Post on 24-Dec-2015
Concentration Measurements of Porphyrin Solutions using the Cavity Ring-Down and Integrated Cavity Output Spectroscopy Techniques
Deirdre O’LearyPY4060 Final Year ProjectMarch 2005
Supervisor: Dr A.A. Ruth
Outline
• Introduction• Techniques
– Cavity Ring-Down Spectroscopy– Integrated Cavity Output Spectroscopy
• The Experiment• Results• Conclusions
Introduction
)Cdε(ot )e(I)(I λλλ
• Beer’s Law:
• Measurement of Concentration :
– Measure Io and It
– Measure d– Knowledge of
Sample
Io() It
d
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:
• Long path lengths– Multi-pass cells– Optical Cavity Methods
• Cavity Ring-Down Spectroscopy• Integrated Cavity Output Spectroscopy
)Cdε(ot )e(I)(I λλλ
The Optical Cavity
EmptyCavity
RR
2non R II
Io
Inte
nsi
ty
Timed
The Optical Cavity
L
EmptyCavity
Additional losses of L per pass
R
R
R
R
2non R II
Io
Io
2n2non L)(1R II
Inte
nsi
ty
Time
Time
Inte
nsi
ty
d
d
Experimental Set-Up
Pulsed laser
Iris
IrisCuvette
OscilloscopeComputer
PM tube
Cavity
HighReflectivity
Mirrors
Cavity Ring-Down Spectroscopy (CRDS)
Detector
Cavity
Laser beam
d
CRDS Measurement Principle
• Measurement of the ‘Ring-Down time’
Empty Cuvette & Solvent Cuvette & Solution
LR)(1c
dτ
R1c
dτempty
cuvcuv LR)(1c
dτ
samcuv
sample LLR)(1 c
dτ
Integrated Cavity Output Spectroscopy (ICOS)
Io 1I 2I …..……….
• Measurement of the total transmitted intensity:
• Transmitted intensity (subject to losses L per pass):
2n2non L)(1R II
..I ....IIII n21ot
22
2
int L)(1R1
L)(1R)(1II
inI
nI
ICOS Measurement Principle
Cavity
d
Laser beam
timein
tens
ityDetector
Iempty
Empty Cavity
R1
R)(1II inempty
ICOS Measurement Principle
Cavity
d
Laser beam Detector
timein
tens
ity
Cuvette and Solvent
cuvetteI
2cuv
2cuv
2
incuv )L(1R1
)L(1R)(1II
ICOS Measurement Principle
Cavity
d
DetectorLaser beam
inte
nsity
time
Cuvette and Solution
solutionI
2samplecuv
2
samplecuv2
insample ))LL((1R1
))LL((1R)(1II
Lambert-Beer Losses
• Knowledge of the losses L may be obtained from either technique thus enabling the calculation of concentration.
• For low losses:exp (-() C d) = 1 - () C d
• Applying Beer’s Law: It = Io (1 - L) = Io exp (-() C d) = Io (1 -() C d) C =
ε(λ)d
L
350 400 450 500 5500.0
0.1
0.2
0.3
0.4
0.5
0.6
Abs
orba
nce
wavelength/nm
Platinum Octaethyl Porphyrin
Absorption Spectrum of PtOEP (xx M)
532 nm
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
Losses due to Cuvette & Solvent
0
2
4
6
8
10
12
14
16
18
Aceto
ne
Aceto
nitrile
Butan
-1-o
l
Chlor
ofor
m
Cyclo
hexa
ne
Dimet
hylsu
lphox
ide
Met
hano
l
n-Hex
ane
Toluen
e
1,4
Dioxa
ne
Solvent
103 L
Consistency of Measurements
• Experiment performed to establish the reproducibility of results
• Large variance in results
• Possible reason:– Inexact alignment of
beam along cavity optical axis and cuvette alignment
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8 9 10
Trial No.
Inte
nsi
ty
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
0 50 100 150 200
0
50
100
150
200
250
EST ICOS CRDS
Ca
lcu
late
d C
on
cen
tra
tion
/ n
M
Estimated Concentration / nM
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