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Are Wax columns always used for essential oils?

  • Jana Rousova
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My last blog examined the analysis of essential oils using GC columns with wax phases. While these columns are commonly used for natural oils, they are not the only option. Another choice that falls on the other end of the selectivity spectrum, in this case, a non or low polar; dimethyl diphenyl polysiloxane stationary phase. The two non-polar columns I’ve compared were Rxi-5MS and Rxi-5Sil MS (both 30x0.25x0.25). The essential oil used was the same as in the previous blog; citronella java oil (Figure 1).

Figure 1: Citronella Java oil (5% in acetone) on Rxi-5Sil MS(#13623, black trace) and Rxi-5MS (#13423, blue trace). Method: 100 °C to 300 °C at 11 °C/min (hold 10 min), carrier gas: He at 1.31 mL/min, split: 100:1. The Rxi-5Sil MS results in a slightly faster overall runtime.

From the two chromatograms we can see that analysis on Rxi-5Sil MS is faster with comparable resolution. We’ve decided to take it a step further and speed up the analysis by using a shorter Rxi-5Sil MS column with a narrower internal diameter, i.e. 20x0.18x0.18 and 10x0.15x0.15 (Fig 2). The methods were translated using the EZGC method translator.

Figure 2: Citronella Java oil (5% in acetone) on Rxi-5Sil MS 20x0.18x0.18 (#43602, black trace) and 10x0.15x0.15 (#43815, blue trace). Method 20x018x0.18: 100 °C to 300 °C at 17.5 °C/min (hold 10 min), carrier gas: He at 1.01 mL/min, split: 100:1. Method 10x0.15x0.15: 100 °C to 300 °C at 45 °C/min (hold 10 min), carrier gas: He at 1.01 mL/min, split: 100:1. Translated conditions with smaller bore columns can significantly decrease runtimes.

There is very little resolution lost between the long and the shorter columns, making it a great choice for fast screening.

Here is a link to the wax column selection blog post and blog post about citral analysis.