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1. 2007
Instrumental Analysis:
Spectrophotometric Methods

2. •Understand interaction between light and matter
(absorbance, excitation, emission, luminescence,fluorescence,
phosphorescence)

•Describe the main components of a spectrophotometer,
(sources, monochromators, detectors, interferometer, grating, ATR, ICP, )
•Make calculations using Beer’s Law
(analyse mixture absorption)
•Understand the mechanism and application of UV-Vis, FTIR, Luminescence,
atomic spectroscopy
By the end of this part of the course, you should be able to:

3. Background knowledge:
What you are expected to know before the course:
Error analysis in quantitative analysis
Solve linear equations
Complementary colour
Exponential and logarithm
What you are recommended to know before the course:
Least square fitting
Basic quantum chemistry
Molecular symmetry
If you have difficulty to understand above topics, find extra reading materials!
Or discuss with me after the lecture.
If you are trying to learn above topics, please let me know.

4. Today’s lecture:
(Instruments based on light interaction with matter)
•Properties of light
•Molecular electronic structures
•Interaction of photons with molecules
•Spectrophotometer components
•Light sources
•Single and double beam instruments
•Monochrometers
•Detectors
•Fluorescence spectroscopy
Next week’s lecture:
•Fourier transformed infrared spectroscopy
•Interferometer
•Atomic spectroscopy
•Quantitative analysis
•Beer’s law
•Method validation
•Dilution and spike

5. Light travelling speed:
in other media: c/n (n = refractive index, generally >1)
in a vacuum: c=2.998 x 10
8
m s
-1
(n=1 exactly, in air n=1.0002926)
c/n= 
Therefore:
Energy is inversely proportional to wavelength
but proportional to wavenumber
And of course, the relationship between energy and frequency:
E = h = hc/ = hc 
h = Planck’s constant (6.626 x 10
-34
J s)
 = wavenumber (most common units = cm
-1
)
~
~
Light is energy in the form of electromagenetic field
Review on properties of light:photon
Wavelength (): Crest-to-crest distance between waves
Frequency (): Number of complete oscillations that the wave makes each second
units: number of oscillations/sec or s
-1
or Hertz |(Hz)

6. Frequency Scanning Techniques: a few definitions
Emission method: source of light is sample
Absorption method: intensities of a source with and without the sample in place are compared
Spectrum: a plot of intensity vs. frequency/wavelength
In quantitative analysis:
common to work at 1 wavelength
running a spectrum is an important initial step (to select best conditions)

7. Fig. 18-2
Regions of Electromagnetic Spectrum-the “colour” of light

8. Electronic structures of simple molecule
S
0
S
1
T
1
Bond length
D
Ground state
Excited state
Singlet
Excited state
Triplet
E
n
e
r
g
y
Dissociated states
V
i
b
r
a
t
i
o
n

s
t
a
t
e
s

9. Interaction between photon and molecule
S
0
S
1
T
1
D
S
0
S
1
T
1
S
0
S
1
transition
AF
I
R
U
V
-
v
is
P

10. Electronic structures
Singlet and triplet
Bond length for ground and excited states
Vibrational structures-infrared absorption/transmission (FTIR)
Internal conversion
Intersystem crossing
Photon adsorption excitation (Beer’s law, UV-vis)
Frank Condon condition and The Stokes' shift
Radionless relaxation and vibration relaxation
Luminescence-fluorescence/phosphorescence
Key concept from energy diagram

11. Type of optical spectroscopy
UV-vis absorption spectroscopy (UV-Vis)
FT-IR absorption/transmission spectroscopy (FTIR)
Atomic absorption spectroscopy (AAS)
Atomic fluorescence spectroscopy (AFS)
X-ray fluorescence spectroscopy (XFS)
What you will learn:
The excitation mechanism
Monochromator design
Instrument principle
Quantitative methods

12. Optical spectrophotometer components
Monochromators
Filters
Grating+slit
prism
Excitation sources
Deuterium Lamp
Tungsten Lamp
Laser
X-ray tube
Mercury lamp
Xenon lamp
Silicon carbide globar
Flame
Furnaces
Plasmas
Hollow-cathode lamp
Detectors
PMT
CCD/CID
Photodiode
Thermocouple
MCT
Pyroelectric detector
UV
UV-vis
X-ray, UV, vis, IR
X-ray
UV-vis
UV-vis
IR
What is the advantage and disadvantage?

13. Design of optical spectrophotometers
Single Beam vs. Double Beam
Fig. 13-12, pg. 315 "Instrument designs for photometers and spectrophotometers”
(a) single-beam design
(b) dual channel design with beams separated in space but
simultaneous in time
(c) double-beam design in which beams alternate between two
channels."
(a)
(b)
(c)
Q: what’s the advantage of double beam spectrophotometer?

14. Light sources
Black-body radiation for vis and IR but not UV
- a tungsten lamp is an excellent source of black-body radiation
- operates at 3000 K
- produces  from 320 to 2500 nm
For UV:
- a common lamp is a deuterium arc lamp
- electric discharge causes D
2 to dissociate and emit UV radiation (160 – 325 nm)
- other good sources are:
Xe (250 – 1000 nm)
Hg (280 – 1400 nm)
( How much in cm
-1
, J, Hz and eV?)
Lasers:
- high power
- very good for studying reactions
- narrow line width
- coherence
- can fine-tune the desired wavelength (but choice of wavelength is limited)
- £££ expensive £££
What is the important properties of a source?
Brightness
Line width
Background
Stability
Lifetime

15. Sample a source containers:
for UV: quartz (won’t block out the light)
for vis: glass [ 800nm (red) to  400 nm (violet)]
for IR: NaCl (to or 15384 nm or 650 cm
-1
)
KBr (to 22222 nm or 450 cm
-1
)
CsI (to 50000 nm or 200 cm
-1
)
Optical transmission coefficient
Best material: diamond, why?
High transmission
Chemically inert
Mechanically strong
Criteria

16. Monochromators
Early spectrophotometers used prisms
- quartz for UV
- glass for vis and IR
These are now superseded by:
Diffraction gratings:
- made by drawing lines on a glass with a diamond
stylus
ca. 20 grooves mm
-1
for far IR
ca. 6000 mm
-1
for UV/vis
- can use plastic replicas in less expensive instruments
Think of diffraction on a CD
http://www.mrfiber.com/
images/cddiffract.jpg
http://www.ii.com/images/prism.jpg
http://www.veeco.com/library/nanotheater_detail.php?
type=application&id=331&app_id=34
10mx10m
Why?

17. Monochromators: cont’d
Polychromatic radiation enters
Second concave mirror focuses each wavelength at
different point of focal plane
Orientation of the reflection grating directs only one
narrow band of wavelengths to exit slit
The light is collimated the first concave mirror
Reflection grating diffracts different
wavelengths at different angles
http://oco.jpl.nasa.gov/images/grating_spec-br.jpg
What is the purpose of concave mirrors?

18. Interference in diffraction
d


d sin()+d sin()=n
n=1, 2, 3 In-phase
n=1/2, 3/2, 5/2 out-phase
>0
<0
Bragg condition
Phase relationship

19. Monochromators: reflection grating

20. Monochromators: reflection grating
Each wavelength is diffracted off the grating at a different angle
Angle of deviation of diffracted beam is wavelength dependent  diffraction grating
separates the incident beam into its constituent wavelengths components
Groove dimensions and spacings are on the order of the wavelength in question
In order for the emerging light to be of any use, the emerging light beams must be in phase
with each other
Resolution of grating: 

=nN
Angular resolution:
As: d sin()+d sin()=n
So: n =d cos() 
Therefore: =n/[d cos()]
What does this mean?
n: diffraction order
N: number of illuminated groves

21. Monochromators: slit
Bottom line:
- it is usually possible to arrange slits and mirrors
so that the first order (n = 1) reflection is separated
- a waveband of ca. 0.2 nm is obtainable
However, the slit width determines the resolution and signal to noise ratio
Large slit width: more energy reaching the detector  higher signal:noise
Small slit width: less energy reaching the detector BUT better resolution!

22. Detectors
Choice of detector depends upon what wavelength you are studying
Want the best response for the wavelength (or wavelength range) that you are studying
In a single-beam spectrophotometer, the 100% transmittance control must be adjusted each time the
wavelength is changed
In a double-beam spectrophotometer, this is done for you!
: Radiation-----charger converter

23. Photomultiplier-single channel, but very high sensitivity
- Light falls on a photosensitive alloy
(Cs
3
Sb, K
2
CsSb, Na
2
KSb)
- Electrons from surface are accelerated towards
secondary electrodes called dynodes and gain enough
energy to remove further electrons (typically 4-12, to 50
with GaP).
- For 9 stages giving 4 electrons for 1,
the amplification is 4
9
or 2.6 x 10
5
)
- The output is fed to an amplifier
which generates a signal
- To minimise noise it is necessary to
operate at the lowest possible voltage
What decide the sensitive wavelength?

24. Photodiode Array-multiplex, but low sensitivity
Good for quick (fraction of a second) scanning of a full spectrum
Uses semiconductor material:
Remember: n-type silicon has a conduction electron – P or As doped
p-type silicon has a ‘hole’ or electron vacancy – Al or B doped
A diode is a pn junction:
under forward bias, current flows from n-Si to p-Si
under reverse bias, no current flows
boundary is called a depletion layer or region

25. Photodiode Array
- Electrons excited by light partially discharge the condenser
- Current which is necessary to restore the charge can be detected
- The more radiation that strikes, the less charge remains
- Less sensitive than photomultipliers  several placed on placed on single crystal
- Different wavelengths can be directed to different diodes
- Good for 500 to 1100 nm
- For some crystals (i.e. HgCdTe) the response time is about 50 ns
Could you compare photodiode with CCD detector?

26. Photodiode Array Spectrophotometer
- For photodiode array spectrophotometers, a white light passes through sample
- The grating polychromator disperses the light into the component wavelengths
- All wavelengths are measured simultaneously
- Resolution depends upon the distance between the diodes and amount of dispersion
No moving parts!
Simple mechanical and optical design, very compact.

27. Photodiode Array Spectrophotometers
vs Dispersive Spectrophotometers
D is pe r siv e S pec t r oph ot om et er :
- on ly a n ar r o w ba nd of wave leng th s r ea ch es t he d et ect o r a t a t im e
- slow sp ect r a l acq uisit io n (ca. 1 m in)
- se ver a l m o ving p ar t s ( gr at ing s, f ilt er s , m ir r or s , et c.)
- r es olut ion: ca. 0 .1 n m
- pr o duc es les s st r ay lig ht  g r eat e r d yna m ic r ang e f or m ea sur ing h ig h ab sor b an ce
- se nsit ive t o st r ay light f r om ou ts id e so ur ce s i.e. r o om ligh t
Photodiode Array
Spectrophotometer:
- no moving parts  rugged
- faster spectral acquisition (ca.
1 sec)
- not dramatically affect by room
light
What are the components 1 to 10?
From: http://www.oceanoptics.com/

28. Property of luminescence spectrum
Fluorescence vs phosphorescence
1.Phosphorescence is always at longer wavelength compared with fluorescence
2.Phosphorescence is narrower compared with fluorescence
3.Phosphorescence is weaker compared with fluorescence
Absorption vs emission
1.absorption is mirrored relative to emission
2.Absorption is always on the shorter wavelength compared to emission
3.Absorption vibrational progression reflects vibrational level in the electronic excited
states, while the emission vibrational progression reflects vibrational level in the
electronic ground states
4.
0
transition of absorption is not overlap with the 
0
of emission
Why?
Why?

29. Fluorescence spectroscopy

30. Fluorescence spectroscopy
Emission spectrum: hold the excitation wavelength steady and measure the emission at
various wavelengths
Excitation spectrum: vary the excitation wavelength and vary the wavelength measured
for the emitted light
Light source
Excitation
monochromator
Reference
diode
8 %
o
f lig h
t
Beam
splitter
sample
Emission
Monochromator
Amplifier
ComputerPMT
Q: why the emission is
measured at 90 relative to
the excitation?

31. Fluorescence spectroscopy: well defined molecules

32. •Describe the main components of a spectrophotometer and distinguish between single double beam instruments
•Describe suitable sources for ultraviolet (UV)/visible (vis), infra red (IR) and atomic absorption (AA) instruments
•Describe and assess advantages and disadvantages of various monochromators e.g. Prism, diffraction gratings
•Explain how to asses the quality of grating
•Explain how photomultipliers and diode detectors work
•Explain the advantage of multiplex detecting
•Describe the luminescence spectroscopy and energy transfer process
•Compare the emission and absorption spectrum
Summary of spectrophotometric techniques

33. FTIR, AS and Quantitative analysis
Instrumental Analysis:
Spectrophotometric Methods II
2007

34. By the end of this part of the course, you should be able to:
•Understand interaction between light and matter
(absorbance, excitation, emission, luminescence,fluorescence, phosphorescence)

•Describe the main components of a spectrophotometer,
(sources, monochromators, detectors, interferometer, grating, ATR, ICP, )
•Make calculations using Beer’s Law
(analyse mixture absorption)
•Understand the mechanism and application of UV-Vis, FTIR, Luminescence, atomic
spectroscopy

35. Last week’s lecture:
(Instruments based on light interaction with matter)
•Properties of light
•Molecular electronic structures
•Interaction of photons with molecules
•Spectrophotometer components
•Light sources
•Single and double beam instruments
•Monochrometers
•Detectors
•Fluorescence spectroscopy
Today’s lecture:
•Fourier transformed infrared spectroscopy
•Interferometer
•Atomic spectroscopy
•Quantitative analysis
•Beer’s law
•Method validation
•Dilution and spike

37. Basic principles before Beer’s law
Qualitative vs quantitative analysis
Sensitivity vs resolution
S/N ratio ratio control
Background correction
Peak shape control
Chemical environment control
Selectivity
What is inside and how much is inside?
Beer’s law is the fundation for quantitative analytical chemistry
How can we balance the sensitivity vs resolution?
Improve the Signal/Noise ratio by repeat.
Control the background (stabilise) and numerically subtract.
Minimum artificial distortion.
Improve the reproducibility.
Improve the uniqueness of the quality analysis.

38. A = bc

39. Beer’s law
Absorption A = bc
whereb =>path length
c =>molar concentration
 =>molar absorptivity
Absorption vs Transmittance
Transmittance T = P/P
o
where T => transmittance
P => power of transmitted radiation
P
o
=> power of incident radiation
%T = (P/P
o)*100
Where %T => percent transmittance
A = - log
10
T = - log
10
(P/P
o
)
and T=10
-A
What is the units for A, b, c, T, P and 
Readout
Absorbance
Source
Detector
b
Sample
A
c
With constant b
A
b
With constant c

40. Cuvets or Cuvettes

41. Error in Beer’s Law
Spectrophotometric measurements involve:
i. an adjustment for P/P
o
= 0 i.e. for no light through
ii. an adjustment for P/P
o
= 100% i.e. for all the light through
iii. an adjustment of P/P
o
with sample in place
concentration(c)100%
Consider the effect of a 1% error in T (P/P
o)
In practice: the measure A should be
between:
1.0 (T = 10%)  0.1 (T = 79.4%)
i. when c is small:
c is also small but it is large
proportion of c 1% error
c
ii. when c is large:
error now corresponds to a large
uncertainty in c
c
1% error
1% error
1% error
C
C
P
/
P
o

%
Scale the spectrometer

42. Quantitative methods:
Part 1. Methods validation:
Specificity: the ability of a method to distinguish the analyte from others in the sample Check resolution
Linearity: How well a calibration curve follows a straight line. Square of correlation coefficient
Accuracy: nearness to truth, check with different methods and spiking
Precision: reproducibility, standard deviation
Range: concentration interval over which linearity, accuracy and precision are all good
Detection Limits: defined by signal detection limit: 3s (standard deviation), minimum concentration:
3s/m, m is the slope of the linear curve.

43. Concentration-dilution formula
A very versatile formula that you
absolutely must know how to use
C
1 V
1 = C
2 V
2
where C = conc.; V = volume
C
conc V
conc = C
dil V
dil
where “conc” refers to the concentrated
solution
and “dil” refers to the dilute solution
How to prepare 100ml of 0.1M NaCl
solution from 2.0M stock?
Calculations:
The total NaCl molecules:
V1x2.0M =100mlx0.1M
So, V1=100mlx0.1M/2.0M
=5ml (needed from stock)
How to do it:
Chef:
Measure 5ml of stock with teaspoon
Add 95ml of water
Chemist:
transfer 5ml stock with a 5ml pipet into a
100ml volumetric flask.
Topup to 100ml mark. Shake not stirred
Can you tell the difference between a chef and a chemist?
Quantitative methods:
Part 2: Dilution

44. a known quantity of analyte add to the sample to test
accuracy and linearity
The unknown sample: V
1, A
1
Spiked with V
2
, c
2
and A
2
.
The solutions are diluted to volume V.
Absorbance difference A
2
-A
1
=bV
2
c
2
/ V
(spike dilution)
So the molar absorbance  can be measured, which will in turn give c
x
Quantitative methods:
Part 3: spike:
V
1C
x
V
1C
x
+V
2
C
2
Diluted to
V
The final concentration:
Dilute unknown: (V
1
c
x
)/ V, absorbance A
1
A
1
=b (V
1
c
x
)/ V
Spiked: (V
1
c
x
+v
2
c
2
)/ V, absorbance A
2
A
2
=b (V
1
c
x
+v
2
c
2
)/ V
Without dilution of the unknown
unknown: (V
1
c
x
), absorbance A
1
A
1
=b (V
1
c
x
)
Spiked: (V
1
c
x
+v
2
c
2
)/ V, absorbance A
2
A
2
=b (V
1
c
x
+v
2
c
2
)/ V

45. Analysis of a mixture and isosbestic point
A solution of mixture of M and N
Pure N
Pure M
Fig. 14-14, pg. 345 Principles of Instrumental Analysis Fifth Edition, by Skoog-Holler-
Nieman
What is in my soup?
A mixture of flavours.

46. Assumption
Analysis of Mixtures of Absorbing Substances
The interaction between substance A and B is so weak that
the presence of A(B) does not affect the molar absorbance
of B(A). A linear addition of individual absorbance is equal to
the total absorbance.

47. With two variables
Need two measurements at wavelength 1 and 2
How to get the answer c
1
and c
2
?
Simple, just eliminate c
2 to get c
1.
In two equations
Solution of Binary Mixture
Wavelength 1
A
m,1 = a
1,1*b*c
1 + a
2,1*b*c
2
Wavelength 2
A
m,2 = a
1,2*b*c
1 + a
2,2*b*c
2

48. Classically, we can solve each pairs of equation to get c
1
and c
2
, then we average
the c
1
and c
2
to reduce the errors in each measurment
With computer, a least square fitting to find the best solution of c
1
and c
2
by
minimising the standard deviation (least square).
If the solution is a mixture of N substances, how many minimum
measurements are required at different energy?
What happens if we do more than two wavelength measurements?
Two vaiables in N equations

49. Analysis of a mixture: Isosbestic point
If absorption curves at different conditions always cross at one wavelength, that
cross point is called isosbestic point.
This suggests:
There are only two principal species in the solution.
At this point, absorption coefficient are equal for different species in the solution.
At this point, total concerntration can be measured.
Changing of experimental conditions
A=
1bc
1+
2bc
2
When 
1
=
2
=
A=b(c
1
+c
2
)=bc
A is a function of c=(c
1+c
2),
but not c
1
or c
2
individually

50. Analysis of mixture: equilibrium constant
A simple classical situation:
P+X PX Equilibrium constant k=[PX]
[P][X]
Complicated considerations:
P, X and PX all have absorbance at wavelength.
Only P and PX have absorbance at  wavelength.
Only PX has absorbance at  wavelength.
A=
1
[P]+
2
[X]+
3
[PX]
With constant total concerntration P
0
P
0
=[P]+[PX]
What we measure: [X], A
Target: get rid off [P] and [PX] with a function of known A and [X]
Adding [X] to achieve different equilibrium (titration).
[P]=P
0-[PX]
A=
1P0- 
1[PX] +
2[X]+
3[PX]

51. k[P][X]=[PX]
A=
1
P
0
- 
1
[PX] +
2
[X]+
3
[PX] [PX]=(A-A
0
-
2
[X])

3
-
1
Here: A
0
= 
1
P
0
Further more: A=A-A
0, 
31= 
3-
1
[P]=P
0-[PX]=P
0-(A-
2[X])/ 
31
At last: k[P]=k{P
0
-(A-
2
[X])/ 
31
}= (A-
2
[X])

31
[X]
Or:
kP
0
-k(A-
2
[X])/ 
31
= (A-
2
[X])

31[X]
x= (A-
2
[X])/ 
31
y= (A-
2
[X])

31
[X]
y
x
Slope=-k
What is the intercept?
Analysis of mixture: equilibrium constant

52. Fourier Transform Infrared Spectroscopy (FTIR)
Traditional dispersive spectroscopy problems:
Low sensitivity in IR
Slow (relatively)
low resolution
FTIR
Large optical throughout, high sensitivity
Fast
And high resolution
Solution:
Interferometer,
mechanical modulation
Jean-Baptiste-Josephde Fourier (1768-1830)

53. Key element of FTIR
Michelson Interferometer
Purpose: incident beam modulation through interference

54. Interference of waves
In-phase: constructive
Out-of-phase: destrictive

55. Michelson Interferometer
-Mirror moves with Velocity V
-Recorded as
intensity as a
function of
distance [I()]
versus the
distance ()
-Two beams recombine before detector
-Monochromatic beam of frequency gives an interferogram (cosine curve with
wavelength proportional to 1/)
-The interferogram contains the spectrum of the source (reference sample) minus the
spectrum of the sample
-V is usually 1.5
cm s
-1
-To distinguish
two frequencies

1
and 
2
:
distance, ,
≥ 1/(
1 – 
2)

56. Fourier Transform Infrared Spectroscopy

57. Fourier Transform Infrared Spectroscopy
Normal spectrum: plot of I() vs 
Intensity as a function of frequency vs. frequency
Fourier transform: plot of I(t) vs t
Intensity as a function of frequency vs frequency (remember: t = 1/)
Called the Fourier Transform of the frequency spectrum
Spectrum may be collected in the frequency domain as function of 
or in the time domain as a function of t
Each version of the spectrum contains the same information
Conversion to one form to the other can be accomplished by a computer

58. Transfer interferogram to absorption spectrum
FFT: Fast Fourier Transformation

59. - sample interferogram is transformed into sample spectrum
Fourier Transform Infrared Spectroscopy
- background spectrum is subtracted
from sample spectrum

60. Beyond stabdard transmission FTIR

61. Atomic spectroscopy
Quantitative and qualitative elementary analysis.
A tool for

62. Atomic Spectroscopy: Overview
Number of spectral lines for each
element can be large!
Hydrogen
Helium
Mercury
Uranium
http://library.thinkquest.org/19662/low/eng/model-bohr.html

63. Atomic Spectroscopy: Overview
-Samples vaporized at 2000 – 6000 K  decompose
into atoms
-Concentration of atoms in the vapor are measured
-Very sensitive: ppm (g/g) to ppt (pg/g) levels
-Can measure up to 60 elements at a time:
Molecular spectroscopy has a bandwidth of at least
10 nm
Atomic spectroscopy has a bandwidth of 0.001 nm
-1 – 2% precision

64. Atomic Absorption, Emission and Fluorescence

65. Hollow-Cathode Lamps
-Hollow-Cathode lamps (HCL) contains the vapor of the element of interest (i.e. Na)
-Positive ions from a noble gas bombard the cathode and give metal atoms by sputtering
-Metal atoms absorb energy by colliding with fast-moving filler gas ions, are elevated to excited electronic states, and return to
ground state (emission)
-Spectrum consist of discrete lines from the metal and gas (chosen so that interference is
minimized)
-The lines have a bandwidth of 0.001 nm
-Atoms in the flame are mainly in the ground state
 Only those HCL lines terminating in the ground state can be used for
absorption measurements
-Lines are separated by a monochrometer
-Sometimes can use the same lamp for 2 – 3 elements (e.g. Ca/Mg)

66. Simplest system: Flame Atomization
-Nebulizer: converts the liquid into aerosol
-Typical temperature of flame = 2200 ºC
-Typical fuel: acetylene (can use H
2
)
-Typical oxidant: air (can use N
2O or O
2)

67. Background correction:
A. Corrections performed by alternate sampling from the HCL & D
2
lamp
D
2 lamp will give the background
B. Alternatively, beam chopping or modulation of the HCL is used to
distinguish between signal from flame (emission) and atomic line of
element
Source is usually modulated (e.g. at 325 Hz)  detector arranged so that
only the modulated signal is recorded
Modulation cuts down noise!
Background Correction

68. Inductively Coupled Plasma Spectrophotometer
-Sample and Ar are aspirated into concentric
quartz tubes surrounded by a lead coil
-Inner tube has sample aerosol and Ar support
gas
-Outer tube has flow gas to keep the tubes cool
-Magnetic field creates an oscillating current in the ions
and electrons of the support gas (a plasma)
-Atoms or ions are almost all excited
 emission rather than absorption is measured
From:
http://www.chemistry.adelaide.edu.
au/external/soc-rel/content/icp.htm
-Oscillating current is produced in induction coil
 oscillating magnetic field is created
-These ions and electrons collide with other
atoms in the support gas
-Temperatures of 6000 – 10000 K are generated

69. Interference: An example
Sn has to major emission lines at 189.927 nm and 235.485 nm
Element added at
50mg/L
[Sn] (

g/L), 189.927
nm emission line
None 100.0
Ca 96.4
Mg 98.9
P 106.7
Si 105.7
Cu 100.9
Fe 103.3
Mn 99.5
Zn 105.3
Cr 102.8

70. Interference: An example
Sn has to major emission lines at 189.927 nm and 235.485 nm
Element added at
50mg/L
[Sn] (

g/L), 189.927
nm emission line
[Sn] (

g/L), 235.485
nm emission line
None 100.0 100.0
Ca 96.4 104.2
Mg 98.9 92.6
P 106.7 104.6
Si 105.7 1029
Cu 100.9 116.2
Fe 103.3 intense emission
Mn 99.5 126.3
Zn 105.3 112.8
Cr 102.8 76.4

Which elements interfere at both wavelengths?
Which wavelength is preferred for analysis?

71. Dealing with Interference: Standard Addition
-For better accuracy add several
different known amounts to
generate a plot
-From Beer’s Law:
A
X = k V
1C
x /V
-Intercept = -V
1
C
x
-Measure the absorbance (A
X
) of sample of unknown concentration (c
X
)
-Add a known amount of standard to give a new concentration (c
S
) and measure new
absorbance (A
T
)
A
T
= k(V
1
C
x
+ V
2
C
s
)/V
V
1
C
x
V
1
C
x
+V
2
C
s
Diluted to
V
[Added analyte]=V
2
C
s0
Slope =k / V
A T
= k(V 1
C x
+ V 2
C s
)/V
A
x

72. Some Practical Considerations
Atomic Absorption Spectrophotometry:
-Slightly more expensive than flame photometry, but can look at more elements
-Good for relatively high concentrations (routine analysis)
-Most AA spectrometers have facilities for recording emission spectra (HCL taken out of circuit)
Flame photometry is used when looking at the most sensitive spectral lines (looks at emission)
> 300 – 350 nm (e.g. Na 589.0 nm, K 766.5 nm and Ca 422.7 nm)
- good for relatively high concentrations
ICP
-Many lines of varying intensity from both atoms and ions are available
-Since dealing with hotter temperatures, can get more interference
-BUT…. ICP gives sharper lines  more elements can be measured at once with several slits and monochromators
-VERY VERY SENSITIVE!!! NEED TO DILUTE  ERROR

73. Some Practical Considerations: Temperature Effects
-Number of atoms in the excited state is temperature dependent
Boltzmann Distribution
N/N
0 = (g/g
0) exp(-E/kT)
N, N
0
, g, g
0
= #’s in and degeneracies of the excited and ground states
DE = energy difference
T = Temperature
k = Boltzmann Constant (1.39 x 10
-23
)
e.g. Na: g
1 = 6, g
0 = 2
-At 2500 K, N/N
0 = 1.74 x 10
-4
-At 2510 K, N/N
0
= 1.79 x 10
-4
 4% difference!
Excited state
Ground state
N
N
0
Absorption Emission
N/N0
Element g/g0 2000 K 3000K
Cs 2 4.44 x 10
-4
7.24 x 10
-3
Na 2 9.89 x 10
-6
5.88 x 10
-4
Ca 3 1.21 x 10
-7
3.69 x 10
-5
Fe 2.29 x 10
-9
1.31 x 10
-6
Cu 2 4.82 x 10
-10
6.65 x 10
-7
Mg 3 3.35 x 10
-11
1.50 x 10
-7
Zn 3 7.45 x 10
-15
5.50 x 10
-10

74. •Beer’s law, absorbance and transmittance
•Explain why the most accurate data are obtained when the absorbance is between 0.1
and 1.0.
•Quantitative methods, validation, dilution and spike.
•Mixture analysis and isosbestic point
•Explain with diagram how a FTIR spectrometer works
•atomic emission and absorption spectrophotometers work
•Describe an Inductively Coupled Plasma (ICP) spectrophotometer
•Explain the use of a deuterium lamp to obtain background corrections in AAS
•Explain the standard addition method
•Explain why higher temperatures give stronger emission lines
What we have learned today:

75. Atomic and x-ray fluorescence
For atomic spectroscopy, the light sources generally needs to be stronger than HCL in order to get a strong fluorescence signal
- lasers commonly used (see http://science.howstuffworks.com/laser.htm for a nice description on how lasers work)
- vacuum discharge vessels also used
Advantages:
- much better sensitivity (up to 1000 times greater that atomic absorption)
- can even count individual atoms (Mark Osborn looks at single molecules using fluorescence spectroscopy)
- good for biological and medical applications
Sample needs to be viewed at an angle (generally 90)
- differentiate between incident light (P) and fluorescence
- prevent swamping out of the detector with the high intensity laser beam
Monochrometer
and Detector
SampleLASER
90
o
or other light source if
looking at molecular
fluorescence
AF
source sample
detector

76. X-ray fluorescence
-Similar to fluorescence, but uses X-rays instead of photons
-Energy of the resulting X-ray is atom dependent
-Number of characteristic X-rays is proportional to the concentration of the element
-Incoming X-ray ejects an electron from an inner-shell
Incoming
radiation
X-ray
1
Excited electron e
-
E
0
e
-
-A lower energy X-ray is released when an electron replaces the lost inner-shell electron
X-ray
2
X-ray
3
e
-
e
-

77. X-ray fluorescence uses
-
N o n-de s tr uc tiv e
-M ul ti -e l em e nt
-F a st
-
M et a llur g ic a l indust r y
-Ge o che m is tr y a nd m i ne r a lo g y : qua lit a ti ve a nd qua nti ta t iv e
-Env ir onm e nt a l s c ie nc e: m e a s ur e m e nt o f se dim ent s , a e r os o ls , w a te r
-
P a int indust r y: le a d a nal y si s
-J e w el le r y : pr ec i o us m et a ls
-F ue l i ndus tr y : c o nta m i na nt m oni to r ing
-F o o d indust r y: to x ic m e ta l a nal ys i s
-A g r ic ult ure : t r a c e m e ta l a na ly si s
-A r c ha e o lo g y
-A r t s: a na ly s is o f pa inti ng s a nd s cul ptur es
-Le a d ana l ys is