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Carrier Gas Alternatives: Considerations when Switching from Helium

Posted by Teledyne Tekmar on Thu, Sep 13, 2012 @ 04:30 PM

With helium increasing in scarcity and price, many laboratories are looking for cheaper and more readily available alternatives. Some have also been restricted or cut off completely from their suppliers due to the demand. In the realm of gas chromatography, this leaves hydrogen and nitrogen as the most viable alternatives for carrier gas. Gas generators are available from a variety of vendors for both hydrogen and nitrogen. Installation of one of these generators can lead to considerable cost savings over its lifetime, especially when changing from helium gas cylinders. New methods are also starting to allow alternative gases to accommodate these supply challenges.

In addition to cost savings of switching from helium cylinders, there are also some potential unique bonuses to using these alternative gases.

Van Deemter Plot of Helium Nitrogen and Hydrogen

Figure 1: Van Deemter Plot of Helium, Nitrogen, and Hydrogen

These sort van Deemter plots are thrown around a lot, but what do they mean?

To simplify things, the areas you should be shooting for to optimize your GC analysis are the minima of each of the curves. These are the optimal ranges, based on the Height Equivalent of Theoretical Plate (labeled H on the figure), for each carrier gas. As theoretical plate height decreases, the number of plates in a given column length increases, translating into higher the column efficiency.

Hydrogen has the “flattest” curve, which means there is a more forgiving range of gas velocities that will give similar performance. Nitrogen is actually the most efficient carrier gas, with its optimal range occurring at the lowest linear velocity (ū), but the working range is very steep. Another drawback is the difference in optimum gas velocity (~10 cm/s vs 40 cm/s) of nitrogen versus hydrogen. This means a chromatographic run using nitrogen would take roughly 4 times longer to achieve the same separation. Inversely, we can theoretically run 4 times faster and achieve the same separation simply by changing to hydrogen as the carrier.

With the continued desire for faster, more efficient, and cheaper analyses, changing carrier gases is definitely an option worth exploring. The optimal linear velocity is just one factor to account for. There are also implications on the separation, detection (FID, MS, etc.), and even overall lab safety. These sort of systematic changes require thorough evaluation to make a smooth transition and avoid excessive instrument downtime.

Our applications group continues evaluating alternative carrier gases, not only to support customer applications, but for day-to-day lab use as well. As we develop additional applications utilizing different carrier gas options, we will highlight some of these factors to assist our customers in potential cost savings of nitrogen and hydrogen versus helium.

Tags: helium, gas alternative, hydrogen, nitrogen

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