To carry out a preliminary test on the effect of gas pressure on the source trap current. Then having established a stable trap current to carry out a preliminary source sensitivity characterisation.


1) Effect of pressure on trap current

Source pressure: 9.34 e -9 mbar
Filament current: 3.35 A
Trap current: 500 micro-amps
HV: 5000 V

The sample bellows were filled with CO2 at 70 mbar pressure and this allowed to pass through the capillary into the source:

Source pressure: 1.18 e -7 mbar
Filament current: 3.35 A
Trap current: 266 micro-amps
HV: 5000 V

To increase the trap current to 600 micro-amps it was necessary to increase the filament current to 3.395 A.

The reduction in the trap current with increasing gas pressure in the source may be related to increased heat loss from the filament by thermal conduction in the gas phase. The mean free path of molecules is greater than the source chamber diameter and so thermal conduction should be proportional to the gas pressure.

2) Source sensitivity

Having isolated 70 mbar of CO2 in the sample inlet micro-volume the signal decay was monitored as a function of time. The raw data can be found here

3) Comment

This test was hampered by the fact that the trap current drifted both up and down during the experiment. I've implemented (16.05.11) a new PID algorithm to control better the trap current and will repeat the test.