Recently I noticed it happen to a few sample for this gap. If I run a standard sample Silica-Alumina for example, the curve is overlapping between a few first point of adsorption and a few end point of desorption. For this isotherm curve, what does it mean? is that not enough degas process?
In N2 adsorption at a standard temperature (BET), why does the adsorption and desorption curve is not overlapping each other?
I think you may find the answer to your question in Bertier et al. "On the use and abuse of N2 physisorption for the characterization of the pore structure of shales", http://www.clays.org/OTHER%20CMS%20PUBLICATIONS/wrksh21-pdf/CMS-WLS-21_12.pdf
It may be a kinetic problem.
Isotherm indicates the behavior of the gas with the surface at a particular temperature. As partial pressure in sample cell is increased gas is adsorbed initially and gets condensed over the surface. N2 isotherms are usually a straight line below 0.3 partial pressure. BET surface area is measured by measuring 'C' and the slope of this straight line. Above 0.3, usually BET isotherm is not a straight line. This occurs due to multilayer adsorption process.
Adsorption Isotherm (red +) appears to be correctly recorded and I feel can be reported as it is. To calculate the BET surface area from this isotherm, kindly select the data points below 0.3 partial pressure. As above 0.3, the isotherm deviates from the straight line and do not follow BET equation. This absorption isotherm also indicates the type of the isotherm.
The desorption isotherm (Red O) which is incomplete (I don't know if the analysis were terminated abruptly by you) can occur due minute gas leak. As equilibrium conditions are not attained quickly show that there was a gas leak. [faulty transducers (control valves) also cause this].
Usually, ice development in Dewar flask can disturb the placement of sample or P0 cell. O-rings also need to be checked. (as analysis started instrument has passed the leak test). The valves which control the pressure of the manifold and sample cell/s in instrument can accumulate fine power over the period from the samples. Thus, instrument needs a service check periodically. If turbo molecular pump/s is/are on continuously system do not face such problems. Apart from all this improper degasing temperature can cause such abrupt results.
I hope this was helpful.
It can be occurred if inappropriate amount of samples or not properly degassing of the samples