A closer look at selectivity and tailing: Essential oils and Wax phases

The most common way to analyze essential oils is to use GC with either a FID or MS. The wax-based columns are the number one choice for many users and following up on an excellent blog post by Corby, I wanted to evaluate two wax in Restek inventory: Stabilwax and Rtx-Wax. The essential oil we chose was Citronella Java oil, which has high levels of citronellal (CAS 106-23-0), β-Citronellol (CAS 106-22-9) and Geraniol (CAS 106-24-1). Figure 1 shows a comparison between the two columns. While the chromatograms are very similar, a couple of things stand out.

 

Figure 1: Citronella Java oil (5% in acetone) on Stabilwax (#10673 , black trace) and Rtx-Wax (#12423 , blue trace). Method: 100 °C to 250 °C at 11 °C/min (hold 10 min), carrier gas: He at 1.31 mL/min, split: 100:1

First, there is a co-elution of geranyl acetate with β-citronellol, which is more resolved on Stabilwax. However, when the same sample is analyzed using a slower method, the peaks resolve on both columns. Moreover, the third compound, δ-cadinene, is revealed on Rtx-Wax (Fig 2).

Figure 2: Citronella Java oil (5% in acetone) on Stabilwax (#10673 , black trace) and Rtx-Wax (#12423 , blue trace). Close-up of β-citronellol peak. Method: 100 °C to 250 °C at 4 °C/min (hold 10 min), carrier gas: He at 1.31 mL/min, split: 100:1

The second apparent difference between analysis on Stabilwax and Rtx-Wax we can see from Figure 1 is the citronellal peak (close-up in Fig 3). The peak tails using the Stabilwax column, whereas, there is no peak tailing when the Rtx-Wax column is used. This is similar to the citral peak in the previous blog. Similar tailing is visible on geraniol peak, however, not as pronounced.

Figure 3: Citronella Java oil (5% in acetone) on Stabilwax (#10673 , black trace) and Rtx-Wax (#12423 , blue trace). Close-up of citronellal peak.. Method: 100 °C to 250 °C at 11 °C/min (hold 10 min), carrier gas: He at 1.31 mL/min, split: 100:1

To conclude, Rtx-Wax seems to perform better with aldehydes present in the essential oils. But keep in mind that the selectivity is different and certain compound pairs may be better resolved on the Stabilwax.

Keep tuned for the next blog post focused on essential oil analysis using “5” phases!

To read the blog about the citral analysis (Choosing Your Citral Column) follow this link.

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