10/2/2008. hc λ. νλ =c. proportional to frequency. Energy is inversely proportional to wavelength And is directly proportional to wavenumber

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1 CH217 Fundamentals of Analytical Chemistry Module Leader: Dr. Alison Willows Electromagnetic spectrum Properties of electromagnetic radiation Many properties of electromagnetic radiation can be described by a classical wave model using wavelength, frequency, velocity and amplitude. This does not take into account phenomena associated with absorption and emission of radiant energy. Electromagnetic radiation needs to be described in the form of discrete particles of energy called photons. The energy of the photon is proportional to the frequency of the radiation Wave properties Wavelength, λ peak to peak distance between waves Frequency, ν - no. complete oscillations per second (units s - 1, or Hz) νλ =c where c is the speed of light ( m s - 1 in vacuum) Particles and energy Each photon carries the energy E, where: Energy is proportional to E = hν frequency Where h is Planck s constant ( Js) Combining these two equations gives: hc E = = hcν % Where % ν = 1 λ Energy is inversely proportional to wavelength And is directly proportional λ to wavenumber and is called the wavenumber Alison Willows, C522, a.d.willows@brighton.ac.uk 5 6 1

2 Energy levels and spectra When a molecule absorbs a photon, the energy of the molecule increases. The molecule is promoted to an excited state If a molecule emits a photon, the energy of the molecule decreases. The lowest energy state of a molecule is known as the ground state Spectrometry - Principles When light is absorbed by a sample the radiant power, or irradiance, P, of the beam of light is decreased Transmittance, T, is defined as the fraction of the original light that passes through the sample: P T = P0 Absorbance is defined as : A P P 0 = log = logt 7 8 When no light is absorbed P = P 0 and A = 0. If 90% of the light is absorbed then 10% is transmitted, P = P 0 /10 and so A = 1. If 1% of light is transmitted then A = 2 Absorbance is dimensionless but sometimes absorbance units is written after the number to clarify what it means Beer s Law (or Beer-Lambert Law) Absorbance is directly proportional to concentration: A = εbc Where: b is the pathlength in cm, c is the concentration (M or moles per litre), and ε (epsilon) is the molar absorptivity with units M - 1 cm -1 Molar absorptivity is the characteristic of a substance that tells us how much light is absorbed at different wavelengths 9 10 Worked Example Find the absorbance and transmittance of a M solution of a substance with a molar absorptivity of 313 M - 1 cm - 1 in a cell with a 2.00 cm pathlength. Absorbance: A=εbc = (313 M -1 cm -1 ) (2.00 cm)( M) = Transmittance: Log T = -A T = 10 log T = 10 -A = = So just 3.16% of the incident light passes through the solution Exercise 1 a) What value of absorbance corresponds to 45.0% T? b) If a M solution exhibits 45.0% T at some wavelength, what will be the percent transmittance for a M solution of the same substance This and additional problems are available in Chapter 18, Harris(2003) Quantitative Chemical Analysis. 6 th edn. W H Freeman and co

3 Exercise 1 answers A = -log(p/p 0 ) = -log T = -log(0.45) = Absorbance is proportional to concentration, so the absorbance will double to 0.694, giving T = 10 - A = = %T = 20.2% Exercise 2 a) A M solution of compound A exhibited an absorbance of at 238 nm in a 1.00 cm cuvette; a blank solution containing only solvent had an absorbance of at the same wavelength. Find the molar absorptivity of compound A. b) The absorbance of an unknown solution of compound A in the same solvent and cuvette was at 238 nm. Find the concentration of A in the unknown. c) A concentrated solution of A in the same solvent was diluted from an initial volume of 2.00 ml to a final volume of ml and then had an absorbance of What is the concentration of A in the concentrated solution? Exercise 2 answers a) ε = M - 1 cm b) c= M c) c = M cm -1 Beer s Law continued Absorbance is proportional to concentration if: the radiation is monochromatic, passing through a dilute solution where the absorbing species is not participating in a concentration dependent equilibrium In concentrated solutions molecules influence one another because of their close proximity, properties such as molar absorptivity change when molecules are close (generally >0.01 M) In concentrated solution a weak acid, HA may be mostly undissociated. As the solution is diluted, dissociation increases. If absorptivity of A - is not the same as HA then the solution will not appear to obey Beer s law Application of Beer s Law to mixtures We can apply Beer s Law to solutions containing more than one type of absorbing substance. If there is no interaction between the species the total absorbance b is: A = A + A A = ε bc + ε bc ε bc total 1 2 n n n general instrument set-up Measurement Single-beam Double beam Emission experiments 17 3

4 Measurement A reference sample is used to record a baseline spectrum The baseline usually shows areas of small positive and negative absorbance rather than being zero at all wavelengths The baseline is subtracted from the sample spectrum to obtain the true absorbance at each wavelength The reference sample is contained in a matched cuvette to compensate for reflection, scattering and absorption by the cuvette and solvent Measurement The appropriate wavelength needs to be chosen for the measurements Usually this is the wavelength where maximum absorbance occurs, At the maximum absorbance the curve is relatively flat. If the monochromator drifts slightly during measurement or if the bandwidth changes slightly there will be less variation in the absorbance. Beer s law is obeyed more closely when the absorbance is nearly constant across the selected waveband. Sensitivity is greatest at maximum absorbance Spectrophotometers work best at intermediate levels of absorbance, A» The sample concentration is adjusted to fall within this range Other sources of error Sample compartments must be tightly close to avoid stray light Position of the cuvette in the chamber must be reproducible Containers must be covered to prevent dust entering as it causes light scattering Cuvettes must be handled carefully to avoid fingerprints. Only touch the opaque sides and not the faces where the light will pass Sample and reference cuvettes should be matched Single-beam instrument One beam of light is used The incident irradiance, P 0, is measured indirectly. A cuvette containing the reference sample is placed in the chamber and the incident irradiance, P 0, is measured This is then replaced with the cuvette containing the sample and P is measured 22 Double-beam beam instrument Emission experiment The light is split to pass alternately through the sample and the reference The beam is chopped several times per second and P and P 0 are compared automatically Compensates for changes of source intensity and detector response with time and wavelength Alison Willows, C522, a.d.willows@brighton.ac.uk 23 The excitation wavelength (λ ex ) is selected through one monochromator and emission is observed through a second, usually at 90 to the incident light. Emission spectrum is produced if the excitation wavelength is fixed and scan though emitted radiation. Excitation spectrum is produced if the excitation wavelength is varied and the emitted light at one wavelength is measured. 24 4

5 Absorption & Emission In atomic spectroscopy samples are vaporised at K and decompose into atoms Concentrations of atoms are measured by either emission or absorption of wavelengths of radiation Atomic spectroscopy uses electronic transitions of electrons not involved in bonding Atomic emission spectroscopy involves emission of photons as electrons relax from excited states back to their ground states Atomic absorption spectroscopy based upon capture of photons as electrons are promoted or even lost in the formation of an ion Atomic spectroscopy - types 1. Emission from a thermally populated excited state 2. Absorption of sharp lines from hollowcathode lamp 3. Fluorescence following absorption of laser radiation Atomisation - Flame Premix burner often used to mix fuel oxidant and sample before introduction to flame A nebulizer creates an aerosol from the sample Large droplets are blocked and drain away Only 5% of sample is contained in the aerosol which reaches the flame Flames emit light which must be subtracted from the total signal to obtain the analyte signal (blank required) In a rich rich (in fuel) flame metal oxides and hydroxides are reduced by excess carbon. This increases sensitivity A lean flame (with excess oxidant) is hotter Max. absorption/emission is observed at different heights in flame depending on element measured Atomic spectroscopy - atomisation a) Premix burner b) End view of flame. Burner head slot is about 0.5 mm wide Atomisation - Graphite furnace requires less sample: μl (flame: 1-2 ml) greater sensitivity: residence time of analyte is several seconds (flame:: fraction of a second) Precision: manual injection 5-10%; automated injection ~1% Requires more operator skill to find good conditions for each type of sample Matrix modifier: added to reduce loss of analyte during charring step (removes matrix). Makes matrix more volatile or sample less volatile. Atomisation - inductively coupled plasma (ICP) Twice as hot as combustion flame Less interference than flames due to high temperature, stability, and chemically inert Ar environment Simultaneous multi-element analysis is routine Initial and running cost is higher than a flame instrument 5

6 Atomic spectroscopy - atomisation Typical ICP, atomises substances at 6000 K. Atomisation - nebuliser Limit of detection can be lowered by use of an ultrasonic nebuliser Sample solution is directed onto oscillating piezoelectric crystal This creates a fine aerosol that is carried by a stream of argon through a heated tube to evaporate the solvent which is then condensed and removed Analyte arrives at flame as dry, solid particles Plasma energy is not required to evaporate solvent so more energy is available for atomisation Also, larger fraction of sample reaches the plasma Sensitivity is improved by using length of plasma for observation rather than diameter Sensitivity also improved by detecting the ions by mass spectrometer rather than optical emission Atomic spectroscopy - atomisation Ultrasonic nebuliser Detection Detectors - Device that indicates existence of some physical phenomenon Transducers special type of detector that converts signals such as light intensity, ph, mass and temperature into electrical signals Photon detectors Photomultiplier l tubes Silicon photodiode arrays Charge coupled device Infrared detectors Thermocouples Ferroelectric material Photoconductive and photovoltaic detectors 34 Photomultiplier tube Photon flux is converted to electron pulses and then amplified Photocathode ejects electrons when hit with EM radiation These electrons strike a dynode which emits 5 5 electrons for each electron hit More dynodes are hit until the required amplification is made Electrical current measured at anode is directly proportional to the amount of radiation reaching the PMT Charge coupled device (CCD) CCD (a) charge generation and storage in each pixel (b) top view showing, simply, how stored charge is read, row by row Alison Willows, C509, a.d.willows@brighton.ac.uk