Improving Peak Shape Using the Automatic Pretreatment Function (Co-Injection) in the SIL-40 Series Autosamplers

Importance of the Topic

The composition of sample solvents can adversely affect chromatographic peak shapes, especially when the solvent’s elution strength exceeds that of the mobile phase. This is critical in quantitative analysis, where distorted peaks undermine accuracy and repeatability. Co-injection offers an automated means to mitigate these effects by diluting or weakening sample plugs within the autosampler, improving data quality in high-throughput and sensitive applications.

Objectives and Study Overview

This technical report demonstrates the use of the automatic pretreatment co-injection function built into the SIL-40 series autosamplers (Nexera™) to enhance peak shape. The study focuses on reverse-phase LC analysis of lipid mediators prepared in high-strength organic solvent, evaluating the performance gains when water is co-injected alongside the sample.

Methods and Instrumentation

• Autosampler: Shimadzu SIL-40 series with built-in automatic pretreatment and co-injection capability.
• Software: LabSolutions workstation for method setup and template configuration.
• Chromatographic Conditions: C8 column (2.1×150 mm, 2.6 µm), mobile phases A: 0.1 % aqueous formic acid, B: acetonitrile; flow rate and gradient per Shimadzu lipid mediator method package.
• Sample: Standard mixture of 196 lipid mediators in 100 % methanol.
• Co-Injection: Water volumes of 5, 10, and 15 µL added simultaneously with sample injection.

Main Results and Discussion

Co-injection of water significantly improved early-eluting peaks by reducing tailing factors (S) from values around 0.69–0.95 up to approximately 1.02–1.07. Resolution between closely eluting lipid isomers increased with higher co-injected volumes, reaching Rs >2.3 at 10–15 µL. The results confirm that a weak elution-strength solvent plug counterbalances strong sample solvents, sharpening peak fronts and enhancing quantitative reliability.

Benefits and Practical Applications

• Enhanced peak shape and symmetry for fast-eluting analytes.
• Improved resolution of isomeric or closely eluting compounds.
• Greater quantitative accuracy and method robustness.
• Easy implementation via LabSolutions templates without hardware modifications.

Future Trends and Potential Applications

Further optimization could explore alternative co-injection solvents, dynamic timing of injection events, and integration with MS-based lipidomics workflows. Extending co-injection to include on-needle derivatization or reagent addition could broaden its utility for diverse sample types in pharmaceutical, environmental, and food analysis.

Conclusion

The co-injection function within the SIL-40 series autosamplers provides a straightforward, software-controlled strategy to counteract strong sample solvents, yielding sharper peaks and higher resolution in reverse-phase LC. This automation enhances the reliability of quantitative assays, particularly in complex lipid mediator analyses.

Reference

• Akita S., Uchikata T., Yasunaga K., Terada H., Hattori T., Matsumoto K. Improving Peak Shape Using the Automatic Pretreatment Function (Co-Injection) in the SIL-40 Series Autosamplers. Shimadzu Corporation, First Edition August 2019.