Screening of Terpenes with Solvent-Mediated Chemical Ionization

Importance of the Topic

Terpenes represent a large class of natural products with critical roles in plant defense and widespread applications in flavors, fragrances, and pharmaceuticals. Gas chromatography–mass spectrometry (GC–MS) is the primary technique for terpene analysis; however, traditional electron ionization (EI) often fails to produce stable molecular ions, complicating identification in complex mixtures. Introducing solvent-mediated chemical ionization (SMCI) using common solvents enhances safety and yields complementary spectral data essential for confident molecular weight determination and rapid screening.

Objectives and Study Overview

The article investigates the performance of SMCI employing methanol and acetonitrile as reagent gases on a Shimadzu GCMS-QP2020 NX platform. A mixture of 21 terpenes, including monoterpenes, oxygenated monoterpenes, sesquiterpenes, and oxygenated sesquiterpenes, was analyzed to:

• Assess the generation of molecular and adduct ions compared to EI.
• Define characteristic ion patterns for each terpene class.
• Demonstrate a rapid screening approach based on selected ions.

Methodology and Instrumentation

A Shimadzu GCMS-QP2020 NX equipped with an SMCI unit was used. Key parameters:

• Column: SH-I-624Sil MS (30 m × 0.25 mm, 1.4 µm).
• Carrier gas: Helium at 1.5 mL/min.
• Injection: 1 µL at 250 °C, split 5:1.
• Oven program: 55 °C (hold), ramp to 130 °C at 25 °C/min (6 min), then to 280 °C at 30 °C/min (2 min).
• Scan range: m/z 50–350; source 200 °C; interface 280 °C.
• Reagent gases: HPLC-grade methanol and acetonitrile.

Results and Discussion

Compared with EI, SMCI enhanced the observation of molecular ions:

• Monoterpenes (C10H16, C10H14) and sesquiterpenes (C15H24) generated strong [M+H]+ ions in both methanol and acetonitrile modes.
• Oxygenated species (C10H18O, C15H26O) predominantly yielded dehydration ions ([M+H–H2O]+) due to ROH2+ fragmentation.
• Using acetonitrile, a unique adduct [M+MIE]+ appeared (m/z 190/188 for monoterpenes, 258 for sesquiterpenes, 208/276 for oxygenated species), reflecting the formation of CH2=C=N+=CH2 reagent ions.

Extracted ion chromatograms demonstrated clear separation and allowed assignment of characteristic ions for each terpene class, facilitating rapid profiling.

Benefits and Practical Applications of the Method

• Eliminates the need for flammable reagent gases by using common laboratory solvents.
• Produces complementary mass spectral data to EI, improving identification confidence.
• Enables quick screening of complex samples through selected-ion monitoring.
• Reduces operational costs and enhances laboratory safety.

Future Trends and Applications

SMCI shows potential for broadening to other volatile organic compound classes and complex matrices. Integration with automated retention time alignment (AART) and high-throughput workflows could streamline routine QC and research applications. Exploring additional solvents or mixed-reagent systems may further enrich spectral libraries and analytical flexibility.

Conclusion

Solvent-mediated CI on the Shimadzu GCMS-QP2020 NX reliably retains molecular and adduct ions of diverse terpenes, overcoming limitations of EI and enabling rapid screening. Its safety, cost-effectiveness, and complementary spectral output position SMCI as a valuable tool for terpene analysis in research and industrial settings.