Investigating Nitrogen as a Substitute for Helium in Gas Chromatography for POP Detection

Scientists from the Environmental Monitoring Division of Los Angeles in California are looking into using nitrogen as a substitute for helium in detecting persistent organic pollutants (POPs) in gas chromatography (GC). The team of researchers published their findings in the Journal of Chromatography A (1).

Low-poly nitrogen gas icon illustration is constructed with chaotic filled triangles. Triangulated nitrogen gas polygonal 2d vector illustration. Nitrogen Gas icon is filled with triangles. | Image Credit: © Viktor - stock.adobe.com

Recently, helium (He) gas supply has become inconsistent and unreliable. According to Peak Scientific, only three sources produced 75% of the world’s helium supply in 2018, leading to a 135% increase in helium prices (2). This presents a problem in several major industries including gas chromatography research, in countries like China, Japan, and the United Kingdom (UK) facing challenges. The world’s helium supply could run out within a decade and plans for preventing another helium shortage are in motion. However, the costs involved in programs for helium recycling have proven to be extremely expensive for several businesses relying on helium for their daily processes.

As such, entities that rely on helium as a mobile phase for GC in monitoring persistent organic pollutants (POPs) in their operations, such as wastewater treatment plants, are searching for reliable alternatives. Some proposed mobile phase alternatives include ammonia (NH₃), dihydrogen (H₂) and dinitrogen (N₂). For residual organochlorine pesticide in wastewater treatment-related samples (influent, effluent, brine and biosolids), a GC system coupled to a pair of halogen-specific electron capture detectors (GC-ECD) is preferred. This is because a GC-ECD system can identify and quantify POPs at the part per billion (ppb) levels that are normally associated with wastewater treatment samples. While this can lead to limited and nonlinear responses, GC-ECD systems are uniquely suited for this role for sensitivity and selectivity towards electronegative elements.

N₂ has been used as a make-up gas in GC-ECD systems for quite some time, and the lower N₂ diffusion rate relative to He, which could lead to poor peak separation, can theoretically be compensated for by reducing N₂ flowrate optimally. For this study, N₂ gas was evaluated in this study as a potential substitute for He in quantifying organochlorine pesticides, polychlorinated biphenyls, chlordane congeners and toxaphene in wastewater treatment related matrices (influent, effluent, benthic sediment, and biosolids/sludge). According to the scientists, “N2 is inert, inexpensive, and requires no additional hardware to incorporate into the basic functions of a GC-ECD” (1).

Following the experiment, the scientists concluded that N2 can substitute He as a carrier gas in GC-ECD analysis of five classes of regulated environmental contaminants (single components organochlorine pesticides, chlordane congeners, technical chlordane, toxaphene, and PCB as Aroclors), regardless of column chemistry and what instrument is used. This conclusion was reached based off several points, including high precision and similarity of retention time between He-based and N₂-based data (low and similar standard deviations), the consistency of N₂-based retention time once the flowrate and oven temperature programs are optimized, and the little difference between He- and N₂-based concentrations of targets commonly associated with the wastewater treatment analysis. Nitrogen, in addition to being safer than H2, the other He alternative other laboratories are considering, is also cheaper, since it can be continuously generated onsite using commercially available N₂ gas generators. However, the scientists did note that a potential downside is longer analysis times, though they claim this is mostly a non-issue in practical terms. More research must be completed, with the scientists saying this can help determine if there is a universal relationship between, He and N₂-base retention times under conditions encountered in a GC analysis, such as different column chemistries and various temperature programs and flow rates.

References

(1) Phan, K.; Afifiyan, N.; Li, L.; Soto, O.; Juma, T.; Otim, O. Nitrogen Can Substitute Helium as a Mobile Phase in the Analysis of Wastewater Treatment Matrices for Persistent Organic Pollutants by Gas Chromatography – Electron Capture Detection System. J. Chromatogr. A 2024, 1728, 465018. DOI: j10.1016/j.chroma.2024.465018

(2) Solutions to the Helium Shortage for Gas Chromatography Applications. Peak Scientific Instruments 2024. https://www.peakscientific.com/helium-shortage/ (accessed 2024-6-13)