Particle analyzers are calibrated for size and concentration accuracy using polystyrene beads which are NIST-traceable and provide an absolute reference for these parameters.
However, particles found in many real-world samples are more transparent than beads. If a particle is sufficiently transparent, the contrast of some or all of the pixels in the particle image may not be sufficient to trigger the system threshold. These particles will then be undersized, seen as fragmented images or missed entirely. An undesirable consequence would be that measurements on the same sample by different instruments may vary depending on small variations in instrument sensitivity.
These effects may be reduced by employing a fixed threshold having the minimum value consistent with optical noise. However it has been found that the range of adjustment of optical field and illumination where accurate and consistent measurements of bead size and concentration can be obtained still allows for significant variations in the measurement of more challenging particle populations. In other words, the polystyrene bead particle standards do not sufficiently challenge the sensitivity of the optical particle analyzer.
A further sensitivity challenge standard is therefore required which allows different instruments to be assessed and adjusted to achieve consistent results when very low contrast particles are present. The challenge standard must be sufficiently stable and repeatable so that long-term and multi-site instrument sensitivity verification is possible.
ProteinSimple has addressed this requirement by identifying a stable mineral-based Sensitivity Reference Suspension (SRS) which has similar optical properties and measurement behavior to challenging sample populations. Individual MFI/DPA instruments are optically configured and verified for achieving a consistent depth-of-field profile using this surrogate. This approach has been found to provide consistent high sensitivity and minimize inter-instrument differences.
