Integrating in the Frequency Domain

[PAvibe]

I took a two sets of time domain data, converted them into the power spectrum(freq. domain) and now I need to integrate that result in order to get the magnitude. The only functions for integration I can find are for time domain measurements. Is there a way to integrate in the Frequency domain?

[Nanxiong_Zhang]

The integration function can be used for both time domain and frequency domain data. The differnce is the result of the integration having different phisycal insight.