Regarding the three most commonly used carrier gases in GC, the Van-Deemter plot can provide valuable insights. This plot illustrates the height equivalent of a theoretical plate (HETP) in relation to the linear velocity of the gas. A smaller HETP value indicates a higher separation efficiency of the system.
Nitrogen exhibits the lowest HETP. However, its separation efficiency significantly declines at higher or lower flow velocities than the optimum. Nitrogen requires a longer time to achieve satisfactory separation results. Moreover, its low diffusion coefficient limits its applicability for time efficient routine analysis.
In contrast, helium has a slightly higher minimal HETP and attains optimum separation efficiency at a higher flow rate compared to nitrogen. Together with its inertness, this characteristic is often why helium is preferred in laboratory and routine settings.
Hydrogen, on the other hand, is comparable in terms of HETP with helium, but its unique properties make it an attractive alternative to helium. With its low viscosity and high diffusion coefficient, hydrogen can achieve much higher linear velocities maintaining excellent separation performance. Moreover, another advantage of hydrogen lies in its availability and the possibility of on-site generation using gas generators, reducing dependence on external suppliers.
However, the use of hydrogen does present challenges. Firstly, hydrogen is highly reactive and flammable, demanding specialized safety precautions in the laboratory. Secondly, the combination of hydrogen with GC-MS (mass spectrometer) can be problematic, as older GC-MS pumps may not be capable of handling the high flow rates required for hydrogen. Although new GC-MS system can manage these hydrogen flow rates, there is a significant loss of sensitivity (typically a factor of 3–10) when using hydrogen as a carrier gas in GC-MS.