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:

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.

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, algebraicsumandabsolute 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, algebraicsumandabsolute 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