Quality control of oligonucleotides with a single quadrupole mass spectrometer

Importance of Topic

Quality control of synthetic oligonucleotides is increasingly crucial in research, diagnostics, and therapeutic development. Reliable confirmation of identity, purity, and yield ensures reproducible performance in downstream applications ranging from molecular biology assays to oligonucleotide-based drugs. Integrating mass spectrometric detection with high-performance liquid chromatography (HPLC) provides orthogonal data that extend beyond simple UV quantification.

Goals and Study Overview

This study demonstrates the effectiveness of a Thermo Scientific ISQ EM single quadrupole mass spectrometer, coupled to a Vanquish Flex Binary UHPLC system and controlled by Chromeleon CDS, for routine oligonucleotide quality control. Two complementary workflows are presented: direct confirmation via selected m/z species and an alternative approach using spectral deconvolution for mass determination.

Methodology and Instrumentation Used

Sample preparation and chromatography were performed as follows:

• Analyte: 52-mer single-stranded DNA (average mass 16,278.6 Da), 10 μM in water
• UHPLC: Vanquish Flex Binary system with Hypersil GOLD 2.1 × 50 mm, 1.9 μm column
• Mobile phases: A = water + 0.1% TEA + 2% HFIP; B = methanol + 0.1% TEA + 2% HFIP; gradient 10–90% B in 2.2 min, total runtime 5 min; flow 0.4 mL/min; 40 °C column temperature
• Detection: UV at 260 nm (2.5 µL flow cell) and mass spectrometry in negative HESI mode
• Mass spectrometer: ISQ EM single quadrupole (300 °C vaporizer, 350 °C transfer tube, −3,000 V spray voltage, sheath gas 75 psig, aux gas 7.5 psig); mass range 600–2,000 m/z; 0.5 s scan time
• Data system: Chromeleon CDS with Intact Protein Deconvolution (ReSpect™ algorithm, 10–20 kDa range)

Main Results and Discussion

Initial source settings produced multiple HFIP adducts and lower signal. Systematic optimization of vaporizer and transfer tube temperatures, gas pressures, and spray voltage increased signal intensity two- to threefold and suppressed adduct formation. Quadruplicate injections showed excellent reproducibility: retention time RSD <0.05%; peak area RSD <4% for both UV and MS.
The apex mass spectra exhibited deprotonated charge states z=12–18 alongside HFIP adduct peaks. Measured m/z values deviated by <0.25 m/z from theoretical values, allowing reliable confirmation using the most intense ion (m/z 1016.41, z=16) and additional confirmation ions.
Spectral deconvolution identified molecular masses of 16,280.19 Da (analyte) and 16,447.99 Da (HFIP adduct) with deviations of 1.59 Da and 1.39 Da, respectively. A minor species at 16,143.19 Da arose with low confidence (<14% abundance).

Benefits and Practical Applications

• Fast, robust QC workflow with 5-min runtime supports high throughput.
• Single quadrupole MS delivers sufficient mass accuracy for identity confirmation.
• Dual detection (UV + MS) provides comprehensive purity assessment, avoiding UV-only overestimation.
• Two flexible workflows accommodate known or unknown charge state distributions.

Future Trends and Potential Applications

Advances may include integration of automated source optimization via custom variables, expansion to modified nucleotides and larger oligonucleotides, coupling with data-driven QA/QC dashboards, and application in regulated laboratory environments. Emerging improvements in single quadrupole sensitivity and deconvolution algorithms will further streamline quality control of therapeutic oligonucleotides.

Conclusion

The ISQ EM single quadrupole mass spectrometer, when paired with optimized source settings and Chromeleon CDS deconvolution, enables rapid, accurate, and reproducible oligonucleotide quality control. Both direct m/z confirmation and spectral deconvolution workflows deliver reliable identity verification and purity assessment.

References

• Thermo Scientific Application Note 72820. Oligonucleotide characterization for quality control and increased productivity by single quadrupole mass spectrometer with extended mass range.
• Thermo Scientific Application Note 72976. Identity confirmation and accurate quantitation of a genotoxic impurity in an active pharmaceutical ingredient by UHPLC-UV coupled to a single quadrupole mass detector.