Depending on the type of gas chromatography system employed, hydrogen is often an attractive replacement for helium as a carrier gas. It is less expensive and more widely available resulting in cost savings. It is also more environmentally friendly since helium is obtained as a byproduct of fractional distillation of natural gas while hydrogen comes from water. In the case of flame ionization detection (FID) systems, hydrogen also lends itself to higher linear gas rates (LGRs), the rate at which gas flows through the columns — potentially increasing the throughput of the laboratory. This is part one of a three part series on the benefits of converting from Helium to Hydrogen as a carrier gas.
Although the switchover from helium to hydrogen is fairly straightforward in many cases (view article on Considerations in Switching from Helium to Hydrogen as a Carrier Gas for Gas Chromatography), those analyses that use temperature programming (such as to obtain sharper peaks at reasonable temperatures in later eluted compounds) may need to undergo some adjustments. These adjustments concern:
- The head pressure of the gas
- The temperature program
- Elution order of peaks
Change in head pressure
The viscosity of hydrogen is about 45% less than helium, however the viscosity of both gases increases with temperature at about the same rate (see Figure 1). This means that for any given temperature the head pressure of hydrogen to run the analysis will need to be about 45% lower than for helium to achieve the same LGR.
Changes in the temperature program
One reason to use hydrogen as a carrier gas is to enable higher flow rates in order to reduce analysis time. A higher flow rate, however, may require raising the temperature rates to match the higher flows. If the temperature program rates are not increased some compounds might be eluted faster but provide broader peaks than desired. If, for example, when doubling the LGR and using a 5˚C/MIN temperature program rate, it might be desirable to increase the program to 10˚C/ min. If, however, the peaks are coming out rapidly and are sharp, without increasing the temperature, then you can take advantage of the lower column temperatures and stop the temperature programming just before or after the last peak is eluted. This might mean a lower final run temperature and will often extend column life.
Changes in linear gas rate and temperature may affect the eluting power of certain compounds and thus the order in which they are used in the analysis. That’s not an issue if you use the same LGR and temperature with hydrogen as you did with helium. However, with polar columns, such as Carbowax or highly polar Cyano phase columns, it may be necessary to change the elution order as some column phases exhibit different polarities or orders of peak elution at different temperatures.
Figure 1: Plot of gas viscosity versus temperature. Reprinted with permission of Sigma-Aldrich Co.
Part One is adapted from an article authored by Reginald Bartram and Peter Froehlich.
This series continues with Part 2: Is Hydrogen a Safe Gas Chromatography (GC) Carrier Gas?
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