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GC Connections columnist, John Hinshaw, answered some of the concerns of LCGC Europe readers on switching from helium to hydrogen selected from the CHROMacademy Forum. If you want to know more about this topic please revisit the CHROMacademy webinar http://www.chromacademy.com/Translating-GC-Methods-from-Helium-to-Hydrogen-Carrier-Gas.html. An extended version of this interview will be published in the next issue of The Column.
GC Connections columnist, John Hinshaw, answered some of the concerns of LCGC Europe readers on switching from helium to hydrogen selected from the CHROMacademy Forum. If you want to know more about this topic please revisit the CHROMacademy webinar http://www.chromacademy.com/Translating-GC-Methods-from-Helium-to-Hydrogen-Carrier-Gas.html. An extended version of this interview will be published in the next issue of The Column..
Would you describe the helium shortage as a crisis or is this hype in the media?
JH: The current helium shortage is real and not just hype. Many contractual helium consumers have received letters from their suppliers in the past few months claiming a 'Force Majeur' condition that prevents the delivery of contracted quantities of instrument-grade helium. This in turn has affected laboratories' ability to produce results and manufacturers' ability to test instruments in production.
Will helium supplies ever return to “normal” levels?
JH: Within a year or two, as the new worldwide capacity for helium production becomes fully available, the supply shortage should abate. Don't expect prices of high-purity helium to drop down to the pre-shortage levels seen before 2006, but some relief should be in the works.
Can most methods be transferred to hydrogen? Are there alternative to hydrogen for specific applications?
JH: Yes, hydrogen carrier gas is suitable for almost all GC methods, except of course for the analysis of hydrogen as a component in a mixture. The GC method has to be converted to conditions suitable for hydrogen, and detectors may require special consideration. For example, a flame ionization detector (FID) also uses hydrogen as a support gas; total FID hydrogen flow should be kept constant and equal to the manufacturer's specified amount. Some electron capture detectors (ECD) support hydrogen carrier gas while others may not; it is best to consult the manufacturer in this case. Thermal conductivity detectors (TCD) will function well, but the size of the peaks will not be the same as with helium carrier.
Hydrogen has a much lower diffusion coefficient than helium. How does this affect the vacuum of a GC–MS instrument?
JH: Higher vacuum pump capacities are needed to obtain the same source pressure at the same hydrogen flow rate as for helium. Often it may make good sense to change to a narrower i.d. column with a lower flow rate when converting GC–MS methods. Some manufacturers of mass spectrometric detectors have recommended minor hardware updates for the best performance with hydrogen carrier. Consult them to see what might be required for your specific model.
How do I get equivalent retention times for my analytes when swapping from helium to hydrogen?
JH: Fortunately for gas chromatographers, the chemical nature of the carrier gas does not affect selectivity. The viscosity of hydrogen is roughly half that of helium, however, so it takes about half the pressure drop to drive the same flow rate or linear velocity with hydrogen through a given column as required for helium. With electronic pressure control (EPC), choose the correct carrier gas type, then set the average linear velocity. The EPC system will calculate the correct pressure drop to achieve the desired velocity, and retention times will be the same for isothermal methods. If temperature programming is used then it gets more complicated, as detailed in the next question. Some short or wide-bore columns may require too low an inlet pressure than is required for the best retention time and area stability.
Will swapping from helium to hydrogen affect the selectivity of my separation? If so, what can be done about this?
JH: Selectivity for peaks of divergent polarities can shift when changing carrier if temperature programming is used for the separation. Care should be taken to ensure that the linear velocities, temperatures and temperature ramps are selected appropriately. It is best to use method translation software to determine new conditions that will yield closely comparable separations. Any method that is adapted for a different carrier gas should receive thorough validation, documentation and review.
Will swapping to hydrogen carrier affect the sensitivity of my method when using a) FID b) ECD c) TCD detection?
JH: Yes and no. A FID, if operated with the same total hydrogen flow, should not be affected. An ECD still requires helium or argon/methane working gas and, if designed for hydrogen carrier applications, its sensitivity may not be affected significantly. In any case, consult the manufacturer for their recommendations. The sensitivity of a TCD, however, is affected by the carrier gas. TCD response depends on a change of thermal conductivity between pure carrier carrier gas and a mixture of carrier gas and a peak as it is eluted. Since hydrogen has about 1.2-times higher thermal conductivity than helium, peaks other than hydrogen or helium would be expected to be that much larger with hydrogen carrier, as long as pneumatic conditions were adjusted so that the peak shapes and positions were the same as with helium.
I've heard hydrogen is explosive in air. How do I convince my HSE team that I don't present a major incident risk?
JH: The small risk of a major incident with hydrogen arises from the total mass of hydrogen stored in an enclosed area. This risk is the same for a tank of hydrogen without regard for its application as FID flame gas or as carrier gas. The risk of personal injury from moving a tank is much greater. If the risks from compressed gas tanks are unacceptable then install suitable gas generators.
Other risks include the accumulation of an explosive mixture inside the column oven, the release of hydrogen at split vent flow rates, and the accumulation of hydrogen in mass selective detectors (MSD). Electronic pressure control (EPC) is highly recommended with hydrogen carrier gas. EPC will detect serious gas leakage from a broken column connection and will interrupt carrier flow in that condition. Also, more recent GCs include flow-limiting hardware in the carrier lines that will reduce the amount of hydrogen that could build up. In-oven hydrogen detectors with a cut-off relay are available options.
Hydrogen flowing at 300 mL/min from the split vent will dissipate into the surrounding air quite rapidly. Nonetheless, always use caution as even a small static discharge on a dry day could ignite flowing hydrogen. Directing the split vent flow to a vent hood or snorkel is good preventative practice.
For a mass selective detector, little hydrogen will accumulate in the vacuum roughing pump oil. However, hydrogen carrier gas flow must be cut off before powering down a mass spec detector to avoid filling the detector and pumping system with hydrogen. And always purge and pump down a MSD before starting carrier gas flow.
A special webinar on the helium shortage — Translating GC Methods from Helium to Hydrogen Carrier Gas – was presented by John Hinshaw and Tony Taylor on our CHROMacademy e-learning platform on 18 October 2012 (11:00am EST /16:00 BST). Listen here http://www.chromacademy.com/Translating-GC-Methods-from-Helium-to-Hydrogen-Carrier-Gas.html
If you have any questions relating to the helium shortage, please post them on the CHROacademy Forum using the link www.chromacademy.com/forum-cm-signup.html
An extended version of this article with more readers’ questions will be published in the next issue of our digital magazine The Column.