UltraPerformance Convergence Chromatography (UPC2) with MS Detection Using Three Different Atmospheric Pressure Ionization Techniques Using Liquid Crystal Intermediates as a Model

Significance of the Topic

Convergence chromatography using supercritical carbon dioxide (UPC²) offers unique selectivity and high throughput for separating nonpolar to moderately polar compounds. When coupled with mass spectrometry (MS), the technique provides powerful qualitative and quantitative capabilities, reducing analysis time and solvent consumption compared to traditional normal-phase or reversed-phase liquid chromatography.

Objectives and Overview of the Study

This study demonstrates the integration of an ACQUITY UPC² system with a Xevo TQD MS platform, evaluating three atmospheric pressure ionization techniques—electrospray (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI)—for the analysis of liquid crystal intermediate compounds. Key goals include:

• Assessing the ease of interfacing UPC² with MS via a splitter and makeup pump
• Optimizing ionization conditions and modifier selection for each ion source
• Comparing sensitivity and selectivity of ESI, APCI, and APPI for model analytes

Methodology and Instrumentation

Chromatographic conditions were established on an ACQUITY UPC² BEH 2-EP column (3.0 × 100 mm, 1.7 µm) at 65 °C, using CO₂ and methanol as mobile phases with a gradient over 5 minutes at 2.0 mL/min. Back pressure was maintained at 2000 psi. Detection included a UPC² PDA for UV monitoring (210–450 nm) and a Xevo TQD MS with interchangeable ESI/APCI/APPI sources. A controlled leak splitter and a Waters 515 makeup pump delivered modifiers—methanol, formic acid, or toluene—prior to MS to enhance ionization. MassLynx v4.1 software managed instrument control and data processing.

Key Results and Discussion

Retention times for six target compounds ranged from 1.03 to 2.97 minutes. MRM transitions and collision energies were optimized via direct infusion. In mixed calibration standards (0.1 mg/mL), all three ionization modes yielded clear chromatographic peaks. ESI (with 1% formic acid in methanol) and APPI (with 10% toluene co-modifier) generally produced the highest signal response, while APCI without makeup solvent showed improved early-eluting peak intensity by increasing initial methanol from 2% to 4%. The universal source platform enabled rapid switching between ion sources without venting, facilitating efficient method development.

Benefits and Practical Applications

• Rapid separation and MS detection within a 5-minute cycle
• Minimal organic solvent usage due to supercritical CO₂ mobile phase
• Enhanced selectivity for structurally related intermediates
• Streamlined source exchange for multi-mode ionization screening
• Improved sensitivity through tailored makeup solvents and modifiers

Future Trends and Applications

UPC²–MS coupling is poised to expand in high-throughput pharmaceutical screening, impurity profiling, and lipidomics. Emerging developments include novel stationary phases for polar analytes, automated modifier blending for on-the-fly optimization, and integration with high-resolution MS for improved structural elucidation. Green analytical initiatives will further benefit from reduced solvent waste and energy-efficient supercritical fluid usage.

Conclusion

The combination of UPC² and Xevo TQD provides a versatile, high-speed analytical platform. By optimizing co-solvent gradients, MS splitter configurations, and ionization conditions across ESI, APCI, and APPI, robust and sensitive methods for liquid crystal intermediate compounds were established. This approach maximizes chromatographic efficiency and MS specificity while minimizing environmental impact.

Reference

• E. Riches, Xevo TQ MS with Atmospheric Pressure Photo Ionization (APPI) Source: The Ionization of Compounds with Diverse Structures Using Vitamins as a Model. Waters Technical Note 720003276en, 2010 Jan.