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GC-MS Leak Detection

Recently a CHROMacademy user contacted our experts as their GC-MS 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).

 

 

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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/z 32) 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.

  2. Cap the transfer line with a blanking nut, pump down the system, and see if the vacuum holds.  Again monitor for peaks at m/z 28 and 32.  If these peaks are not present at the specified values then the leak is not from the source onwards.  If the vacuum holds then the leak is likely to be prior to the transfer line.

  3. Check for leaks at other points in the system.  An easy way to do this is to use a dust removal aerosol (also often known as a leak detector spray, which contains either compressed air or chlorodifluoromethane).  With the mass spec pumped down look at masses between m/z 35 and 75, spray the aerosol (chlorodifluoromethane) and look for peaks to appear at m/z 51, 52, and 67.  If the dust removal spray contains compressed air monitor the nitrogen (m/z 28), and oxygen (m/z 32) ions.  Spray short bursts at fittings in the interface oven, around the separator, and in the GC allowing time for the peaks to show up in the mass spectrum. Tightening or replacing any fittings or ferrules that appear to be leaking should fix the problem.  If helium is being used as the carrier gas a helium detector can be used to check for leaks.  You should also spray around all the seals on the MS - the door, the tune compound valve and vent to the foreline pump are common leaks sites.

  4. When you have pumped down the instrument to carry out step 2, cap the GC inlet and perform a leak test here (either using the MS to detect the leak or using a leak detector).  If the pressure holds then there is not a leak here, however, it may indicate that the column was not properly seated or a new ferrule is required.  As a precaution change the septum and both column ferrules.

  5. Check the GC-MS user manual to see if there are any other easily accessible fittings or seals within the instrument which may need tightened or replaced.

  6. If vespel or vespel blended ferrules are being used these need to be tightened regularly to maintain a proper seal (weekly), therefore, if these ferrules are in places that are easily accessible then a quick tighten may help.
 

Figure 1: Normal air/water background spectrum (top), spectrum obtained with a small leak (middle), spectrum obtained with a large leak (bottom).  
m/z
 18 = H2O and m/z 28 and 32 = N2 and O2/air.

 
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