HALO 1000 Å OLIGO C18

HALO 1000 Å OLIGO C18 — Expert Summary

Significance of the topic

Long-chain oligonucleotide analysis is central to modern nucleic acid therapeutics, CRISPR applications, and advanced biologics quality control. Accurate separation and impurity profiling of long oligos (30–>100 nucleotides) impose stringent requirements on stationary phase pore size, mass transfer, and system backpressure. Columns that combine large pore size with superficially porous particle (SPP) design can improve resolving power and throughput while keeping backpressure manageable for longer column formats and coupling with high-resolution MS for impurity identification.

Goals and overview of the document

The document introduces the HALO 1000 Å OLIGO C18 column and summarizes comparative performance versus conventional wide-pore silica packings. Key aims are to demonstrate: enhanced separation and impurity profiling for long oligonucleotides; improved peak capacity and peak shape; lower backpressure enabling scalable column lengths; and robustness under elevated temperature and pH for demanding oligo analyses.

Methodology and approach

Comparative chromatographic experiments were performed using TEA/HFIP and DiBA/HFIP-based mobile phases under gradient elution. Test analytes included single-stranded DNA up to 90 bases and resolved mixtures up to 100 bases. Column formats compared included HALO 1000 Å SPP (2.7 µm) and commercially available wide-pore sub-2 µm silica packings (300 Å and 120 Å). Conditions varied column length (10–25 cm), flow rate, gradient slope, temperature (60 °C) and mobile phase composition to assess peak width, gradient span, peak capacity, band dispersion, and backpressure. Select analyses were coupled to high-resolution MS for product confirmation and impurity identification.

Used instrumentation

Key LC and MS hardware reported in the study:

• LC system: Shimadzu Nexera X2.
• MS system: Thermo Q-Exactive HF (high-resolution, negative electrospray ionization; MS1 resolution up to 120,000; typical m/z range reported 450–2000).
• UV/PDA detection at 260 nm with 1 µL flow cell for many comparisons; data rates 40 Hz.

Product/column properties specified:

• Stationary phase: dimethyloctadecylsilane (C18), endcapped; USP L1.
• Particle: 2.7 µm superficially porous particle (Fused-Core style).
• Pore size: 1000 Å (designed for long oligonucleotides).
• Carbon load: ~2.4%; surface area: ~22 m2/g.
• Operational limits: pH 2–9, temperature up to ~90 °C at low pH and up to ~85 °C at high pH (column lifetime dependent on conditions).

Main results and discussion

Improved separations for long oligonucleotides

The 1000 Å HALO SPP delivered narrower peak widths and a broader useful gradient span for long oligonucleotides compared with 300 Å sub-2 µm silica packings. These effects combined to produce higher observed peak capacities for challenging samples (for example, a crude 90-mer ssDNA with numerous impurities). Peak shapes for larger oligos also improved when using a more hydrophobic DiBA/HFIP mobile phase rather than TEA/HFIP in some cases.

Reduced backpressure and scalability

Because of the fused-core SPP design and larger pores, the HALO 1000 Å column ran with considerably lower backpressure than competing sub-2 µm wide-pore columns (example: 2.1 × 100 mm HALO at ~241 bar vs 300 Å FPP 1.7 µm at ~540 bar under the reported conditions). Lower pressure enables longer columns (up to 250 mm reported) to be used for enhanced resolution while keeping system limits manageable.

Pore size and flow-rate effects

At constant gradient volume, the larger pore SPP exhibited lower band dispersion, particularly for the 90-mer, and showed better efficiency at higher flow rates compared with smaller pore materials. For smaller oligos (e.g., 30-mer), the larger pore material still provided improved band width at lower flow rates, indicating advantages across a range of chain lengths.

Column length and resolution scaling

Resolution improved when increasing column length from 10 cm to 25 cm using appropriately scaled (shallower) gradients. Observed scaling followed chromatographic theory approximately proportional to the square root of column length (√L) when gradient duration was adjusted, supporting longer columns for resolving complex oligo mixtures to ~100 nt.

MS-based impurity characterization

Coupling the HALO 1000 Å column to a high-resolution Orbitrap allowed distinguishing and characterizing more than 100 impurities in a 90-mer crude ssDNA sample. Use of HRMS with negative ESI and high resolving power was essential for confirming full-length product masses and identifying closely eluting species.

Benefits and practical applications

Principal advantages of the HALO 1000 Å OLIGO C18:

• Enhanced peak capacity and narrower peaks for long oligonucleotides—improves impurity profiling and quantitation.
• Lower backpressure relative to sub-2 µm wide-pore columns—enables longer columns and scalable methods without exceeding instrument pressure limits.
• Robust under elevated temperature and near-neutral to moderately high pH—useful for method optimization and extended lifetimes when care is taken with buffers.
• Compatibility with high-resolution MS—facilitates confident identification of near-isobaric impurities and product confirmation.

Applications include therapeutic oligonucleotide development, CRISPR oligo characterization, quality control of long synthetic RNAs/DNAs, and research workflows requiring high-resolution impurity mapping.

Future trends and applications

Expected directions and opportunities:

• Integration of large-pore SPP materials with automated, scalable workflows for routine QC of therapeutic oligos.
• Further optimization of ion-pairing reagents (e.g., DiBA/HFIP variants) to balance chromatographic performance and MS compatibility.
• Broader adoption of longer column formats for ultra-high-resolution separations of complex oligo pools, enabled by low-pressure SPP technology.
• Method standardization across labs using constant retention or linear solvent strength approaches to compare efficiencies objectively.

Conclusion

The HALO 1000 Å OLIGO C18 column presents a targeted stationary phase solution for long oligonucleotide separations, showing measurable gains in peak capacity, peak shape, and operational flexibility versus conventional wide-pore silica packings. Its SPP architecture and large pore size reduce band dispersion for long chains while maintaining acceptable backpressure—facilitating longer columns and improved resolution. When paired with high-resolution MS, this column format enhances impurity characterization and supports advanced workflows in therapeutic oligo development and analytical QC.

References

Stoll et al., J. Chromatogr. A, 1744 (2025), 465687 — referenced for constant-retention/linear solvent strength methodology.

Instrument and product details referenced in document: Shimadzu Nexera X2 LC; Thermo Q-Exactive HF MS; HALO 1000 Å OLIGO C18 (2.7 µm SPP), product specifications as described in the source material.