925-456-1020 info@acornnmr.com

Using RD to correct a folded spectrum

NUTS HELP

Using RD to correct a folded spectrum

The Rotate Data (RD) command was created to handle digitally filtered data from Bruker (and Tecmag NT) systems.  RD performs a cyclic rotation of a specified number of data points.  This can be used for other purposes.

This is a spectrum with a low-field acid peak.  The window was incorrectly set to be too small, and the acid peak has folded in from the upfield edge of the spectrum, and appears at -2 ppm.  RD can be used to perform a cyclic rotation, so this peak can be displayed at its correct frequency.

 

The number of points to rotate can be determined using the cursor.  Press and hold the mouse button and place the cursor at around -1.5 ppm (for this example).  The data point at this frequency is 14,234.  The total number of data points is 16,384.  So rotating the spectrum by 2,000 points will move the acid peak off the right end of the spectrum, back to the left end where it belongs.

 

Type RD to display the RD parameter box.  Enter Right rotation of 2000 points and click OK.

 

The spectrum now looks correct, except for the chemical shift scale.

 

Setting TMS back to 0 ppm results in a correct spectrum.

Caveats:

Note that integration of the folded peak after this correction may not be accurate, as the peak was outside the window when the data were acquired.

If the data were acquired with parameters such that a significant linear phase correction is required, the folded peak will be out of phase, and cannot be phased with the rest of the spectrum, even after the cyclic rotation.  It is good practice to adjust the pre-acquisition delay to minimize linear phase correction.

Cyclic rotation will un-fold a folded spectrum ONLY when the data were acquired as true quadrature.  Bruker’s “sequential” data acquisition is not, and peaks fold back differently.  A peak which is off the left end of the spectrum folds in from the left end, not from the right end, so this trick won’t work.


Last updated 5/6/99.