Continuous mixing methods sample

This section deals briefly with classical methods based on conventional mixing of the sample and reagents such as the batch mode and low-pressure flow mixing methods, as well as the use of CL detection in continuous separation techniques such as liquid chromatography and capillary electrophoresis for comparison with the unconventional mixing mode. 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

When using the continuous flow method, however, some additional versatility is available in chemisorption measurements. For example, when data is required at an adsorbate pressure of 0.1 atm, a 10 % mixture of adsorbate, mixed with an inert carrier gas, is passed through the apparatus with the sample cooled to a temperature at which no chemisorption can occur. Upon warming the sample to the required temperature, adsorption occurs producing an adsorbate-deficient peak that is calibrated by injecting carrier gas into the flow stream. Equation (15.9) is then used to calculate the quantity adsorbed. This process is repeated for each concentration required. Caution must be exercised to avoid physical adsorption when the sample is cooled to prevent chemisorption. Should this occur, the adsorption peak due to chemisorption can be obscured by the desorption peak of physically bound adsorbate when the sample is heated. 

With a continuous flow method, flowing is the sole way the sample is mixed. Consequently, there may be imperfect mixing. Thus, the concentration of the adsorptive in the flow chamber may not equal the effluent concentration this is because transport and chemical kinetics phenomena are both occurring simultaneously. 

To analyze data using these two methods one must make two assumptions (1) that a sorptive entering the chamber can either be sorbed or remain in solution, and (2) the sample is perfectly mixed i.e., the concentration in the mixing chamber equals the effluent concentrations. With these assumptions, one can then develop an equation for mass balance which can be used to analyze time-dependent data using a continuous flow method (Skopp and McAllister, 1986) ... 

In continuous flow methods, the sample is inserted into a flowing stream, where a number of operations can be performed before it is transported to a flow-through detector. Hence, these systems behave as automated analyzers in that they can perform not only sample processing operations but also the final measurement step. Such sample processing operations as reagent addition, dilution, incubation, mixing, dialysis, extraction, and many others can be implemented between the point of sample introduction and detection. There are two different types of continuous flow systems segmented flow analyzers and flow injection analyzers. 

Second method a continuous flow of sample was mixed with a solution of 2% NaBH4, which was followed by the addition of HCl (3 mol L ). Calibration was by matrix matching, using Na2Se03 calibrant. Final detection was by quartz furnace AAS. 

The most widely used fast mixing method is the continuous-flow method. The reactants flow in separate continuous streams that meet in a mixing chamber and then pass along an observation tube or chamber with detection devices at appropriate points along its length (see Fig. 18.2). The detection devices, which measure the composition of the flowing sample, may be optical, thermal, chemical, electrical, or any other method applicable to a rapidly moving sample. Reactions with halftimes of the order of 10" sec can be observed by this method. 

Menendez Garcia et al.[50] combined on-line liquid-liquid extraction separation with hydride generation gas-liquid separation for the determination of arsenic with ICPES. Arsenic in the aqueous sample is extracted as ASI3 into xylene which is continuously mixed on-line with sodium borohydride in dimethylformamide and acetic acid solutions. Arsine is generated in the organic phase and separated in a gas-liquid separator which prevents most of the xylene vapour from entering the plasma. The method was used to improve the sensitivity and to remove interferences from transition metals in the determination of low levels of arsenic in white metal, cast iron, cupro-nickel etc.. 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

This type of chromatographic development will only be briefly described as it is rarely used and probably is of academic interest only. This method of development can only be effectively employed in a column distribution system. The sample is fed continuously onto the column, usually as a dilute solution in the mobile phase. This is in contrast to displacement development and elution development, where discrete samples are placed on the system and the separation is subsequently processed. Frontal analysis only separates part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the... 

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