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It’s a confusing time for GC/MS analysts

16 Dec 2012

I don’t know about you, but the more information I hear about switching from helium to hydrogen as a carrier gas, the more confused I get. When the helium shortage started, there were articles stating that this was a minor hiccup in the processing/supply chain, and capacity & inventories would be restored by the fall. Well, winter is (almost) here and supplies are still tight.

Then came along articles stating that we all were using much more helium than was capable of being produced, and it should be rationed to those industries whose livelihood depended upon it. Many scientists began calling for the end of helium-filled balloons, and other non-essential uses.

Eventually, slight panic set-in among chromatographers, and laboratories started to consider switching from helium to alternative GC carrier gases. The gases which were considered were nitrogen, argon, and hydrogen.  Unfortunately most mass spec manufacturers will adamantly state that you do not want to use either nitrogen or argon as carrier gas because sensitivity will suffer (due to the large size of the nitrogen and argon molecules), so the only option is hydrogen.

At that time, most articles focused on the positives of hydrogen (faster analysis time, more column efficiency, etc…) with the only negatives I remember reading about were:

A. Safety (as we all know, hydrogen is combustible).

B. If you were using old copper tubing to plumb the carrier gas, it was recommended to switch to new, clean stainless steel tubing (supposedly hydrogen can make older copper tubing brittle). If you were already using stainless steel tubing, but with other gases, it was recommended to replaced the used tubing with new, clean stainless to eliminate the “scrubbing” effect of hydrogen.

OK, so when switching to hydrogen, make sure you do not have any leaks and safely vent the carrier gas into vents/exhausts – check. Don’t use old copper tubing – check. Don’t reuse stainless steel tubing which was used for helium or other gases – check.  All will be well.

Things actually seemed to be headed in the right direction and having hydrogen as a substitute carrier gas seemed like a done-deal. For most GC detectors, it probably was a done deal, but then came along several issues involving mass spec. Customers reported that right after the switch from helium to hydrogen, their baseline was very noisy, and ghost peaks filled their chromatogram. Others reported loss of compound sensitivity, and some reactive pesticides were disappearing altogether. Still others reported that compound peaks were tailing, and column lifetime decreased. Older instruments seemed to have the majority issues, with several not even able to pump-down properly. But, when they switched back to helium, everything returned to normal. Now it started to look like this was not going to be an easy switch.

I am happy to see that instrument manufacturers are taking the helium shortage seriously, and have provided webinars and other useful information to help customers make the switch to hydrogen. Overall, I feel they have done an excellent job. If you haven’t watched any of their webinars, or read any of their published information, I encourage you to contact your instrument manufacturer to see what they have available.

However, several important questions continue to go unanswered. Maybe I’m just an impatient person, and maybe I’m just too detailed-oriented for my own good, but there are a lot of “Why’s” that I still need answered. Maybe, hopefully, that will change in 2013. Below are the most common questions I hear, but still don’t know the answer.

1. Why do they recommend against using methylene chloride as a solvent? I have heard that HCl vapors can be produced in the injection port, and if moisture is present, even trace levels of hydrochloric acid. If this is true, I would think that every instrument would be negatively affected. Why have I only heard about this using mass spec? I have also heard not to use carbon disulfide as a solvent because an end-product (if moisture is present) can be sulfuric acid. Can the same be true for other chlorinated solvents (like chloroform) and sulfur-containing solvents (like dimethyl sulfoxide and carbon disulfide)?

2. It has been stated that no gas traps should be installed on the hydrogen carrier gas line to further purify the gas – why? I have heard of a “scrubbing effect”, but what exactly is getting scrubbed out of these traps (even ones designed for use with hydrogen)?

3. I’ve heard that fragmentation patterns will be different from those published in the NIST library. I have also heard that fragmentation patterns will be the same as those published in the NIST library. If the patterns are different for a particular instrument, is there any way to correct this through mass spec tuning (or other methods), so they once again match the NIST library?

4. I have heard you should use metal columns in your GC (to prevent breakage). I have heard that you should not use metal columns (because of possible arcing issues within the source). Are there certain models of mass specs where arcing is a problem? If so, I would like to know which models.

5. I have heard that injection port liners need to be replaced every day. While this may already be common for certain analysis, I’m getting the impression this will need to be done for all analysis when one switches to hydrogen. If so, why? What is occurring in the injection port that damages the inertness of a liner so quickly?

6. What exactly is getting “scrubbed” from the gas lines, source, injection port, ect… when one switches to hydrogen? I have heard that the fragmentation pattern resembles hydrocarbons. So, is it really hydrocarbons getting scrubbed from the gas lines, or something else?

7. What is causing peak tailing? If there are no leaks, and typical linear velocities are faster with hydrogen, why are certain peaks tailing?

Believe it or not, the last thing I planned was to write yet another post on switching from helium to hydrogen as a carrier gas until I had answers to the questions listed above, but hearing the frustration from our customers has been a difficult pill for me to swallow, because as a member of tech service, it’s my job to have answers. Honestly, I’m not interested in hearing statements like “Here is what I think is happening”, or “I have heard that this is happening”. I am only interested in the cold, hard facts, and the data to back it up. I do realize that no one may have these answers yet, but if that’s the case, just tell us. I would rather have someone state in a webinar “Don’t do that, but we don’t yet know why”, than simply “Don’t do that”.

So in summary, I (we) do feel your frustration. Hopefully we will get some answers soon and end this confusing time for GC/MS analysts. Thanks for reading.