Ex situ monitoring

Dell Orco, P. Diederich, A.M. Rydzak, J.W., Designing for crystalline form and crystalline physical properties The roles of in-situ and ex-situ monitoring techniques Am. Pharm. Rev. 

Biosensors, however, are well suited for ex situ monitoring using FIA. The incorporation of biosensors in FIA offers many advantages such as (i) preconditioning of analyte samples for optimal analytical conditions (for example, pH, buffer capacity) before analysis, (ii) recalibration of sensor to counter the drift, (iii) replacement of nonfunctional or poorly functioning parts of the analytical system such as an inactivated... 

A variety of other techniques have been used to investigate ion transport in conducting polymers. The concentrations of ions in the polymer or the solution phase have been monitored by a variety of in situ and ex situ techniques,8 such as radiotracer studies,188 X-ray photoelectron spectroscopy (XPS),189 potentiometry,154 and Rutherford backscatter-ing.190 The probe-beam deflection method, in which changes in the density of the solution close to the polymer surface are monitored, provides valuable data on transient ion transport.191 Rotating-disk voltammetry, using an electroactive probe ion, provides very direct and reliable data, but its utility is very limited.156,19 193 Scanning electrochemical microscopy has also been used.194... 

Polarons are epr-active by virtue of their unpaired spin. Consequently, another technique to be applied relatively soon after the discovery of conducting polymers was epr. Again the early studies were carried out ex situ on free-standing films, usually by monitoring the epr response (and the conductivity) of the completely reduced film on various exposures to oxidising gases such as I2 or 02 (cf. the UV-visible experiments discussed above). 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

It has been quickly recognized that the individual operando techniques can be combined to yield a more complete picture of the catalytic reaction sequence. In addition, since many reactions of industrial significance occur in the liquid phase, it is important that techniques are developed to probe and monitor those systems under conditions that at least keep the reactants, intermediates and products in their actual operating states or phases. This has resulted in researchers utilizing a multitude of techniques, some in situ and ex situ, to obtain a more complete understanding of the entire catalytic cycle. 

Two of the recognized limitations of in situ technologies are (1) physicochemical restraints (e.g., bioavailability, desorption kinetics), and (2) a need for extended treatment time as compared to ex situ biotreatment approaches. Inherent geological parameters such as permeability, vertical and horizontal conductivity, and water depth can also represent constraints that are critically important to recognize and appreciate (Norris et al., 1993 Norris Falotico, 1994). Another widely recognized limitation inherent to in situ processes is that the systems are difficult to monitor and thus effective and complete treatment is difficult toascertain and validate. 

In this experiment, ex situ soil colloids with diverse mineralogical composition after equilibration with metal solutions of known concentrations were leached through undisturbed soil monoliths exhibiting considerable macroporosity. The colloids (<2 pm) were separated from upper-soil Bt horizons with montmorillo-nitic, illitic, and kaolinitic mineralogy. The equilibration metal solutions contained Cu, Zn, and Pb. Eluents were monitored over ten pore volumes for colloid and metal concentrations. 

Environmental monitoring has also taken advantage of acoustic levitation for the investigation of physico-chemical processes relevant to the troposphere — mainly at temperatures below 0°C. Gas-liquid transfer of H2O2 from the gas phase to the levitated droplet was studied from in situ chemiluminescence measurements. Also, freezing of stably positioned droplets was observed by means of a microscope and a video camera, and the usefulness of this technique for simulation and investigation of cloud processes thus demonstrated. Ex situ microanalysis of sub-microlitre droplets by the use of an optical fibre luminometer also proved an effective means for investigating important physicochemical processes at the micro scale [100]. 

Fig. 15.5 Oxidation and reduction of epitaxially grown polyhedral Rh nanoparticles (mean size 5 nm) on alumina, monitored ex situ by HRTEM. In the as-prepared state, most of the Rh particles were half-octahedra with 111 and 100 surface facets, as revealed by combining results from HRTEM and WBDF (a, d), and SAED (b). Upon oxidation in 1 bar at 723 K, an epitaxial Rh-oxide shell developed on top of a Rh core (c, e). Reduction in 1 bar H at 523 and 723 K led to polycrystalline (f) and rounded crystalline (g) nanoparticles, respectively. The microstructural changes were correlated with changes in catalytic hydrogenolysis activity (see text for details) adapted in part from [20] with permission. Copyright (1998) Elsevier...

For the in-situ studies, an electrochemical cell was designed to hold the nearly perfect copper crystal in contact with a thin layer (20 to 50 fan) of electrolyte. Figures 29 and 31 show the cells employed in the in- and ex-situ experiments, respectively. In addition. Fig. 31 shows the voltammetric traces obtained for the deposition of T1 in the presence and absence of oxygen. In the experiments, they simultaneously monitored the reflectivity and the T1 fluorescence intensity. Figure 32 shows the results for the ex-situ study. 

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