Transport of mobile phase

An eluent container can be a Mariotte flask that produces constant hydrostatic pressure, and thus it can serve for the generation of the gravitational eluent transport in the chromatographs of the first generation. Simple solvent tanks, or even open vessels, are used as eluent containers in the instruments provided with efficient pumping systems. The preferential evaporation effects, however, should be prevented when working with multicomponent, mixed eluents. 

The degassing unit removes gases dissolved in the eluent. The gas bubbles released from the effluent may complicate the detection of sample. Aqueous eluents can be degassed by increased temperature, by application of vacuum or by displacing effect of helium. 

Filters remove dust particles and other mechanical impurities from the eluents and solutions of samples. Filters made of sintered glass, porous polymers, and sometimes also of sintered stainless steel are used for aqueous mobile phases. 

The problem of pulses is solved by means of damping devices that consist of a hydraulic resistor and a hydraulic capacitor. The modern reciprocating pumping systems are equipped with two or more pistons, their motion being mechanically or electronically controlled in such way that both short and long term flow rate variations are substantially reduced. 

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Since recently, monoliths of silica gel or organic pofymers became popular in HPLC of low-molecular substances. They exhibit reduced flow resistance and high mechanical strength. Their internal structure forms the sohd matrix and two kinds of pores. Macropores or charmels allow fast transport of mobile phase, while the separation processes take place in the smaller mesopores. So far, however, most commercially available monoliths possess too small effective volume of the mesopores to compete with particulate colurrm packings in polymer HPLC, especially in SEC. 

But there can be no question of chamber saturation if the TLC plate is then placed directly in the chamber. But at least there is a reduction in the evaporation of mobile phase components from the layer. Mobile phase components are simultaneously transported onto the layer (Fig. 57). In the case of multicomponent mobile phases this reduces the formation of / -fronts. 

An important measure concerning column characterization in LC is the column permeability, which represents the capacity of the support to transport the mobile phase as consequence of a pressure drop occurring over the column. In other words, the permeability of a column determines the required pressure to achieve the desired flow rate. The linear flow velocity (u) across an empty cylindrical column is given by... 

Complex mathematical formulae will be minimized here for the purpose of simplicity since there are numerous texts that deal with detailed theory of mass transport in chromatography1[2,1,22 The flow of mobile phase through a packed column bed is shown schematically in Figure 2.1. There are two transport mechanisms in progress. Firstly, the convectional flow around the particles and secondly, the diffusion in and out of the pores of the stationary phase. 

The auxiliary phase should intimately contact the solid electrolyte to ensure transport of mobile ionic species. This will benefit the performance of sensors under certain conditions. 

Stolyhwo et al. [17] attempted to improve the sensitivity of the detector by using metal spirals wound on wire and stranded wire to increase the surface area of the carrier and thus increase the proportion of the column eluent taken into the detector. The authors claimed a minimum detectable mass of 100 ng of triolein. However, again the exact volume of mobile phase in which the mass of solute was contained was not clear from the publication. If the solute was eluted in a peak 1 ml wide at the base, the concentration at the peak maximum would be twice the average concentration /.e., 2 x g/ml, which, for a transport detector, would be a greatly improved sensitivity. If, however, the same mass was eluted as an early peak in the chromatogram with a band width of only 50 pi, then the sensitivity would be 4 x 10 g/ml, which would be no better than the previously developed transport detectors. This confusion emphasizes the importance of specifying sensitivity in terms of concentration, which allows the direct comparison of one detector with another. 

V], called the void volume, represents the volume of mobile phase necessary to transport a large molecule assumed to be excluded from the pores and Vm is the volume accessible to a small molecule that can enter all the pores of the packing (volume Vj). 

Elution volume Kr, volume of mobile phase required to elute a component, that is, to transport a component through the chromatographic system. 

The common methods of mobile phase transport through the layer are capillary action, forced flow, and electroosmosis. Ease of implementation results in capillary flow... 

The chromatographic methods are primarily subdivided according to the type of mobile phase used, which in addition to its role as a transport medium... 

The common methods of development in TLC employ capillary forces to transport the mobile phase through the layer. These weak forces arise from the decrease in free energy of the solvent as it enters the porous structure of the layer. The velocity at which... 

Simple and inexpensive PTFE horizontal S-chambers for 5 X 5 cm or 10 X 10 cm plates are shown in Figure 7.1. The chambers are covered with a 4-mm-thick glass cover, and a glass frit rod is used to transport the mobile phase to the layer. Other commercial chambers for horizontal TLC include the Camag horizontal development chamber (Section IIIC), the Vario-KS chamber (Section IIIF), and the BN-chamber (no longer available commercially). For construction of a chamber suitable for horizontal TLC, see Stahl (1969) and Dzido (1990). 

Transport detectors are a unique type of solute property detector. In most solute property detectors, the sensing system monitors some property of the solute that is not shared by the solvent or that the solvent has to markedly less extent. It follows that such detecting systems are,to some extent, selective in their detecting capabilities and further restricts the choice of solvents to those that do not possess the property being measured. Thus, the choice of mobile phase is limited, and this can be particularly disadvantageous when employing gradient elution development. The transport type of detector was developed to overcome these limitations. 

A transport detector consists of a carrier that can be for example, a metal chain, wire or disc that continuously passes through the column eluent taking a sample with it as a thin film of mobile phase adhering to its surface. The mobile phase is then... 

The transport system for LC detection was developed to render the detector independent of the choice of mobile phase and allow any solvent to be used without compromise. The column eluent flows over the transporter, which may be a moving wire, chain or disc which takes up all, or a portion of the column eluent. The solvent is then evaporated from the transporter, usually by heating, and the solute is left as a coating on the surface. The transporter then carries the solute into a detection area, where it is sensed by suitable means, such as pyrolysis and subsequently detected by passing the pyrolysis products to a flame ionization detector. The transport detectors, by and large, are not very... 

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