Dear Chris,

Thanks for the willing to help!

So, the instrument is Surfer of Thermo Scientific. The instrument did not work for the summer. The problem appeared when I switched it on after the summer break. Yes, I used filler rod to reduce the dead volume. The answers of your other question you can find in the uploaded file.

Regards,

Lenia

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 the BET isotherm is not a straight line. This occurs due to multilayer adsorption process.

Isotherm (blue-black) 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. Adsorption analysis also indicates the type of the isotherm.

The red isotherm which is incomplete (I don't know if the analysis were terminated abruptly) 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.