Introduction to Optical Spectroscopy: Fundamentals and Instrumentation

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 • http://www.shsu.edu/~chm_tgc/sounds/flashfiles/ICPwCCD.swf • http://www.thespectroscopynet.com/Index.html?/ • http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/ • http://www.chemguide.co.uk/analysis/uvvisiblemenu.html See also individual citations on slides

4. Electromagnetic radiation Spectroscopy = interactions between light & matter E = hn = hcl n = frequency; l = wavelength http://www.spectroscopynow.com/coi/cda/detail.cda?id=18411&type=EducationFeature&chId=7&page=1 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 http://physics.nist.gov/PhysRefData/ASD/lines_form.html

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 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 Mass spectrometries Optical spectroscopies Fluoresc-ence Spectros-copy AAS ICP-MS SIMS AES Others (L. 6) Others See table ICP-OES GFAAS Flame AAS

10. Atomic spectroscopies

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 (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 • 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. AAS Spectrometer Light Source Sample Monochro-mator Detector Read-out/Data system Instrument components 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 http://www.spectroscopynow.com/coi/cda/detail.cda?id=18412&type=EducationFeature&chId=7&page=1

17. Example for a dual beam spectrometer

18. Single beam

19. UV-Vis spectroscopypracticalities: 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 Ar lamp Xe lamp D2 lamp Continuum 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 130-700 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 Gratings 3000 lines/nm 50 lines/nm Interference filters Discontinuous Interference wedge Glass filters

25. Monochromators • Consist of • 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 1000-2000 lines/mm: d = 0.5-1 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 Photographic plate Photomultiplier Phototube Photon detectors Photocell Silicon diode Charge-coupled device (170-1000) 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 http://www.chemistry.adelaide.edu.au/external/soc-rel/content/ccd.htm

37. 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- MX(g) Vaporisation 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 www.spectroscopynow.com (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 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 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 I0 It 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 10-4s) • Device: Graphite furnace