Improved CE/MS Sensitivity by Operating the Triple-Tube Coaxial Sheath-Flow Sprayer Without Appling Nebulizing Gas

Importance of the Topic

Capillary electrophoresis-mass spectrometry (CE/MS) is a versatile and robust analytical technique widely used for the separation and detection of charged and polar compounds in pharmaceutical, metabolomic and quality control applications. Traditional operation of the triple-tube coaxial sheath-flow sprayer relies on a nebulizing gas to establish the electrospray, but this gas introduces a suction effect that degrades separation efficiency, resolution and sensitivity. Optimizing CE/MS to eliminate the nebulizing gas offers the potential to enhance detection limits and improve chromatographic performance.

Objectives and Overview

This study aimed to define operating parameters that allow the Agilent triple-tube coaxial sheath-flow sprayer to function without nebulizing gas and to assess the impact on analytical performance. A design-of-experiments (DoE) approach was used to optimize four critical source parameters: drying gas temperature, capillary voltage, sheath liquid flow rate and capillary protrusion. The optimized no-gas configuration was compared with conventional gas-assisted conditions using a panel of 47 basic compounds, including endogenous metabolites and a model drug (MDMA).

Methodology

• A Box-Behnken design was applied to evaluate parameter effects on MDMA ion intensity.
• Experimental ranges were: drying gas temperature (80–250 °C), capillary voltage (4 000–6 000 V), sheath flow rate (2–10 µL/min) and capillary protrusion (0–11 ×10⁻³ in).
• Separations were performed on a 70 cm × 50 µm fused silica capillary at 30 kV with 10 % acetic acid background electrolyte, injecting 13.7 nL sample volumes.
• Spray stability and repeatability were assessed over 14 consecutive injections without an internal standard.

Used Instrumentation

• Agilent 7100 Capillary Electrophoresis system
• Fused silica capillary (70 cm length, 50 µm i.d.)
• Agilent 6490 Triple Quadrupole LC/MS with electrospray ionization and coaxial sheath-flow interface (p/n G1607B)
• Agilent MassHunter B.08.00 software

Main Results and Discussion

DoE models were significant (p < 0.1) with R² > 89 %. Optimal no-gas source parameters were predicted as: dry gas 200 °C, capillary voltage 5 500 V, sheath flow rate 2 µL/min and capillary protrusion +3 (≈8 × 10⁻³ in). Under these conditions:

• 63 % of compounds exhibited a moderate signal-to-noise (S/N) increase (1.5–4.5×).
• 23 % showed a substantial S/N gain (4.5–11.5×).
• 15 % remained within ±50 % of original S/N.

Elimination of nebulizing gas reduced the suction effect, leading to sharper peaks, improved separation efficiency and resolution. Spray stability remained high and repeatability for MDMA and L-arginine areas was below 5 % RSD.

Benefits and Practical Applications

Operating the triple-tube sprayer without nebulizing gas simplifies the interface, increases sensitivity for a broad range of analytes and enhances separation performance. This approach is directly applicable to metabolomic profiling, pharmaceutical impurity analysis, environmental testing and quality assurance in industrial laboratories.

Future Trends and Opportunities

• Extending no-gas optimization to other CE/MS interfaces and ion sources.
• Evaluating compatibility with gradient focusing and ion mobility separations.
• Developing portable and field-deployable CE/MS systems without gas supplies.
• Integrating automated sample handling and data processing workflows for high-throughput analyses.

Conclusion

Removing nebulizing gas from the triple-tube coaxial sheath-flow sprayer, coupled with targeted optimization of source parameters, significantly enhances CE/MS sensitivity, resolution and robustness. This no-gas operation mode offers a straightforward route to improved analytical performance in diverse research and industrial applications.

Reference

1. Lindenburg P. W. et al. Developments in Interfacing Designs for CE-MS: Towards Enabling Tools for Proteomics and Metabolomics, Chromatographia 2014, 78, 367–377.
2. Mokaddem M. et al. A new insight into suction and dilution effects in capillary electrophoresis coupled to mass spectrometry via an electrospray ionization interface. Part I - Suction effect, Electrophoresis 2008, 29, 1957–1964.
3. Mokaddem M. et al. New insight into suction and dilution effects in CE coupled to MS via an ESI interface. II - Dilution effect, Electrophoresis 2009, 30, 1692–1697.
4. Köhler I. et al. Compatibility of Agilent Jet Stream thermal gradient focusing technology with CE/MS, Agilent Technologies Technical Note 5990-9716EN, 2012.