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.

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Figure 1. The relevant conditions altered for each method. The flow rate and oven programming were determined using a method translator. The acquisition frequency was determined by taking the number of scans in a traditional D8071 run and calculating a new frequency to give the same approximate number of scans in the new runtime.

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.

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Figure 2. Comparison of the RFG 1711 gasoline sample run under ASTM D8071 conditions with helium (top), translated D8071 conditions with hydrogen (middle), and a forced runtime of 33.6 minutes with hydrogen (bottom). The peaks in each chromatogram line up very well despite the differences in method conditions, which means that the data for each run should be similar as well.

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Figure 3. Comparison of a gasoline sample run under translated D8071 conditions (top) and SOF conditions (bottom) using hydrogen. While the peaks generally line up very well between these two runs, some peaks in the SOF run lost chromatographic resolution. Other detectors may not be able to distinguish these peaks very well, but GC-VUV can handle coelutions like these easily.


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Figure 4. Enhanced view of the coelution highlighted in Figure 3. Each compound can be individually identified and quantified despite decreased peak resolution from the SOF conditions.

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.

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Table 1. Data gathered for RFG 1711 gasoline using both with helium and hydrogen. D8071 – original ASTM D8071 conditions; SOF – speed-optimized flow conditions; D8071 trln – pure translation of ASTM D8071 conditions to hydrogen; 33.6 – forced runtime of 33.6 minutes to match the runtime of ASTM D8071. Although there are some slight differences between each of the runs, the data is overall very comparable, meaning that hydrogen can be used effectively as a replacement for helium.

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!

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