NMR phase correction is a processing step to convert NMR signals in absorption/dispersion shapes into pure absorption mode by correcting frequency-independent zero-order (ph0) shifts and frequency-dependent first-order (ph1) shifts.

The need for zero order phase correction stems from an offset in the relative phases of the transmitter pulse and the receiver leading to a mix in real and imaginary parts of the data. This offset affects all parts of the spectrum equally and is therefore frequency independent.

First order phase shift stems from a delay between the pulse sequence and the receiver giving a frequency dependent distortion. These errors are proportional to the chemical shift and so affect different parts of the spectrum differently

To phase a spectrum first choose a large peak toward on side of the spectrum and select as the pivot point. Alter the zero order phase so that this peak is symmetrical and absorptive. Leaving the pivot point in the same place alter the first order phase so that peaks at the opposite edge of the spectrum are in phase. Save the phases. Zoom in to check that all peaks are correctly phase and adjust if required.

Manual phase correction for a 1D spectrum in Bruker Topspin is demonstrated here