40m/z is most likely Argon and 44m/z as above is CO2. If the ferrule on the interface nut has shrunk after heating you will see these as a tiny air leak forms. If the numbers drop as the oven temp rises, then what is happening is the heating of the oven is sealing the leak.
What it appears here is that you have trace CO2 in your carrier gas, and the Argon is coming from the air. As the temperature ramps in the oven you are sealing the leak and the Argon goes away and leaves a higher percentage of CO2. I have seen it before many times.
One way to help prevent this is to keep a few graphite/vespel ferrules in a small beaker in the oven, as you cycle the temperature in normal use the ferrules will shrink and when you use them later you won't have the problem of them forming a leak as they condition on oven cycles.
