Using the Line Fitting (LF) subroutine.
This subroutine performs a Simplex fit to a set of peaks. The user must define a starting set of peaks before starting the fit process.
The parameters adjusted are each peak’s frequency, intensity, width at half height and ratio of Lorentzian to Gaussian lineshape. Starting with NUTS versions dated May, 2000, it is possible to "freeze" chosen parameters so they are not adjusted during the fit process. See below.
Note that LF is not included in NUTS Lite.
Because these 2 peaks overlap, integrating does not give an accurate ratio of their areas.
Start by expanding the spectrum to show just the region of interest. Enter the deconvolution routine by typing LF or by selecting Line Fit from the Tools menu.
Begin by creating a set of peaks that match the real spectrum reasonably well, as a starting point for the fitting routine.
There are 3 different ways to enter a set of peaks.
As shown above, place the cursor on the top of a peak and click once with the left mouse button.
A red Lorentzian line is drawn at the location chosen in the previous step.
A second way to input a peak is to place the cursor below the axis at the chosen frequency. NUTS will find the closest peak maximum and put a peak there.
Notice that the second peak which was entered is red, and the first peak has changed to green. NUTS builds a list of peaks as they are entered, and the "current" peak is always in red.
A third method, not shown here, is to let NUTS select peaks using the same criteria as normal peak picking. To use this method, type P or choose Peak Pick from the Fit menu.
It is possible to select a different peak to be the current peak by moving through the list, by typing F (Forward) and B (Back) commands, which are also available from the Edit menu.
The broader peak has been selected as the current peak, so it is now shown in red.
The default peak width did not match the actual width of this peak. The fit will proceed better if its width is adjusted.
Place the cursor on the peak at half height, and click the right mouse button. The width of the red peak changes.
(Users with a single button mouse should hold down the shift key and use the mouse button when instructions call for use of the right mouse button.)
Once all starting peaks have been created, and line widths adjusted, we are ready to start the fitting process. By default, 4 parameters are adjusted during the fit for each peak: amplitude (intensity, or height), width, position (frequency) and ratio of Lorentzian to Gaussian lineshape. Any of these parameters may be "frozen", so that it is not adjusted during the fitting process (see below).
The fit is performed by typing A or selecting Optimize All Lines from the Fit menu. This is a Simplex optimization. The optimization can be aborted by typing Q
The parameters for each peak can be examined by typing I or selecting Information on Line from the Fit menu. This brings up the box shown below.
Intensity and Area are reported in two ways, as absolute and relative numbers. The relative intensity is relative to the tallest peak in the spectrum equal to 100, so is usually a more convenient number. The absolute numbers can be used to compare different spectra.
Shift is shown in both Hz and ppm.
Each peak is fit as a combination of Lorentzian and Gaussian lineshapes, expressed as Fraction Lorentzian, which is always a number between 0 and 1.
The user can change parameters by entering values in any field except Absolute Area. When a value is changed, and Apply or OK is clicked, related values are updated to reflect the change, and the peak is changed accordingly.
It is usually helpful to set the Relative Area of a chosen peak to a convenient number, which is done by entering the desired value in the box for Relative Area. All area values are then scaled relative to this.
Note the 4 check boxes labeled "Iterate", next to the Intensity, Shift, Width and Fraction Lorentzian values. A check mark in a box indicates that this parameter will be adjusted during the fit. To "freeze" a chosen parameter, remove the corresponding check mark.
The parameters for each peak can be placed into the clipboard by typing T or by selecting Type Peak List to Clipboard from the Edit menu. This can then be pasted into any text editor, or displayed on the screen using Ctrl-B. The resulting table looks like the following:
LINE HZ PPM HEIGHT REL_HT WIDTH AREA REL_AREA FRACTION LORENTZIAN 1 2401.33 8.000 2716201 90.602 2.36 6408714 37.71 1.000 2 2389.61 7.961 1521459 50.750 11.17 16996376 100.00 1.000
The sum of the calculated peaks can be displayed, rather than the individual peaks, by typing S or choosing Sum from the Display menu. The sum is shown above.
The Display menu provides other display options, including the difference between real and calculated peaks.
Last updated: 5/30/00.