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Session 3 Optical Spectroscopy: Introduction/Fundamentals

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

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

3 Useful websites for spectroscopy
See also individual citations on slides

4 Electromagnetic radiation
Spectroscopy = interactions between light & matter E = hn = hcl n = frequency; l = wavelength This primer also contains a wavelength-energy converter

5 Fundamentals Absorption and emission of light by compounds is generally associated with transitions of electrons between different energy levels E2 E1 E0 E2 E1 E0 DE2 excited states DE2 DE = hn = hc/l DE1 DE1 ground state Emission: Sample (in an excited state) produces light/looses energy Absorption: sample takes up energy Consumes light of appropriate wavelength Atomic spectra: line spectra provide specificity: each element has its own pattern, as each element has its own electronic configuration

6 Fundamentals The population of different states is given by the Boltzmann equation: N0: 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 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 Qualitative method Sodium

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

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

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

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

12 Atomic spectra vs molecular spectra:
Lines Bands (nm) Typical atomic spectrum Two typical molecular spectra e.g. acquired by AAS Acquired by UV-Vis spectroscopy Y axes: intensity of absorbed light. Under ideal conditions proportional to analyte concentration (I  c; Beer’s law).

13 Origin of bands in molecular spectra
Molecules have chemical bonds Electrons are in molecular orbitals 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: A band is the sum of many lines LUMO Vibrational substates rotational substates HOMO

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

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

16 UV-Vis spectrophotometer (dual beam)
Monochromator Slit Diffraction grating Mirror Slit Light sources Filter Reference Half- Mirror Detector Mirror Sample

17 Example for a dual beam spectrometer

18 Single beam

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

20 Light sources Continuum Line Ar lamp Xe lamp D2 lamp Tungsten lamp
Nernst glower (ZrO2 + Y2O3) Nichrome wire Globar (SiC) Line Hollow cathode lamps Lasers

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

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

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

24 Wavelength selectors: dispersive elements and filters
Fluorite prism Fused silica or quartz prism Glass prism Continuous NaCl prism KBr Prism 3000 lines/nm Gratings 50 lines/nm Discontinuous Interference filters Interference wedge Glass filters

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

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

27 Echellette grating: Extra pathlength travelled by wave 2 must be multiple of l for positive interference: nl = d(sin i + sin r) for UV lines/mm: d = mm echelle: French for ladder

28 Bandwidth of a monochromator
Spectral bandwidth: range of wavelengths exiting the monochromator 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 l of features

29 Effect of slit width on peak heights

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

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

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

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

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

35 Photodiode arrays: measure several wavelengths at once
linear array of discrete photodiodes on an integrated circuit (IC) chip Photodiode: Consists of 2 semiconductors (n-type and p-type) Light promotes electrons into conducting band: generates electron-hole pair “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

36 Detection in simultaneous ICP-OES:
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 AAS and ICP-OES Sample preparation Interferences Calibration
Lecture 4 AAS and ICP-OES Sample preparation Interferences Calibration

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

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

40 Sample preparation for analysis in solution: Digestion
Digestion in conc. HNO3 and mixtures thereof (e.g. aqua regia) Br2 or H2O2 can be added to conc. acids to give a more oxidising medium and increase solubility Certain materials require digestion in conc. HF Common to use microwave digestion

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

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

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

44 Atomic absorption spectroscopy

45 Atomic Absorption Spectroscopy
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 Qualitative and quantitative analysis of >70 elements Quantitative: Can detect ppm, ppb or even less Rapid, convenient, selective, inexpensive

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

47 Schematic Atomiser I0 It E.g. Hollow cathode lamp Analyte solution
Light Source Monochromator Detector Amplifier E.g. Hollow cathode lamp Analyte solution Atomiser Fuel (e.g. acetylene) Air Nebuliser, spray chamber, and burner

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

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

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

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

52 Interferences in AAS 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 Background correction can be applied Chemical: Formation of compounds that do not dissociate in the flame Ionisation of the analyte can reduce the signal Matrix interferences due to differences between surface tension and viscosity of test solutions and standards Another caveat: Non-linear response common in AAS

53 Physical interferences: Atomic line widths/ line shapes
Very important in atomic spectroscopy Narrow lines increase precision, decrease spectral interferences Lines are broadened by several mechanisms: Natural broadening Doppler effect Pressure broadening Figure taken from

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

55 Doppler Effect Due to rapid motion of atoms in gas phase
Photon detector 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 away from the detector absorbs / emits radiation of longer : detector perceives fewer oscillations

56 Pressure broadening Results from collisions of absorbing/emitting species With analyte atoms or combustion products of fuel Deactivates the excited state – shorter lifetime - broader spectral lines Increases with concentration and temperature 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

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

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 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 Effect on K concentration on measured Sr

60 Specialised applications in AAS: Flameless cold vapour methods
Mercury: has sufficient vapour pressure at RT 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) Reduction by NaBH4 to form volatile hydride (e.g. SnH4) Hydrides carried into light path by argon gas Decomposed into elemental vapour by injection into (electrothermally) heated silica cell

61 Calibration – some practical aspects

62 Principles Recap: Measured quantity must change with analyte concentration in systematic and defined way Can be determined by calibration, using defined standards 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) NEVER extrapolate: concentration of sample must be in same range as standards

63 Calibration in AAS In theory, Beer’s law applies for dilute solutions
In practice, deviation from linearity is usual Small dynamic range Possible to use non-linear curve fitting for calibration 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 Linear range

64 Alternative to matrix-matching: Method of standard additions
Extensively used in absorption spectroscopy, accounts for matrix effects Several aliquots of sample Sample (1): diluted to volume directly 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 Method of standard additions
If linear relationship exists between measured quantity and concentration (must be verified experimentally) then: Vx, Cx: volume and concentration of analyte Vs: variable volume of added standard Cs: concentration of added standard VT: total volume of volumetric flask k: proportionality constant (= єl) Ax, AT: absorbances of standard alone and sample + standard addition, respectively.

66 Method of standard additions
Graphical evaluation slope = m = (єlcs) / VT intercept = b = (єlVxcx) / VT 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 Most simple version of standard addition: spiking
Spiking means deliberately adding analyte to an unknown sample Involves: preparation of sample and measurement of absorbance Addition of standard with known concentration, measurement of absorbance From difference in absorbance, calculate e From reading of sample alone, calculate amount of analyte (use Beer’s law for calculations)

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

69 Atomic emission spectroscopy

70 Atomic emission spectroscopy
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 Developed in the 1970s Higher energy sources than flame or electrothermal methods

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

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

73 ICP-AES Instrumentation

74 Components for sample injection and the ICP torch
Up to 7000°C

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

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

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

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

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

80 Radial and axial observation
Radial. Can achieve higher sensitivity Axial Combined viewing expands dynamic range

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

82 Homework for revision Read

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

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