The majority of laboratories utilize helium as the carrier gas for gas chromatography. Many gas chromatographers believe that helium is the gold standard for gas chromatography as it is safe, inert, and provides reasonable run times.
At the end of the day, laboratories must be profitable in order to survive. Shortages of helium are affecting laboratories everywhere.
Helium is a widespread necessity, but there is not enough to go around. Numerous laboratories have to cope with increasing prices, and some find it difficult to source any helium at all.
Nitrogen and helium are two viable alternatives that are already available. Each of these gases are comparably inexpensive to produce. With some work, a peak resolution that is similar to helium can be achieved. This may lead one to ask why laboratories have not made the switch already, but there are additional factors that must be explored first.
All three gases, nitrogen, hydrogen, and helium, can attain an equal peak resolution, but they achieve peak separation at individual speeds. The example in Figure 1 presents a gasoline sample run on three distinct carrier gases.
Figure 1. Gasoline sample run on helium (top), hydrogen (middle), and nitrogen (bottom). Using the different carrier gases, we can get the same chromatographic peak resolution, but we need different amounts of time to get this resolution. Image Credit: VUV Analytics
While the chromatography appears the same, each gas has taken an individual amount of time to attain it. Figure 1 shows that nitrogen is the slowest, helium is in the middle, and hydrogen is the fastest.
Much more time would be required for each run if nitrogen was the gas of choice. As the runs will be more extensive, the peaks are wider and shorter, resulting in a loss of sensitivity.
The run could be manually sped up by modifying the oven conditions and flow rate. This would decrease the run time and enhance sensitivity, but resolution would be lost as a result of this.
Hydrogen has a clear advantage in regard to its run time. It does have its own limitations, and the most crucial of these is safety.
Hydrogen is flammable, unlike helium or nitrogen, which is of particular concern to partners and customers in the fuels industry.
Special precautions must be taken to avoid explosions in laboratories using hydrogen, particularly when cylinders of pressurized hydrogen are employed.
Not every detector can utilize nitrogen or helium. As an example, GC-MS is ideally suited to helium, and changing to nitrogen or hydrogen could cause issues.
For hydrogen, MS detectors (particularly older versions) may have issues with pumping down, which could negatively influence the quality of the mass spectra gathered.
It has been demonstrated that nitrogen reduces sensitivity as a result of an increase in ion scattering and its ability to ionize more efficiently than helium.
VUV Analytics wanted to discover if there were precautions that could be taken when running GC-VUV to stop any of these issues, in a way that other detectors could not.
A selection of techniques was formulated at first. Three basic methodologies were taken:
- Pure translations – The present ASTM D8071 technique conditions were used and run through a method translator to find the conditions for hydrogen and nitrogen.
- Speed-optimized flow (SOF) conditions – The method was translated to hydrogen and nitrogen under SOF conditions. This provides a more efficient run time with a minimal decline in resolution. These SOF conditions strongly decreased the run times, particularly for nitrogen.
- Forced ASTM D8071 run time – The method translator was used to find hydrogen and nitrogen conditions with a forced runtime of 33.57 minutes
These techniques were then used to measure 6 samples under ASTM D8071 conditions. The given conditions modified by the method translations are detailed in Figure 2.
Figure 2. The relevant conditions changed 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. Image Credit: VUV Analytics
- Produced from materials originally authored by Ryan Schonert from VUV Analytics.
This information has been sourced, reviewed and adapted from materials provided by VUV Analytics.
For more information on this source, please visit VUV Analytics.