of the electric vector at a maximum in the wave.Peroid, p time in seconds required for the passage of successive maxima or minima through a fixed point in space of the radiationVelocity of propagation, c radiation travelling through a vacuumc = 2.9979x108 m/sFrequency, v the number of oscillations of the field that occur per second1 Hz = 1 cyle/sWavelength, the linear distance between any two equivalent points of successive waves. (cm, nm, m) = c/vEnergy content of a photon, E = hv =hc/ Where:h = Plancks Constant = 6.6254 x 10-34 J-sc = 2.9979 x 108 m/s1 J = 6.24 x 1018 eVWavenumber, v reciprocal of the wavelength in centimeters, another way of describing electromagnetic radiation. The unit for v in cm-1 of Kaiser.Monochromatic beam beam of radiation whose rays have identical wavelengths.Polychromatic beam beam is made up of different wavelengths.

B) DIFFRACTION OF RADIATIONDiffraction process in which parallel beam of radiation is bent as it passes by a sharp barrier or through narrow opening.

C) TRANSMISSION OF RADIATIONRefractive Index, n one measure of its interaction with radiation.

Where: n = refractive index at a specified frequency = 1.3 to 1.8 (liquids) = 1.3 to 2.5 (solids)v = velocity of radiation in the mediumc = velocity in a vacuumPolarization temporary deformation of the electron clouds associated with atoms or molecules that is brought about by the alternating electromagnetic field of the radiation.Dispersion the splitting up of white light to seven constituent colors on passing through a transparent medium.

D) REFRACTION OF RADIATIONRefraction this is when radiation passes at an angle through the interface between two transparent media that have different densities, an abrupt change in direction. The beam is observed as a consequence of a difference in velocity of the radiation in the two media.The extent of refraction is given by Snells Law:

E) REFLECTION OF RADIATIONReflection this occurs when radiation crosses an interface between media that differ in refractive index. The fraction of reflected radiation becomes greater with increasing differences in refractive index.For a beam that enters an interface at right angles, the fraction reflected is given by:

Where:I = is the intensity of the incident beamI = reflected intensityn & n = refractive indexes of the two media.Example:Calculate the percentage loss of intensity due to reflection of a perpendicular beam of yellow light as it passes through a glass cell that contains water. Assume that for yellow radiation the refractive index of glass is 1.5, of water is 1.33, and of air is 1.00.

F) SCATTERING OF RADIATIONRayleigh Scattering Scattering by molecules or aggregates of molecules with dimension significantly smaller than the wavelength of the radiation. Its intensity is proportional to the inverse fourth power of the wavelength.Raman Scattering these changes are the results of vibrational energy level transitions that occur in the molecules as a consequence of the polarization process.

G) ENERGY STATES OF CHEMICAL SPECIESThe quantum theory was first proposed in 1990 by Max Planck, a German Physicist, to explain the properties of radiation emitted by heated bodies. The theory was later extended to rationalize other types of emission and absorption processes. Two important postulates of quantum theory include the ff:1. Atoms, ions, and molecules can exist only in certain discrete states, characterized by definite amounts of energy. When a species changes it state, it absorbs or emits an amount of energy exactly equal to the energy difference between states.2. When atoms, ions, or molecules absorb or emit radiation in making the transition from one energy state to another, the frequency v or the wavelength of the radiation is related to the energy difference between the states by the equation

Where: E = the energy of the higher state E = the energy of the lower state

Ground State - lowest energy state of an atom or moleculeExcited State Higher energy state of an atom or moleculeH) EMISSION OF RADIATIONElectromagnetic radiation is produced when excited particles (atoms, ions, or molecules) relax to lower energy levels by giving up their excess energy as photonsExcitation can be brought about by a variety of means, including:1. Bombardment with electrons or other elementary particles, which generally leads to the emission of X-radiation.2. Exposure to an electric current, an ac spark, or an intense heat source (flame, dc arc, or furnace), producing ultraviolet, visible, or infrared radiation.3. Irradiation with a beam of electromagnetic radiation, which produces fluorescence radiation4. An exothermic chemical reaction that produces chemiluminescence(excitation of the analyte by a chemical reaction).I) ABSORPTION OF RADIATIONAbsorption process in which electromagnetic energy is transferred to the atoms, ions, or molecules composing the sample.Absorption Methods:Transmittance, T = P/Po=Psoln/PsolventWhere:Po=Incident Radiant Power measurement before the beam has passed through the medium that contains analyteP= Transmitted Radiant Power measurement after the beam has passed through the medium that contains analyteAbsorbance, A = -log[T] = log [Po/P]=log[Psolvent/Psoln]*In contrast to transmittance, the absorbance of a medium increases as attenuation of the beam becomes greater.

J) BEERs LAWFor monochromatic radiation, absorbance is directly proportional to the path length b through the medium and the concentration c of the absorbing species. These relationships are given byA = abc = log [Po/P]Where:a = proportionality constant, absorptivity, L/g-cmb = width of the cuvette/cell, cmc = concentration, g/LApplication to Beers Law to MixturesBeers Law also applies to a medium containing more than one kind of absorbing substance. Provided that the species do not interact, the total absorbance for a multicomponent system is given by:Atotal = A1 + A2 + + AnAtotal = 1bc + 2bc + + nbcA. Deviations to Beers Law due to Polychromatic Radiation~for the wavelength, P = Po 10-bc~for the wavelength, P = Po 10-bcWhen an absorbance measurement is made with radiation composed of both wavelengths, the power of the beam emerging from the solution is the sum of the powers emerging at the two wavelengths, P + P. Likewise, the total incident power is the sum Po + Po. Therefore, the measured absorbance Am isAm = log B. Deviations to Beers Law due to Stray RadiationStray Light defined as radiation from the instrument that is outside the nominal wavelength band chosen for the determination. These is the result of scattering and reflection off the surfaces of gratings, lenses or mirrors, filters, and windows.A = log Where: Ps = power of nonabsorbed stray radiationPhotometer used for measuring the transmittance and absorbance of aqueous solution with a filtered beam of visible radiation. Here the radiation from a tungsten bulb passes through a colored glass filter that restricts the radiation to a limited band of contiguous wavelengths. The beam then passes through a variable diaphragm that permits adjustments of the power of the radiation that reaches the transparent cell that contains the sample. A shutter can be imposed in front of the diapraghm that completely blocks the beam. With the shutter open, the radiation impinges on a photoelectric transducer that converts the radiant energy of the beam to a direct current that is measured with a microammeter.

| Prepared by Engr. Benedict S. Marzan1