A "Solid" Alternative for Analysing Oxygenated Hydrocarbons — Agilent's New Capillary GC PLOT Column

PLOT columns are often used in GC analyses when it is necessary or desirable to retain one class of solutes in favour of other solutes that have little or limited interactions with the surface of the stationary phase. With a PLOT column, chromatographers can even cause lower boiling point compounds to elute well after higher boiling point compounds, thus providing better qualitative and quantitative separations for the solutes of interest.

Figure 1

GS-OxyPLOT is suitable for ASTM methods that are intended to measure the amount of oxygenates in complex matrices such as gaseous hydrocarbons, motor fuels and crude oil. A sample is injected first into a non-polar DB-1 pre-column, which is connected to a switching valve in front of a GS-OxyPLOT column. During the first several minutes of the chromatographic analysis, the polar oxygenated solutes, which are not well retained in the DB-1 pre-column, are allowed to elute into and become trapped at the head of the GS-OxyPLOT column. The lower boiling point hydrocarbons that elute into the GS-OxyPLOT column are not strongly retained on the stationary phase and elute through the FID very rapidly. At a predetermined time, the valve switches to backflush the higher boiling point hydrocarbons from the pre-column. Pre-defined oven temperature programming causes the trapped oxygenated solutes to chromatographically go through the GS-OxyPLOT column, where they can be quantified without interference from the hydrocarbon matrix. GS-OxyPLOT has an upper temperature limit of 350 °C and exhibits no bleed that is associated with liquid and siloxane coated GC columns. Figure 1 shows a chromatogram of a simulated hydrocarbon stream in which n-Octane (n-C₈) is allowed to elute from the DB-1 pre-column before backflushing the higher hydrocarbons out to vent. This allows for the analysis of C₁ through C₄ oxygenated compounds to be determined by this method.

Conclusion

GS-OxyPLOT column is an excellent alternative to GC columns with gas-liquid phases that are often used for similar applications.

Compound List

1. Dimethyl ether, 2. diethyl ether, 3. acealdehyde, 4. ethyl t-butyl ether, 5. methyl t-butyl ether, 6. diisopropyl ether, 7. propionaldehyde, 8. tert-amyl methyl ether, 9. propyl ether, 10. isobutylaldhyde, 11. butylaldehyde, 12. methanol, 13. acetone, 14. isovaleraldehyde, 15. valeraldehyde, 16. methyl ethyl ketone, 17. ethanol, 18. n-propanol, 19. isopropanol, 20. allyl alcohol, 21. isobutanol, 22. t-butyl alcohol, 23. s-butyl alcohol, 24. n-butyl alcohol, 25. 2-methyl-2 pentanol.

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Allen K. Vickers, Agilent Technologies, Wilmington, Delaware, USA.