GC–MS Leak Detection

Jul 09, 2014
By LCGC Editors

Recently a CHROMacademy user contacted our experts as their GC-MS system was not maintaining vacuum.

Normally, the vacuum was set at 1.2x10-7 Torr; however, it was dropping to 2.3x10-4 Torr. 
The user carried out some maintenance by tightening and then subsequently replacing the nut and ferrule at the transition line which solved the problem initially.

Unfortunately this did not fully remedy the problem. The source was capped and the pressure held, however, this still left the user scratching their head as to why they were seeing gross pressure drops.

The vacuum not holding in a GC-MS system would indicate a leak. Leaks in a GC–MS system can be particularly frustrating; they can cause spurious peaks in the mass spectrum, a decrease in sensitivity, loss of carrier gas (this could be expensive), and damage to the instrument.

Often leaks can occur where the column enters the MS or at point(s) where the vacuum system was disassembled and reassembled (i.e. for any routine maintenance operations). If it is expected that there is a leak in a GC-MS system there are a few things that can be done to try and identify the source of the leak in order to repair it.

1. When the system is running verify that peaks at m/z 28 and 32 are present in the mass spectrum.  These peaks are indicative of nitrogen and oxygen; if they are present in a ratio of 4:1 (nitrogen:oxygen) this would indicate that there is a leak to the atmosphere.  If the peaks for water (m/z 18), nitrogen (m/z 28), and oxygen (m/z32) rise above 10% (relative to m/z 69 peak — typical base peak from PFTBA used during auto-tune) this is also indicative of a leak.

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