iNMR icon

Subtracting a Spectrum from Another One

You can subtract a spectrum from another to highlight the difference. Obtaining the addition, or any other linear combination, is just as simple.

Subtracting B from A to get: C = A - B

Step 1

Bring document A to the fore. Expand the region(s) with the most important peaks; in this way you can monitor what's happening.

Step 2

Choose Format > Overlay.

Step 3

You will see the list of the other windows. Check the box at the left of the name of B. Now the spectrum B is drawn over A.

Step 4

Set the pre-multiplication factor for B (it's reported into the last column, “amplif.”). Set a positive factor for subtraction and a negative factor for addition. You can type the value or modify it with the little arrows at the bottom of the same column.

Step 5

Click Subtract: you'll see the difference A - B instead of A.

B is still on display. You can still change the pre-multiplication factor, if you need. The subtraction too is reversible.

Step 6

When the result satisfies you, close the dialog. B disappears and only the difference spectrum remains.

Step 7

It's not enough to save A, because iNMR never overwrites the original spectrum. If you want to store the difference, choose File > Export and create a new iNMR document.

Not all overlays are suitable for subtraction. The two spectra shall have the same number of dimensions and belong to the same domain (time or frequency). The digital resolution (ratio between the number of points and the spectral width in ppm) must be the same. In case of bi-dimensional spectra, the number of points must correspond in each dimension. When the two spectra are mono-dimensional and their baselines have both been corrected, the difference spectrum will contain the difference of the two baselines corrections. It is not, however, necessary that both baselines have been corrected.

To obtain the highest accuracy, bring the amplification factor near to its optimal value before beginning the subtraction.

Related Topics

Comparing Two Spectra in Two Windows

Suppressing Nasty Signals