Traditionally, helium has been used as the carrier gas for gas chromatographic (GC) separations because it provides good separation efficiency, is inert, and is safe to use. Recently there has been a lot of interest in switching to hydrogen as a carrier gas for GC separations due to helium supply and cost issues. Hydrogen is well suited for use with GC and is even more efficient than helium, but until now, its reactivity and safety concerns have kept it from being the first choice.
Due to these issues, GERSTEL recommends the following steps when considering switching from helium to hydrogen as a GC carrier gas:
- Perform a helium use audit
- Institute helium conservation methods to reduce use by up to 95%
- For GC only – Use nitrogen as carrier gas if GC method has excess resolution
- For GC – Use hydrogen as carrier gas if the GC method does not have excess resolution
- For GC-MS – Use hydrogen carrier gas as a last resort
- For GERSTEL TD systems – Make sure all hydrogen flows can be vented properly
A graphic showing the decision pathway for using hydrogen as a GC carrier gas is shown here:
Special considerations for Switching to hydrogen when a mass spectrometer is used.
- Existing methods developed using helium must be re-validated for every analyte and matrix
- If unknown (non-target) compounds are to be determined, there is significant risk of inaccurate results due both to hydrogen’s reactivity and to much lower library match quality
- Background noise is increased, especially for existing systems. Hydrogen efficiently flushes out chemicals accumulated on surfaces over months of operation. It can take weeks of purging and costly service calls before the system is ready for routine analysis operation.
- The recently introduced Agilent® Technologies 5977C MSD has specifications for Hydrogen. Spectral quality in combination with hydrogen is improved when using a larger extractor lens aperture, which unfortunately also exposes the quadrupole to more contamination
The table below lists some pros and cons of using hydrogen carrier gas.
|Inexpensive, readily available carrier gas||Sensitivity decreases up to 30 % (MSD); no MSD sensitivity specs for H2 carrier for existing instrument versions|
|Ionization source requires less maintenance||NIST database does not contain spectra obtained with hydrogen carrier gas leading to incorrect search results|
|Average 1.5x higher throughput due to higher optimal flow velocity with constant partition height and pressure||Extensive safety precautions and user knowledge required|
|No retention shift if constant linear velocity is used||MS must be re-tuned after changing the carrier gas; some target tunes (e.g., DFTPP) may not work at all with H2|
|Use of gas generators eliminates need to connect, handle, resupply gas cylinders||Increased compound degradation due to Hydrogen reactivity can lead to incorrect results|
|Converting an existing GC-MS system to H2 carrier gas typically results in significant background signal. This can render the system inoperable for weeks, requiring extensive servicing|
Always follow the GC, GC-MS, and TD manufacturers’ safety instructions for using hydrogen carrier gas. In all cases when using hydrogen, adhere to the rules and regulations that apply in your area and establish adequate ventilation and safety procedures.
For more information, and if you have questions pertaining to changing the carrier gas used for your analysis and for your instrument setup, please contact your local GERSTEL representative.