Mass Photometery

Mass photometry measures the mass of individual molecules as they land on a glass coverslip, using interferometric scattering (iSCAT) microscopy. The scattering contrast of each landing molecule is directly proportional to its mass. A few thousand single-molecule events build a mass histogram. Fully label-free, in dilute solution, in minutes.

Experiment Examples

• Resolving co-existing oligomeric populations (monomer/dimer/tetramer) and their relative abundance in one measurement.
• Measuring the mass of an antibody-antigen complex and confirming binding stoichiometry.
• Characterizing the native mass of a detergent-solubilized or nanodisc-embedded membrane protein.
• Checking the quality and homogeneity of a sample destined for cryo-EM.

Sample Requirements

• Standard mass distribution / oligomeric state determination: Very low sample amount; working concentration typically 5 – 40 nM, in volumes of a few microliters. A single measurement may consume only femtomoles to picomoles of protein.
• Complex assembly studies: Mix components at near-stoichiometric, low-nM concentrations; the technique resolves co-existing species in one measurement.
• Mass range: from ~ 50 kDa to several MDa; very small proteins approach the detection limit.
• Buffer constraints: Standard aqueous buffers; very high detergent, glycerol, or sucrose content can raise background. Membrane proteins in detergent/nanodiscs are feasible with care.
• Crucial note on concentration: Because the measurement is at low nM, the sample must remain stable upon strong dilution — weakly bound complexes may dissociate.
• Moderate to low affinity complexes: Microfluidics setup allow rapid dilution before measurement, preventing strong complex dissociation.

Practicalities

• The slides for measurement have a limited shelf life of 6 months after the manufacture date. Once opened, they expire after 1 month. The smallest kit has 5 slides, which allows 30 measurements. Delivery in Switzerland can take 2 – 3 weeks.
• Optimal sample concentration is often determined during measurement. Bring your buffer for dilution and sample at your stock concentration.
• Use only freshly made and filtered buffers (at least 0.2 µm pore size filter).
• Most proteins are fine to be diluted in D-PBS at nanomolar concentrations. Commercial D-PBS from Gibco (can be purchased at Magasin Central) is already filtered and shows low background signal.