Transcript Cavity Ring-Down Spectroscopy

Concentration Measurements of
Porphyrin Solutions using the Cavity
Ring-Down and Integrated Cavity
Output Spectroscopy Techniques
Deirdre O’Leary
PY4060 Final Year Project
March 2005
Supervisor: Dr A.A. Ruth
Outline
• Introduction
• Techniques
– Cavity Ring-Down Spectroscopy
– Integrated Cavity Output Spectroscopy
• The Experiment
• Results
• Conclusions
Introduction
• Beer’s Law:
It ( λ )  Io ( λ )e  ε( λ )Cd
It(l)
Io(l)
Sample
d
• Measurement of
Concentration :
– Measure Io and It
– Measure d
– Knowledge of e(l)
Conventional Techniques
• Measure difference in
intensity between It and Io
• Problem:
– Inherently Weak
Absorptions
– Difficulty in measuring
the difference between
It and Io
Solution
• Returning to Beer’s Law:
It ( λ )  Io ( λ )e  ε( λ )Cd
• Long path lengths
– Multi-pass cells
– Optical Cavity Methods
• Cavity Ring-Down Spectroscopy
• Integrated Cavity Output Spectroscopy
The Optical Cavity
Io
R
Intensity
R
Empty
Cavity
d
I n  I o R 2n
Time
The Optical Cavity
Io
R
I n  I o R 2n
Intensity
R
Empty
Cavity
Time
d
Additional
losses of L
per pass
R
L
d
Io
Intensity
R
I n  I o R 2n (1  L) 2n
Time
Experimental Set-Up
Cavity
Pulsed laser
PM tube
Iris
Cuvette
Iris
High
Reflectivity
Mirrors
Computer
Oscilloscope
Cavity Ring-Down Spectroscopy
(CRDS)
Cavity
Laser
beam
Detector
d
CRDS Measurement Principle
• Measurement of the ‘Ring-Down time’
d
τ
c(1  R)  L
Empty
τ empty
d

c1  R 
Cuvette & Solvent
τ cuv 
d
c(1  R)  L cuv 
Cuvette & Solution
τ sample 
d
c (1  R)  L cuv  Lsam 
Integrated Cavity Output Spectroscopy
(ICOS)
Iin
I n  I o R 2n (1  L) 2n
I o I1 I 2
…..……….
In
• Measurement of the total transmitted intensity:
I t  Io  I1  I 2  ....  I n  ..
• Transmitted intensity (subject to losses L per pass):
(1  R) 2 (1  L)
I t  Iin
1  R 2 (1  L) 2
ICOS Measurement Principle
Empty Cavity
Laser
beam
Detector
intensity
Cavity
I empty
time
d
I empty
(1  R)
 I in
1 R
ICOS Measurement Principle
Cuvette and Solvent
Laser
beam
Detector
intensity
Cavity
I cuvette
time
d
I cuv
(1  R) 2 (1  L cuv )
 Iin
1  R 2 (1  L cuv ) 2
ICOS Measurement Principle
Cuvette and
Solution
Laser
beam
Detector
intensity
Cavity
Isolution
time
d
Isample  Iin
(1  R) 2 (1  (Lcuv  Lsample ))
1  R 2 (1  (Lcuv  Lsample )) 2
Lambert-Beer Losses
• Knowledge of the losses L may be obtained
from either technique thus enabling the
calculation of concentration.
• For low losses:
exp (-e(l) C d) = 1 - e(l) C d
• Applying Beer’s Law:
 It = Io (1 - L) = Io exp (-e(l) C d) = Io (1 -e(l) C d)
C= L
ε(λ)d
Platinum Octaethyl Porphyrin
0.6
Absorption Spectrum of PtOEP (xx mM)
0.5
Absorbance
0.4
0.3
532 nm
0.2
0.1
0.0
350
400
450
wavelength/nm
500
550
The Experiment
• The absorption of various porphyrin solutions
was analysed at 532 nm
• CRD and ICOS techniques were implemented
to calculate the losses due to absorption
• This enabled the calculation of the
concentration of each solution
Intensity versus time plot
12 nMol Porphyrin
Solution
D
im
et
Solvent
l
e
an
o
xi
d
e
H
ex
an
e
To
lu
en
1,
e
4
D
io
xa
ne
n-
et
h
ph
o
M
hy
ls
ul
fo
rm
ol
1-
he
xa
n
or
o
C
yc
lo
C
hl
ta
n-
ile
ne
ni
tr
et
o
et
o
Bu
Ac
Ac
103 L
Losses due to Cuvette & Solvent
18
16
14
12
10
8
6
4
2
0
Consistency of Measurements
– Inexact alignment of
beam along cavity optical
axis and cuvette
alignment
16
14
12
Intensity
• Experiment performed
to establish the
reproducibility of results
• Large variance in
results
• Possible reason:
10
8
6
4
2
0
1
2
3
4
5
6
Trial No.
7
8
9
10
The Cuvette
•
•
•
•
•
•
•
Losses in the cuvette
Cuvette consists of four interfaces between different
media
Reflections occur at each surface
Two configurations for minimal reflection loss:
– Incident Beam normal to the surface
– Beam incident at Brewster’s angle
In this experiment the incident beam was normal to
the surface
Cuvette alignment is critical in this configuration,
huge losses otherwise
Design of Cuvette holder to allow for fine
adjustment of the position of the Cuvette in the
beam of the laser
Other Difficulties
• Fluctuations in the laser intensity
– Renders the ICOS measurements inaccuate
– CRD data is not affected by fluctuations because CRD is
intensity independent
• Experimental Conditions
– Optical Cavity is not closed off to surroundings
– Dust on mirror, scattering in open cavity etc.
• Experimental Methodology
– Systematic approach to recording the data
– Measurements for the reference, empty, and sample
performed over a short time scale.
Results
Calculated Concentration / nM
250
EST
ICOS
CRDS
200
150
100
50
0
0
50
100
150
Estimated Concentration / nM
200
Conclusions
• Successfully implemented CRD and ICOS
techniques
• Difficulties encountered
– Fluctuations in the laser intensity
– Cuvette losses!!!
Outlook
• Many Possibilities….
– Investigation of the effect of dissolved oxygen on
absorption:
• comparison between standard and de-gassed samples
– Obtain an absorption spectrum
– Simultaneous monitoring of absorption and
emission of PtOEP
Acknowledgements
• I would like to thank the following people:
– Dr. A.A. Ruth
– Kieran Lynch
– All members of the Laser Spectroscopy
group