FB — Fix Baseline
This command enters a subroutine which fits the baseline with a 5th order polynomial.
As a starting point, the program selects a set of baseline points as follows: The spectrum is divided into 64 regions and those regions which contain just noise are selected. The data points within each selected region are averaged to give one point for input into the polynomial fit. The 64 selected regions are displayed in inverse video, usually red. The decision as to what constitutes noise is based on the RM multiplier value (same parameter as is used for peak picking). If the user finds that the regions being selected as noise are not appropriate, the RM value may be changed.
The user can also select or un-select individual regions using the mouse. Place the cursor on a region of the baseline to be selected and click once with the left mouse button. This is a toggle process, so clicking on a selected region will un-select it.
To initiate the fitting process, type L to perform Least Squares fitting to the selected baseline points. The equation used is a 5th order polynomial whose coefficients are adjusted to fit the spectrum. The coefficients are displayed on the screen.
The calculated polynomial can be displayed by selecting Draw Polynomial from the Display menu or by typing P (shown in green below). The region selection can be changed and the fit recalculated by repeating the fit (C or L command).
When satisfied with the calculated baseline, typing A will apply the correction and exit the FB subroutine. To abort the process and exit without altering the baseline, type <ENTER>.
The Quick Baseline (QB) command performs the same process, but in an automated manner, without user interaction. It will act only on the currently displayed spectral region.
By default, the FB routine divides the spectrum (or expanded region) into 64 regions (or fewer, if very few data points are displayed). It is possible to override this, and specify the number of points in each region. This can be used to define narrower baseline regions, useful when the spectrum is very crowded, with only small segments of baseline between peaks. The FB command will accept either one argument, or 2 arguments. The first argument specifies the number of data points in each region. If there is a second argument, it is a multiplier for the RM parameter, used in the automatic determination of which regions contain peaks, and which are baseline.
FB n – where n is the number of data points in each baseline region
FB n m – where n is the number of data points in each baseline region, and m is RM multiplier
If an expanded region is displayed at the time the FB subroutine is entered, the fit will be confined to only that part of the spectrum. This allows a spectrum with very a distorted baseline to be corrected in sections. Be sure to execute a Ctrl-F first if the fit should be to the entire spectrum.
If the correction is made on a part of the spectrum, rather than the entire spectrum, there arises the question of what should be done to the rest of the spectrum. If no adjustment is made to the data outside the displayed region, discontinuities may be introduced at the ends of the corrected region. A logical way to address this is to apply a DC correction to the adjacent regions to avoid discontinuities. However, that would result in changing the baseline, and therefore to the integrals, in parts of the spectrum other than the selected region. The best choice in this case depends on the characteristics of the spectrum and the information being extracted from it, so the program allows the user to choose how this situation should be handled.
To keep the baseline continuous, and avoid discontinuities, the following command arguments can be used:
FB -c on
This enables the DC adjustment of the baseline to eliminate discontinuities.
To prevent any changes to the spectrum outside of the displayed region, use the following command:
FB -c off
This disables the DC adjustment of baseline, so that regions outside the current display region are unchanged.
A – Apply the correction
L – Initiate Least squares fit to the selected baseline points
P – Draw the calculated polynomial
See also: Segmented baseline correction
Last updated: 9/5/07