An alternative explanation could be resistive heating of the QCM crystal upon drawing an electron current through it. We explored this by cooling the QCM crystal to 5 °C, using the water-cooling system of the QCM set-up (Fig. ) and exposing it to a continuous flow of plasma electrons. This resulted in a reduced, but not eliminated, frequency shift (Fig. ). Consequently, we assume that, at least partially, the signal perturbation is due to momentary heating by the plasma electrons. It has earlier been shown that even small temperature gradients may generate apparent mass changes in QCM measurements. 26 A plasma power between 5 W and 20 W in combination with a fixed QCM bias voltage of +40 V was used in a period of 120 s.