Chromatography Forum

Hello there. I would like to ask if anyone could help me out in
clarifying the problem with my GCMS machine.

Whenever I tried to do an autotune with a higher column flow of He gas; in
this case 2.0mL/min, I will obtained a high EMV of above 2200V and above.
I tried using a lower column flow(eg 1.8mL/min) and I will obtained a
lower EMV of app. 2000V. In both the case, I''m running under constant
flow as part of the requirement of the testings being conducted.

I would like to ask if this problem arises due to a problematic column or
is there any problem with the foreline pump/electron multiplier. I
suspected that it''s the column being problematic; ie such as deteriorated
column or crushed column due to overtightening column nut..etc..however, I have run
a system leak check and there''s no leak in the system. So, I would like
to ask if anyone could help me in this and is there any way to prevent me
from getting a rather high EMV voltage from autotuning.

FYI, I have also cleaned the ion source recently.Furthermore, I believed
that the ion source is not the problem due to the fact that I was able to
obtain a much lower EMV from lower column flow.

Thank you for your attention and I awaits your kind help.

When you do autotune, MS will be tuned to achieve certain area count for the base peak of PFTBA(m/z 69), and other ions will be tuned against m/z 69. When you have higher carrier gas flow, you will have less PFTBA in the ion source, the EMV will be higher. So I don't think you have anything wrong with your system, but the flow rate you are using is kind of too high for 5973. Usually the column ID for 5973 should be 0.18 mm or 0.25 mm, if that's the case, the carrier flow should be less than 1 ml/mim.

Is your 5973 a turbo pumped or diff pumped system? If diffusion pumped, then you are at the very upper limit of your vacuum capacity at 2 mL/min. I would try to back off a bit in flow, if your method allows. If turbo pumped, you are only half way there.

Electron multipliers don't last forever, I have seen some fail in as little as one year. If there has been a steady increase in the tune voltage, then the electron multiplier may be failing.

You mentioned the source has been cleaned recently. Are you sure the vacuum chamber is sealing properly? Due to the design of the 5973 you can have a significant leak at some points along the long sealing surface that will not shou up in the air and water check. This will cause a sudden rise in the electron multiplier voltage during tuning.

Are you evaluated the ( tune evaluation) in the main menu of the HP 6890/5973 software? what about amus gain? this r a tune operation parameters

The PFTBA ions should meet the following:

m/z amu Ion Abundance Criteria Intensity (counts)
69 Base Peak 100 % 75,000-200,000
219 30 - 60 % of 69 amu 22,500-120,000
502 - 10 % of 69 amu 1750-20,000 [/b]
The electron multiplier voltage is raised when the 69 amu intensity falls below 75,000 counts. Set the intensity count to about 150,000 when adjustment is needed.

Take care of 502 abundace values...mabe the total volume flow is too high!!!

regards