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
Help: Show popup help document
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 |