Foldover, or aliasing

Foldover, or aliasing, refers to the presence of spurious peaks caused by a spectral-window setting that is too narrow for a particular spectrum. If, for example the 1H spectral width is set encompass 0 to 10 ppm, but the sample contains a carboxylic acid proton which resonates at 12 ppm, the out-of-range peak will be "folded over", or "aliased" back into the spectrum, and will appear as an out-of-phase peak at around 8 ppm. Imagine that the entire spectrum is printed on a sheet of paper, but that you fold over the left side so that the crease is at 10 ppm. The peak at 12 ppm will be reversed, but positioned over the 8-ppm range on the chemical-shift scale. In general, an aliased peak will resist all attempts at phase correction; in fact, that trait is one of the hallmarks of such an artifact.