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Gas management system for Gas chromatography laboratory and importance of changing filters regularly.

A clean gas stream is critical to the quality of your GC analysis and the reliability of your analytical results. Carrier gas must contain less than 1 ppm of oxygen, water vapor, and other trace contaminants for stable baselines on all detectors and to prevent column degradation, shortened column lifetime, and increased stationary phase bleed. Clean fuel gases and make-up gases are also essential for stable detector baselines. Your gas management system must deliver a high-purity gas stream from your source to your instrumentation without introducing contaminants. Figure below shows a typical gas management system using a cylinder

Questions to consider when building gas management system for a GC Lab:

  1. What gases do you require and at what purity?
  2. What flow rates and/or pressures are required?
  3. Are you using gas generators or high-pressure cylinders?
  4. What type of tubing and fittings are needed to build your system?
  5. What type of gas purifiers should you install in your system?

This article explains why it is important to replace your gas filters annually instead of waiting for the indicators to change color

A significant number of instrument and column complaints are carrier gas related, which could be caused by breakthrough from filters that are not replaced in time.

The possible consequences of a filter breakthrough are:

  • Gas distribution system behind the filter will be contaminated (fast cleaning nearly impossible, bleeds for months)
  • Instrument gets contaminated, expensive maintenance required
  • Column lifetime reduced, bad analytical results, high cost of ownership, unnecessary changing of column brands
  • MS source gets contaminated, expensive maintenance required, long system shutdown.

Figure showing reduced instrument baseline due to filters

Gas Purifiers

Gas purification is essential in your gas management system. Carrier gases must contain less than 1 ppm of oxygen, water vapor, or any other trace contaminant to prevent column degradation, shortened column lifetime, and increased stationary phase bleed. Contaminants cause ghost peaks to appear during temperature programming and degrade the quality of analytical data. The expense of using high-purity gases in combination with carrier gas purifiers will be offset by longer column lifetime and less instrument maintenance along with better instrument sensitivity. Gas purifiers are available for specific types of contamination (moisture, hydrocarbon, or oxygen) or as a combination of filters that provides broader protection. These purifiers can be installed in-line or using a quick-install baseplate system.

A typical GC and GC-MS laboratory gas filter system contains three types of filtering media:

  • Oxygen Catalyst for absorbing traces of oxygen
  • Activated Carbon for adsorbing hydrocarbons
  • Molecular Sieve for adsorbing moisture

These filter media types can be divided into two groups:

  • Chemically absorbent media:

Like a Venus fly trap, when it comes in contact with a contaminate it absorbs it and does not let it go. Adsorption differs from absorption, which also removes things, but the result is swelling of the media. The media size increase equals the amount of material removed

  • Surface adsorbent media

Like a floor mop, where the contaminant is trapped onto the surface of the media and retained. The material doing the adsorption does not change in size.

Also note that most filters do not provide an indicator for hydrocarbons, so you would never know whether the activated carbon has reached its adsorption capacity.

Filter Media Breakthrough

Both adsorption and absorption media have fixed capacities, meaning they hold just so much, since they are storing the material removed from the gas, not destroying it.

Normal situation

Hydrocarbon breakthrough

Over time, the pores of the activated carbon fill up. The molecules that are adsorbed with higher energy (larger mass) can displace the lower-energy molecules that are less tightly held. This phenomenon, called displacement, may knock the smaller particles off the media, straight to the column and into the instrument.

Moisture breakthrough

The molecular sieve adsorbs moisture until it cannot adsorb anymore. If the humidity level of the gas is lower than in the molecular sieve, it will de-adsorb its moisture until it is in “balance” with the lower humidity level of the gas, which means the filter could increase the amount of moisture in your gas.

Oxygen breakthrough

The media size increase equals the amount of material removed. When the media has reached its absorption capacity it will not release the already trapped contaminants but it will also not absorb any new ones.

When am required to change my gas filter?

Filter Media Breakthrough Indicators

The visual indicators are mainly for urgent situations such as a leak or high amount of impurities breaking through.

Indicators are typically placed behind the filter media bed. When they change color shortly after installation of the filter it usually means there is a leak or that the gas distribution system including the manifold to which the filter is connected was not flushed properly prior to – or during – installation of the filter.

When the indicator changes color during normal operation of the filter it indicates that the filtration media has reached its capacity, and the filter should be replaced immediately to avoid contaminant breakthrough.

Other components such as branched hydrocarbons which are also trapped by the molecular sieve are not shown by the moisture indicator.

Also note that most filters do not provide an indicator for hydrocarbons, so you would never know whether the activated carbon has reached its adsorption capacity.

 

Conclusion

Replace your filter before the indicators start changing color to prevent breakthrough and to avoid high maintenance and repair cost of your instrument.

As explained previously, filters which are not maintained on a regular interval can cause the outgoing gas to become more contaminated than the original source gas. The color indicators used in gas filters are so called ‘last minute’ indicators and require quick action. You can compare it to the engine oil indicator in your car. When the engine oil indicator is blinking on your dashboard, the car should not be driven and ignition should be switched off unless topped with engine oil – In event of taking risk to drive – high probability of engine getting seized causing high expenditure to repair or replace the engine.

The same is valid for gas filters. When one of the indicators starts to change color, the filter should not be used and instrument should be switched off unless the filter is immediately being replaced with a new one – In event of taking risk to continue – there will be a high probability of contaminant breakthrough causing high expenditure on instrument maintenance as compared to planned annual preventive maintenance on your instrument and gas filters.

So what you can do as an end-user is to always buy an additional set of filters to keep on standby in case the indicator starts changing color or to use a good preventive maintenance plan or tool (such as the electronic indicator) to replace your filters at least once a year.

Reference

https://www.peakscientific.com/articles/use-of-traps-in-gc/

https://www.restek.com/Technical-Resources/Technical-Library/General-Interest/general_GNSS1758B-UNV

https://www.shimadzu.fr/sites/shimadzu.seg/files/shimadzu_brochure_filtre_gaz_super-clean_c180e083a_-_2017-06.pdf

Written by Oweh Gabriel,

www.aasnig.com, gabriel@aasnig.com

08035696303

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