For decades, analytical chemistry has treated ensemble measurements as a given. Inject a sample into an HPLC system, monitor the eluent with a UV-Vis detector, analyze a fraction by mass spectrometry — in each case, you're watching billions, trillions, or quadrillions of molecules at once and drawing conclusions from their collective behavior.
That approach has carried the field remarkably far. But it comes with a blind spot.
Beneath the smooth curves of a traditional chromatogram lies a world of molecular heterogeneity. Molecules rarely behave identically. They occupy different conformational states, follow different kinetic pathways, and interact with stationary phases in subtly different ways. Ensemble measurements average all of that away.
Single-molecule spectroscopy (SMS) takes the opposite approach. Rather than observing the crowd, it watches individuals. For the separation science community, SMS is transitioning from a specialized biophysics tool into something genuinely useful for understanding what's actually happening inside a chromatographic column — at the scale where it matters most.
It's worth noting that the term "single-molecule spectroscopy" is used broadly here to encompass both spectroscopic and imaging-based methods that detect and track individual molecules.
What Is Single-Molecule Spectroscopy and How Does It Work?
SMS encompasses a suite of advanced optical techniques designed to isolate and detect signals from individual molecules. In practice, most approaches rely on single-molecule fluorescence microscopy, which offers exceptional sensitivity and low background noise — two qualities that are essential when you're trying to pick out one molecule against everything else in the sample.
The challenge is straightforward to describe but technically demanding to solve: maximize photon signal from the fluorophore of interest while minimizing background scattering from the solvent, support material, and surrounding matrix. Researchers address this using highly sensitive detectors — electron-multiplying charge-coupled devices (EMCCDs) or single-photon avalanche diodes — paired with specialized illumination strategies.
Three approaches are particularly common in chromatographic research:
Confocal microscopy: A laser beam is focused into a diffraction-limited spot, creating a femtoliter-scale observation volume. Individual molecules passing through or dwelling within the focal volume can be detected one at a time.
Total internal reflection fluorescence (TIRF) microscopy: TIRF uses an evanescent field to illuminate only a thin layer roughly 100 nm above a glass interface. By restricting illumination to that shallow region, background fluorescence from the bulk solution drops dramatically — making TIRF especially well-suited for studying how solutes interact with surfaces.
Highly inclined and laminated optical Sheet (HILO) microscopy: Where TIRF is limited to the near-surface region, HILO extends single-molecule imaging deeper into commercial chromatographic particles. That extra depth matters when researchers want to study what's happening inside the pore network, not just at the outermost surface.
Across all three approaches, the payoff is the same: distributions of fluorescence intensity, lifetime, polarization, and emission spectra that are impossible to access through ensemble methods, because they vanish the moment you average across a population.
Why Single-Molecule Spectroscopy Matters in Chromatography
Classical chromatographic theory describes band broadening in macroscopic terms — eddy diffusion, mobile-phase mass transfer, stationary-phase mass transfer, all captured in the familiar van Deemter equation. Those concepts are useful, but they're abstractions. They describe outcomes rather than mechanisms.
SMS provides something different: a direct experimental view of the molecular-scale events that produce those outcomes. That distinction turns out to matter quite a bit.
Unraveling Stationary Phase Heterogeneity
Ask a conventional chromatographic measurement what a stationary phase looks like, and it will give you an average retention factor. That number is meaningful, but it conceals a great deal.
Using single-particle tracking (SPT), SMS reveals something richer: individual molecules don't move continuously through the stationary phase. They undergo a stochastic sequence of adsorption and desorption events, binding to specific sites, dwelling there for varying lengths of time, then releasing and moving on. Measuring those dwell-time distributions — which are simply inaccessible through ensemble methods — gives researchers direct access to the heterogeneity of the adsorption landscape.
What that landscape often looks like in reversed-phase systems is instructive. The C18 surface is not uniform. Scattered across it are localized regions of enhanced affinity — adsorption "hot spots" such as exposed silanol groups or incompletely endcapped areas. A small population of these sites can bind analytes far more tenaciously than the surrounding C18 environment, producing long residence-time events that contribute to peak tailing and broaden retention-time distributions in ways that ensemble measurements simply can't resolve.
Visualizing Mass Transfer and Anomalous Diffusion
How does a protein actually navigate the pore network inside a chromatographic particle? It's a reasonable question that has been surprisingly difficult to answer.
Single-molecule tracking studies have shown that the answer is often: not smoothly. Transport through porous stationary phases can deviate substantially from ideal diffusion models. Individual molecules may become temporarily trapped in dead-end pores, encounter steric barriers, or experience locally restricted motion caused by structural heterogeneity in the pore network. Researchers describe these behaviors as anomalous diffusion — transport that doesn't follow the simple Brownian motion assumptions embedded in many classical models.
By revealing where molecules get delayed within chromatographic media, and for how long, SMS connects nanoscale transport phenomena directly to macroscopic column efficiency and mass-transfer performance.
Monitoring Electrophoretic Dynamics
Single-molecule techniques have also found a place in capillary electrophoresis (CE) and microfluidic separations. By watching individual DNA molecules and proteins migrate through polymer matrices under an electric field, researchers can observe how molecules stretch, contract, hook around obstacles, and relax during separation — events that collectively determine resolution and efficiency, but that are invisible in ensemble measurements.
From Fundamental Research to Practical Chromatography
SMS remains primarily a research tool rather than a routine analytical detector, but its implications for the practice of separation science are real.
Rational Column Design
Researchers have proposed quantitative frameworks — including a metric called the distribution function ratio (DFR) — to relate microscopic adsorption behavior observed by SMS to macroscopic chromatographic performance. These approaches could help guide the development of more homogeneous stationary phases and sharpen our understanding of what actually drives peak tailing.
Optimizing Bioseparations
Large biomolecules, particularly monoclonal antibodies, don't just passively adsorb to chromatographic surfaces — they can undergo conformational changes in the process. SMS can probe the orientation, conformational dynamics, and surface interactions of individual proteins while they're bound, providing information about how purification materials influence molecular behavior and, ultimately, product quality.
This matters beyond mAbs. As biopharmaceutical pipelines expand to include antibody-drug conjugates, oligonucleotides, and viral-vector-based therapies, the interactions between complex biomolecules and chromatographic surfaces become more varied and harder to generalize. Single-molecule approaches offer a way to investigate those interactions at the level of individual molecules rather than at the level of averaged populations.
Ultra-Sensitive Microfluidics
As analytical platforms continue to shrink toward single-cell and organ-on-a-chip formats, the line between separation science and single-molecule analysis grows increasingly blurry. Integrating SMS detection strategies with microfluidic platforms may eventually enable the characterization of ultra-low-abundance biomarkers that are simply out of reach for conventional workflows.
The Future of Single-Molecule Spectroscopy in Separation Science
The deeper contribution of SMS to separation science may be less about any specific finding and more about the kind of questions it makes possible. Instead of asking what molecules do on average, researchers can ask what the full range of molecular interactions within a separation system actually looks like — and then connect that picture to the chromatographic performance they observe at the column scale.
The ensemble average has served analytical chemistry well for over a century. It will continue to. But for researchers trying to understand why a column tails, why a bioseparation yields inconsistent results, or how a pore network slows mass transfer, looking at one molecule at a time is increasingly the most direct path to an answer.
References
Kisley, L., & Landes, C. F. (2014). Molecular approaches to chromatography using single molecule spectroscopy. Analytical Chemistry, 87(1), 83–98. https://doi.org/10.1021/ac5039225
Bishop, L. D. C., Misiura, A., & Landes, C. F. (2021). A new metric for relating macroscopic chromatograms to microscopic surface dynamics: The distribution function ratio (DFR). The Analyst, 146(14), 4268–4279. https://doi.org/10.1039/d1an00370d
Unni, S., Shaik, M., & Kisley, L. (2023). Single-molecule imaging in commercial stationary phase particles using highly inclined and laminated optical sheet microscopy. Analytical Chemistry, 95(4), 2133–2138. https://doi.org/10.1021/acs.analchem.2c03753
