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