I could not find the sampling frequency for fft to sample custom data

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I need to find the FFT of my EEG signal. You can think of the EEG signal as CSV data. I got 600 data in 10 seconds, and 14 cycles in 1 second and I need to specify the sampling frequency to find which frequencies are there. However, the current frequencies are changing according to the sampling frequency. For example; if I sample signals with 1000 Hz, My current frequencies are seen around 200 Hz.

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Y = fft (X);
Fs = 500; % Sampling frequency 
f = (0:length (Y)-1)*Fs/length (Y); % Frequency vector
plot (f, abs (Y))

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If I use 500 Hz, the frequencies with the same amplitudes are shifting to around 100Hz. How can I arrange sampling frequency to find the frequency levels in EEG Data?

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