Common light gas coelutions when using a GC packed column18 Oct 2023
Over the years we have worked with many customers on their packed column gas analysis troubleshooting requests. In some cases, customers were having coelution issues with several of their gases and were hoping that Restek could help. For many of these requests, a different packing was all that was needed to achieve the required separations. In other cases, substituting a similar but more efficient column, such as using a PLOT (Porous Layer Open Tubular) column in place of a less-efficient packed column, resolved the issue. However, achieving certain separations was not as simple as switching GC columns and may have required multiple columns connected to a column switching device, like described here Restek - GC columns – when one is not enough
Below are several of the more common coeluting gases customers ask us about. Hopefully after you have reviewed this post, you will not only be aware of common gas coelutions when using packed columns, but also how you may wish to proceed if you are experiencing these coelutions.
One of the more popular requests we receive is a column recommendation for separating argon and oxygen. A molecular sieve 5A is a popular choice of packing because these two gases will very likely coelute using all other packings. Even though this separation is possible using a 5A packed column, often a very long column and/or cryo-cooling is needed. That is why we recommend a 30-meter Rt-MSieve 5A PLOT column for this difficult separation instead of a packed column, as mentioned in the link below. The additional theoretical plates you will obtain by using a PLOT column instead of a packed column will be noticeable (as long as you do not overload the PLOT column). Restek - Molecular Sieve Packed Columns and Fixed Permanent Gas Analysis
Another difficult separation is helium, neon, and hydrogen. Once again, the molecular sieve 5A is often the go-to packing. If you are not having success separating these compounds using a packed column, your time and energy may be better spent using a 30-meter Rt-MSieve 5A PLOT column, like shown here. Restek - Noble and Permanent Gases on Rt-Msieve 5A This separation may also be possible if using a PLOT or micropacked carbon molecular sieve column, like shown in the following link, as long as the column is not overloaded and the instrument is optimized. Restek - Noble and Permanent Gases on ShinCarbon
Nitrogen/Krypton and Methane/Krypton
You may be asking yourself why nitrogen/krypton and methane/krypton are listed in this blog post. The reason is because many gas analysis customers who use molecular sieve 5A (and 13X) packed columns have noticed that separating nitrogen and krypton can be difficult. Similarly, when using carbon molecular sieve packed columns (like the ShinCarbon), analysts are often surprised that separating methane and krypton can be difficult. Also note, it is not unusual to see an elution order change between these compound pairs when the starting GC oven temperature and/or carrier gas flow rate are changed.
The good news, carbon molecular sieve columns (like the ShinCarbon) will easily separate nitrogen from krypton and molecular sieve 5A and 13X columns will easily separate methane from krypton.
Permanent and Hydrocarbon Gases
This section will discuss challenging gas separations using Hayesep and Porapak porous polymer packed columns. The Hayesep packings reviewed for this post include A, B, C, D, DB, N, Q, S and T. The Porapak packings include N, P, Q, R, S and T.
Please note that each of these packings will often show poor performance separating the lightest gases. For example, unless the columns are very long, separating several of the primary components of air (nitrogen/oxygen/argon/carbon monoxide) is very difficult. In addition, depending upon the dimensions of the column and the concentration of the air in the standard(s)/sample(s), methane (and possibly carbon dioxide) may coelute with a high concentration air peak, even when using a porous polymer PLOT column.
Listed below are challenging hydrocarbon gas separations for certain Hayesep and Porapak packings. Your column may be able to separate these gases without issue, or if there are coelutions, separation may be possible with optimization of your analytical instrument conditions (temperatures and/or carrier gas flow rate). Or, depending upon compound concentrations and your specific instrument, you may need to switch to an alternative column to achieve separation while keeping analysis (run) times reasonably short and/or to avoid the need for cryo-cooling. Because packed column separation performance is not always predicable or consistent (packings and their mesh size range may vary slightly from batch to batch), do not be surprised if the compound elution order (or coelutions) on your instrument differs from those shown in published documentation.
If a compound peak seems to be missing, do not assume column/instrument activity is the issue, especially if the compound is an inert compound like a straight-chain hydrocarbon. Instead, consider that the missing compound may be coeluting with another peak.
Finally, to obtain the most consistent results from column to column, always measure the carrier gas column flow rate exiting the packed column. Do not simply set the head pressure and expect reproducibility. Each packed column is different. You must adjust the head pressure for each column, even for those with the same catalog number, to obtain the proper carrier gas flow. I suggest reviewing this post if you have any questions.
The information below and in this spreadsheet will summarize much of the preceding information in addition to showing the most likely elution order of common gases for several molecular sieve and porous polymer packings. Please note that there are two individual sheet tabs at the bottom of the spreadsheet, one titled “Molecular Sieves” and the other “Porous Polymers – Hydrocarbons”. Much of this data was obtained from retention time/relative-retention indexes that I was able to find using Google, and from application chromatograms generated at Restek and elsewhere. If certain gases are not listed for a specific packing, I was unable to locate retention time or elution order data.
Please remember to always confirm the accuracy of peak identification, as it is not uncommon for incorrect information to be published. It is also possible, and even probable, that closely eluting peaks may switch elution order depending upon instrument analytical method conditions (especially temperatures and carrier gas flow rates).
Thank you for your time. I hope you have found this interesting and useful.
Very likely coelutions when using packed columns
Molecular Sieve 5A/13X: Oxygen/Argon and Helium/Neon/Hydrogen
Carbon Molecular Sieve (ShinCarbon): Oxygen/Argon and Helium/Neon/Hydrogen
All porous polymers: Helium/Neon/Hydrogen and Nitrogen/Oxygen/Argon/Carbon Monoxide
Hayesep A: Propylene/Propane and Iso-butane/n-Butane
Hayesep B: Ethylene/Ethane
Hayesep C: Ethane/Acetylene
Hayesep N: Propane/Propylene and Iso-butane/n-Butane
Porapak N: Propane/Propylene
Porapak P: Acetylene/Ethane and Propane/Propylene
Hayesep Q: Ethylene/Acetylene
Porapak Q: Ethylene/Acetylene
Porapak R: Acetylene/Ethane
Hayesep S: Iso-butane/n-Butane
Hayesep T: Propane/Propylene
Porapak T: Propane/Propylene
Somewhat likely coelutions when using packed columns
Molecular Sieve 5A/13X: Nitrogen/Krypton
Carbon Molecular Sieve (ShinCarbon): Oxygen/Nitrogen and Methane/Krypton
Hayesep B: Ethylene/Ethane/Acetylene
Hayesep C: Ethylene/Ethane/Acetylene and Propylene/Propane
Hayesep D: Acetylene/Ethylene
Hayesep DB: Acetylene/Ethylene