Edit Peak Aliasing

Move Aliased Peaks to Their Underlying Resonance Position

This popup window is used to move aliased ‘ghost’ peaks to their real underlying resonance locations by adding or subtracting a whole number of spectrum widths to the position in one or more peak dimensions. This is used when a resonance, that causes a peak, lies outside the normal recorded bounds of the spectrum but the peak nonetheless still appears within the spectrum, as an aliased signal that re-appears as if wrapped back onto the opposite side of the spectrum.

The minimum and maximum aliased frequency values for a spectrum dimension, as edited in the “Referencing” table of the main Spectra popup, may be set extend the contour display beyond the normal sweep width (ppm range) of the spectrum and thus cover the real ppm position of peaks that have been unaliased. In such instances the contours are extended by tiling; one or more copies of the contours (not mirror image) are made and placed sequentially next to the normal, fundamental region. If a peak is unaliased to a position outside the displayed spectrum limits then the contour display will naturally be extended to cover the new peak position; all peaks will be visible within the spectrum by default. However, the user may at any time reset the minimum and maximum aliased frequency for a spectrum display (see the Spectra popup); deleting all values will reset bounds to the original sweep with, but any values may be chosen, within reason.

A peak can be moved to the unaliased position of its underlying resonances by editing the “Num. Aliasing” column of this popup; double-clicking and typing in the appropriate number. When this number is changed for a peak dimension the peak will be instantly moved to its new location. An aliasing value of zero means that the peak lies within the sweep width of the spectrum. For a ppm scale having a positive aliasing value will reduce the ppm value, placing the peak a number of spectrum widths above or to the right of the spectrum bounds. Likewise a negative aliasing value will place a peak at a higher ppm value; below or to the left. Often aliasing values will be 1 or -1, where a peak has just fallen off the edge of a spectrum. For example a glycine amide nitrogen really at 100 ppm may be just outside the top of a 15N HSQC and be wrapped back into the bottom to appear as a peak at around 135 ppm, which means that the aliasing value should be set to 1, moving the peaks position up by a sweep with of 35 ppm, from 135 ppm to 100 ppm. The sign of the aliasing number may seem to be backwards, but it is perhaps the ppm scale that is ‘backwards’. More complex aliasing is often seen in 3D 13C spectra where the 13C axis can be folded (wrapped) to reduce the sweep width that needs to be recorded, but still avoiding peak overlap because of the way shifts correlate. For example a 13C HSQC NOESY may be recorded with a sweep with that covers the CA, CB range 25-75 ppm but aliased methyl carbons below 25 ppm and aromatic carbons between 110 ppm and 140 ppm will be present; the methyls will have aliasing values of 1, and the aromatics -1 or -2.

It should be noted that picking peaks in the tiled copies of a contour display, i.e. outside the sweep width, will automatically set the aliasing value for the peak to reflect the displayed chemical shift value. Thus, the user does not need to explicitly unalias the peak position. Any peaks that are moved by virtue of being unaliased will have their contribution to the chemical shifts, of any assigned resonances, adjusted automatically. Chemical shift values are always calculated using the underlying resonance positions, not the apparent peak position. Also, if it is known that many peaks share the same aliasing values, i.e. are in the same sweep width tile, then the user can propagate the aliasing value from one peak to many others in a single step via the right-click window menu; “Peak::Unaliasing propagate”.

Main Panel

Peak Dimension Positions

button Help: Show popup help document

button Close: Close popup

Table 1
Dimension The peak/spectrum dimension number
Isotope The kind of isotope measured in the dimension
ppm The position of the peak in this dimension, in units of ppm
Hz The frequency position of the peak in this dimension, in units of Hz
Points The data point position (in the spectrum matrix) of the peak in this dimension
Num. Aliasing Sets the number of spectrum sweep withs to add to the peak dimension position to locate it at its real ppm value. Note an aliasing of “1” moves a peak to a lower ppm (Editable)
Annotation The assignment annotation for the peak dimension

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