Helium to Hydrogen — A Change Would Do You Good

May 19, 2015

Helium to Hydrogen - A Change Would Do You Good

 

Change is always a scary concept but let us dispel some of the fear associated with changing your helium carrier gas to hydrogen by answering some of the most common questions posed to us. John V. Hinshaw, our GC department editor has written a comprehensive article answering the questions below in great detail.  Here, we have highlighted the key points to help get you started. (For a more in depth discussion the full article can be read here »).

Performance
Hydrogen carrier gas has many advantages over helium or nitrogen. Higher plate numbers can be achieved at high linear velocities as well as being able to achieve higher linear velocities while utilising lower pressures.  Extra hydrogen in the system may; however, affect detectors that use hydrogen fuel gas (in this instance, FID) and MS detectors.

Does hydrogen carrier gas affect retention times?
Yes and no.  As linear velocity increases, isothermal retention times decrease in exact proportion, so doubling the velocity will reduce retention times by half. Three particular situations that need to be considered are:

  • Constant velocity - for isothermal operation at constant average linear velocity, retention times are not affected by changing carrier gas.

  • Constant inlet pressure - hydrogen carrier gas will reduce elution times by about half.

  • Constant flow rate - isothermal retention times will be unchanged with hydrogen compared to helium.

For temperature programming:

  • Maintaining constant linear velocity during the temperature program will maintain retention times.

  • With constant inlet pressure hydrogen will cause peaks to be eluted earlier, but their elution temperatures will also be reduced which can change the relative retentions of peaks with divergent chemical characteristics.

  • With constant flow rate a similar situation for constant inlet pressure will be observed, but to a lesser degree.

  • Always revalidate each peak in your chromatogram and optimise the temperature program ramp rate using method translation software (see you may also like below).

How can I manage hydrogen fuel flow in my flame ionisation detector with hydrogen carrier gas?
The simple answer is to reduce the hydrogen fuel gas flow so that the total hydrogen flow through the detector remains close to the manufacturer’s specified optimum flow – usually between 40 – 50 cm3/min.  If the column oven temperature drops during the run the flow rate may decrease.  With narrow bore columns this may be small enough that the ionization detector will remain within the optimum range, however, with wide bore columns the FID sensitivity may be significantly affected. Electronic pneumatics can be used to overcome this problem, by maintaining a constant total flow of hydrogen through the detector.
 

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