Absolute Molar Mass, Oligomeric State & Complex Stoichiometry — Without Column Calibration Standards
Creative Proteomics offers size-exclusion chromatography coupled with multi-angle light scattering (SEC-MALS) services for first-principles determination of absolute molecular weight, oligomeric state, and binding stoichiometry of proteins and their complexes. MALS detection eliminates the need for column calibration standards — every mass measurement is traceable to the physics of light scattering, not to a set of reference proteins that may behave differently from your analyte.
What SEC-MALS Reveals About Your Complex:
Measure mass from first principles. Know your complex, not a calibration curve.
SEC-MALS combines size-exclusion chromatography (SEC) with multi-angle light scattering (MALS) detection to determine the absolute molar mass of macromolecules in solution. Traditional SEC estimates mass by comparing elution volume to globular protein standards — an approach that fails for non-globular proteins, intrinsically disordered regions, glycoproteins, and complexes with elongated shapes. MALS overcomes this by measuring scattered light intensity at multiple angles simultaneously. Combined with a concentration detector (UV or refractive index), the Rayleigh-Debye-Gans light scattering equation yields true molar mass, independent of molecular shape or elution position.
Our system integrates three detectors in series: an 18-angle DAWN MALS detector, an Optilab differential refractive index (dRI) detector, and a UV absorbance detector. This three-detector configuration enables protein conjugate analysis — deconvolving the mass contributions of protein and its conjugated moiety (glycan, PEG, detergent, or drug) from a single SEC run.
If your question requires knowing the absolute mass and composition of macromolecular assemblies in solution — without relying on standards that may not represent your analyte — SEC-MALS is the definitive answer.
Absolute Molar Mass — First Principles, No Standards Needed
MALS determines molar mass from the fundamental physics of light scattering. Unlike calibrated SEC, the result does not depend on column calibration standards, molecular shape assumptions, or whether your protein behaves like a globular standard. An elongated monomer and a compact dimer may elute at the same volume but are unambiguously distinguished by MALS.
Three-Detector Conjugate Analysis
The combination of UV, dRI, and MALS detectors enables protein conjugate analysis — determining the mass of each component in a complex. For glycoproteins: protein mass and glycan mass, separately. For membrane proteins: protein mass and detergent micelle mass. For ADCs: drug-to-antibody ratio. This capability is unique to multi-detector SEC-MALS among chromatographic techniques.
Resolve Heterogeneity That Calibrated SEC Misses
Because MALS measures mass directly rather than inferring it from elution time, it detects species that calibrated SEC misidentifies. A partially unfolded monomer eluting early appears as a dimer by SEC alone — MALS reveals its true mass. Glycoproteins with variable glycosylation show mass heterogeneity invisible to UV alone.
Broad Applicability Across Biomolecular Classes
From small peptides (~5 kDa) to large viral vectors (~7 MDa), from soluble proteins to membrane proteins in detergent, from unmodified antibodies to complex PEGylated or drug-conjugated biologics — SEC-MALS provides absolute mass for virtually any macromolecular analyte that can be separated by SEC.
SEC-MALS is a versatile, quantitative platform for macromolecular characterization. Each service below leverages the technique's unique ability to provide absolute, calibration-free molar mass combined with multi-detector compositional analysis.
Sample Preparation & Filtration
Samples are buffer-exchanged into the SEC running buffer and filtered (0.22 µm) to remove particulates. Concentration is adjusted to the optimal range (typically 0.5–5 mg/mL, 20–100 µL injection volume). The running buffer is degassed and pre-equilibrated.
SEC Separation
Samples are injected onto an analytical SEC column (100 Å, 300 Å, or 500 Å pore size selected based on analyte size). An Agilent 1260 Infinity II HPLC delivers isocratic flow at 0.3–0.5 mL/min. Molecules are separated by hydrodynamic radius.
Multi-Detector Data Acquisition
The eluent passes sequentially through the UV absorbance detector (280 nm), the 18-angle DAWN MALS detector, and the Optilab differential refractive index detector. All three signals are recorded simultaneously for each eluting species.
ASTRA Data Processing
Raw light scattering, UV, and dRI data are processed in ASTRA software. Molar mass is calculated across each peak using the Zimm or Debye fitting method. For conjugate analysis, protein and modifier masses are deconvolved from the combined detector signals.
Peak Integration & Quantification
Peak boundaries are defined and integrated. For each peak: weight-averaged molar mass (Mw), polydispersity (Mw/Mn), mass fraction (%), and hydrodynamic radius (if DLS module included) are reported.
Report Delivery
You receive annotated chromatograms with molar mass overlay, peak quantification tables, conjugate composition analysis (when applicable), and a written interpretation summarizing oligomeric state, stoichiometry, and sample quality — ready for regulatory submission or publication.
Wyatt DAWN 18-Angle MALS Detector + Optilab dRI
– 18-angle light scattering detection for absolute molar mass from ~500 Da to >10 MDa
– Optilab differential refractive index detector for universal concentration measurement
– Agilent 1260 Infinity II HPLC with UV detector (280/260 nm)
– Optional in-line DynaPro NanoStar DLS for simultaneous hydrodynamic radius (Rh)
– ASTRA 8 software for Zimm/Debye fitting, conjugate analysis, and 21 CFR Part 11 compliance
The 18-angle MALS detector provides the angular coverage needed for accurate molar mass determination across a wide range of macromolecular sizes. For complexes that dissociate during chromatographic separation, composition-gradient MALS (CG-MALS) without a column is available upon consultation.
| Parameter | Recommended Specifications |
| Sample Types | Purified proteins, protein complexes, antibodies, ADCs, glycoproteins, membrane proteins (in detergent or nanodisc), AAV capsids, nucleic acids, PEGylated proteins |
| Purity | ≥ 90% recommended; partially purified samples acceptable if the target species is identifiable by SEC elution and mass |
| Concentration | 0.5–5 mg/mL (ideal); concentrations as low as 0.1 mg/mL possible depending on dn/dc and molecular weight |
| Injection Volume | 20–100 µL per run; 100–500 µL total for replicates and multiple concentrations |
| Amount Required | 50–200 µg protein per injection; ~0.5–2 mg total for a full characterization (multiple columns, replicates, conditions) |
| Buffer | PBS, Tris, HEPES, or phosphate buffers preferred. Must be compatible with SEC column chemistry. Degassing recommended. |
| dn/dc Value | Provide if known; otherwise we use standard values (0.185 mL/g for proteins, 0.170 mL/g for glycoproteins) or measure experimentally |
| Shipping | Ship on dry ice or ice packs. Include complete buffer composition, expected molecular weight, and any known oligomeric states |
SEC-MALS requires more sample than mass photometry but provides the richest compositional information of any single-run biophysical technique. Contact us for a feasibility assessment if you are unsure about column selection, buffer compatibility, or sample quantity.
Four techniques provide mass and stoichiometry information — but they differ fundamentally in sample requirements, resolution, and the type of information they provide. The right choice depends on your specific question.
| Feature | SEC-MALS | AUC | Mass Photometry | Native ESI-MS |
| Primary Output | Absolute molar mass per peak, conjugate composition, Rh | Sedimentation coefficient, MW, aggregation, empty/full ratio | Single-molecule mass distribution, oligomeric state | Exact mass, stoichiometry, ligand occupancy, assembly state |
| Measurement Basis | Light scattering + concentration — absolute from first principles | Sedimentation in centrifugal field — matrix-free | Single-molecule iSCAT — counts individual molecules | Mass-to-charge ratio in vacuum — native ionization |
| Conjugate Analysis | ||||
| Sample Amount | 50–200 µg per injection | ~50 µg total | <1 ng; 2–20 µL | 1–5 µg per injection |
| Speed | 20–30 min per run | Hours to days | 2–5 minutes | 30–60 min per sample |
| Mass Range | ~5 kDa – 5 MDa | >500 Da | 30 kDa – 5 MDa | ~10 kDa – >2 MDa |
| Aggregates | Yes — separate and quantify | Gold standard | Yes — detect by mass | Limited — aggregates may not ionize |
| Matrix Interaction | SEC column — may shear fragile complexes | None — true in-solution | None — glass surface landing | Gas phase — requires volatile buffer |
| Best Used For | Absolute MW, oligomeric state, conjugate analysis, aggregate quantification | Aggregation gold standard, AAV empty/full, biosimilarity | Rapid screening, oligomeric state, minimal sample | Exact stoichiometry, ligand occupancy, assembly heterogeneity |
SEC-MALS provides rich compositional information, but it is not the universal answer for every characterization question. Being transparent about its limitations helps you choose the right technique — or combine methods for a complete picture:
| If Your Question Is... | Why SEC-MALS Is Not Ideal | Use Instead |
| My complex dissociates during chromatography (weak/transient, Kd > µM). | SEC dilutes the sample 10–100 fold, causing weak complexes to dissociate on the column | AUC (matrix-free) or CG-MALS (no column) |
| I need kinetic rate constants (ka, kd). | SEC-MALS measures equilibrium states, not real-time binding kinetics | SPR or BLI |
| My sample is extremely limited (<10 µg total). | SEC-MALS requires 50–200 µg per injection | Mass Photometry (<1 ng per measurement) |
| I need residue-level binding interface information. | SEC-MALS reports mass and stoichiometry, not atomic contacts | XL-MS or HDX-MS |
SEC-MALS is often the starting point: confirm oligomeric state and purity in one run, then invest in deeper kinetic, thermodynamic, or structural analysis with confidence. Contact our team to design an integrated characterization strategy.
Absolute Mass Data — From Chromatogram to Interpreted Report
Each SEC-MALS project delivers a comprehensive data package suitable for regulatory submissions, publication, and downstream decision-making.
Annotated Chromatograms with Molar Mass Overlay
SEC-UV chromatogram with the MALS-determined molar mass trace overlaid across each peak. The horizontal mass trace confirms monodispersity within peaks and reveals the true mass of each eluting species.
Conjugate Composition Analysis
Three-detector deconvolution showing protein mass vs. conjugated moiety mass (glycan, PEG, detergent, or drug). Component mass fractions are quantified with associated uncertainties.
How does SEC-MALS differ from regular calibrated SEC?
Regular SEC estimates molecular weight by comparing elution volume to a set of globular protein standards. This approach assumes your analyte has the same shape and column interactions as the standards — an assumption that fails for elongated, disordered, or glycosylated proteins. SEC-MALS measures scattered light intensity and concentration directly, calculating absolute molar mass from the Rayleigh-Debye-Gans equation without any shape assumptions or calibration curves. An elongated 50 kDa monomer and a compact 50 kDa dimer would appear identical by SEC alone but are immediately distinguished by MALS.
What is protein conjugate analysis and when do I need it?
Protein conjugate analysis uses the combined signals from UV, dRI, and MALS detectors to deconvolve the mass of a protein from the mass of anything conjugated to it. This is essential for glycoproteins (what fraction is protein vs. glycan?), ADCs (what is the drug-to-antibody ratio?), PEGylated proteins (how many PEG chains?), and membrane proteins (what is the protein mass vs. detergent micelle mass?). If your protein has any modification that changes its mass or refractive index relative to the unmodified protein, conjugate analysis provides compositional information unattainable by any single-detector method.
Can SEC-MALS analyze weak or transient protein complexes?
Standard SEC-MALS is not ideal for complexes with fast dissociation kinetics (Kd > µM range) because the SEC column dilutes the sample 10–100 fold, favoring dissociation. For such complexes, we recommend composition-gradient MALS (CG-MALS), which performs the measurement without a column by titrating one component into another and measuring the change in light scattering at equilibrium. AUC is another excellent matrix-free alternative. If neither is available, chemical crosslinking to stabilize the complex before SEC-MALS can be an effective workaround.
What is dn/dc and do I need to know it?
dn/dc (refractive index increment) describes how much the refractive index of a solution changes with solute concentration. It is required to convert the dRI signal into concentration, which is then used with the light scattering signal to calculate molar mass. For unmodified proteins in aqueous buffer, a standard value of 0.185 mL/g is accurate to within ~2%. For glycoproteins (0.170–0.180), PEGylated proteins, or samples in unusual buffers, we can either measure dn/dc experimentally using the Optilab detector or use literature values. If you are unsure, we use the standard protein value and note the associated uncertainty.
How much sample does SEC-MALS require compared to mass photometry or AUC?
SEC-MALS requires the most sample of the three mass-measurement techniques: ~50–200 µg per injection, compared to mass photometry (<1 ng) and AUC (~50 µg total). However, SEC-MALS provides the richest compositional data per run — absolute mass, conjugate composition, aggregation quantification, and hydrodynamic radius — in a single 20–30 minute analysis. If sample is abundant, SEC-MALS is the most information-dense option. If sample is limiting, mass photometry is the better first choice.
Can SEC-MALS analyze membrane proteins?
Yes. Membrane proteins solubilized in detergent micelles, amphipols, or reconstituted into nanodiscs can be analyzed by SEC-MALS. The three-detector conjugate analysis method is particularly valuable here: it separates the protein mass from the detergent or lipid mass, providing the true molecular weight of the protein component alone. This is critical for confirming oligomeric state and stoichiometry of membrane protein complexes where the detergent contribution can add 50–200 kDa to the measured mass.
What SEC column should I use for my protein?
Column selection depends on your analyte size: 100 Å pore columns for peptides and small proteins (1–50 kDa), 300 Å for most proteins and antibodies (10–500 kDa), and 500 Å for large complexes, viral vectors, and AAV capsids (100 kDa – 5 MDa). Our team selects the optimal column during project consultation based on your analyte's expected mass, and we can run multiple columns if the oligomeric state is unknown.
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