Phase change detection

A phase reaction is always accompanied by an enthalpy change (section 2.12), and this heat effect can readily be observed if a cooling curve is plotted for the system. In many cases a very simple apparatus can be used. A large glass test-tube, fitted with a stirrer and a thermometer graduated in increments of 0.1 °C and held in a temperature-controlled environment, will often suffice. The temperature of the system is recorded at regular intervals of say 1 min. 

Differential thermal analysis is a method used for observing phase changes and measuring the associated changes in enthalpy. A small test sample, often only a few milligrams, is heated in close proximity to a sample of reference material in an identical container. The reference material, chosen for its similarity to the test sample, must not exhibit any phase change over the temperature range under consideration. 

A good introductory account of the basic principles and practical requirements of a range of modern techniques of thermal analysis is given by Brown (1988). The development of a differential scanning calorimeter, coupled with a personal computer, for the measurement of solid-liquid equilibria, has been described by Matsuoka and Ozawa (1989). 

The dilatometric methods for detecting phase changes utilize volume changes in the same way as the calorimetric methods utilize thermal effects. Dilatometry is widely used in the analysis of melts and particularly of fats and waxes (Bailey, 1950 Swern, 1979). The techniques and equipment are usually quite simple. 

Solids absorb heat on melting and, with the notable exception of ice, expand. They evolve heat when they undergo polymorphic transformation to a more stable polymorphic and contract. Consequently, dilatometric (specific volume-temperature) curves bear a close resemblance to calorimetric (enthalpy-temperature) curves. The melting dilation corresponds to the heat of fusion, and the coefficient of cubical expansion, a, corresponds to the specific heat capacity, c. The ratio cja is virtually a constant independent of temperature. 

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See Reference 40 for another example of a polymer phase change detected by NLO techniques. 

Highly UV absorbing mobile phase. Change detection wavelength taking into account the UV cutoff of mobile phase solvents. 

The application of load in materials produces internal modifications such as crack growth, local plastic deformation, corrosion and phase changes, which are accompanied by the emission of acoustic waves in materials. These waves therefore contain information on the internal behaviour of the material and can be analysed to obtain this information. The waves are detected by the use of suitable sensors, that converts the surface movements of the material into electric signal. These signals are processed, analysed and recorded by an appropriate instrumentation. 

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

The recoil-free fraction depends on the oxidation state, the spin state, and the elastic bonds of the Mossbauer atom. Therefore, a temperature-dependent transition of the valence state, a spin transition, or a phase change of a particular compound or material may be easily detected as a change in the slope, a kink, or a step in the temperature dependence of In f T). However, in fits of experimental Mossbauer intensities, the values of 0 and Meff are often strongly covariant, as one may expect from a comparison of the traces shown in Fig. 2.5b. In this situation, valuable constraints can be obtained from corresponding fits of the temperature dependence of the second-order-Doppler shift of the Mossbauer spectra, which can be described by using a similar approach. The formalism is given in Sect. 4.2.3 on the temperature dependence of the isomer shift. 

A powerful advantage of SFC is that more detectors can be interfaced with SFC than with any other chromatographic technique (Table 4.30). There are only a few detectors which operate under supercritical conditions. Consequently, as the sample is transferred from the chromatograph to the detector, it must undergo a phase change from a supercritical fluid to a liquid or gas before detection. Most detectors can be made compatible with both cSFC and pSFC if flow and pressure limits are taken into account appropriately. GC-based detectors such as FID and LC-based detectors such as UVD are the most commonly used, but the detection limits of both still need to be improved to reach sensitivity for SFC compatible with that in LC and GC. Commercial cSFC-FID became available in... 

Gross phase changes are detectable by eye upon close inspection of products. The package of course gets in the way of such analysis, but if a product is truly suspect, it should be closely examined by opening and inspecting the full contents of the container. Ajar can be opened and its contents probed with a spatula without ruining the container. Close inspection of the... 

CHOICE OF FILTER FOR AUTOMATED PHOTOMETRIC TITRATION. At the end of a photometric titration using the above two indicators the colour of the chloroform phase changes from pink to blue. To choose a filter to detect this end point the visible spectra of the separated chloroform layers of surfactant titrations were recorded before, at and beyond the end point, see Figure 2. At 580 nm there was a greater change in absorbance than at 440 nm, thus the 580 nm filter was preferred. 

They observed abrupt changes in the slope of Arrhenius plots for reactions catalyzed by NADH oxidase and p-lactate oxidase that correlate well with phase transitions detected by the ESR spectra of the nitroxide spin labels bound covalently to the enzymes (Table 5.4). 

It is even possible to convert changes of the fluorophore emission intensity as a function of the analyte level into lifetime-based sensing devices using phase-sensitive detection. The technique is called dual lifetime... 

To give an example both sensitivity coefficients are evaluated for the current sensor (see Sect. 10.3 for details). For the bulk detection of glucose this results in A bulk (rad) = 5.6 x 102 AC (g/ml), whereas for the adsorption of proteins on the sensor surface the overall sensitivity of the sensor is evaluated as A< >layer (rad) = 2.0 x 10 5 Am/A (fg/mm2). Measuring the phase change A< >,-, between any of the two channels i and j can thus give an estimation on the change in analyte concentrations between those two channels. If one channel (e.g., channel N) is used as a reference channel, then ACV = 0 and AmN = 0 and absolute analyte concentrations can be determined. 

The possibility to use the YI sensor for virus detection was explored by monitoring the interaction between a-HSV-1 gG antibody and HSV-1 virus particles. To this end, channel 1 was coated with protein pA as described in Sect. 10.4.2 followed by the immobilization of a a-HSV-1 gG layer on the sensing surface of channel 1. Channel 4 was used as a reference channel. Finally a solution with HSV-1 virus particles at a concentration of 105 particles/ml was added to channel 1. Figure 10.2 shows the phase change measured between channel 1 and reference channel 4, clearly demonstrating the detection of virus particles by the YI sensor (Fig. 10.15). 

Fig. 10.15 Virus detection test. Sensor signal (phase change) measured between channel 1 and the reference channel for the immobilization of anti HSV 1 glycoprotein G monoclonal antibody layer on the sensing surface of channel 1 (A HSV i gG) and the binding of HSV 1 particles to this layer (A IISV i). Reprinted from Ref. 28 with permission. 2008 American Chemical Society...

Fig. 10.16 Measurement of different HSV 1 concentrations and detection in serum, (a) Phase change measured for different concentrations of HSV 1 sample solutions in PBS applied in the measuring channel of the YI sensor (filled triangle). Solid line is a linear fit of the experimental data, ip represents the phase change measured for HSV 1 diluted in serum (see Fig. 16b), dashed line indicates the phase detection limit of the sensor, (b) Sensor response due to the binding of HSV 1 diluted in serum. Final concentration of HSV 1 was 105 particles/ml. The total signal is estimated to be A

Fig. 10.17 Specific detection of HSV 1. Phase changes A

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