Stan’s post is a little old, he may not monitor responses after this time!
Note that removing the DC offset is important before dividing into epochs and applying a window function before running an FFT. If you don’t remove the DC offset, the windowing function will still force the ends of each epoch to go to zero, but the middle of the epoch will still be somewhere around 4200uV, so you will end up with a very strange waveform when it hits the FFT. Not good!