Dear Roberto,

You cannot use MS online analysis to calculate absolute quantities. The MS is sensitive to the gaseous mixture composition (CO2 presence is especially problematic). You have a mixture of different gases that will change in composition during the chemical reaction, therefore, it is wiser to use the MS to report relative amounts of the different gases that are present without trying to compute absolute quantities.

Keeping this in mind, the drop from 1.9*10-6 to 1.8*10-6 Torr would not be a problem,

Hope my answer helped you

Regards

Dear Samer,

I agree with you, I cannot measure absolute quantities but I could still calculate conversion, selectivity and yield referring to variations in compositions, isn't it?

You say that a drop of 0.1*10-6 is not a problem but I'm not sure I can agree with you because it continues decreasing by time so how can I take a value as a reference?

The conversion of any component can be computed relatively to the initial amount.

Now as you have baseline problems, I can only think of the following:

If all partial pressures are droping, it is an apparatus problem and you should contact the company. If it is just one component that is droping and you use different MFCs to prepare the mixture, it might be that one of the MFCs is malfunctionning.

To circumvent this problem, try to leave the MS for a long period of time, either you reach a stable value, or some maintenance should be done.

If you require additional help, provide some details about gas mixture preparation with correct quantities that produce the drop

Regards

Thank you for your kind answer,

I also think it could be a MFC problem, due to the fact that they haven't been inspected in a long time.

However, we've already tried leaving the QMS for a long time (overnight) but it stills has some problems reaching a stable value. We feed a mixture (using MFC) 5%CO/He directly from the tank (50cc/min)