minimum pressure in stand-by mode

Hello Everyone,

I am using a 15m 5HP column for 7820A GC - 5977B MS system. After sample run, I put the system in stand-by mode with the following parameters and found out 'front inlet pressure shutdown'.

1. front inlet temperature, MS quad, MS source - all at 100degC

2. oven temperature : 50degC

3. gas flow : 20mL/min

4. gas saver mode : on

5. mode : splitless

My question is : 

1. what is the ideal pressure and gas flow to be maintained for a 15m column?

2. which is the correct mode - split/splitless/pulsed split/pulsed splitless for a 15m column?

Thanks!

Parents
  • Built into MassHunter Acquisition are some GC Calculators. 

    Using the Pressure/Flow Calculator, 15m column at 50°C and 1 ml/min, the inlet pressure required is only 1.1psi.  It is recommended to always have the inlet at >=5 psi for decent pressure regulation.  This is hard to do with a short column going to vacuum. 5psi needs the oven to be 100°C to be 1.22 ml/min.

    The GCMS benefits by maintaining thermal equilibrium. If you have the ion source at 100°C standby and then want to run, it takes a little while for it to heat up to 230°C - but at least a couple of hours for decent stability.

    I would suggest these parameters for standby.

    Inlet temp = whatever is in your method - typically 250°C or so.
    Inlet mode = split, 20ml/min total flow - do not go lower than this. The inlet needs flow to be able to maintain the inlet pressure which is the column flow.
    Column flow = 1.2 ml/min - this is the optimum flow when running samples as well. The range with helium is 0.5 to 1.5 or so. Higher and lower change the way it works.
    Oven temp = 111°C  --- I use this because it is over 100°C and I can see that 111 is different from across the room. 
    Transferline temp = whatever is in your method - typically 250°C or so
    Source = whatever is in your method - typically 230°C or so....but that's a good discussion topic. I like hotter ion source temps, like 265°C.
    Quad = 150°C.  Never run the quad hotter than 150°C - there's no good reason for essentially all methods.

    Keeping the oven >100°C helps reduce differential expansion differences with the brass transferline nut and the stainless steel transferline.  Leaving the oven cold often results in that nut loosening a bit and causing a leak.

    Why a 15m x 250uM column?  You may want to explore alternative 30m x 250uM or a 20m x 180uM columns to allow you to run at higher inlet pressures.   

  • Thanks a lot for a detailed explanation.

    About the oven temp.and differential expansion differences, the method I am using starts with a oven temp. 50degC and goes upto 280degC with a ramp of 20deg/C - could it cause the nut loosening and leak?

  • The transferline is very hot. If the oven sits cold, like <60 to 100°C  for many hours the brass MS transferline column nut and ferrule may get a bit colder than the transferline, contract, and leak.  Maybe.  It happens overnight or the weekend, typically. The first thing users do is crank down harder on the nut with a wrench to seal it again.  Some do this once or twice and it stops forever - with that column installation.  The better solution is to use the Self-Tightening Column nut and the required short vespel/graphite ferrule.  It has spring washers inside that hold the proper pressure on the ferrule and keep the seal no matter the temperature differential.   G3440-81013 | Agilent  

  •   Thanks a lot for explaining my query. We do have a self-tightening nut but yet to be installed. Till date, I have been using regular ferrule and nuts.

  • Make sure the ferrule on the MS transferline is the vespel/graphite one and not 100% graphite.   The 100% graphite ones are shiny.  The brass transferline nut uses the long ferrule and the self-tightening one uses the short ferrule.

  •    Thanks a lot for explaining further. I believe the ferrule is not 100% graphite, but I will surely doublecheck on that. 

    An off-topic query : It's mentioned in the 'Steps to Shutdown a system' booklet that to "open the vent valve ~1/2 turn counter clockwise" during the pump down of the MSD. I am wondering that it isn't said to turn it back clockwise afterwards -- why so?

  • Which booklet? A link or part number?  

    It surely should say to turn it back!  Make sure that any cables are connected before turning on the power.   Open the vent valve ~1/2 turn.  Turn on the power to start the rough pump.  Hold the analyzer door closed.  When you hear vacuum sucking air in through the vent valve you know that the vacuum is working and the door is shut.  Turn the vent valve all the way closed clockwise.  Wait at least an hour before using the MS to ensure that the air is pumped away.

    It's just metal pressing against an O ring-  the same as an injection port liner O ring -- so tightening it harder does not make it more leak-free.

  • Oh!! can't believe I missed that phrase !!Frowning2 probably the "~1/2" caught my attention and was looking for the same afterwards!!

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