I got some BET results. When i plot the relative pressure (P/Po) against the volume adsorbed the curve goes to negative (below the x axis). What does this means. And after a certain point the desorption curve is below the adsorption curve. It should have been the opposite. Can somebody please help. I can send the xcel files if somebody is interested to help analyse the data.
What is your sample? How did you degas?
Can you show the isotherm?
the reason of having adsorption isotherm in below x-axis could be due to improper degassing, gas evolution from sample during analysis or instrumental error.
The sample is CO3O4. I am not sure how did they degased it. Cause we normally send the sample away to external laboratory.
What surface area are you expecting from your sample? 1 m2/g? 10 m2/g? 100? 1000?
Maybe amount loaded was too low (equipment should require around 1-10 m2 to be loaded).
It is difficult to try to help with such a limited information.
I am trying to upload the xcell file but its failing to upload the xcell files. I can send the files to any of you who are interested to look at the files. Please send me a dummy email at: [email protected]
It can also be caused by a leak on your sample tube. Check it by performing a leak test with your instrument. You can manually look at a leak on your instrument by using an empty sample tube connected to your device. Then put it under vacuum and when the ultimate vaccum is reached isolate the sample tube and look at the pressure: it have to remain stable or go up very slowly.
A proper degassing of your sample is ensured when the ultimate vacuum of your pump is reached during the operation.
This is what i get. Can some body please help to explain whats happening. Its type III i guess meaning the samples are non porous??
I think the comment of David Fairen-Jimenez is the solution for your problem: the amount of sample is not sufficient because it's non porous or macroporous. The amount of gas adsorbed is very low and the pressure transducers of your device are not able to detect such a low variation of pressure. Add more sample and don't forget to perform a free-space calibration with Helium. Check the instructions manual of your instrument and find what is the minimum total area measurable (in m²). For example, if it's 1m², and if your sample has a specific area of about 0,5m²/g, you will use 2g or more of this sample on the sample cell.
Please check the free space setup conditions. Sometimes amount of the sample used is very small in comparison to the volume of the degas tube which may result in negative data.
I think it is experimental error
you can check this from the y plotting the Relative Pressure (P/Po) against 1/[Q(Po/P - 1)] and it should give a straight line with R2 (correlation coefficient), 1
- Badges
- Science topic
- Similar topics
- Phytochemistry
- Extracts