Particle concentration is determined from the number of frames and the measured particle counts in the known volume of fluid captured in each frame. The combination of system magnification and flow-cell depth determine the accuracy of measurement.
his is determined by the requirement to avoid coincidence of particle images and depends on image size. MFI instrument concentration limits are determined using 2.5µm polystyrene beads.
MFI does not artificially generate any particles and, in principle, the lower concentration limit is zero particles per ml. For practical low concentration samples; baseline particle counts, sample transfer and volume of sample tested will determine the accuracy of the concentration measurement. In laboratory tests, MFI has been shown to reliably measure concentrations below 10 particles per milliliter.
Because small portions of the sample fluid lie outside the field of view or pass between successive frames, not all the sample which passes through the flow-cell is analyzed. Therefore the sampling efficiency is not 100%. When analyzing small volumes, investigating rare events and adapting to existing sampling schemes, it is desirable that sampling efficiency be well-quantified and as high as possible without double counting. Proper flow-cell design and selection of appropriate operating parameters can provide a sampling efficiency exceeding 85%. The combination of high sampling efficiency and sensitivity allow samples with only a few particles of interest per ml to be detected and accurately measured.
Flow cells are designed to provide high accuracy and reasonable service life in measuring protein formulations. Since the flow-cell depth and channel dimensions directly affect sampling efficiency and the accuracy of concentration measurement, these have the highest possible accuracy. The inside surfaces have a bio-inert hydrophobic coating.
