Eluent systems modes

The other method to separate polymer mixtures utilizes the full adsorption and desorption of particular components by switching eluent [120-127]. Under appropriate choice of a sorbent and eluent system, one component of the polymer mixture is separated in terms of molecular weight in the conventional SEC mode while the other components are fully retained on the surface of the sorbent. In the next step the eluent is changed so that one of the previously retained polymers is desorbed and analyzed in the SEC mode [32,123]. In the favorable case a blend of more than two components can be fractionated in sequence [124,126]. Figure 14 shows an example of the separation of a four components blend. 

Cyclodextrin phases can be operated in RP and normal-phase modes, as well as in the polar ionic mode (see Table 3). Aqueous eluent systems are better suited for the formation of inclusion complexes of apolar or aromatic moieties in the inner cavity of the macrocycle. The selectivity of the cyclodextrins depends on ring size, pH, ionic strength, the nature and concentration of modifier, and temperature. 

FIGURE 16.5 Broad standard calibration (linear mode) of a semipreparative Sephacryl S-IOOO system (95 x 1.6 cm) with an aqueous mixture of Blue Dextran, Dextran T-SOO, and glucose eluent 0.005 M NaOH V, i = 75 ml. = 162 ml. 

The optimization of preparative and even micropreparative chromatography depends on the choice of an appropriate chromatographic system (adsorbent and eluent), sample application and development mode to ensure high purity, and yield of desirable compounds isolated from the layer. For the so-called difficult separations, it is necessary to perform rechromatography by using a system with a different selectivity. But it should be taken into account that achievement of satisfactory results frequently depends on a compromise between yield and the purity of the mixture component that is being isolated. 

The presence of 10% water in the eluent minimized variations. Two different LC modes were used for the analysis of the extracts a smaller (50 x 2.1 mm) Atlantis C18 column (5 /um particle size) on a Shimadzu liquid chromatograph and a UPLC column (Acquity C18,1.7 fim particle size, 50 x 2.1 mm) on a Waters Acquity system. In the Shimadzu experiment, a gradient from 0.5 min... 

Using the Tomtec Quadra 96 workstation, 0.1 mL of the ethyl acetate layer was transferred to a 96-well collection plate containing 0.4 mL of acetonitrile in each sample well. The solution was mixed 10 times by aspiration and dispersion on the Tomtec. The plate was then covered with a sealing mat and stored at 2 to 8°C until LC/MS/MS analysis. The HILIC-MS/MS system consisted of a Shimadzu 10ADVP HPLC system and Perkin Elmer Sciex API 3000 and 4000 tandem mass spectrometers operating in the positive ESI mode. The analytical column was Betasil silica (5 fim, 50 x 3 mm) and a mobile phase of acetonitrile water formic acid with a linear gradient elution from 95 5 0.1 to 73.5 26.5 0.1 was used for 2 min. The flow rate was 1.0 mL/min for the API 3000 and 1.5 mL/min for the API 4000 without any eluent split. The injection volume was 10 jjL and a run time of 2.75 min was employed. 

The moving wire device has a number of major shortcomings. Due to the small surface area of the stainless-steel wire, such as available from a 0.1 mm diameter wire, the device can only accommodate about 10 pL/min eluent which results in poor sensitivity. The system is difficult to operate in a continuous mode. Modification of the moving wire approach has led to the invention of a continuous moving belt, which offers improved transfer efficiency and therefore higher sensitivity. The moving belt interface is capable of handling up to 1 mL/min of mobile phase. 

A well-known method for peak tracking is based on the phase-system switching idea, " which was developed to solve problems of mobile phase incompatibility in LC/MS target compound analysis. An analytical column is usually connected to a trapping column in tandem mode. A switching valve is placed after the UV detector, and the flow of nonvolatile eluents is directed through the trapping column to waste. When the peak of interest elutes from the analytical column it is... 

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

In IC-MS systems, the core of the equipment is the interface. In fact, inside the interface evaporation of the liquid, ionization of neutral species to charged species and removal of a huge amount of mobile phase to keep the vacuum conditions required from the mass analyzer take place. Two main interfaces are used coupled to IC, namely electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). In the ESI mode, ions are produced by evaporation of charged droplets obtained through spraying and an electrical field, whilst in the APCI mode the spray created by a pneumatic nebulizer is directed towards a heated region (400°C-550°C) in which desolvation and vaporization take place. The eluent vapors are ionized by the corona effect (the partial discharge... 

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