Frequently Asked Questions about Hydrogen Carrier Gas - Chromatographers are choosing hydrogen carrier gas more often for its performance and, in the face of increasing helium prices, cost. In this in

Frequently Asked Questions about Hydrogen Carrier Gas

Chromatographers are choosing hydrogen carrier gas more often for its performance and, in the face of increasing helium prices, cost. In this instalment of "GC Connections", John Hinshaw addresses some of the more frequently asked questions he encounters regarding hydrogen as a carrier gas.

Jan 1, 2009
By: John V. Hinshaw
LCGC Europe
Volume 22, Issue 1

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Many gas chromatographers are considering hydrogen carrier gas instead of helium or nitrogen, but they might hesitate to do so because they have questions about performance, safety, or cost. This month's "GC Connections" focuses on these and other issues related to hydrogen in the gas chromatography laboratory.

Performance

For sheer column performance, hydrogen carrier gas offers some strong advantages over helium or nitrogen. Hydrogen yields higher plate numbers at rapid linear velocities and achieves higher velocities at lower pressures. At the same time, the presence of extra hydrogen can affect flame ionization and other detectors that use hydrogen fuel gas.

Does hydrogen carrier gas affect retention times? The answer is yes and no. Retention times in gas chromatography (GC) are controlled by several factors: the distribution coefficient (K) of a solute in the column, which is not affected by the choice of carrier gas; the column dimensions of length, inner diameter and stationary film thickness, all of which we will keep constant in this discussion; and the average carrier gas linear velocity (ū). As the linear velocity increases, isothermal retention times decrease in exact proportion, so doubling the velocity cuts retention times in half. We will look at the effects of changing carrier gas for three situations: constant velocity, constant inlet pressure and constant flow-rate and then we will see what happens if the column is temperature programmed.

Figure 1

In addition to its dependencies upon the column dimensions and pressure drop, the linear velocity is influenced by the viscosity of the carrier gas. As many readers will know, hydrogen is a bit less than half as viscous as helium or nitrogen at the same temperature, as shown in Figure 1, and so for this reason hydrogen requires a lower pressure drop to achieve the same average carrier gas velocity as for helium or nitrogen. For example, a 50 m × 250 μm column will deliver an average velocity of 60 cm/s at 100 °C with 58.6 psig (404 kPa) of helium or with 27 psig (186 kPa) of hydrogen; the situation is similar for nitrogen. If the velocity is unchanged with a hydrogen versus helium carrier, then retention times remain the same too, while the inlet pressure that is required with hydrogen will be about half as much as with helium.

Conversely, if the same inlet pressure were applied with hydrogen as with helium, then the hydrogen carrier gas would cause peaks to be eluted in less time, because the linear velocity would be faster than with helium. In the previous example, helium at 27 psig would have an average linear velocity of 29.4 cm/s and so with hydrogen carrier at the same pressure, all of the peaks' retention times would decrease in the proportion 29.4/60 ≈ 0.5. The relationship between linear velocity and pressure drop is somewhat nonlinear because of effects stemming from the compressibility of the carrier gas, but in general, the effect of switching from helium to hydrogen on retention time will be to cut retention times roughly in half if the inlet pressure is unchanged.

At constant flow-rates, the situation is intermediate between the effects at constant velocity and those at constant pressure. At 27 psig of helium, the column flow-rate is going to be 1.43 cm³/min and the average velocity will be 29.4 cm/s. The hydrogen carrier pressure required for the same flow-rate will be about 16.3 psig (112.4 kPa), but now the average velocity will increase to 37.5 cm/s. Therefore, in this example, retention times will decrease by a factor of 29.4/37.5 = 0.78 for hydrogen compared with helium at the same column flow-rate.

In summary: for isothermal operation at constant average linear velocity, retention times are not affected by changing the carrier gas. At constant inlet pressure hydrogen carrier will cause peaks to be eluted in about half the time and at constant flow-rate retention times will be about 78% with hydrogen compared with helium.

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