Extrapolated sample baseline

Point A is the intersection of the extrapolated straight-line portion of the low-temperature side of the peak with the baseline, and point B is the inflection point of the low-temperature side. Point C is the extrapolated peak temperature, while D is the extrapolated return to the baseline. The sample and reference temperatures at which the various points occurred during the melting of benzoic acid and the boiling of toluene are shown in Table 7.20. 

Linear extrapolated sample baseline used to calculate the enthalpy of transition... 

Generally, the extrapolated sample baseline is taken as a straight line joining the points of deviation from the sample baseline before T ) and after ) the peak. This procedure is approximate and... 

When measuring the enthalpy of transition by HS-DSC, it is important to establish the most appropriate extrapolated sample baseline. It is generally assumed that the sample baselines observed before and after the transition can be expressed as linear functions of temperature, and can be extrapolated into the transition region. For a system which is strictly two-state (A — B). the apparent specific heats are given by... 

Calculated extrapolated sample baseline for the HS-DSC heating curve, using equation 5.50... 

Extrapolated sample baseline Extension of the sample baseline of a DSC (or DTA) curve into the region of a phase change, used to calculate the characteristic temperatures and enthalpy change associated with the change of phase. 

Onset temperature Transition temperature defined as the intersection between the tangent to the maximum rising slope of a DSC (or DTA) peak and the extrapolated sample baseline. 

Two methods are commonly used to obtain isothermal data from DSC. The first method involves insertion of the sample into the DSC previously equilibrated at the required temperature. In the second method the sample is placed in the DSC cell at ambient temperature and the temperature is then increased at a controlled rapid rate to the required temperature. Small samples are used to ensure the sample temperature remain close to the required value. In both methods there is an initial off-balance signal and the output finally reaches a value corresponding to completion of the reaction. The baseline is usually taken as this final steady state signal, and horizontal negative extrapolation to intersect with the initial exotherm is taken as zero time for the reaction, as shown in Fig. 4. 

The large UV absorptions of nucleic acids and proteins make the optical method a highly sensitive technique, and very small amounts of sample in very dilute solutions can be used. However, the selection of baselines between which the fractional change in absorption is evaluated must be made carefully to avoid error. The best agreement with values obtained from calorimetric measurements is observed when the slopes of both baselines are extended into the transition region, and the determination of x and y are made between the extrapolated lines as shown in Figure 16.6a. 

Using these relationships, the enthalpy and/or entropy changes associated with the occurrence of phase transitions, melting, sublimation or decomposition of the sample can be determined. Such processes generally result in considerable changes in the heat capacity of the sample, which appear on the DSC trace as marked deviations from the extrapolated baseline. 

FIGURE 5.19 Recorded peaks for the reagent (R) and sample (S) solutions in a typical sequential injection system. M = monitored signal IP = iso-dispersion point wD, w wr, = baseline widths of the overlapped zone, sample zone and reagent zone respectively. Note that wD is obtained by extrapolation. For details, see Ref. [97]. Figure adapted with permission from "T. Guebeli, G.D. Christian,. Ruzicka, Fundamentals of sinusoidal flow sequential injection spectrophotometry, Anal. Chem. 63 (1991) 2407". Copyright 1991, American Chemical Society. 

The baseline at the end of the reaction was extrapolated to determine the total area under the exotherm curve and hence the isothermal heat of cure q.. Dynamic DSC analyses were run at a heating rate of 2°C/min. f)e8ween 30 and 200°C on each isothermally cured sample to obtain the residual heat of reaction... 

Figure 5 shows a typical mass loss in a decomposition experiment. The obvious definition would seem to be where the mass loss is steepest, which corresponds to the peak temperature T in the DTG plot. However, this is merely the point where reaction is fastest and does not represent the start of reaction, e.g. where bonds in the compound begin to break. The position of T will depend upon the sample size, packing, and heat flow properties. The point Tjis the initial temperature or onset temperature, but is not easy to identify and depends on the sensitivity of the balance and the amount of drift or noise seen. There may be traces of impurities, which decompose or promote some decomposition ahead of the main reaction. A better definition of start of reaction is the extrapolated onset temperature T. This requires drawing of tangents to the curve at the horizontal baseline and the steepest part of the curve and marking their intersection. For a reaction that starts very slowly and only speeds up later, T and Tj will be very different and a more satisfactory point would be shown as temperature where the fraction reacted a is equal to 0.05, i.e. Tq.05- Another definition of reaction temperature, important in kinetic studies, is when the reaction is half over, that is, when the fraction reacted... 

An alternative method [2] of determining Mi uses the fact that in power compensation DSC the proportionality constant between the transition peak area and Mi is equivalent to the constant which relates the sample heat capacity and the sample baseline increment. By measuring the specific heat capacity of a standard sapphire sample, an empty sample vessel and the sample of interest, from the difference in the recorded DSC curves of the three experiments Mi for the sample transition can be calculated. The advantage of this method is that sapphire of high purity and stability, whose specific heat capacity is very accurately known, is readily available. Only one standard material (sapphire) is necessary irrespective of the sample transition temperature. The linear extrapolation of the sample baseline to determine Mi has no thermodynamic basis, whereas the method of extrapolation of the specific heat capacity in estimating Mi is thermodynamically reasonable. The major drawbacks of this method are that the instrument baseline must be very flat and the experimental conditions are more stringent than for the previous method. Also, additional computer software and hardware are required to perform the calculation. 

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