qNMR Purity Recipe Book (3 – Data Processing)

Data Processing

In this section I will consider just the basics of data processing before describing how to determine the purity. If you do this step carefully you will also have a chance to look closely at the data and satisfy yourself whether or not the spectrum quality is adequate for qNMR. Note that you always want to increase the vertical scaling significantly to see the nitty gritty of your data!

Exponential multiplication

The only Apodisation function you should consider using is Exponential multiplication. The object is to increase the SNR a little, and ensure that all signals decay to ~zero at the end of the FID. Using a “matched filter” is a good place to start. Select “Apodization…” under the Processing menu (keyboard shortcut: “W”), and you will see a dialogue resembling this:

Mnova reports the “Average T2*” at the top: 0.300 s in this case. What we need to do is (a) check the box to ensure Exponential multiplication is used, and (b) enter the same 0.30 Average T2* as the Hz value.

When you click on “OK”, this Apodisation will be applied and the spectrum automatically updated.

Phasing

Mnova will usually import the phase values saved by the spectrometer. If these are hideously incorrect you can use AutoPhase (Processing > Phase Correction > Automatic). But we need the phase to be correct to 0.1°, and you will almost always have to perform a manual phase correction (Processing ? Phase Correction ? Manual Phase Correction).
When you invoke manual phase correction you will see something like this:

The blue vertical line shows the tallest peak – which is where the PH0 will be applied. Whilst looking at the base of this peak, place the mouse cursor on the blue pad and drag the mouse whilst holding down the left mouse button. Hold down the ‘Ctrl’ key whilst dragging the mouse to make finer adjustments. Once the biggest line is correctly phased, repeat this process whilst holding down the right mouse button: the other peaks should be phased.

Baseline correction

It may not be terribly obvious that the baseline needs correction. In the spectrum, below, the lower trace (A) shows one common symptom of poor baseline, here evidenced in the “slope” the integrals – shown with a blue arrow.
The simplest and most robust solution to baseline correction is the “Bernstein Polynomial”, accessed using Processing > Baseline > Baseline correction (Keyboard shortcut “B”). You will see a dialogue like this:

Note that there are more sophisticated baseline correction routines available to you, such as the Whittaker Smoother. This must be used with care as it can play havoc with integrals if used incorrectly. Email me if you want some instruction on using this. In most cases the “Bernstein Polynomial” will be the best choice.

Multiplet determination [1]

The last task before processing task is to determine the multiplets in the spectrum and the number of nuclides for each multiplet.

Automatic Multiplets Analysis

This is invoked by selecting Analysis > Multiplets Analysis > Automatic, and the analysis will be performed automatically.

Automatic Multiplets Analysis with the Molecular Structure

Follow exactly the same process as above, but be sure to include the structures of the analyte and reference compound. This method is more robust in the determination of the number of nuclides.

Checking your procedure

It is very likely that the procedure you adopt will differ a little from what I have described, and you will likely want to perform tests from time to time to ensure that you and your NMR are doing things correctly.
I would therefore suggest that you make a sample of a molecule of high and known purity. Try to use the solvent and internal reference that you use most commonly. The result from this sample will tell you if everything is working well. If you can (and the compound is stable), make perhaps 3 tubes and seal them. This will allow you to check everything from time-to-time. I leave it to you to decide how frequently to do this.

1. Automated purity analysis supported within Mnova NMR, although currently not in the qNMR engine.