Analysis of noble and permanent gases on adsorbent columns

Predicting selectivity characteristics of adsorbent columns is often a wild guess. Yes, we know argon will have less retention than krypton, but how well do adsorbent columns separate noble gases from permanent gases? Can neon and hydrogen be resolved using a porous polymer column or do we have to use more retentive columns, like Molecular Sieves and ShinCarbon? To give the definite answers, we mapped out the selectivity of some of our adsorbent columns for noble gases relative to permanent gases.

The column with the least retention, Q-Bond, separates noble gases from each other, however, helium and hydrogen coelute with neon, and argon elutes at the same time as the main air components oxygen and nitrogen.

Figure 1: Chromatogram showing the noble gases and permanent gases using the Q-Bond column.

 Better performance and resolution was observed on the MSieve 5A column.

Figure 2: Chromatograms generated using the Molesieve 5A column with (A) nitrogen and (B) helium as carrier gases.

When developing the analysis method on the MSieve column, krypton exhibited some interesting behavior. When the starting temperature is isothermal at 30°C krypton elutes from the column before the nitrogen. Increasing the starting temperature of the analysis or, like in the above example, using an aggressive oven ramp will switch the elution order of those compounds where krypton elutes after nitrogen. When developing a method on the MSieve column do not allow the starting temperature of the analysis to be your “kryptonite” and be sure to adjust it (see Figure 3).
Figure 3: Three different isothermal starting temperatures illustrating elution order changes between nitrogen and krypton.

 ShinCarbon columns offer high retention for gases, similar to the MSieve 5A. However, the ShinCarbon column will also elute carbon dioxide. It's main limitation is the inability to adequately resolve argon from oxygen and methane from krypton.

Figure 4: ShinCarbon column chromatograms using both nitrogen (A) and helium (B) as carrier gases. The limitations of this column are poor resolution of argon/oxygen and methane/krypton.

 Before selecting the column that will fit your analysis needs, lets "change” into a superhero of gas analysis on adsorbent columns with a few very helpful tips previously posted in our blog.

• Overloading on an adsorbent column will show as tailing, with the overloaded peak starting at earlier retention time. The end of the peak will remain at the same retention time. For example, if your analysis goal is to determine impurities in neon and the targeted impurity among others is helium, no matter which column you use, the overloaded neon peak will mask the peak directly preceding neon.
• Select a carrier gas which is the same as the balance (matrix) gas. This way your balance gas will “not be detected” and lower concentration compounds eluting before the overloaded peak can be seen. In the case of the analysis of trace impurities in neon, using neon as carrier gas (1) can be an interesting solution.
• Are you using a sample loop to transfer the sample onto your column? Optimizing the ratio between the carrier gas flow rate and sample loop size will reduce the time for the sample to be transferred on to the column which will minimize peak broadening. The peak shape of early eluting compounds is strongly affected by sample transfer on to the column. Late eluters spend more time in the column, and therefore, have time to refocus. Decreasing sample loop size and increasing column flow are only two obvious solutions. Also, consider using pulsed injection or split injection to decrease the time it takes to transfer the sample from the sample loop onto the column.
• For gas injections, often the best option are the 0.53mm ID PLOT columns. They allow maximum injection volumes and provide the smallest injection error.

Literature: 1. Analysis of trace impurities in neon by a customized gas chromatography, J. Chromatogr. A.,1463 (2016), pp. 144-152