Correcting the Baseline (1-D)
The baseline should ideally be flat and lie at zero intensity everywhere; otherwise the values of the integrals are not meaningful. Even when you are not going to integrate your spectrum, you may dislike seeing a rolling baseline. You can correct the spectrum by subtracting a model of its baseline.
This kind of correction is applied after phase correction. The simplest solution is to click the icon for automatic baseline correction (a slope with a gear). If you need finer control, follow these steps:
To Correct the Baseline of a 1-D Spectrum:
- Choose View > Full.
- It is important to observe the baseline, not the peaks. You'll normally move the spectrum upward with the Up-Arrow key and increase the intensity with the Plus key. This preparation is not strictly necessary. The same adjustments can be made while the correction dialog is open. This is always possible on the Mac. To make it possible on Windows, click the word “interact”, inside the baseline dialog.
- Choose Process > Baseline Correction.
When the baseline dialog opens, iNMR displays a red line representing the baseline model that will be subtracted from the spectrum.
By default, this model is obtained by smoothing the baseline in regions that do not contain peaks.
In regions where peaks are detected, smoothing would distort the signal;
therefore iNMR replaces those sections with straight-line segments that connect the surrounding baseline.
The controls in the dialog allow you to modify this model until it follows the baseline satisfactorily.
- A third option is: Process > Straight Baseline, which fits the beginning and the end of the spectrum with a straight line and subtracts it from the spectrum.
- If the red line coincides with your baseline, click OK and you are done. Otherwise, read on.
- After you click OK, the correction proposed by iNMR is subtracted from the spectrum.
- The default correction is a sequence of straight lines (where iNMR has identified a peak)
and sections of smoothed baseline.
- To increase the smoothness of the latter pieces, set Degree = 0.
- To prolong the straight lines, set filter = 128.
- Alternatively, you can fit the baseline with a polynomial function:
check the option at the top of the dialog.
- To improve the approximation of the baseline, set Degree = 5-7.
- Also reduce, gradually, the filter.
- In practice, you can experiment with the parameters until the baseline is modeled satisfactorily.
To remove the polynomial correction, simply choose Process > Clear Correction. Removing the correction performed with the smoothing algorithm is a little more complicated: choose File > Reload, then click the gear icon.
You can try different correction methods and repeat the procedure as many times as you like, although this is rarely necessary.