For operation of GC columns a carrier gas is required. In the first blog of this series we zoomed into the importance of using clean carrier gas. We also have a choice on type of carrier gas. Mostly Helium is used, because of its high optimal velocity and inertness but that gas is loosing its attractiveness for several reasons.
Helium is becoming more and more expensive, increasing cost per analysis. This will not change as the supply of helium is also not secured.
One of the main sources of helium in Amarillo, Texas, will be drying up. We have to consider alternatives.
Alternatives are available for many applications using Hydrogen. One has to deal with safety aspects, but its not as dangerous as people always seem to think. See for details our blog on hydrogen: http://blog.restek.com/?p=3520 risks are very low and we can take precautions.
Helium has also disadvantages. It’s my personnel experience that I found that columns used under hydrogen show a more lasting inertness then used under helium I always blamed low level residues of sulfur compounds to be responsible for that, but I never could proof that.
A big challenge all must know is that helium diffuses through the fused silica. That means that if you take a column out of the GC, in 16 hrs there is no helium present anymore and depending on how good you seal the column, it will be completely evacuated or filled with lab-air, see http://blog.restek.com/?p=568 .
We know that Restek columns filled with lab-air, have no problem when kept at ambient temperature, but you need to be aware of this. That means also, when you install the column in GC you need to flush very well before heating as there is air (meaning water and oxygen)present. Because air contains water, this is also entering the column. For alumina column this means high temperature conditioning as the water will deactivate the alumina.
Fig 1. Leak detection will assure correct column installation and reveil possible septum or leaks in the gas lines. Its an investment that is earned back in very short time
Whatever gas we use, leaks and gas purity are always a big challenge. The higher the temperature of the injector, column and detector, the more problems we can anticipate. Leak detection can be done nowadays with sophisticated instruments like a leak detector. See fig.1. Such a leak detector is measuring on the principle of gas conductivity and is very sensitive for H2 and helium. Its clean and it will give an audio or visual alarm if a leak is detected. Such a device is also practical for checking the septum seal as the septum is probably the biggest leak in the system.(but many do not realize that).
I also list the injector and detector on purpose as many seem to forget that in this part we have high temperatures and activation can be very fast. See the extreme data generated by activity in a 6890 FID.
When you do not use the column and want to keep it in the GC, always maintain a positive flow through the column and keep it at a temperature of 80-90 C. I have seen many times that when the flows are turned off, back diffusion of air can occur even into the gas lines. Keep the split flow to a minimum value of 5-10 m/min to save carrier gas.