Transcript Slide 1
INTRODUCTION TO OPTICAL METHODS Many analytical methods are based on the interaction of radiant energy with matter. THE NATURE OF RADIANT ENERGY Dual nature of electromagnetic energy – behaves as: - waves or - discrete packets of energy (photons) Recall: E h c hc E h = Planck’s constant = 6.62610-34 J s = Frequency = Wavelength c = velocity of radiation = 2.998108 m s-1 through a vacuum Energy All electromagnetic radiation travels at the same speed, c The interactions of radiations with chemical systems follow different mechanism and provide different kinds on information. Atomic/ molecular transitions: Valence electrons Molecular vibrations Molecular rotations INTERACTION OF RADIATION WITH MATTER Electron configuration of Na: 1s2 2s2 2p6 3s1 wavelengths Outer valence electrons can absorb photons and move to higher energy level Ground state Partial energy level diagram for valence electrons in sodium atoms. Irradiated with light containing wavelengths 589.00 and 589.59 nm outer valence electrons absorb photons and transfer to 3p levels excited state absorb photons h Excited electrons have a strong tendency to return to ground state emit photons of definite amount of energy h Ground state Na line (589 nm): 3p 3s transition Analytical application of resonance absorption and radiation = atomic absorption spectrometry Other alkali metals also emit characteristic colours when placed in a high temperature flame. Li line: 2p 2s transition K line: 4p 4s transition EMISSION With 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 regions Analytical application = emission spectrometry 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 level The remainder of the energy is the radiated, returning the molecule to the ground state FLUORESCENCE heat h Lowering of energy to the lowest vibrational and rotational level within the same electronic level The remainder of the energy is the radiated, returning to the ground state Radiation source 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 linear i.e. concentrations must not be too high to prevent curvature - concentrations are high enough to give good signal-to-noise ratios Match 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. NOMENCLATURE Sensitivity: Related to - signal-to-noise ratio - detection limit Resolution: Related to - peak overlap - selectivity ATOMIC ABSORPTION SPECTROMETRY 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 2000-6000 K Signal measured - atomic absorption or emission at characteristic wavelengths High sensitivity – ppm levels 1 ppm = 1mg/kg 1 mg/L for aq solutions High resolution – ability to distinguish one element from another in complex samples Ability for simultaneous multi-element analysis 1-2% precision – not as good as some wet chemical methods FLAME AAS Flame temperature = 2000-3000 K Solution is aspirated into a flame causes solvent to evaporate remaining solid is atomised in flame Some 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 the flame The higher the concentration of a solution the more atoms there are the more radiation is absorbed. Atomic absorption FLAME SOURCE k = absorption coefficient P ln o k b P b = path length Po = intensity of source P = intensity of radiation measured or Po A log k b loge P A = absorbance Therefore: A k concentration Recall: The higher the concentration of a solution the more atoms there are the more radiation is absorbed. INSTRUMENTATION ~10 cm HOLLOW CATHODE LAMP Energetic Ne+ or Ar+ ions accelerated towards and bombards cathode where atoms vapourise and emit radiation Apply potential such that currents of 1-50 mA flow Contains inert gas (Ne or Ar) Inert gas ionises at anode. To create frequencies of radiation that are absorbed by the analyte, the cathode must be of the same element as the analyte. MONOCHROMATOR tuned to a specific wavelength and slit width separates the selected absorption line from other lines emitted from the source DETECTOR measures the amount of light that passes through the flame (the rest is absorbed) NEBULISER AND BURNER Sample 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 gas Sample drawn up in capillary by decreased pressure of expanding gas – Venturi effect Large droplets condense (support gas) 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 temperatures Acetylene-nitrous oxide flame: - reducing flame prevents oxide formation - high temperatures remove many chemical interferences BUT increased ionisation of many elements occurs at higher temperatures: e.g. Na Na+ + e Results in loss of sensitivity (fewer neutral atoms) 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. OPTIMISATION OF SIGNAL Choice of wavelength Ratio of gases in mix Aspiration rate of solution Height of burner position of measurement in flame Monitor absorption while aspirating solution of test element and adjusting conditions. FURNACE ATOMISERS Instead of using a flame to atomise the sample, a furnace can be used. Heating occurs in an inert atmosphere to prevent oxide formation Produces significantly lower detection limits than flame AAS. Much smaller sample size is required. BUT: Interferences are great Precision is poorer Graphite furnace 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 agent e.g. EDTA complexes with Ca2+ Matrix match standards Acids frequently cause depression in signal Matrix match standards Ionization interferences: Some elements ionise easily in the flame e.g. alkali metals cause decrease in no. of atoms in flame decrease in sensitivity Add ionisation suppressant high concentration (~200-1000 ppm) of other easily ionisable elements e.g. Na, K to (suppresses ionisation of analyte element) Matrix match standards Physical interferences: Altering physical properties of sample solution e.g. viscosity affects aspiration, nebulisation etc. INDUCTIVELY COUPLED PLASMA OPTICAL EMISSION SPECTROMETRY (ICP-OES) 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!! Atomic emission ICP DISCHARGE ICP 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). 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 Forms a stable plasma that is extremely hot. Quartz tube Radio frequency load coil 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. 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 QUANTITATIVE ANALYSIS See section under Optical Methods! Internal standards used to minimise effect of variation in instrument response - useful for multi-element techniques Interferences: Some chemical interferences are reduced due to high temperatures of the plasma Spectral interferences: Spectral overlap as light is emitted by many different elements in the sample (at the same wavelength) DETECTION LIMITS OF SOME SPECTROMETRY TECHNIQUES NOTE: GFAAS is more sensitive than FAAS ICP-MS has extremely low detection limits