Ion-Pair-Free Reversed-Phase LC/MS Analysis of siRNA Using a Single Quadrupole Mass Spectrometer

Significance of the topic

The characterization and quality control of oligonucleotide therapeutics, including small interfering RNA (siRNA), is critical for development, manufacturing and regulatory submission. Rapid and reliable confirmation of intact molecular weight supports identity testing, lot release and stability monitoring. Ion-pair reagents commonly used in reversed-phase LC/MS improve chromatographic retention but pose risks of long-term instrument contamination and require dedicated instrumentation or extensive cleaning. Developing ion-pair-free LC/MS workflows that reliably confirm siRNA mass on widely available single-quadrupole MS platforms reduces operational burden and expands accessibility of oligonucleotide analysis in routine labs.

Objectives and study overview

This study evaluated an ion-pair-free reversed-phase LC/MS method to confirm the molecular weight of a model double-stranded siRNA (Patisiran-based sequence) using a Nexera XS inert UHPLC coupled to an LCMS-2050 single quadrupole mass spectrometer. The goals were to demonstrate: (1) chromatographic separation and detection of siRNA components without alkylamine ion-pair reagents, (2) confirmation of molecular weight for sense and antisense strands within acceptable mass error, and (3) functionality of LabSolutions Insight Biologics software to identify multiple oligonucleotide sequences simultaneously.

Methodology and instrumentation

Samples:

• Double-stranded siRNA based on Patisiran sequence. Sense and antisense strands included 2'-deoxy (d) and 5-methyl (m) base modifications; sequence details were configured in the analysis software.

Chromatography and MS:

• LC system: Nexera XS inert UHPLC with a Shim-pack Scepter Claris C18-300 column (50 mm x 2.1 mm, 1.9 μm).
• Mobile phases: A = 10 mM ammonium bicarbonate in water (volatile, ion-pair-free); B = methanol. Gradient from 2% to 40% B over 5 min, quick ramp to 90% then re-equilibration (total cycle ~12 min).
• Flow rate: 0.4 mL/min; injection 1 μL; column oven tested at 25 °C and 60 °C.
• MS: LCMS-2050 single quadrupole with DUIS heated ion source enabling combined ESI/APCI behavior. Positive-ion mode, m/z scan range 800–2000, detecting multiply charged species (observed z = 4–6). Source/desolvation settings: nebulizing gas 2.0 L/min, drying gas 5.0 L/min, heating gas 7.0 L/min, desolvation temp 450 °C, DL temp 200 °C.
• Data analysis: LabSolutions Insight Biologics used to create oligonucleotide sequence definitions (bases, ribose/linker types, modifications) and to perform component identification by combining MS1 spectra, charge states and isotopic patterns. Multiple sequences (sense and antisense) were analyzed in a single run.

Main results and discussion

Chromatography:

• Under the chosen ion-pair-free conditions with ammonium bicarbonate, the double-stranded siRNA eluted as two chromatographic peaks in the UV (260 nm) trace at both 25 °C and 60 °C column temperatures. The two peaks are consistent with thermal or mobile-phase-induced dissociation to single strands.

Mass spectrometry and identification:

• Component chromatograms generated by Insight Biologics matched the UV peaks: the first peak corresponded to the sense strand and the second to the antisense strand when the column was held at 60 °C.
• Multiple charge states (4+ to 6+) were observed in the MS1 spectra within the instrument scan range, enabling deconvolution to intact molecular weight.
• Measured masses for both strands were within 1 Da of theoretical monoisotopic molecular weights, demonstrating that a single-quadrupole MS coupled to an ion-pair-free RP method can confirm siRNA mass with acceptable accuracy for identity testing.

Interpretation and limitations:

• The ion-pair-free volatile buffer (ammonium bicarbonate) enabled chromatographic retention and ionization without introducing alkylamine contaminants; DUIS ion source supported robust ionization of multiply charged oligonucleotide ions.
• Single-quadrupole resolution sufficed to confirm overall intact mass but cannot replace high-resolution MS for detailed sequence-level impurity mapping, positional isomer differentiation or low-level modification identification. For applications requiring precise mass accuracy or MS/MS structural data, high-resolution instruments are recommended.

Benefits and practical applications

• Reduced instrument contamination risk: avoiding alkylamine ion-pair reagents lowers carryover and need for dedicated LC/MS systems or extensive cleaning cycles, improving lab throughput.
• Broad instrument compatibility: the method runs on a single-quadrupole MS, making oligonucleotide mass confirmation accessible to more QC and development labs.
• Efficiency and multiplexing: LabSolutions Insight Biologics allows storage of oligonucleotide definitions and parallel identification of multiple sequences in a single run, simplifying sequence verification workflows.
• Expedient identity testing: the short gradient (~12 min) and small injection volume enable relatively high sample throughput for routine lot release or stability checks.

Future trends and potential uses

• Integration with high-resolution MS: combining ion-pair-free LC separations with high-resolution/accurate-mass instruments would extend capability to impurity profiling, modification mapping and confirmation of near-isobaric species.
• Automation and higher throughput: further miniaturization, multiwell autosampler workflows and software automation will help scale oligonucleotide QC for larger pipelines.
• Method generalization: adapting volatile-buffer, ion-pair-free RP methods to other oligonucleotide modalities (e.g., GalNAc-conjugated siRNA, mRNA fragments) and to non-denaturing versus denaturing conditions will broaden applicability.
• Software development: enhanced deconvolution algorithms and sequence-aware identification tools will improve confidence in low-level impurity detection and in distinguishing closely related sequences or modifications.

Conclusion

This work demonstrates that ion-pair-free reversed-phase LC/MS using ammonium bicarbonate as a volatile buffer, combined with a DUIS-equipped single-quadrupole LCMS-2050 and LabSolutions Insight Biologics software, can reliably confirm the intact molecular weight of siRNA strands. The siRNA dissociated into single strands under the conditions used and both sense and antisense were identified with mass errors under 1 Da. The approach reduces contamination risk associated with alkylamine ion-pair reagents and makes routine identity testing more accessible to laboratories equipped with single-quadrupole MS systems. For in-depth structural characterization or trace-level impurity analysis, complementary high-resolution MS workflows remain advisable.

Instrumentation used

• Nexera XS inert UHPLC system.
• LCMS-2050 single quadrupole mass spectrometer with DUIS heated ion source (ESI/APCI capability).
• Shim-pack Scepter Claris C18-300 column (50 mm x 2.1 mm I.D., 1.9 μm).
• LabSolutions Insight Biologics software for sequence definition and component identification.

Acknowledgments

This research was supported by AMED under Grant Numbers JP21ae0121022, JP21ae0121023 and JP21ae0121024 (Project leader: Satoshi Obika).

References

1. Ion-Pair Reversed-Phase LC/MS Analysis of GalNAc-siRNA Conjugates under Denaturing and Non-Denaturing Conditions. Application News No. 01-01177-EN.
2. Reversed-Phase Ion-Pair LC/MS Analysis of siRNA under Denaturing and Non-Denaturing Conditions. Application News No. 01-00915-EN.
3. Efficient Method Development for Separation of Capped mRNA Fragments. Application News No. 01-00898-EN.
4. Shimadzu Corporation. Ion-Pair-Free Reversed-Phase LC/MS Analysis of siRNA Using a Single Quadrupole Mass Spectrometer. Application News No. 01-01180-EN. First Edition: May 2026.