In the fast-moving world of biopharmaceutical analysis, yesterday's archives sometimes hide the keys to solving tomorrow’s complex analytical puzzles. This is precisely the case for slalom chromatography (SC)—a separation mode that researchers first described in the late 1980s, only to largely set it aside for decades.
Today, the rapid growth of genomic medicine, mRNA therapeutics, and gene delivery vectors drives a renewed interest in reviving and modernizing this unique chromatographic technique. Fabrice Gritti, Research Distinguished Scientist at Waters Corporation, discusses the history, intricate physics, and modern engineering developments that have turned a historical curiosity into a viable analytical tool for the 21st century.
The Historical Arc: From Theory to Practice
The story of slalom chromatography represents a journey of rediscovery. "When I started working in the field, I hadn't heard about slalom chromatography," Gritti recalls. "But in 2010, an older colleague of mine, Edouard Bouvier—who had been a student of the liquid chromatography pioneer Csaba Horváth—mentioned the name. He remembered it as a distinct retention and separation mode from the late 1980s."
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At the time, the market lacked a clear analytical demand to justify a major column development program. That equation changed dramatically with the onset of the COVID-19 pandemic. The rapid development and authorization of mRNA vaccines in 2021 and 2022 sent biopharmaceutical developers searching for more efficient ways to separate and characterize massive nucleic acid sequences, including large double-stranded DNA (dsDNA) intermediates and single-stranded mRNA.
Initial development programs in 2022 to apply slalom chromatography to these large biomolecules stumbled. "To summarize, we were not successful at first because we did not deeply understand the technique from a fundamental, physical perspective," says Gritti.
Recognizing that engineering cannot outrun basic science, researchers took a step back. In 2023, a shift in focus back to fundamental research allowed Gritti to decode the physics of this separation mode. A year of fundamental research yielded the necessary non-equilibrium and hydrodynamic models, enabling the successful design of research prototypes. By 2025, this research culminated in the commercial launch of a dedicated Slalom chromatography column: the GT xResolve Slalom column.
The Physical Mechanism: The "Coil-to-Stretch" Transition
To understand how slalom chromatography functions, chromatographers contrast it with its analytical counterparts, size-exclusion chromatography (SEC) and hydrodynamic chromatography (HDC).
SEC is a standard method for separating biomolecules ranging from small proteins to objects of roughly 50 nanometers (500 Angstroms) in size. Operators run SEC at low flow rates and low pressures to maximize resolution, eluting larger molecules first because the pores of the packing material exclude them.
Slalom chromatography operates on opposite physical principles, targeting a complementary size domain: molecules spanning from 50 nanometers to over 1 micrometer.
"We don't do direct comparisons between SEC and slalom chromatography because they address entirely different molecular size domains," Gritti explains. "But the fundamental difference lies in how operators run them. Slalom chromatography requires very high flow rates and pressures because we actively rely on hydrodynamic shear forces."
In slalom chromatography, there is no chemical absorption or physisorption of the analyte onto the stationary phase beads. Instead, the retention and separation occur entirely within the interparticle volume under laminar flow conditions. In the absence of turbulence, the narrow, twisting interstitial pathways between the packed microparticles create steep local velocity gradients.
When a large, flexible macromolecule travels through these gradients, it experiences strong extension and shear forces. This triggers a coil-to-stretch transition:
- The spherical coil: At rest or under low flow, a flexible polymer naturally exists as a random, roughly spherical coiled conformation. In this state, steric hindrance prevents it from approaching the surface of the packed particles too closely.
- The stretched filament: Under high-velocity gradients, the hydrodynamic drag forces the coil to unfold and stretch into a long, thin filament.
- The boundary layer trap: Once stretched, the molecule behaves like a flexible thread. It no longer experiences exclusion from the boundary layer near the particle surfaces. Under the fluid dynamics of a "no-slip" boundary condition, the local fluid velocity at the solid particle surface is exactly zero.
- Retardation: Because the stretched filament enters this low-velocity zone, the boundary condition significantly slows its migration.
Because longer polymer chains have a higher probability of stretching and getting caught in these zero-velocity boundary zones, slalom chromatography yields an elution order opposite to SEC and HDC: smaller molecules elute first, while larger, more easily stretched molecules undergo selective retardation and elute last.
Is the Stretch Non-Destructive?
A common point of skepticism for researchers is whether these high shear forces damage fragile genomic analytes. Gritti points out that biophysicists map this threshold precisely: "The force required to covalently break the phosphodiester backbone of a double-stranded DNA molecule is roughly 1,000 piconewtons. The shear forces generated inside our slalom columns range from just zero to ten piconewtons. We are orders of magnitude below the damage threshold."
Furthermore, this mechanism highlights why slalom chromatography is highly selective for double-stranded DNA, whereas single-stranded mRNA separates via a different pathway. The double-helix structure of dsDNA endows it with a high "persistence length" (inherent rigidity), meaning it forms a loose, easily unfolded coil at equilibrium.
Conversely, single-stranded RNA is extraordinarily flexible and folds into highly compacted, structurally intricate random coils. The entropic cost to stretch ssRNA is too high for the average two to three piconewton forces of liquid chromatography to overcome. Consequently, ssRNA remains coiled, experiences exclusion from the boundary layer, and separates solely by HDC, whereas dsDNA undergoes true slalom separation.
Modern Engineering: Why SC is Viable Today
If researchers documented the physics of slalom chromatography in the late 1980s, why did commercialization take nearly forty years? The answer lies in the limitations of 20th-century materials science and instrument engineering. Four major contemporary technological pillars make slalom chromatography viable today:
1. High-Purity Type B Silica
Early slalom experiments suffered from severe sample loss and poor reproducibility of column performance. Modern columns utilize ultra-pure Type B silica particles that are virtually free of residual heavy metals. Eliminating these metal impurities is crucial because they initiate strong, irreversible chelating interactions with the negatively charged phosphate backbones of DNA and RNA. Additionally, they offer high reproducibility for users.
2. Sub-2 Micron Stationary Phases and UHPLC
Achieving the shear forces necessary to stretch DNA requires high linear velocities and compact particle packing. The arrival of ultra-high-performance liquid chromatography (UHPLC) in 2004 changed the paradigm. Today, slalom columns pack stable 2.5 micron particles that routinely operate at pressures up to 15,000 psi (1 kbar backpressure), generating the precise, reproducible shear environment needed for the slalom effect.
3. Surface Passivation and Bio-Inert Hardware
Even with high-purity silica, some residual non-specific hydrophobic and electrostatic surface sites can strongly interact, which actively bind phosphorylated compounds. Column manufacturers solve this by chemically modifying the surface of the stationary-phase particles (typically using hydroxy-modified polyethylene oxide). This shielding prevents non-specific electrostatic and hydrophobic adsorption, dramatically improving sample recovery. Additionally, standard stainless-steel column hardware is rich in iron oxides, which also strongly bind to phosphorylated compounds. This problem is fully resolved by chemical vapor deposition of a thin protective layer onto the metal surface, which prevents sample loss by eliminating the direct contact between the DNA/RNA analytes and the system metal hardware.
4. Ultra-Precise Fraction Collectors
Because analysts run high linear velocities in chromatography, peak elution widths remain incredibly short—often lasting only a few seconds. Isolating pure target analytes for downstream workflows requires high-speed analytical fraction collectors that actuate with ten millisecond precision.
Solving Key Genomic Quality Challenges
The commercialization of modern slalom columns offers solutions to several of the most stubborn quality control challenges in genetic medicine manufacturing.
1. Plasmid Purity: Supercoiled vs. Linear DNA
In the production of mRNA vaccines and gene therapies, circular plasmid DNA serves as the template. Ensuring complete linearization of these plasmids is a critical quality control step.
Because circular plasmids cannot stretch easily under shear forces, while linearized plasmids stretch completely, slalom chromatography exhibits massive selectivity differences between the two topologies, resolving them into distinct, widely separated peaks in under 3 minutes.
Resolving open circular (OC) and supercoiled (SC) plasmids is historically more challenging. While researchers demonstrate that recycling chromatography (passing the sample back and forth between twin columns) can achieve this, the process requires 10 to 12 cycles and takes over 40 minutes.
To make this practical for high-throughput labs, chromatographers designed a hybrid method: hyphenating slalom chromatography with anion-exchange chromatography (AEX).
AEX separates molecules based on charge density, and the highly twisted supercoiled form has a much higher charge density than the relaxed open circular form. By combining an active salt
gradient with a high flow rate that leverages slalom stretching forces, this method completely resolves linearized, open circular, and supercoiled plasmids in just 10 to 15 minutes.
2. mRNA monitoring during IVT reaction
Because the coiled single-stranded mRNA are easily and rapidly separated from the stretched linearized plasmid in IVT reactors, it is possible to follow and quantify in real time the production of mRNa.
3. Double-Stranded RNA (dsRNA) Impurity Profiling
During in vitro mRNA transcription, the reaction often generates double-stranded RNA (dsRNA) as an unwanted byproduct. Because dsRNA triggers a strong, potentially dangerous immunogenic response in humans, regulatory bodies require manufacturers to monitor and minimize dsRNA impurities to picogram levels.
Because ssRNA remains coiled and elutes rapidly, while dsRNA stretches and undergoes retardation, slalom chromatography theoretically offers a suitable option. However, a major hurdle emerged: "The limit of detection for standard UV optical chromatography is roughly 100 picograms," explains Gritti. "But manufacturers must prove their vaccines contain less than one picogram of dsRNA. We face a two-orders-of-magnitude sensitivity gap."
To bridge this gap, researchers design an inline affinity pre-concentration column. By running a large volume of the mRNA sample through a column that selectively captures dsRNA while letting the target single-stranded mRNA flow to waste, analysts can accumulate and concentrate the impurities before eluting them into the slalom column. This technique already pushes the chromatographic detection limit down to 10 picograms, with ongoing research aiming for the 1 picogram threshold.
4. DNA sample preparation for downstream processes and NGS
DNA sample preparation by gel extraction is usually long, tedious, and typically causes 50% DNA loss. This problem is resolved by coupling the GTxResolve Slalom Column to the ultra-precise fraction collectors. 95% pure DNA fractions are collected while preserving the sequence integrity and its biological activity. This purification is key for next-generation sequencing such as the long-read Oxford nanopore sequencing technique.
The Economics: Replacing the Gel
For the average laboratory manager, migrating away from traditional agarose gel electrophoresis is as much an economic decision as a scientific one. While gels have been the standard for decades, they are labor-intensive, slow, and semi-quantitative.
Gritti outlines a clear case for transitioning to UHPLC-based slalom chromatography:
Attribute | Agarose Gel Electrophoresis | Slalom Chromatography Column |
Analysis Time | 1 hour (longer if band extraction is needed) | 3 minutes |
Data Output | Semi-quantitative band visualization | Highly precise concentration and size (plus or minus 50 base pairs for a 6,000 base pair target) |
Consumables Cost | Agarose, buffers, staining reagents | Standard mobile phase (reduced by a factor of four) |
Fixed Equipment Cost | Low (less than 1,000 US dollars) | High (approximately 70,000 US dollars for UHPLC; approximately 3,000 US dollars per column) |
Labor/Automation | Manual pouring, loading, and imaging | Fully automated, high-throughput autosampling |
"The entry barrier is undoubtedly the fixed cost," Gritti admits. "If you do not already have a quality control (QC) liquid chromatography lab, investing in a high-end UHPLC instrument and column is a major capital expenditure. However, if you are running dozens of samples daily, the four-fold reduction in consumable costs, combined with a workflow that takes three minutes instead of an hour, yields an incredibly fast return on investment by slashing labor costs."
The Next Frontier
While genomic medicine remains the primary driver for slalom chromatography, research already looks toward broader horizons in polymer chemistry. The physics governing modern slalom columns is universal: slalom chromatography can separate any macromolecule provided its molecular weight is sufficiently high (typically greater than one megadalton) and it possesses an inherent backbone rigidity (persistence length) comparable to double-stranded DNA.
"We are preparing to publish a paper demonstrating slalom chromatography on non-genomic polymers," Gritti reveals. "Specifically, we focus on a modified natural polymer: hydroxyethyl cellulose (HEC). Despite its highly heterogeneous length distribution and chain branching, we successfully fractionate its distinct molecular populations by pairing a slalom column with multi-angle light scattering (MALS) detection."
Though polymer characterization represents a more niche application compared to the critical needs of biopharma QC, it serves as a powerful validation of the underlying fluid-dynamics coil-to-stretch mechanism. By proving that highly reproducible physical laws govern the separation mechanism of Slalom chromatography, researchers establish a robust, future-proof separation platform. As the size and complexity of therapeutic molecules continue to grow, slalom chromatography is poised to transition from a resurrected classic to an indispensable cornerstone of modern analytical science.
