Injection valves time-based sampling with

Fig. 7a FI manifold of gas-diffusion preconcentration system using multifunctional valve with time-based sampling (valve in injection position). Experimental conditions are for the preconcentration of ammonia. S. sample C, carrier (water) R], reagent for generation of volatile species (base) R2, acceptor reagent stream (buffered acid-base indicator) V.

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

On the one hand, in the determination without preconcentration, valve 2 is switched On while valve 3 is switched Off to inject an appropriate volume of sample (time-based injection) without passing through the SPE column, and at the same time valve 4 is actuated to inject thiocyanate. The red complex thus formed is measured with the spectrophotometer. On the other hand, in order to increase sensitivity the determination can be carried out with preconcentration. Valve 2 and valve 3 are switched On to inject an appropriate volume of sample (time-based injection) passing through the column. Then valve 1 and valve 2 are switched Off while valve 3 is kept in On position in order to... 

Flow injection analysis (FIA) is based on the injection of a known amount of sample in a flowing carrier solution stream via an injection valve (IV) the flowing carrier transports the sample to the detector. The main advantages of FIA compared with continuous flow systems are the operational simplicity and the lower consumption of sample and carrier (Bryce et al., 1995). However, the reduction in the amount of analyte deposited on the electrode surface as a result of the decrease in the contact time between the sample and the working electrode and the disprersion of the sample in the flow manifold results in a decrease of the analytical signal. A typical FIA system is illustrated in Fig. 3. 

Typical Response Peaks. Figure 2 shows some typical response peaks for the HPLC separation of the saturates and aromatics in a typical vacuum gas oil. The curve in the upper portion of the figure is the response for the sample using the RI detector. After the saturates peak appeared, the backflush valve was switched and the attenuation changed as indicated in the figure. The peaks were very sharp and symmetrical and appeared in less than 10-min lapsed time from the point of injection. Base line drift was minimal and the areas of the response peaks were obtained with a ball and disc integrator on the strip chart recorder. 

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