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Crimes Against GC Gas Filters

Are you guilty of any of our most commonly committed ‘crimes’ against GC gas filters?

1. Using the wrong gas traps, or using the correct traps but in the wrong order

These are probably the most common ‘crimes’ our technical team comes across in laboratories. 
Use our handy chart below to check that you have the correct gas traps connected in the correct
order on your GC instrument.   

Key:

Oxygen Filter – removes oxygen

Moisture Filter – removes moisture

Charcoal Filter – removes hydrocarbons

 

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Nitrogen Phosphorus Detector (NPD)    
     

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Electron Capture Detector (ECD)    
     

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2. Installing GC gas filters incorrectly and in the wrong place

Depending on the number (and location) of GC instruments, there are two trap configurations:

  • Install the traps on the main line, thus all GC systems are fed with gas purified by the same traps
  • Install a set of traps at each GC system (each GC system has its own set of traps)

In the first case, it may be cost effective to install a set of large capacity traps in order to minimize the frequency of replacement.
The traps should be installed in accordance to your supplier’s indications (vertical position, required fittings, etc.). Some best practice protocols that can also be followed are:

  • Install gas traps close to the GC instrument.
  • A positive flow should be maintained through the filter at all times to avoid back diffusion of air through regulators on instruments that are powered off.
  • Change filters on a regular basis as part of routine maintenance.  Color indicating filters will easily show when they need changed, BUT remember to check them routinely.
  • Indicating traps should be installed closest to the GC instrument in order to indicate when to change the traps that are upstream.
  • Keep the number of fittings in gas line to a minimum – the more fittings there are the greater the risk for ingress of oxygen, moisture etc.

 


3. Using traps under the wrong operating conditions
Make sure you know the recommended operating conditions for your trap.  However, in general terms, most GC traps are designed to operate at:

  • Room temperature
  • Top flow rates not exceeding 1-2 mL/min
  • Maximum operating pressure below 100-200 psi

Trap capacities depend upon manufacturer, size, etc.  For more information consult your favorite GC trap manufacturer or provider.

 


4. Not using GC-grade tubing

Stainless steel or copper are the preferred materials for GC tubing.  Regular metal tubing can be contaminated with high levels of hydrocarbons form greases used in the manufacturing process.  This can lead to ghost peaks in the chromatogram.  Make sure that GC-grade tubing is installed on all lines.  Plastic or rubber tubing should not be used for any of the gas lines in a GC system.  Teflon, nylon, polyethylene, polypropylene or polyvinyl chloride (PVC) contain contaminants that degrade gas purity.  Plastics are permeable to oxygen and moisture, which will contaminate the carrier gas.  Furthermore, most plastic tubing cannot withstand the high pressures that are used in GC systems, rendering them unsafe to use. 

hydrocarbons

 


 

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