Double bond position in unsaturated hydrocarbons

The double-bond position of unsaturated hydrocarbons detected in crude extracts can be assessed by GC-MS after derivatisation with dimethyl disulfide (DMDS) (Carlson et al., 1989). Given its importance, the method will be described in detail.

To derivatise a few µg of HC in 200 µl of hexane, use the following protocol:

  1. Add 200 µl of DMDS and 100 µl of iodine solution (60 mg in 1 ml of ether) to catalyse the reaction.
  2. Keep at 40 °C for 4 hours.
  3. Dilute with hexane with 5% sodium thiosulfate to neutralize the iodine.
  4. Collect the organic phase which contains the DMDS adducts of the unsaturated hydrocarbons.
  5. Remove moisture from the solution by adding a small amount of anhydrous Na2SO4 to it.
  6. Reduce the solvent under a stream of nitrogen to the desired concentration.

Derivatised alkenes give a single chromatographic peak with a characteristic spectrum composed of two prominent fragments. These are related to the cleavage of the bond between the carbon atoms carrying the methyl sulphide substituent, originally the location of the double bond. The m/z values of these fragments are identified by the series 61+n14; for example, the alkene 9-heptadecene (CH3(CH2)8=CH (CH2)6CH3) will give a peak with two prominent fragments at 159 (CH3(CH2)6CHSCH3) and 173 m/z (CH3(CH2)7CHSCH3); the molecular ion is normally seen (in the case of 9-heptadecene, this has m/z 332, that is the sum of the two fragments described above).