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Thesis Final Final Version © 2009 Erhan Ilkmen ALL RIGHTS RESERVED INTRACAVITY OPTOGALVANIC SPECTROSCOPY FOR RADIOCARBON ANALYSIS WITH ATTOMOLE SENSITIVITY By Erhan Ilkmen A dissertation submitted to the Graduate School – Newark Rutgers, The State University of New Jersey in partial fulfillment of the requirements for the degree of Doctor of Philosophy Graduate Program in Applied Physics Written under the direction of Professor Daniel E. Murnick and approved by Newark, New Jersey October 2009 ABSTRACT Intracavity Optogalvanic Spectroscopy - Radiocarbon Analysis With Attomole Sensitivity By Erhan Ilkmen Thesis Director: Professor Daniel E. Murnick Carbon-14 (radiocarbon) is a naturally occurring radioactive isotope of carbon, having an extremely low natural abundance in living organisms ( 14 C/C ~ 10 -12 ) and a long half life of ~ 5730 years. These properties make it an ideal organic tracer for various applications in biological, pharmaceutical and environmental sciences as well as carbon dating. Today, the state of the art radiocarbon quantitation technique is Accelerator Mass Spectrometry (AMS) which is based on ion counting using a several megavolt tandem electrostatic accelerator as a mass spectrometer. Although AMS sets the standard for high sensitivity detection, its size, cost and complexity as an analysis system, limits its wide and routine use especially in laboratory or field applications. In this thesis, a new ultra- sensitive laser based analytical technique that can quantify attomoles of 14 C in submicrogram samples is demonstrated. The new system exhibits similar or better measurement capabilities as AMS, in sensitivity ( 14 C/C ≤ 10 -15 ), precision ( ≤3%) and accuracy ( ≤5%). Additional advantages include non destructive analysis capability, small size, being a table top instrument, high sample throughput capability via flow processing and the potential to be coupled to GC/LC instrumentation. The developed Intracavity Optogalvanic Spectroscopy (ICOGS) system is based on measuring changes in electrical properties of a weakly ionized glow discharge placed 14 inside the cavity of a periodically modulated high power (50 W) CO 2 laser. This new ii configuration enabled improvement of the signal detection sensitivity by about six orders of magnitude compared to the conventional external cell optogalvanic spectroscopy method. The signal enhancement mechanism is similar to, but with key differences from the well studied optical detection method Intracavity Absorption Spectroscopy (ICAS). Measurement capability of this new system is demonstrated with calibration curves relative to AMS measurements with a dynamic range of more than five orders of magnitude. The systems studied exhibited saturation effects with laser power and measurement time and also non-linearities in response with samples having enrichments greater than 12 Modern. (1 Modern = 1.10 -12 14 C/ 12 C ratio.) However, standard operating procedures were developed for accurately measuring unknown samples. For a more thorough quantitative understanding of the enhancement mechanism, a physical rate equations model has been outlined. iii ACKNOWLEDGEMENTS I would like to express my gratitude to those who helped me with my graduate career as well those who showed greatest support and made sacrifices throughout my life. First, I would like to thank my advisor Prof. Daniel Murnick for his guidance and support throughout my years at Rutgers. I would like to acknowledge Dr. Ozgur Dogru, Dr. Karina Schafer and Mark DeGuzman who have contributed in various ways throughout this project. I also appreciate the contributions of my thesis committee members Prof. Martin Schaden, Prof. Zhen Wu, Prof. Gene Hall and Prof. Robert Barat with their insightful comments and questions. I would like to extend my deepest gratitude to my family for their emotional and financial support to pursue my education in the United States. Finally, I am deeply grateful to my wife Yasemin who was there beside me with her love and confidence which made this thesis possible. iv TABLE OF CONTENTS ABSTRACT........................................................................................................................ ii ACKNOWLEDGEMENTS............................................................................................... iv TABLE OF CONTENTS.................................................................................................... v LIST OF FIGURES .......................................................................................................... vii Chapter 1 : Introduction................................................................................................. 1 1.1 Properties of 14 C.................................................................................................. 1 1.2 Background......................................................................................................... 1 1.3 14 C as a Biomedical Tracer ................................................................................. 3 1.4 Radiocarbon Detection and Analysis Techniques .............................................. 4 1.4.1 Isotope Ratio Analysis by Optical Methods ............................................... 7 1.4.2 Indirect Detection of Absorption and Optogalvanic Spectroscopy .......... 11 Chapter 2 : Optogalvanic Effect As A Detection Tool................................................ 13 2.1 Overview........................................................................................................... 13 2.1 History and Applications .................................................................................. 13 2.3 Optogalvanic Effect in CO 2 RF Discharge ....................................................... 17 2.3.1 Mechanisms of the Optogalvanic Effect................................................... 17 Chapter 3 : Experimental Setup And System Optimization ........................................ 26 3.1 Overview........................................................................................................... 26 3.2 Experimental Design......................................................................................... 26 3.3 Discharge Excitation and Optogalvanic Signal Detection................................ 29 3.4 Data Collection and Processing ........................................................................ 30 3.5 Laser Frequency Stabilization........................................................................... 32 3.6 Internal and External Analysis Cells and Sample Introduction Methods......... 34 3.6.1 Sample Introduction Methods................................................................... 35 3.6.1.1 Batch Mode Measurement Procedure...................................................... 37 3.6.1.2 Flow Mode Measurement Procedure ........................................................ 38 3.6.1.3 Sample Preparation .................................................................................. 40 Chapter 4 : Calibration Studies.................................................................................... 42 4.1 Overview........................................................................................................... 42 4.2 Lineshape Analysis ........................................................................................... 42 v 4.3 Signal Enhancement via Intracavity Detection Technique............................... 49 4.4 System Calibration............................................................................................ 55 Chapter 5 : Dynamics Of Intracavity Optogalvanic Spectroscopy................................... 61 5.1 Overview........................................................................................................... 61 5.2 Signal Dependence on Laser Modulation Frequency and Laser Power ........... 61 5.3 Theoretical Model............................................................................................. 75 5.4 Effects of Nitrogen as a Carrier Gas on OG Signal Enhancement ................... 79 5.5 Summary........................................................................................................... 82 Chapter 6 : Conclusions............................................................................................... 85 6.1 Summary........................................................................................................... 85 6.2 Future Work...................................................................................................... 86 References......................................................................................................................... 88 vi LIST OF FIGURES Figure 1.1 AMS system at Lawrence Livermore National Laboratory17 ............................ 6 Figure 1.2 CRDS setup 25 ................................................................................................... 10 Figure 1.3 ICAS system for direct detection of absorption .............................................. 11 Figure 2.1 The vibrational energy level diagram for 12CO2 laser transitions together with the excitation and de-excitation pathways ................................................................ 19 Figure 2.2 Energy level diagram of the rotational levels of CO 2 molecule are shown with the vibrational levels and laser transitions for the 00 01-10 00 band........................... 20 Figure 3.1 Experimental Setup of ICOGS for high sensitivity radiocarbon quantification. The OGE cell inside the cavity has
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