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Thickness Correction details

Thickness Correction (Thickness pathlength correction) is used to remove a pathlength variation from a given set of spectra. To successfully perform a thickness correction a baseline needs to be drawn under a peak and the peak will be integrated over this baseline. The whole spectrum will be divided by the resulting area. As a prerequisite all samples will have to feature an isolated band which does not vary in concentration. Normalizing the full spectrum to this band will effectively remove the pathlength variation. This method is sometimes referred to as "internal standard".

The Thickness correction command employs a two-point baseline correction for the thickness band baseline and subsequently calculates the thickness correction with the selected thickness band. The command offers a variety of options to define baseline limits and the actual thickness band. In particular these are:

Baseline selection methods (equivalent to Peak area selection)

These particular ranges are always defined by a lower and upper limit. The lower and upper limit may in turn be defined by different methods. Therefore these selection methods always show two selectors. For the method "single" these selectors are two discrete vertical lines. For all other methods these selectors are two rectangle selection areas.  The different method are described in detail in the following:

Single

Both limits (StartX, EndX) are defined by a single numerical value. This value may be entered graphically by moving the vertical lines by grabbing the red boxes with the mouse or by directly entering numerical values into the fields StartX and EndX.

Average

Both limits will be calculated by the average of a certain area. These areas may be entered graphically or by directly entering numerical values into the fields Start Minimum, Start Maximum, End Minimum and End Maximum. Graphically the area position can be defined by moving the position of the colored selection rectangles with the mouse. The area range can be adjusted by dragging the gray tracker boxes next to the rectangles. The actual lower limit is then calculated as the average of all points in the lower limit area and actual upper limit is calculated as the average of all points in the upper limit area. These values are shown as red boxes in the corresponding area rectangle.

Maximum

Both limits will be calculated by the maximum of a certain area. These areas may be entered graphically or by directly entering numerical values into the fields Start Minimum, Start Maximum, End Minimum and End Maximum. Graphically the area position can be defined by moving the position of the colored selection rectangles with the mouse. The area range can be adjusted by dragging the gray tracker boxes next to the rectangles. The actual lower limit is then calculated as the maximum of all points in the lower limit area and actual upper limit is calculated as the maximum of all points in the upper limit area. These values are shown as red boxes in the corresponding area rectangle.

Minimum

Both limits will be calculated by the minimum of a certain area. These areas may be entered graphically or by directly entering numerical values into the fields Start Minimum, Start Maximum, End Minimum and End Maximum. Graphically the area position can be defined by moving the position of the colored selection rectangles with the mouse. The area range can be adjusted by dragging the gray tracker boxes next to the rectangles. The actual lower limit is then calculated as the minimum of all points in the lower limit area and actual upper limit is calculated as the minimum of all points in the upper limit area. These values are shown as red boxes in the corresponding area rectangle.

The following pictures show a few examples of the different methods:

This example shows a Thickness Correction with normalization by intensity and baseline selection with the method "average". The intensity can be selected by moving the red vertical line by dragging it with the red square. The baseline limits are selected by moving the position of the blue rectangles. The limit areas (width of the rectangles) are increased or decreased by using the gray tracker boxes of the selected rectangle.

This example shows a Thickness Correction with normalization by "peak area" and method "single". Therefore the actual peak area for normalization is selected by the two vertical red lines. Again the blue rectangles define the baseline limits. The actual limits calculated by averaging all points inside the rectangle are shown by the red squares inside the rectangles.

Thickness correction algorithm

Normalization

The following normalization methods are available:

By Intensity

The thickness band will be selected by intensity. The user selects the desired peak by either graphically moving the red vertical line or by directly entering a numerical x-value.

By Spectrum Area

The whole spectrum area is used for normalization. The user additionally needs to select an integration method for the peak area calculation. Available options are trapezoid, algebraic sum and absolute sum.

By Peak Area

The thickness band is defined by a peak area. The peak area (thickness band) selection is available via four different methods and is identical to the baseline selection method described above. Essentially the user will have four selection rectangles to graphically select the baseline area (two blue rectangles) and the peak area (two red rectangles). The pictures below shows an example.
Again the user additionally needs to select an integration method for the peak area calculation. Available options are trapezoid, algebraic sum and absolute sum.

Peak Area Calculation

If a peak area is selected for normalization, an additional calculation method needs to be chosen. The options are:

  • Trapezoid
    This method calculates the peak area as half of the sum of all intensities multiplied by the data point distance (resolution) accordingly

  • Algebraic Sum
    This method considers the area under a peak. It is calculated as the sum of all intensities multiplied by the data point distance (resolution) accordingly

  • Absolute Sum
    This method calculates the peak area as the sum of all intensities within peak range