Niels Bohr Institutet > Kalender - det sker på NBI > 2009 > Defense of master thes...

The Effect of Non-Ionic Surfactants on the Potentiometric Response of Ion Selective Electrodes

This thesis investigates the interfering effects of certain non-ionic surfactants on the potentiometric response of polymer-membrane based Ion Selective Electrodes (ISE). It is focused on the surfactant type ``ethoxylated acetylenic diol''.

The interfering effect of certain surfactants has been observed by researchers at Radiometer Medical Aps.. They have measured a change in potential of 5mV, corresponding to a change in ion concentration of 20\%, when changing surfactant concentration. This is fundamentally interesting, since the surfactant itself is not charged.

The technology of ISE's for Blood Gas measurement is introduced and fundamental theoretical models of ISE systems are introduced. General properties of non-ionic surfactants in relation to ISE technology are introduced. A model describing the interfering effect through the complexating behavior of surfactants is introduced and adapted.

Experiments are performed to investigate the influence of the surfactant on the standard parameters of a sensor, i.e. ion sensitivity and selectivity, and investigates the direct potential change caused by a change in surfactant concentration, i.e. the surfactant sensitivity. The experiments are performed on magnesium-selective sensors and dummy-sensors with the same membrane composition but without ionophore.

The sensors with membranes without ionophores demonstrate a very high sensitivity towards changes in surfactant concentration.

For new sensors without previous exposure to surfactant, a nernstian response to changes in surfactant concentration is observed, and fitted to a theoretical model.

For pre-exposed sensors a linear relation between potential an concentration is observed. This is surprising since all theoretical models predict a logarithmic relation.

The results indicate that the interference is caused by complexation of surfactant and ions at the surface of the sensor membrane. The data can be fitted to a model model describing this behavior.

The existence of two phenomena with different timescales is documented.

The thesis is based on a cooperation with the company Radiometer Medical Aps., where the experiments have been performed. Radiometer uses ISE technology for blood gas measurements. Their experimental platform is used for the experiments in this thesis.

Supervisor: Thomas Heimburg