Aliasing

Aliasing is the appearance of resonances at somewhere other than their natural chemical shift in the indirect dimension(s) of multi-dimensional spectra. Aliasing can be used to reduce the time taken to acquire spectra without sacrificing resolution or sensitivity.


Modern spectrometers use digital filters to eliminate signals outside the desired sweep width in the detected dimension, but signals in indirect dimensions wrap back in. With judicious choice of sweep widths and parameters this can be taken advantage of. The unaliased spectrum of strychnine shown below was collected with a ¹³C sweep width of 160 ppm centered at 75 ppm. The aliased spectrum was collected with the ¹³C sweep width reduced to 80 ppm and centered at 50 ppm. The number of slices collected in the aliased spectrum was reduced to half that of the unaliased spectrum, resulting in a spectrum collected in half the time, yet with the same resolution and signal to noise.

The aromatic resonances in the aliased spectrum appear around δ_C 45 ppm rather than δ_C 125 ppm. The true chemical shifts can be recovered by adding the sweep width to the observed chemical shift. In this case, adding 80 ppm to the observed δ_C 45 ppm gives the true value of δ_C 125 ppm. In this case I have only folded downfield (large δ_C) resonances, but it is also possible to fold upfield resonances, and to fold both upfield and downfield resonances simultaneously.

When setting up an experiment with intentionally folded dimensions it is often useful to test a few different values for the sweep width and offset until you get the peaks just where you want.

Aliasing occurs in spectra recorded using the States quadrature method. The TPPI quadrature scheme "folds" resonances rather than aliases them.  Where aliasing wraps the resonances around to appear on the opposite side of the spectrum, folding places the resonances close to the edge that they were excluded from. See Figure 7.4 in Protein NMR Spectroscopy by Cavanagh et al for a nice schematic of the difference. All of the standard parameters in use at the Skaggs NMR Facility use quadrature methods that will alias resonances rather than fold them.