Recommendation for GC flow, inlet and MS temp for He conservation.

Given the He shortages, our institute is trying to preserve the He use during idle instrument time. We do not want to vent the systems but have instead opted to reduce the column flow during idle times so it will be ready to use at a moment’s notice. Our column flow is typically set at 1-1.4mL. We want to reduce it to 0.5 mL when idle so some carrier gas flows through the systems. What would you recommend we keep the inlet and MS temp (both currently at 250°C) at to lessen adverse effects on the column? or does it even matter?

  • I leave everything hot and flow 0.5 mL/min through the column, 0.5 mL/min for the septum purge and 4 mL/min to the split vent trap. Depending on your instrument it might spazz out and shut down, increase flow as needed until your inlet holds pressure.

  • The GC Split/Splitess and MMI inlets need 15-20 ml/min of total flow to be able to effectively control the inlet pressure - the column flow.  20 ml/min is only 0.33 ml per second -- it's hardly any gas  flow.   The MS sucking on the end of the column means that if your inlet pressure is too low, which should never be less than 5psi anyway, and the septum purge or split vent flows too low that air WILL back diffuse into the inlet, kill your column, and contaminate the MS.  I've seen this many times over the last 40 years and the fix is quite expensive. A new column, gold seal and liner, a full ion source cleaning, and the time required.

    The simplest solution is to plumb a three way valve behind the GC for hydrogen or nitrogen.   Don't reconfigure the GC for the gas, though, set the column flow by checking the MS vacuum and the septum purge and split vent flows by measuring with a flow meter.   Make sure that the MS is ~2x10^-5 or so and that there is ~3 to 5 ml/min out the septum purge and ~15-30 ml/min out the split vent.  Then save whatever pressure and flow numbers give you those measurements with the standby gas as your STANDBY_H2 or STANDBY_N2 method.   Before you leave every evening that the instrument is not going to be running, load the standby method, switch the valve, and go home.   In the morning, load your normal run method, switch the valve to helium, and get a coffee...  The inlet subsystem and column will be filled with helium in a few minutes and you'll be ready to go.

    That gets you the maximum helium savings you can do without actually switching your run methods to hydrogen.  I am not a fan of running samples on a GCMS with hydrogen carrier gas for many reasons, but it's a great standby gas.

    Make sure and make that valve easily accessible and really obvious, too. Then follow all of the hydrogen safety guidelines in the instrument manuals.

  • If I'm pulling air I should be seeing it in my tune, correct? These two have been idling at 5 mL/min total flow for two weeks with no injections and they look okay to me.

    I have seen water climb to around 10% for some instruments running purge and trap concentrators whereas others are just fine. Ovens older than the 7800 series need something more like 8 mL/min total flow to keep the inlet from shutting down but the newer ones can run less.

    I copied the sleep methods from our LRN-C instruments but those things get a lot more funding and I'm not too keen on venting and swapping columns.

  • <50ppm oxygen can damage the column....It's cumulative, too, so even incredibly small quantities over time will cause column damage.  it might not be significant. It might not even affect your methodology, but why chance it?   The left plot says 10,348 counts of O2 and the right one says 11,088.  Since there are applications where a peak of 11,000 counts is considered significant, I think you should try to reduce yours further if possible.   

    The SSL inlet tubing sizes have not changed in a very long time - since 5880s. The EPC modules have gotten better with tighter controls, but the flow dynamics at very low flows and pressures are related to physics and tubing sizes.  The spilt vent line is 1/8" copper to the trap, that one little piece of 1/16" stainless to the valve, then the valve itself. If the outward flow is too low, air back diffuses into it.

    20ml/min total flow is the default gas saver setting and I try not to go lower than that.

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