Baseline construction

As mentioned, the calorimetric applications of DSC involve the conversion of a peak area into the energy associated with a chemical reaction or with a physical process (e.g., fusion, vaporization). Because the trace of the peak does not by itself define an area, an appropriate baseline must be found. The complexities of the baseline construction and their influence on the measurement of heat by... 

W. E Hemminger, S. M. Sarge. The Baseline Construction and its Influence on the Measurement of Heat with Differential Scanning Calorimeters. J. Thermal Anal. 1991, 37, 1455-1477. 

Fig. 4. Representation of DSC output in the isothermal mode for an exothermic reaction, showing baseline construction...

Figure 2.11 Baseline constructions for PA-FTIR analysis of ethylene-propylene...

The choice of the reference band (or bands), however, influences less the results than the reproducibility of the baseline construction and the correct reading of the maxima. Modern software in FTIR spectroscopy are useful in automatic and reproducible location of baselines, maxima, integral values, and baseline-corrected absorbances. 

The start and end points of the polymer peak are defined and a baseline constructed on the chromatogram. 

It is usual in quantitative infrared spectroscopy to use a baseline joining the points of lowest absorbance on the peak, preferably in reproducibly fiat parts of the absorption line. The absorbance difference between the baseline and the top of the band is then used. A baseline construction is shown in Figure 5.2a. 

Baseline determination is one of the most difficult emd important tasks in peak-area allocation, since the location of the baseline greatly influences the value of the areas. Except for some simple algorithms, there are few details in the literature about baseline construction. 

Hemminger, W.F. and Sarge, S.M. (1991) The baseline construction and its irrfiuence on the measurement of heat with differential... 

Signal to noise ratio 1 % v/v ethylbenzene in carbon tetrachloride Measure the spectrum over range 62 to 5 ppm. Measure the peak amplitude, A, of the largest peak of the methylene quartet centred at 62.65 ppm and the peak to peak amplitude of the baseline noise between 64 and 55 ppm. The amplitude is measured from a baseline constructed from the centre of the noise on either side of the quartet and at a distance of at least 1 ppm from its centre. The S/N ratio is given by 2.5 A/hi) which has to be greater than 25 1 for a mean of five measurements... 

Integrate the stored detector signal twice, using the baseline construction parameters as directed below. 

Figure 1.7 DSC peak and two possible baseline constructions (a and b). Also shown is the effect of impurity (curve c) which reduces the melting point and broadens the melting profile.

Figure 34 Overall latent heat on heating, AWheaiing, and cold crystallization enthalpy, AWcc. as functions of the previous cooling rate measured for pure PCL and PCL mixed with 0.2 wt.% CNT (carbon nanotubes) functioning as added heterogeneous nuclei. The error bars include the influence of baseline construction on the results of integration. Reproduced with permission from Zhuravlev, E. Schmelzer, J. W. P. Wunderlich, B. Schick, C. Po/ymer2011, 52,1983-1997. ...

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