Anomalous behaviour of the quartz crystal microbalance in the presence of electrolytes

Quartz crystal microbalance (QCM) impedance analysis was used as a microgravimetric tool for thiol adsorptions in a flow injection system. The presence of electrolyte was found to interfere with the proper measurement of the deposited mass and resulted in an error of more than 70%. Aqueous solutions of 1∶1 electrolytes, of up to 6 mM can cause mass errors as high as 60 ng cm−2, that is the equivalent of a monolayer of medium sized molecules. 1∶2, 1∶3, and 2∶2 electrolytes yield even higher errors. The interference is mainly due to the mechanical elasticity of the Gouy Chapman diffuse double layer and cannot be excluded even by using sophisticated QCM impedance methods.