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!

  • 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.

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