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Is Hydrogen the Best Carrier Gas for GC?

Is hydrogen the best carrier gas?  And if so what benefits can we gain from using it?

The van Deemter curves (Figure 1) provide us with information on the optimum linear velocity at which a carrier gas can be used to provide the best efficiency.  These velocities are at the minima of the curves.  The curves are different due to the different molecular weights, viscosities, and diffusivities of the three gases.  N2 provides optimum efficiency when used at ~12 cm/s, He  at ~25 cm/s, and H2 at ~40-50 cm/s. 

One of the benefits of H2 is that it exhibits the widest range of optimum linear velocities at which it can operate without losing efficiency.  This is particularly important when we are using temperature programmed GC; we find that linear velocity will change at higher temperatures due to the change in gas viscosity (viscosity increases with temperature), at higher temperature the linear velocity will decrease unless we increase the pressure applied at the head of the column.  This change in linear velocity can affect efficiency, retention, and resolution of a separation. 

Constant flow mode can be used to remedy this problem.  However, the use of hydrogen is beneficial in that if we were not using constant flow mode we would not see much change in the separation as the temperature increases because hydrogen displays this much wider range of optimum velocities in comparison to the other carrier gases.

Figure 1: Van Deemter plot for nitrogen, helium, and hydrogen carrier gases.

Hydrogen is the fastest carrier gas, with an optimum linear velocity of 40 cm/sec, and exhibits the flattest Van Deemter profile.  Hydrogen’s high optimal linear velocity results in the shortest analysis times. Also, the wide range over which high efficiency is obtained makes hydrogen the best carrier gas for samples containing compounds that elute over a wide temperature range.

  • Increased speed: increasing the linear velocity allows for shorter run times, thereby increasing the throughput of your laboratory (Figure 2)
  • Lower temperature separations: at the faster elution times, it may not be necessary to increase the column temperature ramp rate. It may even be possible to lower the maximum temperature needed for the analysis
  • Longer column life: lower temperatures lead to less column bleed, which in turn can mean a longer column life. In addition, hydrogen is a reducing gas and can remove potential acidic sites off the column, further increasing column life
  • Availability: Hydrogen is readily available through the electrolysis of water and with gas generators it can be generated on demand
  • Hydrogen gas is already being used in the laboratory for a variety of purposes: It is the fuel gas for the most commonly used detectors (FID) and therefore already present in most GC Labs

Figure 2: Increased analysis speed with hydrogen vs. helium carrier gas.

The major concerns over using hydrogen carrier gas are safety and its inherent reactivity (especially with GC-MS methods).  In regards to safety, the use of high pressure cylinders is often the primary concern; however, as mentioned previously most laboratories which are currently using a GC-FID system will already be using hydrogen safely as the fuel gas.  Many of the explosion and manual handling concerns can be addressed by the use of hydrogen generators which produce hydrogen on demand via the electrolysis of water.  Finally, any explosion or leak risk can be remedied by having leak detectors installed in the GC oven (many modern systems have this as standard) as well as having detectors installed in the laboratory area.  It should also be noted that when using hydrogen as the carrier with a GC-FID system nitrogen (or helium) will be requires as the makeup gas.

Another concern is the use of hydrogen for GC-MS applications and EI library searching.    Hydrogen is a little under half as viscous as helium.  This makes it slightly more difficult to pump away for high vacuum equipment.  Secondly, hydrogen shows minimum plate heights at higher linear velocities than helium.  Thirdly, when the outlet pressure drops to almost zero (i.e. a vacuum) the flow of hydrogen into the instrument may be large.  A large volumetric gas flow into the instrument when using a direct interface will mean a lower vacuum level is attainable.  This may increase the number of background molecules which can collide with the ions formed, leading to a potential reduction in sensitivity and a change in the relative abundance of ions within the mass spectrum.  Care should be taken when analyzing fragile compounds, compounds at trace levels, and reactive compounds (alcohols, aldehydes, ketones). 

This all being said, if we use smaller internal diameter columns (0.15, 0.18 or 0.20 mm), higher linear velocities are achievable at lower volumetric flow rates – thus we can maintain the vacuum levels within the system and there should be no wholesale changes in the appearance of the spectra.  Hydrogen is a reactive gas whereas helium is not, and there is always an exception to the rule which states that we do not typically see ion/carrier gas reactions in GC-MS, however, one that regularly appears is the dehalogenation of chlorinated compounds.  There are also many applications in the literature which successfully utilize hydrogen carrier gas for GC-MS.

 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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