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Backflush is a must in gas analysis! Find out what it is used for and how it works.

 

In gas chromatography, there are different analytical columns allowing the separation of different species. One chooses its columns according to the compounds that one wishes to separate and measure.

One of the best known columns in Micro GC is the molecular sieve also called Molsieve 5A or MS5A. It was specifically designed for the separation of the lightest gases available (permanent gases).

Column MS5A Capillary

It is a very popular column for many applications in industry and research. Indeed, it allows the measurement of hydrogen, studies in catalysis, analysis of natural gas and biogas.

In particular, the molecular sieve allows the analysis of the following gases: helium (He), hydrogen (H2), oxygen (O2), nitrogen (N2), methane (CH4) and carbon monoxide (CO). Depending on the length and efficiency of the column, it is even possible to measure oxygen and argon (difficult separation), without sub-ambient temperature.

MS5A chromatogram: 65s analysis of H2, O2, N2, CH4 and CO

Nevertheless, the molecular sieve has a major flaw. Indeed, it retains a lot of gases heavier than permanent gases such as carbon dioxide, water vapor, ethane, propane, hydrogen sulfur etc…

Carbon dioxide and water vapor are found in the air around us and in the majority of gas samples that we want to analyze in Micro GC.

Without protection of the MS5A, these heavy compounds will accumulate in the column. In the long run, they will cause performance losses until the complete loss of chromatographic separation..

MS5A chromatogram: loss of performance, O2 and N2 separation becomes difficult.

Now we come to the main topic of our article, the Backflush.

What is the purpose of the Backflush?

Firstly, and this is the main reason for the backflush in MicroGC, it is used to protect the column and to inject only gases compatible with the MS5A to stabilize its performance. We will detail and illustrate this use during this article.

Secondly, rather in conventional GC, the backflush also allows to reduce the analysis time when the light compounds of interest are in a sample with a complex matrix, composed of heavier compounds.

How does the Backflush work?

A guard column is used with a stationary phase allowing the retention of water, CO2 and hydrocarbons upstream of the molecular sieve column.

In a first step, this guard column will allow us to retain the undesirable compounds and to inject only the permanent gases into the MS5A. In a second step, with the help of a valve, we will backflush the guard column to remove the retained gases and prevent an accumulation of heavy compounds in the guard column.

It is with this type of setup that we can protect a column and perform analyses on a gas mixture that would contain compounds too heavy for the stationary phase.

Now you know how the Backflush works and its role in an analytical system.

In our next article, we will explain how to easily set the Backflush time in order to protect your spine.

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