Chemistry

1. Atomic absorption/ emission
spectroscopies

2. 2
Fundamentals
 The population of different states is
given by the Boltzmann equation:
kT
ΔE
0
1
0
1
e
g
g
N
N 

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

3. 3
Define the following terms:
(a) releasing agent (g) sputtering
(b) protective agent. (h) self· absorption
(c) Ionization suppressor (i) spectral interference
(d) atomization (j) chemical interference
(e) pressure broadening (k) radiation buffer
(f) hollow cathode lamp (L) Doppler broadening

4. 4
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

5. 5
Atomic Spectroscopies - Synopsis
Optical
spectroscopies
Techniques for determining the elemental composition of a
sample by its electromagnetic or mass spectrum
Mass spectrometries
ICP-MS SIMS
AAS
AES
Fluoresc-
ence
Spectros-
copy
Flame AAS GFAAS
ICP-OES
Others
Others

6. 6
Atomic spectroscopies
 Common principles:
 Sample introduction:
Nebulisation, Evaporation
 Atomisation (and excitation or
ionisation) by flame, furnace,
or plasma
 Spectrometer components:
 Light source
 Sample
 Wavelength selctor
 Detector

7. 7
Atomic spectra vs molecular spectra:
 Lines Bands
(nm)
Typical atomic spectrum Two typical molecular spectra
Y axes: intensity of absorbed light. Under ideal conditions proportional
to analyte concentration (I  c; Beer’s law).
e.g. acquired by AAS Acquired by UV-Vis spectroscopy

8. 8
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 and accessible through absorption:
many transitions possible:
A band is the sum of many lines
Vibrational substates rotational substates
HOMO
LUMO

9. 9
Instrument components
AAS Spectrometer
ICP-OES Spectrometer
Monochro-
mator
Sample =
light
source
Detector
Read-
out/Data
system
Light
Source
Monochro-
mator
Sample Detector
Read-
out/Data
system
Light
Source
Monochro-
mator
Sample Detector
Read-
out/Data
system
UV-Vis Spectrometer:

10. 10
Wavelength(nm) 100 200 400 700 2000 4000 7000 10,000 20,000 40,000
Spectral region
VAC UV Visible Near IR IR Far IR
Light sources
Continuum
Line
Ar lamp
Xe lamp
D2 lamp
Tungsten lamp
Nernst glower (ZrO2 + Y2O3)
Nichrome wire
Lasers
Hollow cathode lamps
Globar (SiC)

11. 11
Example of a continuum source:
Output from Tungsten lamp
Widely applied in UV-Vis spectrometers

12. 12
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

13. 13
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

14. 14
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

15. 15
Effect of slit width on peak heights

16. 16
Detectors
Wavelength(nm) 100 200 400 700 2000 4000 7000 10,000 20,000 40,000
Spectral region
VAC UV Visible Near IR IR Far IR
Photographic plate
Photomultiplier
Photocell
Phototube
Silicon diode
Charge-coupled device (170-1000)
Photoconductor
Thermocouple
Golay pneumatic cell
Pyroelectric cell
Photon
detectors
Thermal
detectors

17. 17
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

18. 18
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

19. 19
AAS and ICP-OES
Sample preparation
Interferences
Calibration

20. 20
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+

21. 21
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

22. 22
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

23. 23
Microwave digestion
Supplied with dedicated vessels
Closed vessel digestion minimises sample contamination
Faster, more reproducible, and safer than conventional
methods
Rotor

24. 24
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

25. 25
Atomic absorption
spectroscopy

26. 26
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
H
Li
Na
K
Rb
Cs
Fr
Be
Mg
Ca
Sr
Ba
Ra
Sc
Y
La
Ac
Ti
Zr
Hf
V
Nb
Ta
Cr
Mb
W
Mn
Tc
Re
Fe
Ru
Os
Co
Rh
Ir
Ni
Pd
Pt
Cu
Ag
Au
B
Al
Ga
In
Tl
C
Si
Ge
Sn
Pb
N
P
As
Sb
Bi
O
S
Se
Te
Po
F
Cl
Br
I
At
Ne
Ar
Kr
Xe
Rn
He
Zn
Cd
Hg
H
Li
Na
K
Rb
Cs
Fr
Be
Mg
Ca
Sr
Ba
Ra
Sc
Y
La
Ac
Ti
Zr
Hf
V
Nb
Ta
Cr
Mb
W
Mn
Tc
Re
Fe
Ru
Os
Co
Rh
Ir
Ni
Pd
Pt
Cu
Ag
Au
B
Al
Ga
In
Tl
C
Si
Ge
Sn
Pb
N
P
As
Sb
Bi
O
S
Se
Te
Po
F
Cl
Br
I
At
Ne
Ar
Kr
Xe
Rn
He
Zn
Cd
Hg

27. 27
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

28. 28
Schematic
Light Source Monochromator Detector Amplifier
E.g. Hollow
cathode lamp
Analyte solution
Atomiser Fuel (e.g. acetylene)
Air
I0 It
Nebuliser, spray
chamber, and
burner

29. 29
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

30. 30
Structure of a flame
 Relative size of
regions varies with
fuel, oxidant and
their ratio

31. 31
Electrothermal atomisation: GFAAS
 Provides enhanced sensitivity
 entire sample atomised in very
short time
 atoms in optical path for a second
or more
 Device: Graphite furnace

32. 32
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

33. 33
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

34. 34
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
http://www.cem.msu.edu/~cem333/
Week03.pdf

35. 35
Natural linewidths
 Width of an atomic spectral line is
determined by the lifetime of the excited
state
 For example, lifetime of 10-8 seconds (10
ns) yields peak widths of 10-5 nm

36. 36
Doppler Effect
 Due to rapid motion of atoms in gas phase
 Atom moving toward the detector absorbs / emits radiation
of shorter l than atom moving perpendicular to detector.
 Atom moving away from the detector absorbs / emits
radiation of longer l: detector perceives fewer oscillations
Photon detector

37. 37
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

38. 38
Background correction in AAS
 High energy Deuterium background corrector
Deuterium lamp
Hollow
cathode
lamp
Beam
combiner
Sample
Detector
Lamps are
pulsed out of
phase with
each other

39. 39
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

40. 40
Specialised applications in AAS:
Flameless cold vapour methods
 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

41. 41
Calibration – some practical
aspects

42. 42
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

43. 43
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

44. 44
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.
T
s
s
T
x
x
T
V
c
kV
V
c
kV
A 


45. 45
Method of standard additions
slope = m = (єlcs) / VT
intercept = b = (єlVxcx) / VT
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

46. 46
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)

47. 47
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

48. 48
Atomic emission
spectroscopy

49. 49
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

50. 50
ICP-AES/OES
 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
Inductively coupled plasma-atomic emission spectroscopy
(or optical emission spectroscopy)

51. 51
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

52. 52
ICP-AES Instrumentation

53. 53
Components for sample injection
and the ICP torch
Up to 7000°C

54. 54
Atomisation / Ionisation
 In plasma, sample moves through several zones
 Preheating zone (PHZ): temp = 8000 K:
Desolvation/evaporation
 Initial radiation zone (IRZ): 6500-7500 K: Vaporisation,
Atomisation
 Normal analytical zone (NAZ): 6000-6500 K: Ionisation

55. 55
Advantages of plasma
 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
 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

56. 56
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

57. 57
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