Quartz crystal microbalance (QCM) impedance analysis was used as a microgra
vimetric tool for thiol adsorptions in a flow injection system. The presenc
e 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 meth
ods.