Changing from Helium to Nitrogen and Maintaining the Separation Efficiency in the Same Analysis Time
by Jaap de Zeeuw1 and Jack Cochran,2
1Restek Corporation, Middelburg, The Netherlands and 2Restek Corporation, Bellefonte, USA.
For several reasons, there is interest in replacing helium with a different carrier gas in gas chromatography. Hydrogen is the obvious choice but there is a concern on safety and reactivity. It is also possible to use nitrogen but this is often not considered because it has a low optimal flow and velocity. Here we will show that it is possible to replace helium for nitrogen as the carrier gas while separation, peak elution order, analysis time, response and even oven temperature conditions can be kept the same.
By using method translation and chromatogram modelling it became clear that the loss of efficiency using nitrogen could be perfectly compensated by using a smaller ID capillary of a shorter length. By replacing a 30 m x 0.25 mm column for a 20 m x 0.15 mm column, it is demonstrated that separations under nitrogen are almost exactly the same as obtained with helium, in the same analysis time, while using exactly the same conditions for oven programming. Even the inlet pressures are very close.
The only price that has to be paid is a loss in loadability, which means that this concept will not work for every application but for many it will. Besides the guaranteed availability, using nitrogen offers a big advantage in the cost and consumption volume of carrier gas, meaning cost per analysis will also benefit.
In gas chromatography there are mainly three carrier gases used: nitrogen, helium and hydrogen. Figure 1 shows the van Deemter plots, showing the column efficiency versus the linear gas velocity (one can also use flow here). Nitrogen is considered a slow carrier gas with an optimum of 11-13 cm/sec. Helium is about 2 times faster and hydrogen is the fastest gas. Because of concerns of safety and reactivity, hydrogen is often avoided and helium is the preferred carrier gas. Also there are several detection systems that prefer helium because of the detection principle. Think here about the mass spectrometer and the pulse discharge based detectors.(PDD, HID, BID).
Helium as carrier gas, has some challenges. The biggest challenge that developed in the last decade is availability. There is a limited amount and present resources cannot guarantee delivery. Used helium cannot be recycled as it disappears in space. Apart from the availability the price is also rapidly increasing. Helium is already 5x the price of nitrogen and it will only get more expensive.