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