Cleaning the n-alkane fraction for subsequent GC-IR-MS analysis.
- Rheta E. Lanehart, Ph.D.
- Mar 8, 2019
- 2 min read
Updated: Mar 22, 2022
After much frustration with returned samples sent for GC-IR-MS analysis (gas chromatography-isotope ratio-mass spectrometry) because of unwanted compounds (e,g, branched chain alkanes, etc.) mixed with the n-alkane fraction, I have experimented with a protocol for removing these unwanted compounds from the sample.
A protocol for cleaning n-alkanes after lipid extraction adapted from previous research by Grice, et al., 2008 & Plet, et al., 2018.
Activate 5A sieves for 8 hrs. @ 250 ◦C in a muffle furnace
Dilute the aliphatic fraction(s) in 2 ml cyclohexane
Transfer solution to a 2 ml reaction vial ¾ filled with activated 5A molecular sieves
Place vial(s) in oven @ 80◦C overnight
Rinse sieves in a Pasteur pipette (chromatography column) with 4 ml cyclohexane (yields the branched/cyclic fractions)
Fully dry 5A molecular sieves at RT
Place the molecular sieves in a GMCS vial (2 ml) containing 12% cyclohexane in n-pentane and seal the vial.
Heat sample on an aluminum block (or oven) @ 80◦C for 8 hours
Filter the sample through a Pasteur pipette plugged with cotton wool pre-rinsed with n-pentane
Wash the sieve with 1 ml n-pentane to recover n-alkanes
Analyze the 5A included fraction via GCMS & GC-IR-MS
GC-MS Analysis of the Branched Cyclic Fraction (Step 5)
Since this was my first attempt at cleaning the alkanes, I wanted to analyze the branched cyclic fraction eluted in Step 5 via GC-MS. Although the chromatogram of the branched fraction appears almost identical to that of the n-alkane fraction, I was curious enough to pursue further analysis using GC-IR-MS for my peaks of interest (C15 and C17).
Results from the GC-IR-MS analysis for the n-alkane Fraction (Step 11) indicated that the cleaned ding sherd sample sent for analysis did not produce signal with enough sensitivity to detect the peaks of interest.
Therefore, it's back to the drawing board!
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