Due to the cost of helium and increasing supply limitations, some have estimated that it may no longer be available for use in the chromatographic market within a relatively short space of time. Gas chromatography (GC) laboratories are now looking at alternative carrier gases such as hydrogen and nitrogen as possible alternatives to reduce helium reliance.
In this SelectScience® roundtable discussion, now available on demand, a panel of experts discussed how the consolidation to helium as the GC carrier gas of choice arose. The panel also discussed why helium reliance may cause a significant impact on the analytical testing market, and how GC-MS methodologies can be adapted to use nitrogen as a carrier gas, without sacrificing analytical performance.
Read on for highlights from the Q&A, or watch the full roundtable discussion on demand, at a time that suits you.
CE: We have evaluated nitrogen as a carrier gas on an electron ionization (EI) mass spectrometer. The first thing that we did in the evaluation was look at the tuning compound, perfluorotributylamine (PFTBA), under different electron impact voltages. We started with 70 electron volts (eV) and then moved down to 10 eV. The idea behind that was to minimize the ionization in a similar way to an atmospheric pressure gas chromatography system where we have a soft ionization on a corona discharge with nitrogen flow going through.
The problem that we saw was that we didn't have the sensitivity needed. Not only did it affect sensitivity, but it also affected the ions above 219 m/z. We then looked at different flows translating from a 30 m x 0.25 mm column platform to a 20 m x 0.15 mm column platform. And if we look at that at 10 cm a second, that equates down to about 0.2 mL a minute. That flow into the source still appeared to be too high and created excessive source pressure, which caused those collisional effects with the compounds and lack of ionization in the source.
On the other hand, for GC-MS analyses with atmospheric pressure ionization (API), nitrogen is an excellent choice of carrier gas, and it does not result in a negative impact on sensitivity. If the chromatographic column is correctly scaled it is also possible to get the same quality separation as quickly as one would using helium.
FD: When we compare something like EI to an atmospheric pressure ionization source, the atmospheric pressure gas chromatography (APGC) system has considerably higher sensitivity, and so we're usually extending that to lower levels to either incorporate split injection or reduce sample preparation. And so, the capacity of the column is not something that we ever really encounter in practice when we're talking about the switch from an EI tandem to an APGC tandem system. We've done studies on a Restek semi-volatile column and have observed essentially equivalent responses under helium versus nitrogen even while scaling the column dimension. The loss in capacity is easily handled through the methodological parameters and allows us to take more advantage of the increase in sensitivity.
CE: If you're doing EI and your flow rates are very low, some compounds will look different, especially nitrogen-containing compounds, and most likely thermally labile compounds which are going to have effects on the collision with nitrogen in the source. What we've seen just from the initial evaluation is a drop in the match on the NIST/Wiley libraries from 95% confidence down to 75% confidence.
FD: Under APGC conditions, you do get a different spectrum. We tend to see the spectral match decrease. But unlike EI it's not as dramatic to the point where it makes the library unusable. The libraries are still useful because they can point you toward precursor product ions to look at. As this is not a technique that we've applied to single quadrupoles running in full scan mode, it hasn't made the development of a library a high-priority item, at least not yet.
FD: In general, secondary purification is almost always a good idea just before actual entry into the instrument, especially because there can be trace levels of compounds that get into the lines even when changing cylinders. It's always a good idea for the carrier gas to have some sort of filtration system on there.
JZ: Purification is always important. We have combination filters that we use for mass spec. Those are filters that remove oxygen, water, and carbon impurities. It's always recommended to use those combination filters in front of a mass spec as you always get better results.
Want to learn more about how GC-MS methodologies can be adapted to use nitrogen as a carrier gas, watch the full on-demand roundtable discussion here>>
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