Analysis of Double Bond Position in Olefins by Solvent-Mediated Chemical Ionization (SMCI)

Significance of the Topic

Olefins are key unsaturated hydrocarbons widely used in materials, coatings and emerging from chemical recycling streams. Their physicochemical behavior depends not only on chain length but critically on the position of the carbon–carbon double bond. Conventional chromatographic analysis requires authentic standards for each isomer, which are often unavailable. The development of a mass spectrometric approach that can accurately locate double bonds without reference compounds is therefore of high practical importance.

Objectives and Study Overview

This work presents a strategy using solvent-mediated chemical ionization (SMCI) coupled with GC-MS/MS to pinpoint double bond positions in straight-chain olefins. A standard mixture of eight olefins was analyzed to:

• Demonstrate the formation of specific adduct ions via acetonitrile‐derived reagent ions.
• Use product ion scans to generate diagnostic fragments that reveal double bond location.
• Validate the method across various chain lengths (C10–C14) and bond positions (α, β, γ, etc.).

Methodology and Used Instrumentation

Sample Preparation: A standard solution containing eight olefins (50 µg/mL each) was injected (1 µL, splitless).

Instrumentation:

• GC-MS/MS System: GCMS-TQ8040 NX with SMCI unit.
• Autosampler: AOC-20i+s Plus.
• Column: SH-I-5MS (30 m × 0.25 mm I.D., 0.25 µm film thickness).

GC Conditions: Injection at 250 °C; carrier gas controlled for linear velocity of 42.5 cm/s; oven ramp from 60 °C to 300 °C.
MS Conditions: Interface 250 °C; ion source 200 °C; SMCI reagent: acetonitrile; scan range m/z 50–300; collision energy 2 V in product ion scan mode.

Main Results and Discussion

In scan mode, adduct formation of acetonitrile reagent ion (MIE, m/z 54) with the olefin yields [M+54]+, confirming unsaturation. Product ion scans of [M+54]+ generate characteristic fragments: α-, β- and γ-olefins all produce distinct ions at m/z 96, 110 and 124, respectively, irrespective of chain length. For internal double bonds (e.g. 5-decene, 7-tetradecene), two complementary fragment series arise from dual orientations of MIE addition. Observed fragment ions correlate directly with cleavage adjacent to the double bond, enabling unambiguous localization.

Practical Benefits

• No requirement for high-pressure flammable gases; SMCI uses liquid acetonitrile.
• Eliminates need for authentic isomeric standards.
• Combines molecular weight confirmation with structural elucidation in a single GC-MS/MS run.

Future Trends and Potential Applications

Advances in SMCI reagent chemistries could extend this approach to polyunsaturated and functionalized lipids. Integration with high-resolution MS may improve sensitivity for trace analysis in environmental and bioprocess monitoring. Automated data interpretation algorithms could further streamline routine QA/QC workflows in petrochemical and recycling industries.

Conclusion

The described SMCI-GC-MS/MS method reliably identifies double bond positions in straight-chain olefins without standards. It offers a robust, safe and versatile alternative for structural analysis of unsaturated hydrocarbons.

References

• Van Pelt, C.K. and J.T. Brenna, Anal Chem, 1999. 71(10): p. 1981–1989.