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Session 3 Optical Spectroscopy: Introduction/Fundamentals Atomic and molecular spectroscopies Instrumentation.

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1 Session 3 Optical Spectroscopy: Introduction/Fundamentals Atomic and molecular spectroscopies Instrumentation

2 Overview Physical basis of absorption and emission Physical basis of absorption and emission Atomic spectra Atomic spectra Molecular spectra Molecular spectra Instrumentation: components of optical systems for spectrometers Instrumentation: components of optical systems for spectrometers Common techniques in atomic spectroscopy: AAS and ICP-OES Common techniques in atomic spectroscopy: AAS and ICP-OES Calibration Calibration

3 3 Useful websites for spectroscopy http://www.shsu.edu/~chm_tgc/sounds/fl ashfiles/ICPwCCD.swf http://www.shsu.edu/~chm_tgc/sounds/fl ashfiles/ICPwCCD.swf http://www.shsu.edu/~chm_tgc/sounds/fl ashfiles/ICPwCCD.swf http://www.shsu.edu/~chm_tgc/sounds/fl ashfiles/ICPwCCD.swf http://www.thespectroscopynet.com/Inde x.html?/ http://www.thespectroscopynet.com/Inde x.html?/ http://www.thespectroscopynet.com/Inde x.html?/ http://www.thespectroscopynet.com/Inde x.html?/ http://teaching.shu.ac.uk/hwb/chemistry/t utorials/molspec/ http://teaching.shu.ac.uk/hwb/chemistry/t utorials/molspec/ http://teaching.shu.ac.uk/hwb/chemistry/t utorials/molspec/ http://teaching.shu.ac.uk/hwb/chemistry/t utorials/molspec/ http://www.chemguide.co.uk/analysis/uvvi siblemenu.html http://www.chemguide.co.uk/analysis/uvvi siblemenu.html See also individual citations on slides

4 4 Electromagnetic radiation http://www.spectroscopynow.com/coi/cda/detail.cda?id=18411&type= EducationFeature&chId=7&page=1 This primer also contains a wavelength-energy converter E = h = hc = frequency; = wavelength Spectroscopy = interactions between light & matter

5 5 Fundamentals Absorption and emission of light by compounds is generally associated with transitions of electrons between different energy levels Absorption and emission of light by compounds is generally associated with transitions of electrons between different energy levels E2E1E0E2E1E0 E = h = hc/ Emission: Sample (in an excited state) produces light/looses energy Absorption: sample takes up energy Consumes light of appropriate wavelength http://physics.nist.gov/PhysRefData/ASD/lines_form.html Atomic spectra: line spectra provide specificity: each element has its own pattern, as each element has its own electronic configuration ground state excited states E 2 E 1 E2E1E0E2E1E0 E 2 E 1

6 6 Fundamentals The population of different states is given by the Boltzmann equation: The population of different states is given by the Boltzmann equation: N 0 : number of atoms in ground state N1: number of atoms in excited state g1/g0 : weighting factors Note: Equation contains temperature: Excitation can be achieved by providing thermal energy

7 7 Atomic emission: Flame spectroscopy Observation Caused by... Persistent golden-yellow flame Sodium Violet (lilac) flame Potassium, cesium carmine-red flame Lithium Brick-red flame Calcium Crimson flame Strontium Yellowish-green flame barium, molybdenum Green flame Borates, copper, thallium Blue flame (wire slowly corroded) Lead, arsenic, antimony, bismuth, copper Lithium Cesium Sodium Qualitative method

8 8 A simple spectroscope Spectroscope: Device for qualitative assessment of a sample Spectroscope: Device for qualitative assessment of a sample E.g. used in flame analysis E.g. used in flame analysis E.g. used in gemmology E.g. used in gemmology

9 9 Atomic Spectroscopies - Synopsis Optical spectroscopies Techniques for determining the elemental composition of an analyte by its electromagnetic or mass spectrum Mass spectrometries ICP-MSSIMS AAS AES Fluoresc- ence Spectros- copy Flame AAS GFAAS ICP-OES Others See table Others (L. 6)

10 10 Atomic spectroscopies TechniqueAtomisation/Excitation Sample etc Arc/sparke Electric arc/spark Solid sample on carbon electrode Laser microprobe eLaser Solid sample on support Glow discharge e Glow discharge lamp Solid sample disc ICP-OESe Electromagnetic induction Liquid sample, sprayed into gas plasma Flame photometry (atomic emission) eFlame Liquid sample, sprayed into flame AASa UV/Vis light Liquid sample, sprayed into flame or furnace Atomic fluorescence fe UV/Vis light Liquid sample, sprayed into flame or furnace X-ray fluorescence feX-radiation Solid or liquid ICP-MS-n/a Liquid sample, sprayed into gas plasma

11 11 Atomic spectroscopies Common principles: Common principles: Sample introduction: Nebulisation, Evaporation Sample introduction: Nebulisation, Evaporation Atomisation (and excitation or ionisation) by flame, furnace, or plasma Atomisation (and excitation or ionisation) by flame, furnace, or plasma Spectrometer components: Spectrometer components: Light source (can be sample itself - Only AA requires external light source Light source (can be sample itself - Only AA requires external light source Optical system (or mass spectrometer) Optical system (or mass spectrometer) Detector Detector

12 12 Atomic spectra vs molecular spectra: LinesBands LinesBands (nm) Typical atomic spectrumTwo typical molecular spectra Y axes: intensity of absorbed light. Under ideal conditions proportional to analyte concentration (I c; Beers law). e.g. acquired by AAS Acquired by UV-Vis spectroscopy

13 13 Origin of bands in molecular spectra Molecules have chemical bonds Molecules have chemical bonds Electrons are in molecular orbitals Electrons are in molecular orbitals Absorption of light causes electron transitions between HOMO and LUMO Absorption of light causes electron transitions between HOMO and LUMO Molecules undergo bond rotations and vibrations: different energy sub-states occupied at RT and accessible through absorption: many transitions possible: Molecules undergo bond rotations and vibrations: different energy sub-states occupied at RT and accessible through absorption: many transitions possible: A band is the sum of many lines A band is the sum of many lines Vibrational substates rotational substates HOMO LUMO

14 14 Quantitative analysis by molecular absorption: Colorimetry Because absorption spectroscopy is widely applicable, sensitive (10 -5 -10 -7 M), selective, accurate (0.1-3% typically), and easy: Because absorption spectroscopy is widely applicable, sensitive (10 -5 -10 -7 M), selective, accurate (0.1-3% typically), and easy: 95% of quantitative analyses in field of health performed with UV/Vis tests 95% of quantitative analyses in field of health performed with UV/Vis tests Hemoglobin in blood Hemoglobin in blood First step in analysis: establish working conditions First step in analysis: establish working conditions Select Select Selection, cleaning and handling of cells Selection, cleaning and handling of cells Calibration: determine relationship between absorbance and concentration Calibration: determine relationship between absorbance and concentration

15 15 Instrument components AAS Spectrometer ICP-OES Spectrometer Monochro- mator Sample = light source Detector Read- out/Data system Light Source Monochro- mator SampleDetector Read- out/Data system Light Source Monochro- mator SampleDetector Read- out/Data system UV-Vis Spectrometer:

16 16 UV-Vis spectrophotometer (dual beam) Diffraction grating Slit Mirror Light sources Slit http://www.spectroscopynow.com/coi/cda/detail.cda?i d=18412&type=EducationFeature&chId=7&page=1 Filter Mirror Half- Mirror Sample Reference Detector Monochromator

17 17 Example for a dual beam spectrometer

18 18 Single beam

19 19 UV-Vis spectroscopy practicalities: Referencing Matrix (solvent, buffer etc) might also have absorbance: Must be taken care of Matrix (solvent, buffer etc) might also have absorbance: Must be taken care of In dual beam: In dual beam: Simultaneous measurement of reference cell eliminates absorbance of background Simultaneous measurement of reference cell eliminates absorbance of background Recording of baseline recommended Recording of baseline recommended Single beam: Single beam: Requires measurement of reference spectrum, can be subtracted from sample spectrum Requires measurement of reference spectrum, can be subtracted from sample spectrum Preferentially in same cuvette Preferentially in same cuvette

20 20 Light sources Continuum Line Ar lamp Xe lamp D 2 lamp Tungsten lamp Nernst glower (ZrO 2 + Y 2 O 3 ) Nichrome wire Lasers Hollow cathode lamps Globar (SiC)

21 21 Example of a continuum source: Output from Tungsten lamp Widely applied in UV-Vis spectrometers

22 22 Hollow cathode lamp Used in AAS Used in AAS Filled with Ne or Ar at a pressure of 130-700 Pa (1-5 Torr). Filled with Ne or Ar at a pressure of 130-700 Pa (1-5 Torr). When high voltage is applied between anode and cathode, filler gas becomes ionised When high voltage is applied between anode and cathode, filler gas becomes ionised Positive ions accelerated toward cathode Positive ions accelerated toward cathode Strike cathode with enough energy to "sputter" metal atoms from the cathode to yield cloud with excited atoms Strike cathode with enough energy to "sputter" metal atoms from the cathode to yield cloud with excited atoms Atoms emit line spectraAtoms emit line spectra

23 23 Example: Output from iron hollow cathode lamp Small portion of spectrum from Fe hollow cathode lamp Small portion of spectrum from Fe hollow cathode lamp Shows sharp lines characteristic of gaseous atoms Shows sharp lines characteristic of gaseous atoms Linewidths are artificially broadened by monochromator (bandwidth = 0.08 nm) Linewidths are artificially broadened by monochromator (bandwidth = 0.08 nm)

24 24 Wavelength selectors: dispersive elements and filters Fluorite prism Fused silica or quartz prism Glass prism NaCl prism KBr Prism Interference filters Interference wedge Glass filters Continuous Discontinuous Gratings 3000 lines/nm50 lines/nm

25 25Monochromators Consist of Consist of Entrance slit Entrance slit Collimating lens or mirror Collimating lens or mirror Dispersion element (prism or grating) Dispersion element (prism or grating) Focusing lens or mirror Focusing lens or mirror Exit slit Exit slit Czerny-Turner grating monochromator: Czerny-Turner grating monochromator: Mirrors Common in UV-Vis spectrometers

26 26 Dispersers Separate polychromatic light into its components Separate polychromatic light into its components Prism Prism Diffraction grating: patterned surface which diffracts light Diffraction grating: patterned surface which diffracts light Blazed diffraction grating Holographic grating Prisms

27 27 Echellette grating: Extra pathlength travelled by wave 2 must be multiple of for positive interference: Extra pathlength travelled by wave 2 must be multiple of for positive interference: n = d(sin i + sin r ) n = d(sin i + sin r ) for UV 1000-2000 lines/mm: d = 0.5-1 m for UV 1000-2000 lines/mm: d = 0.5-1 m echelle: French for ladder

28 28 Bandwidth of a monochromator Spectral bandwidth: range of wavelengths exiting the monochromator Spectral bandwidth: range of wavelengths exiting the monochromator Related to dispersion and slit widths Related to dispersion and slit widths Defines resolution of spectra: 2 features can only be distinguished if effective bandwidth is less than half the difference between the of features Defines resolution of spectra: 2 features can only be distinguished if effective bandwidth is less than half the difference between the of features

29 29 Effect of slit width on peak heights

30 30 Components of optical system in an ICP-OES spectrometer spherical and cylindrical lenses spherical and cylindrical lenses flat and spherical mirrors flat and spherical mirrors parallel planes parallel planes optical path under vacuum or controlled nitrogen atmosphere (necessary for wavelengths <200 nm; air absorbs far UV light) optical path under vacuum or controlled nitrogen atmosphere (necessary for wavelengths <200 nm; air absorbs far UV light) Disperser(s) Disperser(s)

31 31 Old models: Sequential type Can only measure one wavelength at a given time: Slow

32 32 Newer: Simultaneous type Echelle cross disperser (polychromator): Consists of Echelle grating and prisms/ echellette: separates lights in 2 dimensions CCD detector: 2D detector This combination allows high-speed measurement, providing information on all 72 measurable elements within 1 to 2 minutes

33 33Detectors Photographic plate Photomultiplier Photocell Phototube Silicon diode Charge-coupled device (170-1000) Photoconductor Thermocouple Golay pneumatic cell Pyroelectric cell Photon detectors Thermal detectors

34 34 Photomultiplier: detects one wavelength at a time Based on photoelectric effect Based on photoelectric effect Photocathode and series of dynodes in an evacuated glass enclosure Photocathode and series of dynodes in an evacuated glass enclosure Photons strike cathode and electrons are emitted Photons strike cathode and electrons are emitted Electrons are accelerated towards a series of dynodes by increasing voltages Electrons are accelerated towards a series of dynodes by increasing voltages Additional electrons are generated at each dynode Additional electrons are generated at each dynode Amplified signal is finally collected and measured at anode Amplified signal is finally collected and measured at anode

35 35 Photodiode arrays: measure several wavelengths at once linear array of discrete photodiodes on an integrated circuit (IC) chip linear array of discrete photodiodes on an integrated circuit (IC) chip Photodiode: Consists of 2 semiconductors (n-type and p-type) Photodiode: Consists of 2 semiconductors (n-type and p-type) Light promotes electrons into conducting band: generates electron-hole pair Light promotes electrons into conducting band: generates electron-hole pair Concentration of these electron-hole pairs directly proportional to incident light Concentration of these electron-hole pairs directly proportional to incident light a voltage bias is present and the concentration of light- induced electron-hole pairs determines the current through semiconductor a voltage bias is present and the concentration of light- induced electron-hole pairs determines the current through semiconductor

36 36 Detection in simultaneous ICP-OES: http://www.chemistry.adelaide.edu.au/external/soc-rel/content/ccd.htm CCD: Charge-coupled device Also integrated-circuit chip Contains an array of capacitors that store charge when light creates electron-hole pairs Accumulated charge is read out at given time interval Each wavelength is detected at a different spot Much more sensitive than photodiode array detectors

37 37 Lecture 4 AAS and ICP-OES Sample preparation Interferences Calibration Calibration

38 38 Crucial steps in atomic spectroscopies and other methods Adapted from www.spectroscopynow.com (Gary Hieftje) Solid/liquid sample Solution Molecules in gas phase Sample preparation Nebulisation Atomisation= Dissociation Vaporisation Desolvation Atoms in gas phase Ions Excited Atoms Laser ablation etc. Sputtering, etc. ICP-MS and other MS methods AAS and AES, X-ray methods Ionisation Excitation M + X - MX(g) M(g) + X(g) M+M+

39 39 Sample Introduction: liquid samples Often the largest source of noise Often the largest source of noise Sample is carried into flame or plasma as aerosol, vapour or fine powder Sample is carried into flame or plasma as aerosol, vapour or fine powder Liquid samples introduced using nebuliser Liquid samples introduced using nebuliser

40 40 Sample preparation for analysis in solution: Digestion Digestion in conc. HNO 3 and mixtures thereof (e.g. aqua regia) Digestion in conc. HNO 3 and mixtures thereof (e.g. aqua regia) Br 2 or H 2 O 2 can be added to conc. acids to give a more oxidising medium and increase solubility Br 2 or H 2 O 2 can be added to conc. acids to give a more oxidising medium and increase solubility Certain materials require digestion in conc. HF Certain materials require digestion in conc. HF Common to use microwave digestion Common to use microwave digestion

41 41 Microwave digestion Supplied with dedicated vessels (e.g. PTFE) Closed vessel digestion minimises sample contamination Faster, more reproducible, and safer than conventional methods Rotor

42 42 Sample preparation and sample handling for trace analysis As always – sample preparation is key As always – sample preparation is key Ultra-trace: Contaminations introduced during sample processing can seriously limit performance characteristics Ultra-trace: Contaminations introduced during sample processing can seriously limit performance characteristics Points to consider: Points to consider: Purity of reagents Purity of reagents Chemical inertness of reaction vessels and any other material samples come into contact with Chemical inertness of reaction vessels and any other material samples come into contact with Working environment Working environment Preparation of standards and blanks crucial Preparation of standards and blanks crucial Also measure a process blank: Also measure a process blank: Important for determination of LOD and LOQ Important for determination of LOD and LOQ

43 43 Common Units in trace analysis ppm, ppb, ppt, ppq…..: parts per million etc. ppm, ppb, ppt, ppq…..: parts per million etc. ppm: mg/kg; often also used as mg/L ppm: mg/kg; often also used as mg/L ppb: g/kg ppb: g/kg ppt: ng/kg ppt: ng/kg ppq: pg/kg ppq: pg/kg

44 44 Atomic absorption spectroscopy

45 45 Atomic Absorption Spectroscopy Flame AAS has been the most widely used of all atomic methods due to its simplicity, effectiveness and low cost Flame AAS has been the most widely used of all atomic methods due to its simplicity, effectiveness and low cost First introduced in 1955, commercially available since 1959 First introduced in 1955, commercially available since 1959 Qualitative and quantitative analysis of >70 elements Qualitative and quantitative analysis of >70 elements Quantitative: Can detect ppm, ppb or even less Quantitative: Can detect ppm, ppb or even less Rapid, convenient, selective, inexpensive Rapid, convenient, selective, inexpensive

46 46 Hollow cathode lamps with characteristic emissions Hollow cathode lamps available for over 70 elements Can get lamps containing > 1 element for determination of multiple species Nebuliser and Spray chamber Flame fuelled by (e.g.) acetylene and air Burner Flame AA Spectrometer

47 47 Schematic Light Source MonochromatorDetectorAmplifier E.g. Hollow cathode lamp Analyte solution Atomiser Fuel (e.g. acetylene) Air I0I0 ItIt Nebuliser, spray chamber, and burner

48 48 Flame atomisation: Laminar flow burner - components Nebuliser: converts sample solution into aerosol Nebuliser: converts sample solution into aerosol Spray chamber: Aerosol mixed with fuel, oxidant and burned in 5-10 cm flame Spray chamber: Aerosol mixed with fuel, oxidant and burned in 5-10 cm flame Fuel: Acetylene or nitrous oxide Fuel: Acetylene or nitrous oxide Oxidant: Air or oxygen Oxidant: Air or oxygen Burner head: Laminar flow: quiet flame and long path- length Burner head: Laminar flow: quiet flame and long path- length But: poor sensitivity (not very efficient method, most of sample lost) But: poor sensitivity (not very efficient method, most of sample lost) from: Skoog

49 49 Structure of a flame Relative size of regions varies with fuel, oxidant and their ratio

50 50 Electrothermal atomisation: GFAAS Provides enhanced sensitivity Provides enhanced sensitivity entire sample atomised in very short time entire sample atomised in very short time atoms in optical path for a second or more (flame 10 -4 s) atoms in optical path for a second or more (flame 10 -4 s) Device: Graphite furnace Device: Graphite furnace

51 51 Sensitivity and detection limits in AAS Sensitivity: number of ppm of an element to give 1% absorption. Sensitivity: number of ppm of an element to give 1% absorption. Limit of detection: dependent upon signal:noise ratio: Limit of detection: dependent upon signal:noise ratio: S/N Light intensity reaching detector S/N=3.2

52 52 Interferences in AAS Broadening of a spectral line, which can occur due to a number of factors (Physical) Broadening of a spectral line, which can occur due to a number of factors (Physical) Spectral: emission line of another element or compound, or general background radiation from the flame, solvent, or analytical sample Spectral: emission line of another element or compound, or general background radiation from the flame, solvent, or analytical sample Background correction can be applied Background correction can be applied Chemical: Formation of compounds that do not dissociate in the flame Chemical: Formation of compounds that do not dissociate in the flame Ionisation of the analyte can reduce the signal Ionisation of the analyte can reduce the signal Matrix interferences due to differences between surface tension and viscosity of test solutions and standards Matrix interferences due to differences between surface tension and viscosity of test solutions and standards Non-linear response common in AAS Another caveat: Non-linear response common in AAS

53 53 Physical interferences: Atomic line widths/ line shapes Very important in atomic spectroscopy Very important in atomic spectroscopy Narrow lines increase precision, decrease spectral interferences Narrow lines increase precision, decrease spectral interferences Lines are broadened by several mechanisms: Lines are broadened by several mechanisms: Natural broadening Natural broadening Doppler effect Doppler effect Pressure broadening Pressure broadening Figure taken from http://www.cem.msu.edu/~cem333/ Week03.pdf

54 54 Natural linewidths Width of an atomic spectral line is determined by the lifetime of the excited state Width of an atomic spectral line is determined by the lifetime of the excited state Consequence of the Heisenberg uncertainty principle Consequence of the Heisenberg uncertainty principle For example, lifetime of 10 -8 seconds (10 ns) yields peak widths of 10 -5 nm For example, lifetime of 10 -8 seconds (10 ns) yields peak widths of 10 -5 nm

55 55 Doppler Effect Due to rapid motion of atoms in gas phase Due to rapid motion of atoms in gas phase Atom moving toward the detector absorbs / emits radiation of shorter than atom moving perpendicular to detector. Atom moving toward the detector absorbs / emits radiation of shorter than atom moving perpendicular to detector. Atom moving away from the detector absorbs / emits radiation of longer : detector perceives fewer oscillations Atom moving away from the detector absorbs / emits radiation of longer : detector perceives fewer oscillations Photon detector

56 56 Pressure broadening Results from collisions of absorbing/emitting species Results from collisions of absorbing/emitting species With analyte atoms or combustion products of fuel With analyte atoms or combustion products of fuel Deactivates the excited state – shorter lifetime - broader spectral lines Deactivates the excited state – shorter lifetime - broader spectral lines Increases with concentration and temperature Increases with concentration and temperature E.g. in flame, Na absorbance lines broadened up to 10 -3 nm. E.g. in flame, Na absorbance lines broadened up to 10 -3 nm. Doppler and pressure effects broaden atomic lines by 1-2 orders of magnitude as compared with their natural linewidths Doppler and pressure effects broaden atomic lines by 1-2 orders of magnitude as compared with their natural linewidths

57 57 Background correction in AAS particularly important in GFAAS particularly important in GFAAS Use beam chopper to distinguish the signal due to flame from desired atomic line at the same wavelength (old method) Use beam chopper to distinguish the signal due to flame from desired atomic line at the same wavelength (old method) Lamp and flame emission reach detector Only flame emission reaches detector Resulting signal

58 58 Background correction in AAS High energy Deuterium background corrector High energy Deuterium background corrector Deuterium lamp Hollow cathode lamp Beam combiner Sample Detector Lamps are pulsed out of phase with each other

59 59 Minimising the effect of Matrix Interferences The term "matrix" refers to the sum of all compositional characteristics of a solution, including its acid composition The term "matrix" refers to the sum of all compositional characteristics of a solution, including its acid composition Calibration standards and samples must be matrix-matched in terms of composition, total dissolved solids, and acid concentration of the solution Calibration standards and samples must be matrix-matched in terms of composition, total dissolved solids, and acid concentration of the solution Also advisable for ICP-OES and -MS Also advisable for ICP-OES and -MS Effect on K concentration on measured Sr

60 60 Specialised applications in AAS: Flameless cold vapour methods Mercury: has sufficient vapour pressure at RT Mercury: has sufficient vapour pressure at RT Hydride generation technique for determination of As, Sb, Bi, Se, Te, Ge, Pb, and Sn Hydride generation technique for determination of As, Sb, Bi, Se, Te, Ge, Pb, and Sn Generation of volatile metal hydrides (As, Sb, Bi, Se, Te, Ge, Pb, and Sn) Generation of volatile metal hydrides (As, Sb, Bi, Se, Te, Ge, Pb, and Sn) Reduction by NaBH 4 to form volatile hydride (e.g. SnH 4 ) Reduction by NaBH 4 to form volatile hydride (e.g. SnH 4 ) Hydrides carried into light path by argon gas Hydrides carried into light path by argon gas Decomposed into elemental vapour by injection into (electrothermally) heated silica cell Decomposed into elemental vapour by injection into (electrothermally) heated silica cell

61 61 Calibration – some practical aspects

62 62 Principles Recap: Measured quantity must change with analyte concentration in systematic and defined way Recap: Measured quantity must change with analyte concentration in systematic and defined way Can be determined by calibration, using defined standards Can be determined by calibration, using defined standards Stock solutions of standards can either be prepared or purchased Stock solutions of standards can either be prepared or purchased Working solutions are best prepared by weighing the amounts of stock solution and matrix (rather than using volumetric ware) Working solutions are best prepared by weighing the amounts of stock solution and matrix (rather than using volumetric ware) NEVER extrapolate: concentration of sample must be in same range as standards NEVER extrapolate: concentration of sample must be in same range as standards

63 63 Calibration in AAS In theory, Beers law applies for dilute solutions In theory, Beers law applies for dilute solutions In practice, deviation from linearity is usual In practice, deviation from linearity is usual Small dynamic range Small dynamic range Possible to use non-linear curve fitting for calibration Possible to use non-linear curve fitting for calibration Reasons: Self-absorption: Reasons: Self-absorption: excited atoms emit light that can also be absorbed instead of that of source: on average, less light per number of atoms is absorbed excited atoms emit light that can also be absorbed instead of that of source: on average, less light per number of atoms is absorbed Linear range

64 64 Alternative to matrix-matching: Method of standard additions Extensively used in absorption spectroscopy, accounts for matrix effects Extensively used in absorption spectroscopy, accounts for matrix effects Several aliquots of sample Several aliquots of sample Sample (1): diluted to volume directly Sample (1): diluted to volume directly Samples (2,3,4,5…): known amounts of analyte added before dilution to volume Samples (2,3,4,5…): known amounts of analyte added before dilution to volume BUT: Only makes sense if the added standard closely matches the analyte present in the samples chemically and physically if simple, dissolved ions are analysed

65 65 Method of standard additions If linear relationship exists between measured quantity and concentration (must be verified experimentally) then: If linear relationship exists between measured quantity and concentration (must be verified experimentally) then: V x, C x : volume and concentration of analyte V x, C x : volume and concentration of analyte V s : variable volume of added standard V s : variable volume of added standard C s : concentration of added standard C s : concentration of added standard V T : total volume of volumetric flask V T : total volume of volumetric flask k: proportionality constant (= єl) k: proportionality constant (= єl) A x, A T : absorbances of standard alone and sample + standard addition, respectively. A x, A T : absorbances of standard alone and sample + standard addition, respectively.

66 66 Method of standard additions slope = m = (єlc s ) / V T intercept = b = (єlV x c x ) / V T Graphical evaluation Limitations The calibration graph must be substantially linear since accurate regression cannot be obtained from non-linear calibration points. Caution: The fact that the measured part of the graph is linear does not always mean that linear extrapolation will produce the correct results It is also essential to obtain an accurate baseline from a suitable reagent blank

67 67 Most simple version of standard addition: spiking Spiking means deliberately adding analyte to an unknown sample Spiking means deliberately adding analyte to an unknown sample Involves: Involves: preparation of sample and measurement of absorbance preparation of sample and measurement of absorbance Addition of standard with known concentration, measurement of absorbance Addition of standard with known concentration, measurement of absorbance From difference in absorbance, calculate From difference in absorbance, calculate From reading of sample alone, calculate amount of analyte From reading of sample alone, calculate amount of analyte (use Beers law for calculations) (use Beers law for calculations)

68 68 Other uses for spiking Add spike at beginning of sample preparation Add spike at beginning of sample preparation Process sample with and without spike Process sample with and without spike Difference should correspond to amount spiked Difference should correspond to amount spiked Deviation allows to calculate recovery factor Deviation allows to calculate recovery factor

69 69 Atomic emission spectroscopy

70 70 Atomic emission spectroscopy Historically, many techniques based on emission have been used (See Table on p. 4) Historically, many techniques based on emission have been used (See Table on p. 4) Flame and electrothermal methods now widely superseded by Inductively-Coupled Plasma (ICP) method Flame and electrothermal methods now widely superseded by Inductively-Coupled Plasma (ICP) method Developed in the 1970s Developed in the 1970s Higher energy sources than flame or electrothermal methods Higher energy sources than flame or electrothermal methods

71 71 ICP-AES/OES Offer several advantages over flame/electrothermal: Offer several advantages over flame/electrothermal: Lower inter-element interference (higher temperatures) Lower inter-element interference (higher temperatures) With a single set of conditions signals for dozens of elements can be recorded simultaneously With a single set of conditions signals for dozens of elements can be recorded simultaneously Lower LOD for elements resistant to decomposition Lower LOD for elements resistant to decomposition Permit determination of non-metals (Cl, Br, I, S) Permit determination of non-metals (Cl, Br, I, S) Can analyse concentration ranges over several decades (vs 1 or 2 decades for other methods) Can analyse concentration ranges over several decades (vs 1 or 2 decades for other methods) Disadvantages: Disadvantages: More complicated and expensive to run More complicated and expensive to run Require higher degree of operator skill Require higher degree of operator skill Inductively coupled plasma-atomic emission spectroscopy (or optical emission spectroscopy)

72 72 Modern ICP-OES spectrometer Over 70 elements (in principle simultaneously) Over 70 elements (in principle simultaneously) Including non-metals such as sulfur, phosphorus, and halogens (not possible with AAS) Including non-metals such as sulfur, phosphorus, and halogens (not possible with AAS) ppm to ppb range ppm to ppb range Principle: Argon plasma generates excited atoms and ions; these emit characteristic radiation Principle: Argon plasma generates excited atoms and ions; these emit characteristic radiation

73 73 ICP-AES Instrumentation

74 74 Components for sample injection and the ICP torch www.cleanwatertesting.com/news_ NR149.htm www.midwestrefineries.com /refiningandassaying.htm Up to 7000°C

75 75 Meinhard nebuliser Caution: The capillary is easy to block and difficult to unblock

76 76 ICP torch d=2.5 cm water cooled induction coil powered by RF generator (2 kW power at 27 MHz) concentric quartz tubes 11-17 L/min

77 77 Torch Ignition Sequence Ionisation of Argon initiated by spark from Tesla coil After leaving injector, sample moves at high velocity Punches hole in centre of plasma Switch on RF powerStart gas flow Plasma generated

78 78 Atomisation / Ionisation In plasma, sample moves through several zones In plasma, sample moves through several zones Preheating zone (PHZ): temp = 8000 K: Desolvation/evaporation Preheating zone (PHZ): temp = 8000 K: Desolvation/evaporation Initial radiation zone (IRZ): 6500-7500 K: Vaporisation, Atomisation Initial radiation zone (IRZ): 6500-7500 K: Vaporisation, Atomisation Normal analytical zone (NAZ): 6000-6500 K: Ionisation Normal analytical zone (NAZ): 6000-6500 K: Ionisation

79 79 Advantages of plasma Prior to observation, atoms spend ~ 2 sec at 4000-8000 K (about 2-3 times that of hottest combustion flame) Prior to observation, atoms spend ~ 2 sec at 4000-8000 K (about 2-3 times that of hottest combustion flame) Atomisation and ionisation is more complete Atomisation and ionisation is more complete Fewer chemical interferences Fewer chemical interferences Chemically inert environment for atomisation Chemically inert environment for atomisation Prevents side-product (e.g. oxide) formation Prevents side-product (e.g. oxide) formation Temperature cross-section is uniform (no cool spots) Temperature cross-section is uniform (no cool spots) Prevents self-absorption Prevents self-absorption Get linear calibration curves over several orders of magnitude Get linear calibration curves over several orders of magnitude

80 80 Radial and axial observation http://las.perkinelmer.com/content/relatedmaterials/brochures/bro _atomicspectroscopytechniqueguide.pdf Axial Radial. Can achieve higher sensitivity Combined viewing expands dynamic range

81 81 Applications ICP-OES used for quantitative analysis of: ICP-OES used for quantitative analysis of: Soil, sediment, rocks, minerals, air Soil, sediment, rocks, minerals, air Geochemistry Geochemistry Mineralogy Mineralogy Agriculture Agriculture Forestry Forestry Fornensics Fornensics Environmental sciences Environmental sciences Food industry Food industry Elements not accessible using AAS Elements not accessible using AAS Sulfur, Boron, Phosphorus, Titanium, and Zirconium Sulfur, Boron, Phosphorus, Titanium, and Zirconium

82 82 Homework for revision Read Read http://las.perkinelmer.com/content/ relatedmaterials/brochures/bro_ato micspectroscopytechniqueguide.pdf

83 83 Lab Experiment 3 Analyse a Chromium complex for [Cr] in three ways: Analyse a Chromium complex for [Cr] in three ways: UV (absorbance & extinction coefficient) UV (absorbance & extinction coefficient) Titration (moles Cr and charge) Titration (moles Cr and charge) AAS (Cr standard curve and unknown concentration) AAS (Cr standard curve and unknown concentration) AAS data analysis AAS data analysis Fit standards to quadratic equation Fit standards to quadratic equation A=a[Cr] 2 + b[Cr] + c A=a[Cr] 2 + b[Cr] + c Use a, b, and c to calculate unknown concentration Use a, b, and c to calculate unknown concentration


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