change the column gc/ms without stopped turbo pump

Question

Hello !! I have a question. how I can change the column in the gc/ms without stopped turbopump ? It is possible? regards Azrael ?

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shawntravis · Answer

I believe what @ mhardacker is referencing with the flow calculation is that as soon as the restriction capillary is cut, the EPC is no longer controlling the amount of flow to the MSD, i.e. atmospheric pressure is acting as the EPC supplying about 15psi to the front of the 1m x 0.25mm restriction capillary and at 30C, that will allow about 15mL/min of atmosphere into the MSD. As long as that capillary is open to the atmosphere for only a short time, then one would assume it is a minor issue to apply such a large gas load to the MSD's vacuum system. In my opinion the concerns would be: how much atmosphere enters the system (oxidation) and for how long (which may cause the user to wait some time before use to pump out the atmosphere), repeated loading and unloading of the turbo pump due to large changes in flow to the MSD (relative to the 'normal' column flow during analysis) caused by plugging and unplugging the restriction capillary with a septum and connection to different sources of gas (EPC with helium versus atmosphere). The turbo pump is spinning very fast under normal operating conditions and it is designed to pump at a certain rate which in the case of the MSD is assumed to be a reasonably low and stable amount of gas (load), see excerpt from the 5975C MSD data sheet below. What is not clear are the affects of rapidly changing the amount of gas to which the turbo pump is exposed >>someone with more knowledge about materials and the dynamics of changing forces would need to chime in here<<. For novice users one should be concerned with a catastrophic event whereby the restriction capillary cannot be reconnected quickly to whatever column union device is being used. Allowing the restriction capillary to be open to the atmosphere for an extended period is likely to result in the MSD system venting itself as a failsafe due to the high flow. A possible preventative measure would be to increase the length or reduce the diameter of the restriction capillary such that the maximum possible atmospheric flow rate is much lower. Another would be to reduce the temperatures of the MSD source and quadrupole to reduce the likelihood of oxidation prior to the column change. To address all the concerns, Agilent recommends venting your MSD as well as cooling any heated zones connected to the column(s) with carrier gas still flowing. Once the MSD is vented and the affected zones are cool, mitigate the GC's failsafe of shutting down the inlet (or EPC) supplying gas to the column being changed (this is best done by turning the pressure setpoint 'off' for the affected EPC, either in the software or from the GC itself). The loss of pressure at the inlet or the EPC by removing the column will be seen by the GC as an unmet setpoint, so turning the setpoint 'off' [do not change the setpoint to be zero since the EPC modules are not designed to maintain a setpoint of zero] will tell the GC to not worry about controlling the pressure, no matter what it is. Once the column change is completed, turn the pressure back on so the GC will try to attain and maintain the previous setpoint to help decide if there is now a leak. Once the carrier flow has been reestablished for a few minutes to allow the column to have a few volumes of carrier gas flow through it, one can begin heating the zones back up. Refer to your SoP for specifics, but generically one needs to confirm that the leak rate (if any) is acceptable, then confirm function has been restored. Agilent recommends waiting 2 hours at operating temperatures to establish thermal equilibrium so repeatable results can be obtained. A quick swap device or similar addresses many of these concerns, but as with anything, there are tradeoffs.

Result/summary	*Dimensions of restriction capillary [ID µm L]**	Air Flow into MSD, (without main column)	Idea: where & who to use? (not mandatory to do)
BAD, too high flow	250 * 1.0 m	14 mL

OK	250 * 2.5 m	6 mL	if main column has ID 250µm. extend the main column by 2.5m
OK	200 * 1.0 m	6 mL	if main column has ID 200µm. extend the main column by 1 m

change the column gc/ms without stopped turbo pump

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