Difluoromethane by GC-MS

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

7 posts Page 1 of 1
Bit of an urgent one... I was testing a multi-component refrigerant mixture when I realised I should be seeing more than the three components I was. Alas, difluoromethane is one stated on the composition but I can't see it in my chromatogram...

I'm assuming it's flying out too fast for me to catch. Conditions are:

DB-624, helium, Agilent 6890A/5975, inlet temp 150 deg. C
Hold at 28 deg. C for 10 mins (can't get the oven any cooler than that), then heat up to 250 deg. C for a couple min (to purge out anything left over).
Scanning 30-200 m/z
For reference, pentafluoroethane comes out at around 1.5 mins. So no idea how I'm going to see difluoromethane with that setup...

Any suggestions as to something I can knock together in the next 24 hours?
With a boiling point of -50C you are going to want cryocooling.
I have heard of dropping a coil of column in a beaker of liquid nitrogen to start each cycle. I have never seen it done and it doesn't sound reproducible but is a makeshift way to get cold enough.
I normally use a 40m 0.18mmID column in Rtx624 or Rtx502.2 for such analytes. But we use purge and trap so that can better focus them before going on column, direct injection may be more difficult.

You may have to try it without any solvent delay and can possibly see it that way. Also try lowering the flow in the column as much as possible. I have been able to take 0.25 columns down to 0.6ml/min before, and with such light compounds you don't need much flow to get them through the column.
The past is there to guide us into the future, not to dwell in.
Many thanks for the suggestions guys.

Steve Reimer wrote:
With a boiling point of -50C you are going to want cryocooling.
I have heard of dropping a coil of column in a beaker of liquid nitrogen to start each cycle. I have never seen it done and it doesn't sound reproducible but is a makeshift way to get cold enough.

I had a feeling I might need to go cryogenic. Unfortunately not likely to happen in our lab, but something I'll keep in mind...

James_Ball wrote:
You may have to try it without any solvent delay and can possibly see it that way. Also try lowering the flow in the column as much as possible. I have been able to take 0.25 columns down to 0.6ml/min before, and with such light compounds you don't need much flow to get them through the column.

I tried it without the solvent delay and still nothing unfortunately. it must have virtually no retention. I didn't try lowering the column flow all the way down but will need to try that next time...
Is replacing the column possible ?
eg. https://www.agilent.com/cs/library/appl ... A01110.pdf
If you need an automated solution, PerkinElmer supports sub-ambient conditions w/ CO2 or LN2 dewar connection. I'm sure Agilent has a similar solution, but I'm not as familiar with Agilent GCs.

Steve Reimer wrote:
With a boiling point of -50C you are going to want cryocooling.
I have heard of dropping a coil of column in a beaker of liquid nitrogen to start each cycle. I have never seen it done and it doesn't sound reproducible but is a makeshift way to get cold enough.

I've done this before. It works well enough for a few samples :-)
Maybe try a 100% dimethlypolysiloxane column or any non-polar column you may have on hand. If you have an agilent GC, it should have came with an HP-5 column which may work.

Nonpolar columns are adequate for the retention of methane at ambient temperatures and approx. 2 mL/min helium flow. Might be worth considering. If the other components in the mixture have a decent BP spread, it may be appropriate for the other components as well.

Also worth considering your sampling technique. If they are liquified refrigerants, the best injection technique would be via a vici liquid sampling valve. However, that may be inconvenient for your lab. So, make sure you are using a gas tight syringe. Headspace sample or direct. Or a step further, cooling down samples with acetone/dry ice or liq. N2 before collection.
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