Can hydrogen be used as a carrier gas in a GC/MS?15 Nov 2012
Tech Service receives this question quite often, and the simple answer is yes; however, there are a few items to consider prior to connecting your instrument to a hydrogen line.
First, consult your instrument manufacturer. Some GC/MS manufactures recommend hardware upgrades, such as magnets, draw out lenses, and/or filaments. Additionally, there are some older model GC/MS instruments that are not capable of supporting hydrogen as a carrier gas. The manufacturer should be able to help you with your specific model.
After you know that your instrument is capable of using hydrogen, you are ready to plumb the instrument. Start with a source of clean hydrogen. A hydrogen generator is extremely economical and provides an (almost) endless supply of hydrogen, but cylinders will do. Use brand new chromatography quality stainless steel tubing to plumb the gas lines. Hydrogen has a “scrubbing effect” on tubing. If you reuse your old helium tubing, contamination may be an issue as hydrogen washes the crud out of the old lines. Copper tubing exposed to hydrogen can become brittle over time and break, so stainless steel tubing is a safer and more reliable option. As always, make sure you use a leak detector and check all of your connections.
Since the pumping capacity for hydrogen is less than helium, you may need to switch column dimensions when moving to hydrogen. In general, 20 m columns with a 0.18 mm internal diameter are a good place to start. You will likely need to increase the split flow while decreasing the column flow. There are some method conversion programs floating around the internet, which can be helpful in maintaining elution order and setting flow rates. As a warning, you may experience overloading because these smaller bore columns have less capacity. The solution is likely to decrease the linear range of your calibration curve or dilute your samples.
Solvent selection can also be important when using hydrogen. Although I have not seen any data, I have heard that methylene chloride and carbon disulfide can form HCl and H2S. Since hydrogen is not inert like helium, it does make sense that acids can form in a hot injection port as solvents vaporize. Previously, my colleague, Alan Sensue, discussed how extreme pHs can be a Capillary GC Column Killer. It doesn’t take much acid to destroy a column.
Title "Capillary GC Column Killers – Part 1"
In addition to the theory of creating acids in the injection port, hydrogen has the potential to hydrogenate some unsaturated compounds. For instance, styrene will convert to ethylbenzene in a hot injection port (250°C). By lowering the injection port temperature, this conversion can be reduced.
The key to successfully convert your GC/MS system from helium to hydrogen is to first do your homework. By far, the tips listed above do not cover everything to consider when switching. I haven’t even touch upon methods that have specific tune criteria which can be problematic when using hydrogen. There is still a great deal to learn/discuss in regards to fragmentation, spectra, sensitivity, etc.
I hope this post helps make your conversion to hydrogen a little easier. If interested, Jack Cochran also posted some very useful information in regards to using hydrogen as a carrier gas.
Title "GC-MS with Hydrogen Carrier Gas"