Looking to use alternative carrier gas from Helium


I am putting together a proposal for the owner of the company I work for to make flavors. I cannot purchase the GC/MS system until I can determine if I can use an alternative carrier gas with good resolution,selectivity, etc. We are looking at buying 7890A GC with a 5975C msd as a detector. In the past at another company I used same equipment with helium as a carrier gas and built a mass spec library including retention times, etc. Using the column - DB-5ms 40m x 0.18mm x 0.18um. Now that Helium is difficult to get, when we buy this equipment from Agilent I want to start off using a carrier gas that is in constant supply. I understand I will have to update retention times for thousands of peaks so I need to know what you guys think I should use as a carrier gas other than helium. Should I not use Nitrogen? I remember reading in the Agilent class books from classes I took a few years ago that nitrogen will give some overlapping peaks at some point in the column depending on phase.Is it still not a good alternative to helium now that we may not be able to get helium at times. Is hydrogen ok to use with mass spec using a hydrogen generator? Could I use a combination of two gases?


Also, is there a newer method translation software so I could translate the old method and column to using a new type of column and carrier gas that may keep retention times somewhat close to original method. If I have to start over on the retention times then that is fine.

Thank you,

Dan Lingo

  • A GCMS system is typically left running all the time as it takes some hours to pump down and achieve good vacuum and thermal stability.   A 0.18 mmid column will typically be used at a column flow rate of ~0.75 ml/min or so.  The GC Inlet should be run with Gas Saver ON at 20 ml/min after 2 minutes -- the default Gas Saver parameters. A 244 ft^3 helium tank is 6909 liters, but it's not a good idea to run the tank dry, so let's use a round number of 6500 liters.  6500 liters at 20 ml/min is something like 225 days.   Use an electronic leak checker on every connection outside of the GCMS !


    The site preparation document says 99.9995% or better helium.  Even if you are running a raw material, high concentration sample, you do not want to contaminate the GC inlet subsystem, the column, the MS ion source, and the entire MS vacuum chamber with low grade gas.  Any problems caused by contaminated gas are not covered by warranty nor service contracts.  Use high quality gas with high quality gas filters, like the GasClean filter, to keep your system clean.


    Another setting that will save you troubles....

    I recommend making a Standby.M method.  Have the inlet and transferline temperatures the same as you use in your run method. Have the source and quad temperatures in the tune file the same as your run method.  Have the oven temperature set above 75 degrees - I like 111 as I can read that from across the room.  If you are using the original brass column nut on the MSD transferline with a low standby oven temperature, the differential expansion of the brass to the MSD will sometimes lead to a leak there.  Leave the oven hotter than 75 all the time and this won't be a problem.  The fingertight nut for the transferline will help avoid any leaks there, too.

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