This can be tricky, because there is no way to know whether your setup has a problem. Since your plasma is generated through an RF source, that can contribute noise to the signal, but it could also generate a non-Maxwellian distribution. You would have to find some literature about Langmuir probe measurements in RF plasmas and then determine what is relevant to your setup. For example, you might find something useful in Chen, Plasma Sources Sci. Technol. 18, 035012 (2009). Best of luck!