Offline conductivity

An offline measurement apparatus is usually not directly mounted on the reactor, but is fed with samples withdrawn from it manually or automatically. This is the typical case of chromatography, a widely used measurement device for gas and liquid composition. Both gas and liquid chromatographies are based on the separation of the sample by means of selective adsorption on a solid substrate posed in a fixed bed column, and on the detection of the change of a suitable property of the (gas or liquid) carrier, usually thermal conductivity. 

A group of scientists from Peking University [44,45] proposed conducting CE-IA in a thermally reversible hydrogel added to the buffer as a replaceable packing material. Antibodies were immobilized in the gel and a mixture of tracer and analyte was injected. The presence of gel enhanced the peak shapes and resolution, whereas immobilized antibody suppressed dissociation during the CE run. The latter makes this assay a kind of a mixture between on-line and offline analysis modes. 

Fearful that they may lose their reputation in the community, WAA requested a government audit of their operations to see if the problem could be internal. Indeed, the auditor concluded that the problem was likely due to a contaminated distilled water supply. It turns out that WAA had previously been using a conductivity meter to continuously monitor the quality of the water as it elutes from the distiller and had taken it offline some time earlier because it was apparently malfunctioning. Besides always indicating that the water was highly conductive despite the use of an expensive distillation and deionizing system, it also seemed to be the source of unusual colorations and odors found in the water. As soon as it was taken offline, the colorations and odors ceased. 

Cylinders of the flashed gas were analysed for hydrocarbon gas composition on a Carle gas chromatograph (GC) system equipped with both a thermal conductivity detector (TCD) and a flame ionization detector (FID). An offline preparation system and dual inlet mass spectrometer (MS) were used to analyse the carbon and hydrogen isotopic values of hydrocarbon components. A customized Gow Mac GC was interfaced with a vacuum/combustion system to separate hydrocarbons from other components and combust to CO2 and water that were purified and sealed into Pyrex tubes for isotopic analysis. The CO2 was analysed directly on one of three dual inlet mass spectrometers Finnigan Delta S, Finnigan Delta + XL or VG SIRA II. The water was reacted with zinc turnings and converted to hydrogen gas, which was analysed on either the Delta S or Delta + XL MS. 

The starting point for the development of graphic 3D simulation systems was the problem of planning the use of robots and offline programming. Independent modules or simulation modules integrated into CAD system were created. To enable a simulation to be conducted, the planned or real robot system must first be generated and depicted as a model in the computer. The abstraction level of the simulation model created must be adjusted to the required imitation as detailed as necessary, as abstract as possible. Once the model has been completed, an infinite number of simulations can be carried out and modifications made. The objective is to improve the processes and eliminate the possibility of planning mistakes. 

Offline separation can be conducted in a system consisting of two mechanically stirred extraction cells (Figure lA) attached on both sides of the flat liquid membrane. One of the cells is filled with the sample solution while the other is filled with the receiver solution. Similar extraction systems have also been used in determining the permeability of SL and PL membranes and in studying the extraction mechanism and kinetics involved in membrane separation processes. 

Heating units are somewhat susceptible to failure. If a unit is bent, twisted, or dropped, the element inside the unit can be damaged and an open circuit will result in failure. Plastic buildup on a heater band can also lead to a failure. Finally, if the unit is on and there is an air gap between the heater and the metal surface, a hot spot can occur on the heater and it will fail. A heat transfer paste can sometimes be used to ensure good conduction to the metal. If a heating element is suspected to have failed, it can be easily checked offline with a continuity tester. A short or open circuit means that the heating unit needs to be replaced. 

While MALDI is a typical off-line ionization technique/interface, one can also implement classical interfaces to carry out off-line (or at-line) analyses of reaction mixtures. Aliquots can be obtained from a reaction mixture (a dynamic sample/matrix) at specific time points, and injected to the ion source of a mass spectrometer for analysis (e.g., [49-54]). In some cases, quenching is conducted [49, 50]. If the transient intermediates (e.g., radicals) are to be detected, it is important to assure that the reaction has not finished at the time of ionization [55]. Short-lived intermediates can be reacted with auxiliary compounds in order to enable subsequent measurements in the methodology referred to as spin-trapping (see, e.g., [56, 57]). The off-line analyses based on aliquoting of dynamic matrices, and subsequent separation, provide temporal resolutions in the order of a few minutes (see, e.g., [58]). Nonetheless, they are uncomplicated, and - in some cases - they may enable offline sample treatment prior to detection by MS. For instance, inorganic ions, present in the reaction mixture, can readily be removed on an exchange resin to render the collected samples compatible with MS [59]. 

Sample preparation can also be conducted by direct online extraction, which offers advantages over offline extraction in that it involves minimized sample preparation, reduces loss of the analyte during extraction, and decreases the chance of artifacts, contamination, and dilution during the extraction [16,103]. A typical configuration for online extraction consists of an autosampler, two sets of binary pumps, an extraction column, an analytical column, and a divert valve. In the loading/ extraction stage, a sample is injected onto the extraction column, and the analytes are retained on the head of the extraction column, while the undesired matrix components such as proteins and salts are removed and... 

In addition to the online analyses described, offline tests are also required. Specifically, standard release tests such as a product s dissolution profile and any degradation products must be assessed. If not obtained online via PAT, the assay and content uniformity of extrudate must be measured via off-line lab tests. Any residual solvents (including water) should be measured as they can have a plasticizing effect potentially influencing both extrudate physical and chemical stability. Finally, tests such as differential scaling calorimetry (DSC), X-ray diffraction (XRD), or others must be conducted before and after holding the product at controlled environmental conditions, to ensure that the phase state of the extrudate is understood and controlled over time. 

When real water is chosen (option 1), one must decide on what pretreatment to use to remove particulate matter, biological species, and organic pollutants. Because ionic mixtures are used in options 1 and 2, the effluent freshwater produced must be analyzed using offline individual ion detection, such as inductively coupled plasma in combination with optical emission spectroscopy or mass spectroscopy. However, in option 3, when using only single salt solutions, the measurement of conductivity is sufficient. In single salt solutions, we have to consider that the diffusion coefficients of the anion and cation may be (almost) equal (KCl) or are different (NaCl). 

Operations that are conducted while the RO system is offline are just as important as on-line operations in keeping an RO system functioning well. Off-line operations covered in this chapter include system flush, membrane cleaning, and membrane lay-up. 

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