3.1. BASELINE CONSTRUCTION

Generally the application of straight-line form for the baseline is incorrect [4]. The recorded signal results not only from the heat of the reaction but is additionally affected by the change of the specific heat of the mixture reactant-products during the progress of the reaction.

The construction of the baseline is done by drawing the appropriate tangents at the beginning or the end of each curve.

Recommendations:

Please always use the button ‘zoom’



to zoom the area at the beginning and at the end of the signal before drawing any tangent. This approach enables one to draw the tangents as precisely as possible.

Construction of the tangent at the beginning of the signals:

Once the area at the beginning of the signal has been zoomed, activate the mouse pointer to draw the tangent at the left by clicking on the button:



Then move the mouse to the beginning of the signal. Click on the left mouse button and hold the left mouse button by dragging the mouse towards the right for drawing the left tangent as precisely as possible. Once the tangent is accurately positioned, release the left mouse button to fix it. If the tangent is not accurately constructed, this operation can be repeated by clicking on the button ‘Undo’.





Click on the button ‘Unzoom’ and now draw the tangent at the end of the signal.

Construction of the tangent at the end of the signals:

The construction of the tangent at the right of the signal is done in a similar way.

Once the area at the end of the signal has been zoomed, activate the mouse pointer to draw the tangent at the right by clicking on the button:



Then move the mouse to the end of the signal. Click on the left mouse button and hold the left mouse button by dragging the mouse towards the left for drawing the right tangent as precisely as possible. Once the tangent is accurately positioned, release the left mouse button to fix it. The baseline appears in red. If the tangent is not accurately constructed, this operation can be repeated by clicking on the button ‘Undo’.



If necessary, the slope of the tangent at the beginning of the signal can be slightly modified by clicking on the button ‘Modify left tangent’,



and then by clicking on the left mouse button at a new position in the plot.

Similarly, the slope of the tangent at the end of the signal can be modified by clicking on the button ‘Modify right tangent’,



and then by clicking on the left mouse button at a new position in the plot.

Once both tangents are drawn, the baseline automatically appears in red on the plot.

Then click on the button ‘Unzoom’ to get the complete visualization of the baseline with both its tangents at the beginning and at the end of the signal.


Figure: DSC heat flow curve and the baseline calculated for the heating rate of 8°C/min (Units of S(T) = [mW/mg]).

The construction of all baselines has to be done successively for all curves. Use the spinbutton



to switch between the curves.

Repeat the drawing of the tangents at the beginning and at the end of each signal. The notation ‘Done’ appears in blue at the right of the spinbutton ‘Curve selection’ once both tangents at the beginning and at the end of the currently selected curve are drawn (see red arrow). When all tangents of all signals have been drawn, the notation ‘Ready for Kinetics’ appears in blue on the right top corner of the plot (see red arrow).



Just click on the tabbar ‘Kinetics’ (see red arrow) to continue the evaluation. This operation subtracts the baseline from the signal and normalizes it. For DSC signal types, the heat of reaction is now displayed in the group box ‘Mean signal area’.


Figure: DSC heat flow curve after baseline subtraction and normalization.

=> (units of S(T)_Normed = [1/K])

With:
B(T) the baseline,
S(T) the differential signal,
the reaction progress x(T) can be expressed as


The tangential area-proportional baseline is the most universal type because of its correction possibilities. It is created at x(T) => 0 and at x(T) => 1 by the appropriate tangents at the beginning or the end of the measured DSC signal. It allows compensation of not only changes of the c_p values of the reactant and product, but also of changes in their temperature dependency. These types of baselines can be described by the following equation:

B(T) = (1-x(T))*(a1+b1*T) + x(T)*(a2+b2*T)

With

(a1+b1*T) : tangent at the beginning of the signal S(T).
(a2+b2*T) : tangent at the end of the signal S(T).

B(T) can be calculated iteratively. The convergence is achieved as soon as the relative average deviations between two iterations are smaller than an arbitrarily chosen value (for example 1e-6). In AKTS-Thermokinetics software the area-proportional baseline is calculated using arbitrarily 300 iteration loops.

It is obvious that the baseline determination can significantly influence the determination of the kinetic parameters of the reaction. Moreover, the correct baseline determination should be intimately combined with the computation of the kinetic parameters for the investigated reaction. Advanced mathematical procedures are therefore necessary for an objective calculation of the most appropriate baseline for each DSC signal.