Most NMR users know that phase correction is a very important adjustment that needs to be made to a spectrum. Practically speaking, the phase of a signal is related to the amount of signal observed above and below the baseline. Phase correction works to provide a signal in pure-absorption mode, which means a signal totally above and/or totally below the baseline (e.g. DEPT-135).
Phase correction involves adjusting both zero (ph0) and first-order (ph1) phases. The zero-order phase arises because the relative phase of the transmitter pulse and receiver are offset. This results in a mixing of the desired real part of the spectrum with a portion of the corresponding imaginary part, so one side of the base of each peak is observed to dip below the baseline (Figure 1). Zero-order phase correction undoes this mixing. The zero-order phase shift affects all frequencies in the same way, and is said to be frequency-independent.