High countrate real-time FCS using F2Cor

We present a fluorescence correlation spectroscopy setup based on a software correlator. The setup can process autocorrelation curves in real-time at countrate as high as 8 MHz, with time resolution of 1 µs. It uses the F2Cor autocorrelation algorithm, a low cost counting board and a desktop computer. Symmetrical normalization, which improves the signal to noise ratio of the FCS curve for large values of the lag-time, is adapted to the F2Cor algorithm. A new acquisition mode, which we call oscilloscope-mode, is presented. It takes advantage of the flexibility F2Cor, and proves to be very useful for optical setup adjustment. As an application of this setup, we performed FCS measurements on a reference tetramethylrhodamine solution at high concentration, up to 2.5 µM, which extend to the micromolar range the concentration applicable in FCS, using a conventional optical setup. At such high countrates the FCS curves need to be corrected for dead-time of the photo-detector, which was done successfully.