Pixabay/Michael Gaida: Advantages and disadvantages of substitution of helium as carrier gas in gas chromatography by hydrogen. Part II. - Retention time and selectivity.
Gas chromatography often uses helium, hydrogen, and nitrogen as carrier gases. The most common carrier gas is helium since it provides good separations, is inert, and is safe to use. In recent years, the availability of helium has decreased and its cost has increased significantly. So, it is necessary to choose a suitable substitution. This article is focused on the chromatographic parameters such as retention time and selectivity after switching to the use of hydrogen or nitrogen.
The availability of helium has decreased and its cost has in-creased significantly, and chromatographers have therefore considered switching to the use of another suitable gas (Horák et al., 2013).
In this article the use of helium and hydrogen in gas chromatography is compared and practical aspects of switching to the use of hydrogen as carrier gas in gas chromatographic methods are dis-cussed especially in brewing analysis applications.
The number of theoretical plates (N) or the equivalent of column plate height (H = L/N, L means the column length) can express the efficiency of a chromatographic column. The equivalent of column plate height is the function of the speed average of carrier gas (ū) which is described by the Golay-Giddings equation (Golay, 1958a; Giddings et al., 1960). This dependence is the function of the column diameter (dc), film thickness (df), capacity factor (k) and diffusion coefficients of a compound in mobile and stationary phases. Different H versus ū curves for different gases are obtained for a given column and a given compound (Fig. 1).
Fig. 1 H versus ū curves for carrier gases hydrogen (ring), helium (square), nitrogen (rhombus). (Korytár, Matisová, 2001)
Several important facts ensue from these dependences.
For all carrier gases the value H is almost independent of the type of carrier gas. So when an optimal average speed of carrier gas is adjusted, all three types of gases give similar efficiency and resolution.
The optimal carrier gas speed is the highest for hydrogen in com-parison with helium or nitrogen. For current capillary columns (length 10–50 m, internal diameter 0.25–0.32 mm and film thickness 0.1–0.5 µm) and using the optimal speed, hydrogen is 1.5 times faster than helium and 3.3 times faster than nitrogen. Importantly, the equivalent of column plate height is approximately the same.
The slope of the H versus ū curves drops in the order nitrogen – helium – hydrogen. This means that, when nitrogen is used in a speed higher than the optimal speed, fast degradation of efficiency is attained. On the other hand, in the case of hydrogen the gas speed can be increased without significant loss of efficiency. Helium is characterized by the middle slope (Fig. 1). With decreasing column diameter the slope of the curve drops, the curve becomes flatter and the optimal gas speed increases. Due to this a substantial speed of carrier gas can be used without considerable degradation of efficiency (David, Sandra, 1999; Horák et al., 2009).
For above reasons it is clear that hydrogen is more suitable as helium substitution than nitrogen.
Generally, the chromatographic parameters such as retention time, efficiency, resolution, and selectivity are influenced by the choice of mobile phase. However, in gas chromatography this effect is much less important that in liquid chromatography. This is because the carrier gas in gas chromatography rarely interacts chemically with the stationary phase or with the analyte. The main role of carrier gas is to transport the analyte in the gas phase through the column and act as the second phase in the partitioning mechanism.
Two properties of the gas play a significant role in the gas chromatographic process – diffusivity and viscosity. The diffusivity of hydro-gen and helium is roughly the same but hydrogen is slightly less than half as viscous as helium at the same temperature. For this reason hydrogen requires a lower pressure to achieve the same average carrier gas velocity than helium.
Due to the compressibility of the carrier gas the relationship be-tween linear velocity and pressure is somewhat nonlinear, but in general, the switching from helium to hydrogen will reduce retention times roughly in half if the inlet pressure is unchanged.
Retention times in gas chromatography are influenced by several factors. The distribution coefficient of a solute in the column, which is not affected by the choice of carrier gas; the column parameters such as length, internal diameter and film thickness; and the average carrier gas linear velocity (Chromacademy, online).
The linear velocity is influenced by the column dimension, pressure drop, and viscosity of the carrier gas. Hydrogen is slightly less than half as viscous as helium or nitrogen at the same temperature. For this reason hydrogen requires a lower pressure drop to achieve the same average carrier gas velocity as for helium or nitrogen. If the velocity is unchanged with hydrogen versus helium carrier, then retention times remain also the same, while the hydrogen inlet pressure will be about half as much as with helium.
On the other hand, if the same 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. Retention times at the same pressure would decrease by about half.
At constant flow rates, the situation is intermediate between the effects at constant velocity and those at constant pressure (Chromacademy, online).
The above tenets can be applied also to isothermal operations. If column temperature program is used the situation is a little changed.
When a constant linear velocity can be maintained during the temperature program by an electronic pneumatic unit, the retention times will not change. But the control of linear velocity is not available for all pneumatic systems of gas chromato-graphs.
In the constant inlet pressure mode, hydrogen will cause peaks to be eluted earlier. Simultaneously their elution temperatures will also be reduced and this can change the relative retention times of peaks with diver-gent chemical characteristic of determined compounds.
The same effects, but to a lesser degree, will occur when constant column flow-rate mode is used (Chromacademy, online).
Following a very careful description of the theory of gas chromatography, a method translation software was developed for easy translating of operating conditions when carrier gas is changed (Snyder, et. al., 1992; Quimby et. al., 1995). Thanks to this, no complicated and time consuming search for new acceptable conditions for the separation with a new carrier gas is necessary. One of these forms of translating software is e.g. the GC Method Translation software free available on Agilent Technologies web sites (Agilent, online).
Using this software one can demonstrate the changing separation conditions for some beer flavors during switching from helium to hy-drogen or nitrogen carrier gas.
Tab. 1 shows the changes during analysis of volatile beer flavors on J&W DBWAX, 60 m, 0,32 mm, 0,25 µm column.
If the goal is to keep the retention time constant, it is necessary to maintain the same linear velocity for all carrier gases. In this case the inlet pressure decreases for hydrogen carrier gas about half as much as with helium. But the nitrogen average velocity of 36.4 cm/s is above the minimum equivalent of column plate, as can be shown from van Deemter’s curve (Fig. 1). So a decrease of efficiency can occur when nitrogen carrier gas is used.
Tab. 1 Chromatographic conditions for separation of selected volatile beer flavours on J&W DBWAX, 60 m, 0.32 mm, 0.25 μm column after switching from helium to hydrogen or nitrogen carrier gas in two model modes – A) constant retention times; B) “translation” of original method by GC Method Translation Software in “translate only” mode
Tab. 1 also includes the results of calculated parameters of the original method “translated” by GC Method Translation Software. “Translated” hydrogen average velocity is about 1.5 times faster than with helium but nitrogen average velocity is about 2.3 times lower than with helium. So, by using the calculated temperature program a 1.5 times faster analysis could be obtained by using hydrogen carrier gas in comparison with helium carrier gas. On the other hand, using nitrogen carrier gas leads to a 2.3 times slower analysis.
Faster gas chromatography has recently became more popular. The changes in separation conditions of selected low volatile beer flavors on a faster gas chromatographic column J&W DB-WAX, 10 m, 0,18 mm, 0,18 µm are shown in Tab. 2.
The same average velocities for all carrier gases are necessary for the unchanged retention times, specifically 84 cm/s. Due to using column with 0.18 mm internal diameter, van Deemter’s curves become more flat and this high linear velocity can be applied for helium or hydrogen carrier gas. But this high linear velocity cannot be recommend-ed for nitrogen carrier gas without a strong efficiency degradation.
Another part of Tab. 2 demonstrates the results of calculated parameters of “translated” original methods using GC Method Translation Software. Due to the shallow slope of van Deemter`s curve for hydrogen for this column internal diameter it is possible to use high linear velocity (129 cm/s) for hydrogen carrier gas. So, another acceleration of analysis is reached (1.53 times relative to helium). When nitrogen carrier gas is used, linear velocity of 39 cm/s is in the optimal area of separation column efficiency but the run time increases about 2.1 times in comparison with helium carrier gas.
Chromatographic conditions for separation of selected low volatile beer flavours on J&W DB-WAX, 10 m, 0.18 mm, 0.18 μm column after switching from helium to hydrogen or nitrogen carrier gas in two model modes – A) constant retention times; B) “translation” of original method by GC Method Translation Software in “translate only” mode
Hydrogen is the best choice when switching from helium to an-other carrier gas in gas chromatography. Due to the advantageous course of van Deemter’s curve hydrogen provides the highest separation column efficiency at the highest linear velocity in comparison with helium or nitrogen. Hydrogen therefore provides faster analysis at lower inlet pressures than any other compared carrier gases.