If you are ever doing an experiment where you need to know the original concentration of your sample (i.e. counting bacteria, absolute CD4+ counts) you will love the concentration calculator feature of FCS Express. Volume can be determined by either of two methods, either comparing with a known amount of reference beads or by time. You first set the region where the beads are. Then set the region where your sample is and press the calculate button. FCS Express does the rest!

The concentration calculator is being used to calculate the concentration of the cells in region 2. The sample was spiked with 5x10^5 reference beads that are in region 1. Alternatively, the acquisition time could have been used to calculate the volume.

You can even have the concentration shown directly on the statistics window of any plot.