The “negative staining polystyrene bead protocol” (Monroe and Brandt, 1970, PMID: 4098102) is used to estimate the concentration of particles in a sample. The sample is mixed with a known concentration of polystyrene beads, negatively stained and a sufficiently large dataset of images is acquired to ensure appropriate particle counts. A senior analyst manually identifies and scores particles and polystyrene beads until the required total number of particles has been counted. The ratio of particles vs. polystyrene beads of known concentration is then converted to an estimate of the number of particles/ml.
These analyses help answer questions like:
What is the estimated number of particles of interest in a defined volume, e.g. number of particles/ml?