The sample frequency will be determined by how long your signal is in e.g. seconds. This then gives a hard link between frequency in Hz and your sample length.
If you don't know what the length of your signal is in time then you can still just work with normalised frequency, but it depends what you want to interpret from it. If you want to look at, for example, how much power is in your signal between 20Hz and 30Hz then you will need the sample frequency to map from normalised frequencies onto the Hz range as
frequencyHz = normFrequency * sampleRate;
Where normFrequency runs from 0 to 1, 1 being your sample frequency, 0.5 being the Nyquist frequency. Or -0.5 to 0.5 if you are going to mess with negative frequencies too I guess.
