Considerations for Switching from Helium to Hydrogen for Gas Chromatography Carrier Gas

As the cost of helium increases and its availability decreases, an increasing number of Gas Chromatography (GC) professionals are considering switching to hydrogen gas for their carrier gas.

Hydrogen is an inexpensive and more efficient alternative carrier gas. It also offers GC practitioners the ability to speed up their processes without sacrificing quality.

Run Time

The carrier gas a GC practitioner chooses can have significant influence on analysis speed. The speed depends on the column pressure drop (p_d). When p_d is compared with the lower outlet pressure (p_o) typically in a shorter column, the optimum average linear velocity (u_opt) is proportional to the molecular diffusivity (D) of a solute in the gas (2).

As Table I shows, at a low p_d, helium is 20 percent slower than hydrogen. And, at a high p_d compared to p_o, in a narrow, longer column in which speed is a crucial aspect to the performance of the separation, helium is relatively much slower than hydrogen.

Table I: Relative speeds of analysis based on D for typical carrier gases at low pd (run time is inversely proportional to the speed) (1)

Column Efficiency

Hydrogen carrier gas has the greatest column efficiency. The most efficient columns allow analytes to spend optimal time in the stationary phase at u_opt, measured by the height equivalent to a theoretical plate (HETP). HETP specifies the column length necessary when the partitioning of analytes between the carrier gas and the stationary phase is at equilibrium.

Hydrogen has the lowest HETP of any carrier gas. This can be plotted against u_opt on a Golay curve (2), where optimal linear velocity is specified at the point where the curve is the lowest. On such a curve, hydrogen produces the flattest curves compared to helium (Figure 1), thus hydrogen can operate at a higher optimal linear velocity than helium, without sacrificing HETP.

Figure 1: Helium and hydrogen Golay curves on a 0.1 mm i.d. column (2)

Results: Improved Peak Shape and Resolution, in Less Time

Better speed and efficiency means better peak resolution (narrower peak width) and improved peak shapes in less time. This is illustrated when separating polynuclear aromatic hydrocarbons (PNAs) (3). Comparing helium and hydrogen carrier gases in a pair of experiments, hydrogen creates a narrower peak width (Figure 2) and, during the critical separation of a PNA, peak shapes are improved with a hydrogen carrier gas.

Figure 2: Improved separation of benzo(g&h)flouranthene (A) and improvement of peak shape for Indeno(1,2,3-cd)pyrene and benzo(g,h,i)perylene (B) (3)

References

(1) International Organization of Standardization, Basic Considerations for the Safety of Hydrogen Systems, ISO/TR 15916:2004(E)

(2) E.N. Fuller, P.D. Schettler, and J.C. Giddings, A new method for prediction of binary gas phase diffusion coefficients, Ind. Eng. Chem. 58, 19–27 (1966).

(3) M.J.E. Golay, Gas Chromatography (Butterworths, 1958).

(4) J. Butler, N. Semyonov, and P. O'Brien, Fast GC-MS Analysis of Semi-Volatile Organic Compound: Migrating from Helium to Hydrogen as a Carrier Gas in US EPA Method 8270, p 4 (Thermo Scientific, 2013).

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