Protect SINE OUT BNC fron SR830 lockin

Hi,

I am trying to use SR830 lock amplifier at scientific experintal.

I need to apply the signal from SINE OUT BNC output in a thin ceramic sample. My sample is an plane disk with electrodes in opposite faces.  This sample is isolante material, but sometimes the voltage can damage the sample and cause a short circuit between the electrodes and also a short circuit between pin signal e GND from SINE OUT BNC. I need 5Vrms from Lockin.

My question:
1) How can I protect SR830 not modify or affect my overall result?
2) Can I put an resitor 10Mohm in serie with my sample for to limite electrical current? For exemplo:  
     [signal pin]-----{resistor 10M}-----{sample}-----[GND]

3)This circuit would change my results if the sample remain undamaged?

Thanks,

Acosta