Published Ryan Schonert on November 25, 2019
Last time, we established that helium can be replaced with hydrogen or nitrogen, but that change comes with some particularly important considerations. We had a hunch that GC-VUV might have some distinct advantages here, so we conducted an experiment comparing different gasolines run on different carrier gases – for more experimental details, check out Part 1. Let’s take a look at hydrogen first. The relevant run conditions for helium and hydrogen are listed in Figure 1.
Figure 2 shows some chromatographic comparisons between a few of the runs for sample RFG 1711. Each sample analyzed had very similar chromatography despite the different method conditions and different carrier gases. Although the SOF methods do sacrifice resolution for decreased runtime, causing some peaks to be compressed and coelute, GC-VUV’s ability to spectrally deconvolve coelutions allows us to maintain accurate PIONA data. For example, the peaks highlighted in Figure 3 lose chromatographic resolution when speed optimized flow conditions are used. However, even though we’ve lost some resolution, we’ve bought that resolution back using spectral deconvolution, as shown in Figure 4.
Table 1 shows these results numerically. Not only do the pure translations give comparable numbers, the SOF and forced 33.6 runtime methods also give similar data.
If your lab has the capacity to use hydrogen as a carrier gas, it may well be worth it, as you can get comparable results in a same or shorter runtime than a method using helium. Although hydrogen can’t yet be used for ASTM D8071 fuel certification, it can be used for R&D and other steps preceding fuel certification.
As I mentioned in Part 1, hydrogen has a couple of caveats to consider before switching. One of them is safety, as hydrogen is flammable and can be dangerous. However, technology has improved by leaps and bounds since the old stories of GC ovens exploding from hydrogen leaks. There are several gas sensors and gas delivery systems that prevent the buildup of hydrogen to the point where an explosion is possible, so with some careful planning, hydrogen can be used quite safely. The other caveat is detector compatibility: thankfully, GC-VUV seems to be fully compatible with hydrogen! Because GC-VUV operates at atmospheric pressure, we don’t need to worry about pumping hydrogen out like a mass spectrometer does, and hydrogen is spectrally invisible in the VUV range, so we can use it without spectral interference. Although we haven’t done any long-term studies looking at hydrogen’s effects on the VGA-100 or VGA-101, we currently believe it’s a viable replacement for helium given the proper precautions. However, if you are still uncomfortable using hydrogen, nitrogen is also an option. Stick around for Part 3, and we’ll take a look at our nitrogen test results!