Instrument lag

Quadrupoie. Quadrupoie mass spectrometers, sometimes known as quadrupoie mass filters (QMF), are currently the most widely used mass spectrometers, having displaced magnetic sector mass spectrometers as the standard instrument. Although these instruments lag behind magnetic sector instruments in terms of sensitivity, upper mass range, resolution, and mass accuracy, they offer an attractive and practical mix of features that accounts for their popularity, including ease of use, flexibility, adequate performance for most applications, relatively low cost, non-critical site requirements, and highly developed software systems. 

The development status of process control instrumentation lags that of the quality control instruments significantly Nuclear density gauges function in the coal preparation plant environment The slurry concentration meter has application in the intermediate and fine sized coal cleaning circuits and needs to be tested in a preparation plant Other devices, such as ash monitors to control the operation of heavy media baths or jigs are not available and instruments developed for other process industries are not suitable for use in coal preparation plants Modeling studies of the various unit operations are required in order to ascertain the fundamental parameters required to automate the control of these systems Primary process control instrument needs include ash, sulfur, and moisture monitors secondary needs include an on-line washability and ash fusion measurement ... 

When using continuous DSC for purity determination, the data must be corrected for instrument lag and F must be corrected for the omitted portion E as shown in Fig. 4.42 for testosterone. Computer programs exist to optimize the fit to a linear curve. Over-correction would give a downward deviation instead of the upward deviation. This purity determination is only applicable if there is solubility of A and B in the melt, but no solubility of B in crystals of A (eutectic system). 

Measurements at different heating rates may lead to different amounts of instrument-lag, i.e., the temperature marked on the DSC trace can only be compared to a calibration of equal heating rate and baseline deflection. A simple lag correction makes use of the slope of the indium melting peak when plotted vs. sample temperature as a correction to vertical lines on the temperature axis, hi some commercial DSCs this lag correction is included in the analysis program. It must be considered, however, that different samples have different thermal conductivities and thermal resistances so that different lags are produced as shown, for example, in Fig. 4.94, for an analysis with TMDSC. 

In the melting range, the separation of revering and non-reversing effects is only qualitative and hindered by instrument lag (Figures 4.38 and... 

Figure 4.67[A] shows a typical isothermal experiment carried out with a DSC. Similar experiments could be carried out with isothermal calorimeters, dilatometry and other teehniques sensitive to crystallinity changes. After attainment of steady state at point 0, the experiment begins. At point 1, the first heat flow rate is observed, and when the heat flow rate reaches 0 again, the transition is complete. The shaded area is the time integral of the heat flow rate, and if there is only a negligible instrument lag, it represents the overall kinetics. In case of an excessive heat flow-rate amplitude, lag calibrations with sharply melting substances of similar thermal conductivity may have to be made (see Figure 4.22). Processes faster than about 1 min...

To overcome this large instrument lag, the sample mass must be reduced. This can be done, as discussed before, by changing the sample holder radius by a factor of 0.1, which means going to a quantity of material of approximately 1 mg. In this case, the time of change to the new steady-state is compressed from 150 to 1.5 seconds and the temperature range to 0.5 K, quite acceptable for the study of a glass transition. The temperature difference A 7 caused by the transition, shown in the drawing of Fig. 4.15 to be 2.6 K, is now reduced to 0.03 K. Measurements with smaller samples have to be much more sensitive. 

A special case arises at concentration E, the composition of the compound 3. In this case no transition activity occurs until the melting point of the compound is reached. The compound melts sharply. The breadth of the melting peak indicated by the upper curve E is determined by the instrument lag. Thermodynamics calls for a perfectly sharp transition. 

For heats of fusion measured over a wide temperature range, not only is it necessary to eliminate the base line effects, as indicated by Eq. (2) of Fig. 5.25, but the temperature dependence of A/ff must also be considered. In the absence of instrument lags, Eq. (2) describes the upper curve of Fig. 5.25. The first two terms describe the appropriate base line, as discussed above. The third term represents the fusion. The temperature dependence of the heat of fusion is available through Eq. (3). If the heat of fusion is absorbed over a wide temperature range (i.e., more than 20 K), Eq. (3) must be integrated and inserted into Eq. (2) to come up with a precise analysis of crystallinity and an equilibrium value for AHf at the equilibrium melting temperature. 

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