Application of an optical immersion-gel in a flow cytometer with horizontally oriented objective

In certain flow cytometry systems, it is desirable to use immersion optics to obtain optimum fluorescence yield. This is important when propidium iodi de and other DNA fluorochromes are used that have weaker fluorescence emiss ion compared to DAPI, when a lamp is used instead of a laser and when the D NA concentrations are low. Our Partec PA II with a horizontally oriented ob jective and a vertically oriented flow chamber precludes using a liquid imm ersion medium. The problem was solved using an optical gel with appropriate characteristics. This gel is commercially available and commonly used for connecting glass fiber cables, but has never been used for microscopy befor e. Compared to the manufacturer's objective (40 x, aperture 0.8), the fluor escence yield was improved approximately four-fold using the optical gel an d a 40 x glycerol objective (aperture 1.25). This innovation widens the app licability of flow cytometers with horizontally oriented objectives and ver tical flow chambers. We expect it to facilitate the use of propidium iodide as a DNA stain, especially when interspecific genome size comparisons are to be done and base ratio dependent bias must be avoided.