Wednesday, October 24, 2012

Helium Shortage and Hydrogen as a Carrier Gas for GC-MS

Aviv Amirav and Tal Alon, Tel Aviv University and Aviv Analytical.  

Another post on this topic, discussing a viable alternative, is also available here.

Recently the subject of helium shortage and the possible use of hydrogen as an alternative carrier gas in GC-MS became a hot topic in various forums, blogs and GC-MS vendor seminars. Thus, we wish to further illuminate this topic based on our experience. 

Is the helium shortage real? is it really that expensive and how much more expensive can it get? Can hydrogen or nitrogen serve as an alternative? What can we do to cope? Keep on reading to find out.

Helium shortage, is it a real and long lasting problem?        
The short answer is no. In general, some disturbances in the helium supply will continue to occur as helium from certain helium-rich gas wells dwindle while new wells are being developed. Helium has ample supply in the atmosphere in which it is found at 5.2 ppm (volume fraction) - thus, at worst it will be produced like all other noble gases from air. With the development of new deep-oil drilling technology, gas wells are being found at a higher rate, and since deep old oil is more likely to be associated with gas, these wells are more likely to contain helium due to production from millions of years of exposure to radioactivity. Thus, it is my opinion that we are far from depletion of helium sources, and at worst it will be produced from air.   

Helium price, is it expensive? 
Helium gas is not expensive. With the current price range of $10-40 per cubic meter, its cost is negligible compared to the salary of the GC-MS operator and/or depreciation of the GC-MS hardware. Even with gas saver setting at 20 ml/min and average usage of 28 ml/min 24/7 (considering a high split flow rate), a standard A1 helium cylinder of 10 cubic meters will last (without leaks) for 8 months. At a price of $300 per cylinder this is $1.25/day (<$1/day with 20 ml/min consumption). Currently, a hydrogen generator costs more than 20 years use of helium from cylinders, thus the hydrogen generator might be an economic solution only if it also serves for a GC with FID or few systems.   

How much can the Helium price go up?
If helium will be fully produced from air, a scenario that is not going to happen in the near future, we can safely assume that its price will be close to that of neon, which is found at 18.2 ppm in air, and thus helium cost will increase about 20 times. Although neon is 3.5 times more abundant than helium, the much higher needed quantity of helium is likely to reduce its cost of production to that of neon. Thus, we predict that in the distant future the price of helium will increase no more than 20 times and a current equivalent maximum price of $25/day is still far smaller than the GC-MS operator salary and GC-MS depreciation. Otherwise, the price will continue to be related to the price of energy (oil). Currently, the price of helium is mostly based not on the price of raw helium gas but rather on the price of energy required for its purification and cylinder shipping. Note that today the cost of certain MS grade LC-MS solvents is more than 10 times that of helium and no one complains. In addition, one can use the gas saver mode at 8 ml/min instead of 20 ml/min with minimal impact on performance.  

What is the best solution for short-term problems in the helium supply?            
The simplest and most effective way to cope with short-term helium supply disturbances is to purchase another spare cylinder (or two cylinders). Since a helium cylinder typically lasts for 8-12 months (if one GC-MS is used and without leaks), an additional spare cylinder should give ample time to replace the used cylinder and solve the He short-term supply disturbance.  

Hydrogen as a GC-MS carrier gas, what are the main problems with it? 
We used hydrogen in the past as a GC and GC-MS carrier gas and can indicate the following major problems with it: A) Hydrogen is a reactive gas and it degrades certain compounds without a simple indication to what these compounds could be. The major adverse effect of hydrogen is on the GC injector liner activation which can catalytically degrade samples such as certain simple pesticides. For a literature references please read our paper A. Amirav and H. Jing, Anal. Chem. 1995, 67, 3305-3318 (see Figure 13 and its associated text) and also G. Wells, J. High Resolut. Chromatogr. Chromatogr. Commun. 1983, 6, 651. B) Hydrogen reduces metal oxides at the ion source and exposes bare and highly active metal surfaces at the EI (and CI) ion source. Thus, many compounds are degraded at the ion source, lose their molecular ions and are harder to identify by the library. C) The obtained signal-to-noise ratio is poorer with hydrogen compared with helium via both lower signal and higher noise. D) Some people reported reduced filament lifetime with hydrogen use. E) The injector pressure for 1 ml/min hydrogen flow rate is somewhat too low due to the lower hydrogen viscosity (compared with helium), hence the column and method should preferably be changed and a column with smaller ID such as 0.2 or 0.18 mm should be used or longer solvent tail might be created. Thus, hydrogen is not recommended for use with GC-MS despite its contribution to faster analysis, accept in certain stable hydrocarbon analyses.  

Can nitrogen serve as an alternative carrier gas for GC-MS? 
The simple answer is no. While nitrogen is inert hence its use does not result with the activity problems of hydrogen, its main downside is that its ionization cross section is more than 7 times larger than that of helium, and thus ion source space charge problems (and scattering) significantly reduce the GC-MS sensitivity with nitrogen. In this regard 1 ml/min of nitrogen behaves like helium with a flow rate of 7 ml/min. PerkinElmer explored the use of nitrogen as a carrier gas in GC-MS and reported that it lowers the sensitivity (increases LOD) by a factor of ~20. Nitrogen also slows the analysis by about a factor of 1.5, and at the same method temperature program rate, the GC separation (peak capacity) could be reduced by up to a factor of ~1.2, which is tolerable for most applications.     
  
Aviv Analytical 5975-SMB GC-MS with Cold EI, how is its compatibility with hydrogen?  
The Aviv Analytical 5975-SMB GC-MS with Cold EI uses 60 ml/min helium as a make-up gas (for sample cooling in order to enhance the molecular ions) in addition to the injector use, thus it approximately triples the helium consumption when in use. Considering average Cold EI use of 12 hours per day, the overall average consumption about doubles. This doubled consumption cost of $2.5/day for He is negligible, while an additional spare He cylinder can solve short-term supply disturbances. However, with the 5975-SMB GC-MS with Cold EI one can uniquely bypass the helium shortage problem via the use of hydrogen as a make-up gas and nitrogen as the column carrier gas. In fact, the use of 8% nitrogen in hydrogen is gas dynamically equivalent (same average gas mass) to helium and can replace helium for sample cooling. Thus, we found that we can use N2 as the carrier gas with up to 5 ml/min column flow rate with only little effect on the sensitivity performance. With hydrogen make-up gas, the sensitivity per mA emission current is increased while the upper emission current that can be used is reduced due to both a change of the filament work function and space charge issues. Thus, the overall sensitivity and usability is practically unchanged. 

In conclusion
It is good to know about alternatives to helium but our advice is to stay with helium, and at most to purchase an additional spare helium gas cylinder if you suspect that its supply could suffer from short-time shortage periods.

6 comments:

  1. How can someone get 6 months out of a cylander? I go through one every 3.5-4.5 weeks. That is with only one inlet active at one time with the gas saver set to 15 ml/min and an additional 1-2ml/min for a MSD quickswap.

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  2. This doesn't make sense Steve, you probably have a leak somewhere.

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  3. What type of autosampler do you have? We had a Gerstel MPS with the added SPME unit. The SPME purge unit had a leak and we went through an He tank in a couple days. It must be a leak if it only lasts 4 weeks. Maybe you have a leak at the quickswap (I'm assuming this is a 3 way union as I have installed).

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  4. can one use helium cylinder till the internal pressure of the indicator of the regulator reaches to 20 kg/cm or should i just stick to 30 kg/cm and then change to a new stand by cylinder.

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  5. Personally, I see no harm in letting the pressure drop to 20atm before replacing a cylinder, but you don't gain much by doing so.

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  6. It's interesting to read the real reason why nitrogen is not used for GC/MS

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