2 THE NATURE OF RADIANT ENERGY Many analytical methods are based on the interaction of radiant energy with matter.THE NATURE OF RADIANT ENERGYDual nature of electromagnetic energy – behaves as:- waves or- discrete packets of energy (photons)Recall:h = Planck’s constant = 6.62610-34 J s = Frequency = Wavelengthc = velocity of radiation = 2.998108 m s-1 through a vacuum
3 All electromagnetic radiation travels at the same speed, c EnergyAll electromagnetic radiation travels at the same speed, c
4 The interactions of radiations with chemical systems follow different mechanism and provide different kinds on information.Atomic/ molecular transitions:Valence electronsMolecular vibrationsMolecular rotations
5 INTERACTION OF RADIATION WITH MATTER Electron configuration of Na: 1s2 2s2 2p6 3s1wavelengthsOuter valence electrons can absorb photons and move to higher energy levelGround statePartial energy level diagram for valence electrons in sodium atoms.
6 Irradiated with light containing wavelengths 589. 00 and 589 Irradiated with light containing wavelengths and nm outer valence electrons absorb photons and transfer to 3p levelsexcited state -absorb photonsh
7 Excited electrons have a strong tendency to return to ground state emit photons of definite amount of energyhNa line (589 nm):3p 3s transitionGround stateAnalytical application of resonance absorption and radiation= atomic absorption spectrometry
8 Li line: 2p 2s transition K line: 4p 4s transition Other alkali metals also emit characteristic colours when placed in a high temperature flame.Li line:2p 2s transitionK line:4p 4s transition
9 EMISSIONWith a highly energetic source, many electrons (not only outer electrons) can be excited to varying degrees Resulting radiation contains many discrete and reproducible wavelengths Mostly in UV-Vis regionsAnalytical application = emission spectrometry
10 FLUORESCENCE FLUORESCENCE The energy gained by a molecule on the absorption of a photon does not remain in that molecule, but is lost by several mechanisms.For example:Part of the energy is converted to heat, lowering the net energy of the molecule to the lowest vibrational and rotational level within the same electronic levelThe remainder of the energy is the radiated, returning the molecule to the ground stateFLUORESCENCE
11 Lowering of energy to the lowest vibrational and rotational level within the same electronic level heatThe remainder of the energy is the radiated, returning to the ground stateh
13 QUANTITATIVE ANALYSIS The intensity of the response for each analyte must be calibrated with standard solutions of known concentration of each analyte.A calibration curve of signal vs concentration of analyte is then drawn for each analyte.Use concentration range where:- calibration is lineari.e. concentrations must not be too high to prevent curvature- concentrations are high enough to give good signal-to-noise ratiosMatch standards to samples as far as possible.Intensity will depend on instrument parameters, therefore need to calibrate each time instrument is turned on or the setting are changed.If a large batch of samples are being analysed at once, check signal of standard periodically to ensure the is no drift in the signal.Analyse reference materials to check accuracy.
14 NOMENCLATURE Sensitivity: Related to - signal-to-noise ratio - detection limitResolution:Related to- peak overlap- selectivity
16 At sufficiently high temperatures most compounds decompose into atoms in the gas phase. Electronic transitions can then occur when energy is absorbed or emitted.In atomic spectroscopy:Samples vapourised at KSignal measured - atomic absorption or emission at characteristic wavelengthsHigh sensitivity – ppm levels1 ppm = 1mg/kg 1 mg/L for aq solutionsHigh resolution– ability to distinguish one element from another in complex samplesAbility for simultaneous multi-element analysis1-2% precision – not as good as some wet chemical methods
17 FLAME AAS Flame temperature = 2000-3000 K Solution is aspirated into a flame causes solvent to evaporate remaining solid is atomised in flameSome of these atoms can absorb radiant energy of a characteristic wavelength and become excited to a higher electronic state.In atomic absorption, energy from a light source is absorbed the radiant power decreases as it is transmitted through theflameThe higher the concentration of a solution the more atoms there are the more radiation is absorbed.
19 k = absorption coefficient b = path lengthPo = intensity of sourceP = intensity of radiation measuredA = absorbanceorTherefore:A k concentrationRecall:The higher the concentration of a solution the more atoms there are the more radiation is absorbed.
21 HOLLOW CATHODE LAMPEnergetic Ne+ or Ar+ ions accelerated towards and bombards cathode where atoms vapourise and emit radiationApply potential such that currents of 1-50 mA flowInert gas ionises at anode.Contains inert gas (Ne or Ar)To create frequencies of radiation that are absorbed by the analyte, the cathode must be of the same element as the analyte.
22 MONOCHROMATOR DETECTOR tuned to a specific wavelength and slit widthseparates the selected absorption line from other lines emitted from the sourceDETECTOR measures the amount of light that passes through the flame(the rest is absorbed)
23 NEBULISER AND BURNERSample must be in the form of small droplets when it passes into the flame – done by the nebuliser.(Larger drops)Droplets are mixed with combustion gasSample drawn up in capillary by decreased pressure of expanding gas – Venturi effectLarge droplets condense(support gas)
24 Maximum flame temperatures: !!!Air-acetylene flame – most common, BUT…- Some elements need hotter flame to atomise fully- Some elements form refractory oxides in the flame which are not atomised at the lower temperaturesAcetylene-nitrous oxide flame:- reducing flame prevents oxide formation- high temperatures remove many chemical interferencesBUT increased ionisation of many elements occurs at higher temperatures: e.g. Na Na+ + e- Results in loss of sensitivity (fewer neutral atoms)
25 NB: Careful when lighting and turning off the burners NB: Careful when lighting and turning off the burners – the order is important!For example:First light and air-acetylene flame, then convert to nitrous oxide-acetylene flame. Reverse order for turning off.Use the correct burner for the type of flame used hotter flame, narrower and shorter slot.
26 OPTIMISATION OF SIGNAL Choice of wavelength Ratio of gases in mix Aspiration rate of solution Height of burner position of measurement in flameMonitor absorption while aspirating solution of test element and adjusting conditions.
27 FURNACE ATOMISERSInstead of using a flame to atomise the sample, a furnace can be used.
28 Heating occurs in an inert atmosphere to prevent oxide formation Produces significantly lower detection limits than flame AAS.Much smaller sample size is required.Graphite furnaceBUT:Interferences are greatPrecision is poorer
29 QUANTITATIVE ANALYSIS See section under Optical Methods!Interferences:There are a range of interferences which can affect the absorption signal which could lead to erroneous results.A few of these are mentioned here.Chemical interferences:Analyte element combines with other elements and production of neutral atom in flame is decreased.e.g. Ca2+ combines with PO43- to produce calcium pyrophospate in the flame Add releasing agente.g. EDTA complexes with Ca2+ Matrix match standardsAcids frequently cause depression in signal Matrix match standards
30 Ionization interferences: Some elements ionise easily in the flame e.g. alkali metals cause decrease in no. of atoms in flame decrease in sensitivityAdd ionisation suppressant high concentration (~ ppm) of other easily ionisable elements e.g. Na, K to(suppresses ionisation of analyte element) Matrix match standardsPhysical interferences:Altering physical properties of sample solutione.g. viscosity affects aspiration, nebulisation etc.
32 EMISSION SPECTROMETRY Excitation sources powered by electrical energy(we will consider the ICP source)Excitation source transforms the sample to a plasma of atoms, ions etc. that can be electronically excited.Deactivation of these excited states produces radiation which are sorted by wavelength.Recall: Every element has characteristic spectra.Simultaneous multi-element determinations!!
34 ICP DISCHARGEICP discharge is caused by the effect of a radio frequency field on a flowing gas.Coil is energised by radio frequency generator (5-75 MHz).Ar(g) flows upward and transports sample through a quartz tube inside a copper coil or solenoid.The radio frequency signal causes a changing magnetic field inside the coil in the flowing Ar(g).
35 The changing magnetic field induces a circulating (eddy) current in the Ar(g) which in turn heats the Ar(g).Coolant gas to protects quartz tube from hot plasma
38 Solution droplets formed in the spray chamber. The solvent is evaporated from the solution droplets.Only dried particles flow with the argon to the plasma.
39 NOTE:There are other sources of radiation other than ICP that are used in emission instruments, e.g.:- AC or DC arc- Spark- Microwave plasma dicharge- Laser microprobe
40 QUANTITATIVE ANALYSIS See section under Optical Methods!Internal standards used to minimise effect of variation in instrument response- useful for multi-element techniquesInterferences:Some chemical interferences are reduced due to high temperatures of the plasmaSpectral interferences:Spectral overlap as light is emitted by many different elements in the sample (at the same wavelength)
41 DETECTION LIMITS OF SOME SPECTROMETRY TECHNIQUES NOTE:GFAAS is more sensitive than FAASICP-MS has extremely low detection limits