The object of the procurement is a mass photometry system. Biocenter Oulu seeks a label-free optical measurement system which is capable of nanoparticle, including protein, molecular mass and mass distribution determination in native state (without surface immobilisation) with single molecule detection sensitivity/resolution, including possibility to measure high concentration samples. The system is used to study oligomerization state, biomolecular interactions, polymerization, sample composition, and purity. The mass photometer device must be able to measure mass of various biological nanoparticles including proteins and nucleic acids up to 20 nm size and with low sample amount. The system must enable measurement of different kinds of samples including high concentration (tens of micromolar) protein solutions. The mass photometry device must utilise attenuated optical interferometric back-scattering microscopy with reduction of signal to noise or signal to background ratio to determine mass of individual particles and mass distribution of a sample. When calibrated against known masses of same refractive index, the mass of unknown sample with similar refractive index is calculated from the interference between the light scattered from the molecule above the surface and the reflected light. The measurement is done on a slide or a chip that are not sample specific. Wide-field imaging camera is used for signal detection. The system will be part of the Structural Biology core facility infrastructure of Biocenter Oulu. The core facility has an open access policy, and the instrumentation and software must be easy to use and maintain. The system must have a user-friendly software to control the instrument(s) and analyse the measurement data. The associated software is expected to come with detailed documentation of all features and software upgrades are to be made available regularly. The instrument(s) and the software must be well suited to an academic multi-user environment. The contracting entity has a maximum sum of 260 000 EUR (excluding VAT) available for the procurement and cannot accept tenders exceeding this amount. The procurement is partially funded by the European Regional Development Funds (ERDF), BioParts project (A81573).