one embodiment computer 23 may be pre programmed with a upper conductivity limit should the conductivity of solvent 13 exceed the pre programmed limit an alarm will be tripped and either cassettes 15 16 and 17 will be removed from the solvent or no additional cassettes will be admitted to solvent tank 11 until the solvent bath is changed or modified in a mannered to reduce its conductivity in another embodiment shown in fig2 a sensor 191 may be utilized to monitor the conductivity of a solvent flow stream 31 which bathes wafers 17 the conductivity may be analyzed by analyzer electronics 21 and the output of analyzer electronics 21 provided to computer 23 for appropriate comparison with predetermined values for example in the cleaning of layers containing charged chemical species e g during post halogen based reactive ion etch cleanup such as in the etching of aluminum alloys the conductivity of the solvent will increase as the concentration of charged species enter the fluid stream fig4 shows a graph of the conductivity of fluid stream 31 as a function of time when an appropriate end point is reached at point a the clean up may be terminated alternatively the apparatus of fig3 may be used flow stream may be.