I have a question.
how I can change the column in the gc/ms without stopped turbopump ? It is possible?
regards Azrael ?
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…
There is a product for this.
Good points. I agree that the quick-swap is the best solution if the expense can be justified. As to press-fit vs metal ferrules, it's a hard decision. They both require care on the part of the person doing the column swap, and the ferrules cost twice as much as the press-fit. One has to find what works best for the situation.
I do have one comment in regards to item 3:
I have never considered that final length of capillary to be a restrictor, but an extension to the column (making sure that it's the same ID as the column). Then I just add the length of the capillary to the length of the column in the column definition and the EPC takes care of keeping the flow rate at the desired value. Not that it will make a major difference in any production environment. As long as you get the chromatographic resolution and run length that you need/want, the actual flow rate should be irrelevant as long as it's within the instrument's capabilities. The 14.5 mL/min are the maximum that the 1 m length of 0.25 mm capillary can transfer, but that volume isn't available because the EPC is controlling the flow from the column, so it's only that much volume that gets into the MS.
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.
Ah, I didn't think of that. Yes, it makes sense...
That being said, after 30 years of doing it this way I haven't hit any problems that I could trace to having the short capillary open to the atmosphere for the time that it takes to make the changes. Depending on the ability of the person doing it, this can range from some 30 seconds to several minutes. It is likely that some oxidation might occur. But in a production environment (in my case, commercial environmental testing labs), one has to clean the source quite often due to the amount of "dirt material" that gets injected into the system, that perhaps that's why I haven't noticed ill effects. We clean sources once a month on the average (about 800-1000 sample extract injections, not counting standards or instrument QC). Sometimes more often.
But you are absolutely right that when dealing with inexperienced users, one should make it as safe and easy as possible, and you can't beat the quick-swap for that.
Good morning everybody,
About CFT products.
I have this CFT purged union connected to an EPC doing nothing in the oven of the GC-MS (most of the hardware was installed before I started to work with the equipment).
Could I use it to swap columns without venting the MS?
The setup would be: injector - column - CFT purged union - 2.5mx250µm deactivated capillary going into the MS transfer line.
Helium would be allowed to flow through the purged union (at what pressure?) when columns would be swapped. That way, there would be a 'blanket' of He preventing air from getting in the line.
Other advantages would be, besides avoiding venting the MS:
- saving the thread of the MS transfer line from damages caused from opening/closing the connection and eliminating this potential point for leaks,
- if both ends of the columns have CFT connections, it could be stored by connecting those ends to a CFT union therefore avoiding septum particles to get in the column and leading to having to redo connections (cutting column, using ferrules,...).
Could this solution be applied?