Masters thesis defense by Camilla Marie Jensen

Title: A new continuous detection technique for sodium in ice cores using an ion selective electrode.

Abstract: This projects deals with the development of a new sodium detection technique using a setup with a sodium ion selective electrode (ISE) in the continuous flow analysis (CFA) system. The report describes the process all the way from building a simple setup, optimizing all parts of the setup and all the way to carrying out ice core measurements with the optimized detection technique. The sodium selective electrode is the PerfectION™ comb Na+ combination electrode manufactured by Mettler Toledo. The ion selective electrode measures an electric potential difference over the membrane, caused by the activity of sodium ions moving in and out of the membrane. The electric potential is translated into a concentration using the so-called Nernst equation. The initial setup consisted of a peristaltic pump, pumping the melted ice core sample from the melting device and trough a plastic flow cell, where the electrode was placed for measuring the electric potential every second with a continuously flowing sample stream flowing. The setup was optimized and extended with further components to improve the working conditions for the electrode. The final setup included an ionic strength adjuster (ISA), keeping a constant ionic strength and pH in the sample solution, and thereby stabilize the baseline. The optimizations resulted in an ISE detection technique with a lower detection limit of 1.92+- 0.01 ppb and a response time between 98 and 339 seconds for steps of 5 to 100 ppb. The optimized setup was used as a part of the Copenhagen CFA system during the Summit melting campaign in December 2016. A section of 21.5 metres ice core drilled at Summit in 2013, was successfully analysed using the ISE detection technique together with the absorption spectroscopy detection (AD) technique, currently used to measure the sodium concentrations in ice cores. Comparisons between the performance of the two detection techniques show that the ISE technique has some advantages such as being less demanding regarding the number of chemicals added, having fever components within the setup and performs to be more stable over hours and days than the AD technique. Also, air bubbles in the system affect the AS technique significantly more than the ISE technique. However, a repeatedly lower amplitude of the signal for the narrow peaks in the ISE record compared to for the AD record, indicate problems with the long response time of the ISE technique. To test if the high frequency features hidden in the ISE signal could be reconstructed, a simple deconvolution technique was applied to the signal, where some of these features were successfully reconstructed so that the signal became significant more consistent with the AD signal.