I am using a Micromeretics ASAP2020 machine. The sample is ~25mg of reduced graphene oxide based film. The problem is I am getting adsorption isotherms showing negative volumes of adsorbate (N2) entering the tube as P/P0 increases. The sample was degassed prior to testing at 250°C for 3 hours then backfilled with N2. As a side note, TGA says the sample retains >99% mass up to 400°C. Any ideas kind people?