5977b GC-MS Autotune failing. Low count for mass 502 m/z

Question

Hello,

The tune evaluation on the 5977B GC-MS is failing. The reason is 502/69 mass ratio is smaller than 2.4 (reading = 1.5-2.0), more specifically the 502 m/z count is around 7000-9000, instead of 20000 which is seen in previous autotunes.

I cleaned the ion source, changed the filament and the next possible error that is showing is leaks or bad vacuum.

I can get H2O down to ~3% and N2 to ~2%. Previously, we been able to get N2 down to 0.8-1.0%, but I only have the vacuum on for ~24 hours to achieve 2%.

Any other possible things I can do to troubleshoot?

chemist23 · Accepted Answer

Yes! Cleaned the source again. Replaced insulators and filaments with new ones. Thanks

graettin · Answer

One possibility is that the source radiator (the hunk of metal that you install the source into) has become contaminated. While it is possible to clean this, it involves careful removal and requires that the user uninstall the magnet that comes with the radiator. The magnet must be reinstalled with the same orientation and problems can occur if the magnet is placed on a metal table, sonicated, dropped etc. Best to just replace the source radiator (as it comes with a new, clean magnet.)

If your method has a large amount of matrix, high boiling components etc it is possible this radiator has become contaminated. Contact technical support for the part number and if you don't feel comfortable replacing it yourself schedule a visit with a field service engineer.

I have resolved many "low 502" problems this way, typically on pesticide analysis in complex matrix or pyrolysis type applications.

graettin · Answer

Here is the part number for the source radiator with the magnet: G7002-67037 – Source radiator with magnet When you remove the radiator be careful not to remove the white insulator behind it, if this insulator is removed the quadrupole will drop down and could become damaged. When this ceramic becomes contaminated and requires replacement there are ways to remove it without having the insulator without damaging the quad (remove the whole analyzer from the instrument, orient it vertically to the source is facing the sky. Remove/replace the insulator, then complete reinstallation of the radiator and source. Two person job.) You're correct, there are a few screws holding the radiator onto the analyzer assembly. Once you remove the screws remove the radiator while keeping a gloved finger or two on that white insulator to make sure it doesn't come off with the radiator. Your cleaning process is exactly what I would have done.

gchaplai · Answer

From the looks of the tune, it looks like you still have a fair few smaller peaks. try baking hte ion source, upto 320 degrees and quad to 200 for a few hours.

Another reason could be the method, have you checked the GC method is the correct one? If the flow rate is higher this can and will affect your tune.

An Etune is better for higher masses. A normal Atune just pushes/repels the ions int eh source (Repellers purpose). an Etune uses the repeller to push but also uses an extractor plate to pull the ions through meaning greater sensitivity to higher masses hence the 502 ion abundance has risen greatly.

Last thing is the insulator on the ion source. These are bored holes on either side of the insulator, hoever if these holes are too big the insulator will loosen the extractor plate and potentially offset the ion beam that would normally enter the quadrupole. So ensure this is fixed with no cracks.

5977b GC-MS Autotune failing. Low count for mass 502 m/z

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