Accurate Phase and Baseline Corrections

Here we show a general method for correcting both phase and baseline of a 1D spectrum, based on the semi-automatic routines of iNMR. We assume that the reader has never used the program. She can download this sample spectrum, unpack it with a double click and repeat our step-by-step instructions. She will discover that manual corrections are just as easy as automatic corrections, but more accurate.

Launch iNMR. When it asks you to select a file, choose any file internal to the downloaded folder. When you see the FID, press the “gears” icon. It applies the FT and automatic phase correction.

We want to observe clearly the phase and baseline anomalies of our spectrum. All the routines for automatic phase correction seem fantastic because spectra are miniaturized. With iNMR it is possible to see many more details, defects included. Press the plus key, on the keyboard, more times. On my iMac (with a 20” monitor) and this particular spectrum, I would press the plus key 9 times:

At the usual reproduction scales, this would appear as a good enough phase correction. Only a multiplet is really in phase, the one that falls between 3 and 4 ppm. Move the mouse near to it. While pressing the Cmd key, click at the frequency of 3.5 ppm, thus creating a vertical red mark. It will represent a request (to the phase correction module) to preserve the phase of the signal at this frequency. Click the icon like the one reproduced here at right to open the phase correction module.

Ideally you should never touch the slider at the left. Move the right slider until the peak at 7.13 ppm is in phase. The numerical values at the top of the module will read 11.20 and 41.44. Now move the red mark from where we created it to the singlet at 7.13 ppm. We are going to keep its phase stable while correcting the rest. This “stabilization” convention only applies to the right slider. This is why we advise not to touch the left one. Move the middle slider to the upper position (“Fine”): this makes easier the interactive adjustment of the phase. What we need now is to make the tails of the signal at 1 ppm symmetric and also to simplify the baseline. Look at latter:

It seems broken in two parallel branches. This is a typical sign of a wrong phase correction (it is better, instead, to have two convergent branches or no branches at all). Move the right silder again, until the number at the top-right (“first order”)reads 43.74. The spectrum should look better. Close the phase module. To remove the red mark, Cmd-click on it (the same operation that creates it, deletes it).

While manual phase correction is generally more accurate than the automatic method, with baseline correction the situation is reversed. Click the icon like the one here at left. A dialog appears under the window title and a red line appears over the baseline: it is the automatic correction proposed by iNMR. We are going to choose a more traditional polynomial. It is a less efficient approximation but with two operative advantages: it can be easily removed (with the command “Process/Clear Correction”) and it allows for further phase correction, should you need it. Select the option “Fit with a Polynomial” and the 7th degree as shown here below:

The red line changes every time you choose a different combination of parameters. Normally the fit improves gradually to a maximum and then decreases. When fitting with a polynomial, the best result is often given by a high degree. Press “OK” to subtract the red line from the spectrum.

Another thing you may need to correct is the ppm scale. If you already know the exact frequency of a peak, for example the TMS peak, ctrl-click on it. A dialog appears. Enter the known frequency of that peak. You may have noticed that all the dialogs contain a pinkish help button. It opens the the corresponding page of the manual. It contains more in-depth information than this introductory tutorial.

In conclusion we have shown that iNMR can correct both the phase and the baseline with great accuracy. Even an absolute beginner should aim for perfection, combining manual with automatic methods and paying attention to a few telling details.