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How to Increase the Life Time of your GC Columns? Part I: The Carrier Gas

6 Oct 2011

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A question we get many times is , how one get more analysis out of a column?. It will depend on the application but there are a number of factors that play a role here. In this series of blogs we will focus on a number of contributions we can neglect or take care off.

The purity of carrier gas is always a discussion as I get users that swear they do not believe in filters to customers that cannot live without them.  The basic question is: what does carrier gas do that contains higher levels of moisture, oxygen or hydrocarbon impurities?The answer is:  it depends..  on the temperatures we are using, the components we are looking for and the type of detector.

Impurities in the carrier gas can be coming from the gas purity  itself, reducing systems, connections and tubing.

Hydrocarbon impurities will not damage the column but they are a pain as they may elute at the same time when analytes are eluting. This is a big problem when running FID as hydrocarbon peaks may interfere with our analytes.  If MS is used, one can deconvolute by selecting masses, but it would be better to remove the hydrocarbons using a charcoal type filter.

Oxygen is also a known impurity. Practically Oxygen is not a big challenge as most phases can deal with oxygen up to temperatures of 150C. Some applications are even run using air as the carrier gas.. but in this case  its “dry” air. Oxygen sensitivity depends on phase type. The new arylene stabilized phases like is offered in the Rxi-line of columns(fig 1), are very oxygen tolerant, but poly-ethylene glycol phases have less resistance. Once oxidation starts, the phase activity will increase resulting in tailing of polar compounds.


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Fig 1 New Generation phases that combine Low bleed with Inertness

Water in the carrier gas can cause hydrolization. This is a more aggressive process as this process also takes place at lower temperatures. Once hydrolization is started, the phase starts to break down and bleed will increase, because of formation of terminal silanol groups.(fig 2).  Water removal via water-traps is highly recommended.

An area where people most of the time forget to think of, is the part of the column that is in the injection and the detection system.  That part is always at high temperature. and will therefore immediate react with water and oxygen impurities.  On the injection side we still have a focusing effect from the column, so we will not see a big impact. Detection port liner can be a bigger challenge. Sometimes detection ports are very active which cause peaks to decrease and become a-symmetric. 

If you do not need high inlet or detector temperatures for you analysis, set these at lower values.

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Fig 2: water cust siloxane chains and this starts the bleed formation

Usually the problems with carrier gas purity will be taken seriously when customer detects “ghost” peaks or when semi- or polar compounds have to be analyzed at lower concentrations. Also when higher temperatures are needed suddenly the bleed becomes an issue. Its better to prevent these problems.

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Fig. 3 Filter systems that eliminate risk of hydrolization and oxydation and that are very simple to use and replace

Its important to use cleanest possible carrier gas. To secure that, one can buy very pure gases. There are even solutions where a filter is inside the cylinder. Such systems do not take away the risks of small leaks of couplings in the gas distribution systems as well as the valves that are used.

A more practical solution is to use in-line gas filtration systems where the filters are used as replaceable cartridges. Such filter systems are nowadays very easy to maintain as filter replacement only takes seconds and there is no down-time as the gas system remains pressurized. [fig 3].

One can position such filters close to a GC that we want to optimize and have the system secured. Typically a filter needs replacement once a year.

If you are in the position that can do you analysis without this optimization, be happy and continue to do so. If you run one day into challenges causing down time, think about carrier gas purity.