minimum pressure in stand-by mode

Question

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!

paulsalverda · Accepted Answer

Built into MassHunter Acquisition are some GC Calculators.

Using the Pressure/Flow Calculator, 15m column at 50&deg;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&deg;C to be 1.22 ml/min.

The GCMS benefits by maintaining thermal equilibrium. If you have the ion source at 100&deg;C standby and then want to run, it takes a little while for it to heat up to 230&deg;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&deg;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&deg;C --- I use this because it is over 100&deg;C and I can see that 111 is different from across the room. Transferline temp = whatever is in your method - typically 250&deg;C or so Source = whatever is in your method - typically 230&deg;C or so....but that's a good discussion topic. I like hotter ion source temps, like 265&deg;C. Quad = 150&deg;C. Never run the quad hotter than 150&deg;C - there's no good reason for essentially all methods. 
 Keeping the oven >100&deg;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.

paulsalverda · Answer

The transferline is very hot. If the oven sits cold, like <60 to 100&deg;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

paulsalverda · Answer

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.

minimum pressure in stand-by mode

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