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Deirdre O’Leary PY4060 Final Year Project March 2005 Supervisor: Dr A.A. Ruth

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

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Deirdre O’Leary PY4060 Final Year Project March 2005 Supervisor: Dr A.A. Ruth

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  1. 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

  2. Outline • Introduction • Techniques • Cavity Ring-Down Spectroscopy • Integrated Cavity Output Spectroscopy • The Experiment • Results • Conclusions

  3. Introduction • Beer’s Law: • Measurement of Concentration : • Measure Io and It • Measure d • Knowledge of e(l) Sample Io(l) It(l) d

  4. Conventional Techniques • Measure difference in intensity between It and Io • Problem: • Inherently Weak Absorptions • Difficulty in measuring the difference between It and Io

  5. Solution • Returning to Beer’s Law: • Long path lengths • Multi-pass cells • Optical Cavity Methods • Cavity Ring-Down Spectroscopy • Integrated Cavity Output Spectroscopy

  6. The Optical Cavity R R Empty Cavity Intensity d Time

  7. The Optical Cavity R R Empty Cavity Intensity d Time R R L Additional losses of L per pass Intensity Time d

  8. Experimental Set-Up Cavity Pulsed laser PM tube Iris Cuvette Iris High Reflectivity Mirrors Computer Oscilloscope

  9. Cavity Ring-Down Spectroscopy (CRDS) Cavity Laser beam Detector d

  10. CRDS Measurement Principle • Measurement of the ‘Ring-Down time’ Empty Cuvette & Solvent Cuvette & Solution

  11. Integrated Cavity Output Spectroscopy (ICOS) …..………. • Measurement of the total transmitted intensity: • Transmitted intensity(subject to losses L per pass):

  12. ICOS Measurement Principle Empty Cavity Cavity Laser beam Detector intensity time d

  13. ICOS Measurement Principle Cuvette and Solvent Cavity Laser beam Detector intensity time d

  14. ICOS Measurement Principle Cuvette and Solution Cavity Laser beam Detector intensity time d

  15. Lambert-Beer Losses • Knowledge of the losses L may beobtained 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 =

  16. Platinum Octaethyl Porphyrin Absorption Spectrum of PtOEP (xx mM) 532 nm

  17. 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

  18. Intensity versus time plot 12 nMol Porphyrin Solution

  19. Losses due to Cuvette & Solvent 103 L Solvent

  20. 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 Intensity Trial No.

  21. 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

  22. 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.

  23. Results

  24. Conclusions • Successfully implemented CRD and ICOS techniques • Difficulties encountered • Fluctuations in the laser intensity • Cuvette losses!!!

  25. 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

  26. Acknowledgements • I would like to thank the following people: • Dr. A.A. Ruth • Kieran Lynch • All members of the Laser Spectroscopy group

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