Complementary LC/MS and UV‑Vis Spectrophotometry for siRNA Quality Control and Thermal Stability Assessment

Significance of the topic

Small interfering RNAs (siRNAs) are an emerging class of therapeutic agents that require rigorous analytical characterization to ensure sequence integrity, purity, and functional stability. Reliable orthogonal analytical workflows are essential in development and QC to confirm duplex formation, detect single-strand contaminants or conjugates, and to evaluate thermal stability under formulation and analytical conditions. Combining chromatographic mass confirmation with spectroscopic thermal profiling delivers complementary data that inform design, process development, and routine release testing of siRNA therapeutics.

Objectives and study overview

This application study aimed to demonstrate an integrated analytical strategy for siRNA quality control by combining ion-pair reversed-phase liquid chromatography with mass spectrometry (IP-RP LC/MS) and UV-Vis thermal melt (Tm) analysis. Specific goals were to (1) resolve and confirm molecular masses of antisense (AS), sense (SS), and duplex species under non-denaturing and denaturing chromatographic conditions; (2) identify optimal LC column temperatures that preserve duplex integrity; and (3) quantify duplex thermal stability (Tm) in physiologically relevant buffer (PBS) and in LC elution solvent to show the impact of solvent composition on stability.

Methodology

Analytical approach and sample preparation:

• Materials: Inclisiran single strands and duplex purchased commercially and prepared in RNase-free water for LC/MS (10 pmol/µL) and at ~20 µg/mL for UV-Vis Tm measurements.
• Buffers: PBS (0.01 M phosphate, 0.0027 M KCl, 0.137 M NaCl, pH 7.4) used as an aqueous physiological matrix; LC elution buffer comprised 15 mM triethylamine (TEA), 100 mM hexafluoroisopropanol (HFIP) in water with 25% methanol.
• Concentration and blanks: Duplex concentration ~20 µg/mL for UV experiments; corresponding buffer blanks measured.

Chromatography and mass spectrometry details (selected):

• Column: Altura Oligo HPH-C18, 2.1 × 50 mm, 2.7 µm.
• Mobile phases: Solvent A = 15 mM TEA/100 mM HFIP in water; Solvent B = Solvent A:methanol 50:50. Gradient from 10% to 70% B over 10 min; flow 0.5 mL/min.
• Column temperatures evaluated: 30, 40, 50, and 60 °C to probe on-column duplex stability.
• MS: Agilent JetStream ESI, negative polarity, m/z 700–3000 profile, detailed source and acquisition parameters optimized for oligonucleotides; deconvolution performed with OpenLab CDS spectral deconvolution.

UV-Vis thermal melt (Tm) protocol:

• Instrument: Cary 3500 Multicell UV-Vis spectrophotometer; pathlength 10 mm, sample volume 800 µL, mineral oil overlay to prevent evaporation.
• Measurement: Absorbance at 260 nm, temperature ramp 5 °C/min from 25 to 90 °C, data interval 0.5 °C; Tm determined from the first derivative of absorbance vs. temperature.
• Replicates: All melting experiments performed in triplicate.

Used instrumentation

The experimental workflow employed the following instruments and software:

• Agilent 1290 Infinity II Bio LC System (pump, multisampler, multicolumn thermostat, variable wavelength detector).
• Agilent InfinityLab Pro iQ Plus mass detector (JetStream ESI source).
• Agilent Cary 3500 Multicell UV-Vis Spectrophotometer with temperature probe and quartz semimicro cuvettes.
• Data systems: Agilent OpenLab CDS v2.8 and Cary UV Workstation software v1.6.

Main results and discussion

Chromatography and on-column duplex stability:

• At column temperatures of 30 and 40 °C, the siRNA duplex remained intact under IP-RP conditions, allowing baseline separation of duplex, AS, and SS species. AS eluted earlier than SS because the SS bears a GalNAc conjugate that increases hydrophobic retention.
• Increasing column temperature shortened retention times and improved mass transfer; onset of duplex melting occurred between 40 and 50 °C. At 60 °C the duplex was effectively denatured and chromatograms showed primarily single-strand signals.
• Broadened siRNA peaks under non-denaturing conditions reflect partial separation of diastereomeric phosphoramidate species.

Mass confirmation and spectral deconvolution:

• MS deconvolution produced observed molecular weights matching theoretical values within laboratory-appropriate tolerances: SS theoretical 8,642.5 Da observed 8,641.14 Da; AS theoretical 7,699.6 Da observed 7,697.79 Da. Duplex-related signals were also identified (observed masses near theoretical duplex values).
• The intact siRNA mass spectrum was congested due to overlapping charge-state distributions, partial duplex dissociation under ESI, and adduct formation. Advanced deconvolution algorithms in OpenLab CDS resolved overlapping envelopes and adduct-rich signals to permit reliable component identification.

Thermal stability (UV-Vis Tm):

• Tm in PBS (physiological buffer) averaged 82.5 °C (triplicate), indicating a highly stable duplex under aqueous conditions.
• Tm in LC elution buffer (15 mM TEA/100 mM HFIP + 25% methanol) averaged 46.8 °C, demonstrating a pronounced destabilizing effect of organic solvent and ion-pairing eluent composition on duplex stability.
• UV-Vis Tm trends were consistent with LC observations: the lower Tm in elution solvent explains on-column denaturation at elevated temperatures and illustrates sensitivity of duplex integrity to mobile phase composition.

Benefits and practical applications of the method

The combined IP-RP LC/MS and UV-Vis Tm approach offers practical advantages for siRNA analytics:

• Orthogonal confirmation: Chromatographic separation and accurate mass confirmation validate sequence integrity and distinguish duplex vs single-strand species.
• Method optimization: Systematic evaluation of column temperature and mobile phase reveals conditions that preserve duplex structure for accurate QC analyses.
• Formulation insight: Tm profiling in relevant buffers provides essential data on duplex stability under storage and administration conditions, informing formulation design and comparability assessments.
• Robust deconvolution: Advanced spectral processing allows interpretation of complex oligonucleotide spectra with overlapping charge states and adducts, making MS-based identification reliable for routine workflows.

Future trends and potential applications

Key directions and opportunities for broader application include:

• Expanded orthogonal panels: Integrating additional techniques—e.g., ion-mobility MS, native MS, capillary electrophoresis, and CD spectroscopy—can deepen structural and conformational characterization.
• High-throughput Tm screening: Multicell UV-Vis platforms facilitate rapid formulation screening to rank stabilizing excipients and optimize buffer composition for clinical formulations.
• Standardized acceptance criteria: Community-driven guidelines for mass accuracy tolerances and Tm comparability would improve interlaboratory reproducibility for therapeutic oligonucleotides.
• In-line monitoring: Development of LC conditions that preserve native duplexes coupled with gentle ionization could permit more routine direct analysis of intact siRNA assemblies during manufacturing control.

Conclusion

This study demonstrates that combining IP-RP LC/MS under controlled temperature conditions with UV-Vis thermal melt analysis provides a comprehensive, orthogonal workflow for siRNA quality control. The approach enables separation and mass confirmation of strands and duplexes, identifies chromatographic conditions that preserve duplex integrity (30–40 °C for the tested system), and quantifies solvent-dependent thermal stability (Tm 82.5 °C in PBS vs 46.8 °C in LC eluent). Together these methods support method development, comparability studies, and routine QC for siRNA therapeutics.

Reference

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4. Agilent Technologies. Interview: Evaluating an Innovative Analytical ID Testing Strategy for Oligonucleotides. Agilent Technologies case study, publication number 5994-5144EN, 2022.