Calculate moments for Power Spectral Density

I calculated Power Spectral Density(PSD) with multitaper method (pmtm.m).

[y, fs]=wavread(filename);
[Pxx,f] = pmtm(y,4,[],fs);

Does anyone know how to calculate the central moments of PSD? I used matlab moment function, but the results are not correct. The first moment m1 is the mean found through the below

sumAmp = 0;
sumFreq = 0;
for j = 1:Nf
     sumAmp = sumAmp + Pxx(j);
     sumFreq = sumFreq + f(j)*Pxx(j);
end
m1 = sumFreq/sumAmp;

, but I can not figure out the rest, through searching on the internet.

Thanks a lot.