Plasma properties can be characterised with a multitude of diagnostic techniques which are as varied as the many industrial applications that employ them, from etching microchips to surface treatment and cleaning. A particular manufacturing technique may require a certain plasma characteristic at a given condition. In order to get the necessary properties, other characteristics must be fine tuned and monitored. In some cases there is an offset in the degree of control and what can be measured accurately without disturbing the overall process, thus the ability to monitor and control these parameters is crucial. Of these parameters, the plasma density, n0, the electron temperature, Te, the ion bombarding energy, Ei, and the ion current density, Ji, are the most important quantities for processing plasma applications. Monitoring plasma parameters is often difficult due to poor access or operating condition limitations and so there is an ever-increasing need for improved techniques. Electrical properties of plasmas can be measured relatively easily over a broad range of conditions however ion mass on the other hand is generally only measured using mass-spectrometry. A novel electrostatic probe, arranged so that a cylindrical probe is oriented normal to a planar probe with a retarding potential applied to the normal and a dc pulse to the other is investigated with a view to inferring ion mass. A Retarding Field Energy Analyser (RFEA) is used to investigate Ion Energy Distributions (IED's) and to indicate the average ion mass in gas mixtures. The goal of this thesis is to investigate methods for measuring mass and to use this as feedback for process control.