Electrical sensing zone (ESZ) analysis
Coriolis Pharma is a leading contract laboratory providing analytical service of subvisible particles including the electrical sensing zone method (Coulter Counter), an orthogonal, non-light based technique for subvisible particles in the nm and µm-size range.
The electrical sensing zone (ESZ) method, also known as Coulter principle is based on the increase in the electric resistance by particles in an electrical field passing an aperture or pore between two electrodes. The method enables size and concentration determination for particles from submicron to visible size (~400 nm to 1600 μm) depending on the used system. It needs to be considered that different settings (multiple apertures) are required to cover this size range.
Sufficient buffer conductivity (about 20-150 mM NaCl, depending on the size range, e.g. 150 mM NaCl for sizes between 0.5 and 20 μm) is required, making formulation adjustment necessary for many non-salt based formulation, which needs to be considered.
One main advantage is that ESZ does not depend on optical properties of particles/formulation and is considered as a valuable orthogonal technique to the traditionally light based techniques (light obscuration, flow imaging microscopy). Care should be taken when comparing sizes obtained from different techniques for the same sample: ESZ, which measures the equivalent spherical diameter based on the solid part of the particle tends to measure higher counts at smaller size bins, as for example light obscuration or Micro Flow Imaging, which deliver the equivalent circular diameter based on the image including solid and liquid part of the particle.
Overall, the electrical sensing zone (ESZ) method can be seen as a valuable orthogonal R&D method to the more established light-based techniques light obscuration and Micro-Flow Imaging.