Particle counting apparatus, electronic

Particle counting of electron micrographs is performed on a computer screen with an automatic counting apparatus. In order to minimize errors, at least 2500 (but preferably 10000) particles are counted. 

The apparatus is an electronic, liquid-bome particle-counting system that uses a light-obscuration sensor with a suitable sample feeding device. It is the responsibility of those performing the test to ensure that the operating parameters of the instrumentation are appropriate to the required accuracy and precision of the test result. 

Painstaking attempts to identify the size of the infectious unit have in the past been necessitated for plant viruses, and also for animal viruses for which hitherto no method of assay for single virus particles has materialized. These include chemical fractionation, physical fractionation by means of separation cells in the ultracentrifuge and electrophoresis apparatus, and inactivation of the virus (Lauffer, 181,182,184,299). Because of electron microscopy and the excellent linearity of the plaque count, these methods have not proved necessary with the bacteriophages. 

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