Dear Flowers, We have encountered a perplexing problem with our flow cytometer, a FC500. We have noticed that the fluorescence intensity of back ground controls, such as isotype controls or unlabelled cells appear to increase rapidly over time, in the order of doubling every 20 seconds. This is quite troubling when setting negative background, especially when positive staining is a shoulder rather than a distinct population. We notice that it occurs on fl1 and subsequently fl2, however it is not obvious on fl4. We are told our 488 laser is behaving correctly. We have tested epithelial cell line, lymphocytes and cyto-comp (Beckman Coulter) lymphocytes and find large cells, but NOT small beads, exhibit this "fl1 drift" over time. We have also tested buffer ph and resuspending cells in isoton. We find that MFI fluctuates when the same samples are re-read and that this problem occurs most often in the evening, but not always. Often we do not seem to have a problem at all. The flow cytometer room is air-conditioned reasonably well and would not get over 27 during the summer. What could be the problem? All suggestions welcome. Thank you Katrina Walsh School of Dental Science University of Melbourne. email: kawalsh@unimelb.edu.auReceived on Wed May 9 13:38:00 2007
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