Alice Givan wrote: >We have found that using distilled/de-ionized water in the sheath is >noisier than >buffer on our cytometers. After puzzling over the question of >contaminants in the >water and after some discussion with a few gurus, we have concluded that >the noise >is not from contaminated water but may result from light bouncing off the >refractive >index change at the water (sheath): buffer (sample core) interface. That's correct; the buffer/sheath refractive index mismatch usually isn't a problem for eukaryotic cells but definitely will be for bacteria or really small (0.5 um) beads and may be for platelets. The noise is predominantly in the scatter channels; fluorescence measurements should not be affected significantly although variable scattering of excitation and emission at the interface might be expected to increase CV's slightly. However, don't be too quick to put blame on "refractive index mismatch" when the noise is due to particles in the sheath fluid. We use distilled/de-ionized water most of the time, with an in-line 0.22 um filter cartridge in the sheath line to supplement the 0.22 um filter on our Milli-Q deionizer. -Howard
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