Baseline instrument

Clearly the accurate measurement of the final (infinity time) instrument reading is necessary for the application of the preceding methods, as exemplified by Eq. (2-52) for the spectrophotometric determination of a first-order rate constant. It sometimes happens, however, that this final value cannot be accurately measured. Among the reasons for this inability to determine are the occurrence of a slow secondary reaction, the precipitation of a product, an unsteady instrumental baseline, or simply a reaction so slow that it is inconvenient to wait for its completion. Methods have been devised to allow the rate constant to be evaluated without a known value of in the process, of course, an estimate of A is also obtainable. 

Kinetic data may be collected in which the final instrument reading is unreliable or unavailable. Perhaps excessive time would be needed, or a slow secondary reaction sets in, or the instrument baseline slowly drifts. Nowadays, with readily available nonlinear least-squares programs, one may simply treat as a floated variable, along with k. 

A basic assumption in DSC kinetics is that heat flow relative to the instrumental baseline is proportional to the reaction rate. In the case of temperature scanning experiments the heat capacity of the sample contributes to the heat flow (endothermic), and this is compensated by the use of an appropriate baseline under the exo- or endothermic peak produced by the reaction. It is also assumed that the temperature gradient through the sample and the sample-reference temperature difference are small. Careful control of the sample size and shape, and the operating conditions are necessary in order to justify these assumptions. 

Figure 10.8 Illustration of sample baseline and instrument baseline.

Figure 10.13 illustrates the DSC curves for heat capacity measurement. Figure 10.13a shows an ideal instrument baseline and a DSC line with sample. The displacement between the baseline... 

Correct determination of the instrument baseline is important for accurate heat capacity measurement. The instrument baseline is not likely to be recorded as a horizontal line in experiments. The displacements closed to T and 7i in Figure 10.13b should not be used for measurement because the heating conditions are not stable there. We should note that the measured heat capacity is more likely to be a function of temperature, particularly when a thermal event occurs in the measured temperature range (Figure 10.13c). 

For the determination of molar absorption coefficients, s, care must be taken to use clean glassware and solvents and the instrumental baseline must be recorded or calibrated prior to measurement. Absorbance measurements are most accurate in the range 0.1 < A < 1.5 lower absorbances are prone to baseline errors and those exceeding 1.5 should be avoided, because the amount of light passing the sample cell becomes very small and the readings are... 

Figure 7.56. Typical DSC polymer melting curve and instrumental baseline 1156).

It is recommended to scan the DSC under the proposed experimental conditions to check the curvature and noise level of the instrument baseline before analysing samples. There are several reasons why the instrument baseline may be curved and/or display a high noise level. Trace amounts of residue from a previous experiment may be attached to the sample holder. Decomposed or sublimated compounds frequently condense on the sample holder, distorting the shape and quality of the instrument baseline. If the instrument baseline is still not satisfactory, following cleaning with ethanol or acetone, there may be a problem with the purge gas flow. The linearity of the instrument baseline will be... 

The instrument baseline recorded for successive scans, under the same conditions, should be identical. 

The gas flow rate influences the characteristics of the instrument baseline. When the flow rate is too high the instrument baseline becomes unstable. Fluctuations in the flow rate alter the baseline gradient. A purge gas flow rate in the range 20-50 ml/min is recommended. Measurements under reduced pressure are generally only performed with simultaneous TG-DTA instruments and are discussed in... 

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. 

Instrument baseline DSC (or DTA) curve recorded in the scanning mode when there is no sample or reference present. 

Standards must be prepared accurately from high-purity materials so that the concentration of analyte is known as accurately as possible. A series of standards covering an appropriate concentration range is prepared. The standards should include one solution with no added analyte the concentration of analyte in this standard is zero. This solution is called the reagent blank and accounts for absorbance due to impurities in the solvent and other reagents used to prepare the samples. It also accounts for the instrumental baseline. The absorbance of the reagent blank and each standard is... 

Determine the instrument baseline by repeating the two steps above without a specimen present. Determine the instrument baseline by repeating the two steps above without a specimen present. 

The measured change in length of the specimen should be corrected for the instrument baseline. The measured change in length of the specimen should be corrected for the instrument baseline. 

Despite the complexity of the reaction mechanism involved in the cure of epoxy resins by polyfunctional amines, the overall reaction rate can be expressed by means of a relatively simple kinetic equation. Under the assumption that the heat flow relative to the instrumental baseline is proportional to te reaction rate, the fractional conversion i s expressed by equation 1 ... 

A linear instrument baseline is easily obtained because the relatively large furnace heats the atmosphere surrounding the sample holder unit. However, the time required to stabilize the instrument for an isothermal measurement is considerable for both heating and cooling experiments. 

Compared with a heat-flux DSC, higher scanning rates can be used with a power compensation DSC, with a maximum reliable scanning rate of 60 K min. Maintaining the linearity of the instrument baseline can pose problems at high operating temperatures or in the sub-ambient mode. 

Excellent baseline stability helps give the Perkin Elmer DSC 2910 the sensitivity to detect very small T. Polypropylene (PP), for example, has been difficult to characterise by DSC because its is small and its heat capacity changes greatly with increasing temperature. The Tg of PP is clearly observed with the Perkin Elmer DSC 2910 instrument. Baseline stability and sensitivity also make it possible for the 2910 to detect the Tg of highly filled or highly crystalline polymers. 

Specular surfaces can also be measured by using at integrating sphere at 8° incidence or collection geometry. The measurement technique is identical to the near-normal technique described previously. A reference mirror is used to establish the instrument baseline, and then the sample mirror replaces the reference and a scan is obtained. The reflectance of the sample is the product of the measured value for sample times the actual reflectance for the reference. This technique is quite convenient to use and is as accurate as the multiple mirror method. Care must be taken, however, to ensure that the incidence or collection angle is not 0°, because a specular-excluded measurement of a mirror will certainly give the incorrect answer for reflectance ... 

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