There are a wide variety of carrier gases potentially available for use with gas chromatography (GC) systems, including helium, hydrogen and nitrogen. Your range of choices, however, may be limited by a number of factors that include detection method, application, and the design of a particular system. For example, if the separation requires a helium detector, then you will need to use helium. Likewise, thermal conductivity detection (TCD) typically employs helium for the best thermal conductivity differences between the carrier gas and other compounds that may be detected. Depending on the system’s design, the use of mass spectrometry may allow the use of hydrogen or not. In the case of flame ionization detection (FID) systems, chromatographers typically do have a choice between helium and hydrogen — as they also do with other flame-based detection methods such as where phosphorous or nitrogen detection is employed for separation.
Hydrogen offers several advantages over helium in these systems that chromatographers may wish to consider, including the fact that it is much more widely available and less expensive. It can also allow for higher linear flow rates, hence shorter run times, thereby increasing the throughput of a laboratory.
Switching from Helium to Hydrogen
Making the switch from helium to hydrogen is a little more involved than simply swapping one gas for another. Specifically with respect to the GC system itself, factors that may need adjustment are the linear gas rate (LGR) — i.e., the rate at which gas flows through the column — as well as the split ratio and the amount of flow of fuel gas to the FID system. Also to be addressed is the type of tubing used to deliver the carrier gas. Over time copper tubing will oxidize and become brittle, so it may break if bumped. A better choice is stainless steel, preferably of GC quality to avoid contamination.
How you adjust for the other factors depends on the type of system you have. Many modern GC systems include electronic pneumatics so all you need to do is indicate the gas used and the system automatically adjusts for the differences in the density effects of the gas. For example, hydrogen flows faster (has a higher LGR) than helium under the same amount of pressure.
Some older and simpler systems do not automatically adjust for LGR and for them you will need to adjust the head pressure. To achieve the same LGR for hydrogen that you used for helium, you need to set the head pressure to approximately 45% of the pressure used for helium.
Hydrogen’s lower density could also make column head pressure fall below 10 psig in systems with a short column or wide bore column. That means you might have to change the flow controller to allow flow control below 10 psig — if your system controls LRG using a combination of flow controllers and pressure controllers. Also when using packed columns or large-bore capillary, you might need to reduce the fuel gas so that the combination of carrier and fuel gas is at that rate previously used for separation. Table 1 shows typical detector fuel flow ranges. In general, flame detectors need an optimum flow of hydrogen to optimize the flame sensitivity: typically 30-40 cm3/min of hydrogen. Also, if you use hydrogen as the make-up gas, you should also consider this flow with the carrier gas and fuel gas to optimize detector sensitivity. Be sure you don’t saturate the detector with too much hydrogen, as this will affect both baseline noise and sensitivity.
When switching the carrier gas to hydrogen it may also be necessary to adjust the split ratio, which is the ratio of sample and carrier gas that enters the column versus how much is vented. Always measure the column flow and vented carrier gas before changing the gas to make sure you know the split ratio you were using. After changing the gas, you should measure and adjust your split vent flow to allow for the same split ratio you previously used.
None of these changes are difficult to make, nor in the case of flame-based detectors do they require replacing your existing systems. As supplies of helium continue to become more scarce and expensive, it is likely that more laboratories will be converting.
Table 1: Typical Flame Ionization and Nitrogen Phosphorous Detector Fuel Gas Flow Rates.
Note: Consult your GC manufacturer on your specific unit flow requirements.
Information in the post was adapted from an article by Reginald Bartram and Peter Froehlich: Considerations on Switching from Helium to Hydrogen. View article.
This post was contributed by the Gas Generation Technology Blog Team, Parker Hannifin Gas Separation and Filtration Division.