Backflush is a must in gas analysis! Find out what it's for and how it works.
In gas chromatography, there are different analytical columns allowing the separation of different species. Its column(s) are chosen according to the compounds that are to be separated and measured.
One of the best known columns in Micro GC is the molecular sieve also called Molsieve 5A or MS5A. It has been specially designed for the separation of the lightest existing gases (permanent gases).
It is a very popular column for many applications in industry and research. It enables the measurement of hydrogen, catalysis studies, the analysis of natural gas and biogas.
The molecular sieve allows for 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.
Chromatogram MS5A: 65-s analysis of H2, O2, N2, CH4 and CO
Nevertheless, the molecular sieve has a major defect. Indeed, it retains a lot of gases heavier than permanent gases such as carbon dioxide, water vapour, ethane, propane, hydrogen sulfur etc...
However, carbon dioxide and water vapour are found in the air around us and in the majority of the gas samples that we want to analyze in Micro GC.
Without MS5A protection, these heavy compounds will accumulate in the column. In the long run they will cause performance losses until the complete loss of chromatographic separation.
Backflush, what's the point?
We come to the main subject of our article, the Backflush.
First, and this is the main reason for backflush in MicroGC, it is used to protect the column and inject only MS5A-compatible gases to stabilize these performance. We will detail and illustrate this use in this article.
Second, rather in conventional GC, backflush also reduces the time of analysis when light interest compounds are in a sample with a complex matrix composed of heavier compounds.
Backflush, how does it work?
A guard column with a stationary phase is used to retain water, CO2 and hydrocarbons upstream of the molecular sieve column.
As a first step, this guard column will allow us to retain undesirable compounds and inject only permanent gases into MS5A. In a second step, using a valve we will return the guard column to remove the retained gases and prevent accumulation of heavy compounds in the guard column.
In mode « foreflush », the columns are in series and the gases coming out of the precolumn enter instantly into the analysis column.
In mode « backflush », the components that have remained in the precolumn are backbalayed because the flow is inverse. The column is protected if the switching time is set correctly. The other compounds continue their path in the analysis column whose direction of flow does not change.
It is with this type of assembly that one can protect a column and perform analyses on a mixture of gases which would contain compounds too heavy for the stationary phase.
Now you know the working principle of the Backflush and its role in an analytical system.
In our next article, we will explain how to easily set backflush's time to protect its column.
