Spectrophotometer Design • Based on • Characteristics of the radiation source: – Single beam – Double beam • Characteristics of the wavelength selector – Non-dispersing – Dispersing • Characteristics of the detector – Single channel – Multi-channel 1 Single Beam and Double beam Spectroscopic Methods 2 Spectroscopic Methods 3 Types of Optical Spectrometers • Three types: – Single channel – Multi-channel – Multiplexing spectrometers. • Single Channel: Give information at a single wavelength or record a spectrum sequentially. 4 types: – Fixed wavelength spectrometers with no scanning mechanism – Conventional scanning spectrometer – with a motor to turn a prism or grating scanning. – Rapid scanning spectrometer – with mechanism to oscillate the prism or monochromator very rapidly complete scan in second or milliseconds; requires use of v. Fast transducer, e.g., photomultiplier. – Double-Beam spectrometer – compensates for source drift and changes in monochromator efficiency with wavelength. Spectroscopic Methods 4 Multi-channel Spectrometers • Measure the spectral information at multiple s simultaneously • Consist of a dispersive system coupled to either – A Polychromator with multiple exit slit and detectors, or – A spectrograph with a multichannel detector, e.g., diode array. Spectroscopic Methods 5 Multiplexing Spectrometers • Note the majority of analytical techniques are based on the wavelength as the basis for information selection. • Multiplexing spectrometers have additional criteria to improve selectivity, e.g., – Time-division multiplexing – allows time sharing of a single optical path or single detector, by multiple optical signals, e.g., Double-beam-in-time spectrometer. – Frequency-division multiplexing – Here all s strike a single-channel detector simultaneously. The spectral information at each is encoded in such a way that it can be resolved using mathematical transform techniques, e.g., Fourier Transform Spectroscopy. Spectroscopic Methods 6 Analytical Atomic Spectroscopy Spectroscopic Methods 7 Techniques for Analytical atomic spectroscopy • Atomic Absorption Spectroscopy – Flame – Electrothermal • Atomic Emission Spectroscopy – Flame – Plasma • Atomic Fluorescence Spectroscopy – Flame – Laser Spectroscopic Methods 8 Techniques for Analytical Atomic Spectroscopy (cont.) • Atomic Ionization Spectroscopy – Laser enhanced ionization spectroscopy – Atomic Mass spectroscopy • Atomic X-ray Spectroscopy – X-ray Fluorescence – X-ray absorption – X-ray diffraction – Electrone micro-probe Spectroscopic Methods 9 Atomic Absorption Spectrometry (AAS): Background • Developed by Walsh in 1955 – Ref. A. Walsh, Spectrochimica Acta, 7, 108(1955); 353, 643(1980). • Now the most widely used technique for elemental analysis. Spectroscopic Methods 10 AAS Principle • E.g., for Na, electronic formula: 1s 2 2s 2 2 p 6 3s1 3 p o 4s o 3d o 4 p o 4d o • Ground stateHigher levels, empty • Let the energy difference between the ground state and a higher energy level be E, then from quantum mechanics (1) E h – Where h = Planck’s constant – = the frequency of the radiation = c/ – C = velocity of electromagnetic radiation – = wavelength of the radiation. Spectroscopic Methods 11 Implications of Eq. 1 • It is possible to excite an electron from the ground state of an atom to a higher energy level by irradiating the atom with radiation whose wavelength is given by hc • (2) E • Provided certain selection rules are satisfied. • The process is represented thus • Na + h Na* Spectroscopic Methods 12 AAS Experiment hv Io Free atoms It b Spectroscopic Methods 13 AAS Experiment (cont.) • Imagine some free atoms in a suitable container which is irradiated with radiation of hc E • Let the intensity of the incident radiation be Io, and that of the transmitted radiation be It. • It can be shown that abNo I t I o 10 • (3) • Or log I t I o abNo abC (4) Spectroscopic Methods 14 Symbols in Eqs 3 & 4 • a = a constant characteristic of the atom being irradiated • b = the path traversed by the radiation through the container of free atoms • No = The number of free atoms per unit volume, i.e., the number density C, the concentration of free atoms in the container, • Note Eqs 3 & 4 are statements of the Beer-Lambert Law Spectroscopic Methods 15 Beer-lambert (or Beer’s) Law • For monochromatic radiation, absorbance, A, is directly proportional to the path length, b, through the medium, and the concentration, c, of the absorbing species, i.e., • A abC (5) Spectroscopic Methods 16 Basis for quantitative atomic absorption spectrometry • From Eq. 4, a plot of log Io/It vs No (or C) is a st. line A (Log Io/It) C Spectroscopic Methods 17 Thermal excitation of Atoms • Note excitation of atoms can also be accomplished through application of heat • Excited states produced thermally can also relax to the ground state radiatively • When this happens, the atom is said to EMIT radiation • The energy emitted is referred to as EMISSION • Atomic Emission Spectrometry (AES). Spectroscopic Methods 18 Atomic Emission Spectrometry • In AES, the emission intensity, IE , is directly proportional to the population of the excited state, i.e., • IE Nu • This is the basis of quantitative atomic emission spectrometry. Spectroscopic Methods 19 AES (cont.) • Note in thermal excitation, Nu is a function of temperature, and is related to No by the Boltzmann population distribution law for electronic bound states: g N u N o u e E / kT go • gu and go = statistical weights of excited and ground states respectively • K = Boltzmann constant = 1.380662x10-23 JK-1 • T = absolute temperature. Spectroscopic Methods 20 Atomization Methods • Note before atomic spectrometric measurements can be made, FREE ATOMS of the element have to be generated. • Numerous methods have been developed for generating atomic vapours: – – – – – – – Flame methods Electrothermal methods Direct current arcs AC arcs Microwave plasmas Radio frequency plasmas Laser based methods Spectroscopic Methods 21 Instruments for Atomic Absorption Spectrometry (AAS). Power Supply Radiation Source Atomizer Wavelength selector Transducer Signal Processor & readout Sample introduction system Atomic Spectroscopy. 22 Instruments for Flame Atomic Absorption Spectrometry (FAAS). Power Supply (modulated) Hollow Cathode Lamp Atomizer Monochromator Detector Signal Processor Readout A = abC Sample introduction system Source: Christian & O’Reiley. Atomic Spectroscopy. 23 Performance of the AAS Spectrometer - Depends on: • • • • • • Stability of power source. Type of radiation source Efficiency of atomization process Efficiency of the monochromator Sensitivity of the detector Design of the spectrometer. Atomic Spectroscopy. 24 Common radiation source for AAS: The Hollow Cathode Lamp Anode Fill gas Ne or Ar (1 – 5 torr) 300 – 400 V 3 – 25 A Quartz window Socket Glass Shield Hollow Cathode Glass envelope Source: Christian & O’Reiley. Atomic Spectroscopy. 25 • It looks like any other lamp but its uniqueness lies in the fact that its cathode is made of the same element which you have to determine in your sample This means that you have to use a different lamp for every element whose analysis is required. Spectroscopic Methods 26 Efficiency of the HCL – depends on: • Its geometry; – cylindrical configuration of the cathode concentrates the radiation in a limited region of the metal tube/cup. – Enhances the probability of redeposition of sputtered metal in the metal cup, rather than the glass walls. • Operating potential – High potentials high currents greater intensity of emitted radiation. – Note high currents greater vaporization increased number of ground state atoms in atomic cloud, capable of absorbing some of the radiation emitted by excited atoms. – This self-absorption leads to reduced intensities Atomic Spectroscopy. 27 HCL: Advantages & disadvantages • Advantages: – Available for most elements – Produce very narrow emission lines, high specificity for each element. • Disadvantages – The need for a different HCL for each element, HCL based AAS essentially a single element technique. – Self-absorption at high currents – Doppler broadening of emitted lines at high currents. – Optimum lamp current different for different elements. Atomic Spectroscopy. 28 The Flame Atomiser: Types. • Direct injection or total consumption burner – Fuel and oxidant mixed externally to the burner – Sample solution sprayed directly into flame centre. – Produces turbulent flames • Premixed or Laminar-flow burner – Sample, oxidant, fuel mixed before introduction into flame. – Provides relatively quiet flame, enhances sensitivity. Atomic Spectroscopy. 29 The Flame Atomiser: Laminar Flow Burner. Flame Burner Auxiliary oxidant Fuel Nebulizer Impact bead Blowout plug Capillary tube Spray Chamber Sample solution Nebulizer oxidant To Drain Flow spoilers Atomic Spectroscopy. 30 Laminar Flow Flame Atomizer. • Consists of – Sample aspiration tube – A concentric tube nebulizer – A spray chamber – A slot-type burner Atomic Spectroscopy. 31 Sample introduction in FAAS • Sample must be solution • Can be aqueous solution, usually acidified to keep the metal in solution. • Organic solution, e.g., after complexation and extraction of the organic complex • Sample introduced via the nebulizer – most commonly the pneumatic concentric tube nebulizer. Atomic Spectroscopy. 32 FAAS Spectrometer Design • On basis of optics – Single beam – Double beam • Single beam: 3 steps involved in taking absorbance or emission readings – Zero reading with shutter in front of transducer – 100% T (or Io) taken while aspirating blank solution into flame – Unknown T (or Io) while aspirating sample solution into flame • Disadvantages: Flame conditions, e.g., temperature, can change in between the 3 steps, leading to incorrect readings. Atomic Spectroscopy. 33 Double beam FAAS • Beam from the HCL is split by a half-mirrored chopper – ½ passes through to flame (Sample beam) – ½ directed around the flame (Reference beam) • The 2 beams are recombined after the flame by a ½-silvered mirror, before passing through the monochromator to the detector. • Detector alternately senses Ref. Beam and sample beam – Ref. Beam 100%T (or Io ) – Sample beam T (or IT ). Atomic Spectroscopy. 34 Double beam FAAS (cont.) • Advantages: – Io and IT measured simultaneously • Disadvantages: – Ref. Beam does not pass through the flame, therefore does not correct for absorption or scattering of radiant power by the flame itself. – Methods for correcting for these loses have to be incorporated into the instrument itself or into the method. Atomic Spectroscopy. 35 Background correction methods • Built into Instrument: – Continuum source correction method – Zeeman background correction method – Pulsed Hollow Cathode Lamp method • Method based: – Two-line method – Radiation buffer method – Standard addition method. Atomic Spectroscopy. 36 Continuum Source Background Correction method • Employs a Deuterium lamp – provides continuous radiation throughout the UV region. • Radiation from the D2 lamp and HCL are alternately passed through the flame. • The absorbance of the radiation from D2 lamp Abg broad band absorption or scattering by sample matrix • Absorbance of beam from HCL AT = Analyte absorbance + background absorbance. • Analyte absorbance, Aan = AT - Abg Atomic Spectroscopy. 37 Two-line Method • Uses a second line from the source Reference line – – – – – Ref. line should be as close as possible to analyte line Should not be absorbed by the analyte Can be another analyte line from HCL Can be impurity line in HCL Can be line from HCL filler gas, He or Ar • If these conditions are met, then any decrease in intensity of Ref. Line – Abg = Absorption &/or scattering by sample matrix – Aan = AT - Abg – Assumption: Background absorbance is the same at both wavelengths. Atomic Spectroscopy. 38 X – RAY FLUORESCENCE SPECTROSCOPY 3/12/2013 CH 208 Spectroscopic Methods 39