Negative adsorption is desorption, i.e. removal of nitrogen from the surface. Your organic waste decomposes and releases nitrogen. Desorption occurs, which can be seen on the graph as negative adsorption.
Marcella Bini Thanks for your suggestion. degassing condition was 140-degree Celsius for 14 hr. Should I try increasing the temp/time! Samples were dried garden waste. Thanks.
BET instrument is very sensitive to processing the sample for N2 adsorption and desorption. Various parameters are responsible for a negative result. you cross-check one by one:
-keep the sample in the oven for 18-24 h to completely dry, before processing BET analysis.
-pre-treatment condition varies with different time and temperature interval. Sometimes, temperature and time intervals are not enough to remove all contaminants (moisture, solvent).
-fill proper liquid nitrogen in dewar.
-in the case of synthesized product, wash thoroughly 3-4 times and oven dry before BET analysis.