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Environmental

Techniques for Optimizing GC Analysis of Ethylene Glycol in Water

The analysis of ethylene glycol in water is a very common test in environmental laboratories. Many of these samples originate from water runoff at airports, where ethylene glycol is used as a de-icing agent for airplanes during winter months. Because ethylene glycol is highly soluble in water, it is not easily concentrated by purge and trap. Therefore, the most frequently used sample introduction technique is direct aqueous injection. The direct aqueous injection of ethylene glycol can be challenging because, if not done properly, it can be difficult to attain reproducibility and good peak shape. The large expansion volume of water can cause backflash, carryover can cause inconsistent results, and excess water can extinguish the FID flame. These problems can prevent achieving the detection limit for ethylene glycol, which may vary in the 1-10 ppm range.

Poor Peak Shape
With a column head pressure of 10 psig and an injection port temperature of 250 °C, a 1 μL injection of water will expand to 1,420 μL of vapor. This large vapor cloud exceeds the volume of most inlet liners, causing backflash. If backflash occurs, the vapor cloud can expand out of the liner and injection port and result in poor sample transfer onto the column. Also, the glycol compounds are not focused in a narrow band but, instead, are focused in the condensed water that beads onto the column walls, so the compounds of interest can elute as split peaks. This peak splitting effect is most apparent when performing a splitless injection because of the solvent focusing required. Split peaks and backflash compromise the analysis by causing irreproducible peak shapes.
One technique to reduce the effect of vapor expansion and poor solvent focusing is the use of a Uniliner® or Drilled Uniliner® injection port liner. This liner forms a leak-free connection with the column end (Figure 1), thereby ensuring a complete sample transfer. The column sealed at the bottom of the liner minimizes sample backflash, eliminating the potential for ghost peaks. By using a Uniliner® liner, the aqueous ethylene glycol sample is vaporized and quantitatively transferred to the column in a focused band, thereby achieving reproducible peak areas. Uniliner® liners are available for conversion of packed column injection systems and for split/splitless injection systems.

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