10 Replies Latest reply on Mar 25, 2019 8:09 AM by gchaplain

    Air leak 5973




      I have some problems with my MSD 5973 (diffusion pump). Nitrogen was between 10-18% and EMV between 2400-2600. I bought supplies and i did a maintanance: I changed He trap, MSD column Nut and ferrule, diffusion pump oil, filaments, interface, analyzer, PTFBA valve and vent valve o-rings.


      The first days, my EMV decreased to 2100, however, it increased to 2400 again, and N2 maintain between 10-18%. I isolated the MSD and it continued with high N2. I used air duster for checking leaks and i didnt found.


      The vacuum in ion gauge is 1.0x10 -5 with 1.1 mL/min of He flow. When I start the manual tune, 69 219 and 512 abundances decrease a lot (30% aproximatly) and it stabilizes. When I start the manual tune with PTFBA closed and later I open it, N2 abundance decrease 15%.


      I think that PTFBA valve has problems. Any recommendation or what other problem can it be?



        • Re: Air leak 5973

          One way to test if it is the PFTBA valve is to use a common solvent like dichloromethane and perform a manual tune but look for the ions 49 and/or 84 whislt dropping one drop onto the vent valve next to the PFTBA.


          Also check if your PFTBA is empty or not. Quite often when that is running out the EM volts are increased to compensate for the lower PFTBA response and in turn increases the nitrogen/oxygen seen.


          Another common area for leaks in the MSD is the MS door O-ring. this may need pushing back into place. But DO NOT put solvent on this part as there are high voltage electrical connections on this section.

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            • Re: Air leak 5973

              Thank you, last week i found the leak in the analyzer door, i cleaned few times the o-ring and N2 decreased to 3 %. However, the EMV increases a lot, very close to 3000 V. I have to change the EM and maybe the ptfba valve.

              1 person found this helpful
              • Re: Air leak 5973

                Hello gchaplain,

                thank you for your interesting post.

                We have a similar MS-air-leak-problem.

                I just want to ask exactly which O-ring is meant.

                Please look at my Picture.

                Which step have you met exactly?


                Thank you in advance.





                  • Re: Air leak 5973

                    That o-ring can leak yes, but the one I was referring to here is wrapped around the PFTBA bulb. There is a o-ring there that can leak and on the vent valve hand tight screw to vent the GC. Both these use the same o-ring as the agilent liners.

                • Re: Air leak 5973

                  Hello *****,


                  To check if you have a real leak at the MS, before changing the EM, you have to check the abundance of ion 31 compared to the ion 32, when the PFTBA valve is open.

                  If 31 is ~70% or more abundant than the 32, your system doesnt have a relevant leak. If 31 is very small  (less than ~ 70%of 32) compared to the 32 then a leak is present.

                  31 is fragment of the PFTBA, and 32 is obviously O2.

                  The ratio of 31 and 32 has nothing to do with the age of the EM.

                  Lets find first the leak.

                  By the way, it is not a bad idea to have a new EM, ready to be changed when needed, if it has never been changed.


                  When the leak has been found (can be at the PFTBA valve, by the way), then the decrease of the PFTBA response can be explained by the capillary of the valve being partially blocked. In that case, the valve need to be replaced.

                  Best regards,



                  1 person found this helpful
                  • Re: Air leak 5973


                    Looks like you found the leak at the oring. Great job!!!. That's a typical place for a leak especially if you had the shipping screws tightened. I think replacing the EM is a good start. Try rotating your horn 90 degrees. Sometimes it will buy you some time and definitely point in the direction of the horn.

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