Sample valve injection high-pressure

For liquid chromatography, a sample of the mixture solution is injected through a loop injector which allows a quantity of the solution to be placed in a small tubular loop at atmospheric pressure. By manipulating a valve, the high-pressure flow of solvent to the column is diverted through the loop, carrying the sample with it (Figure 35.5). 

Figure4.62 Experimental set-up for liquid/liquid experiments (a) reservoir for the substrate in n-heptane (b) water reservoir (c, d) high-pressure liquid pumps (e) HPLC injection valve with sample loop for catalyst injection (f) micro mixer ...

Figure 5.28 Schematic of the experimental set-up. Water/ethylene glycol/SDS reservoir (a) high-pressure liquid pumps (b) catalyst/ substrate HPLC injection valve with 200 pi sample loop (c) hydrogen supply, equipped with mass flow controller (d) micro mixer (e) heating jacket (f) tubular glass or quartz reactor (g) back-pressure regulator (h) [64],...

Injectors introduce the sample into the mobile phase under high pressure. There are several approaches to injection in HPLC, such as syringe injection via septum, a combination of a septum and syringe or a valve injection. Valve injection is the method of preference in up-to-date HPLC instrumentation. 

Fig.6.1.5a-c Connections and positions of the three high-pressure valves used for columnswitching HPLC of oxidized pterins. Position a 0-2 min, injection of sample with precolumn and analytical column in series. Position b 2-12 min, gradient elution of fast-moving compounds from the precolumn into the detector. Position c 12-32 min, elution of slower-moving compounds from the analytical column into the detector, with simultaneous cleaning of precolumn... 

Figure 3.16 Schematic representation of (A) NCE with negative pressure pinched sample injection. SP, syringe pump V, 3-way valve, HV, high voltage power supply. (B) View of the connection from the three-way valve to the sample waste reservoir SW [106].

In SFC, the mobile phase is delivered by a high-pressure pump. The sample is usually injected as a solution by means of a high-pressure injection valve. The column may either be a packed column, comparable to a high-performance liquid chromatographic (HPLC) column, or an open capillary column, comparable to a capillary gas chromatographic (GC) column, but with somewhat smaller internal diameters (50-100 / m). Detection is performed either on-line, (i.e., UV-VIS) or after the expansion of the fluid [i.e., flame ionization detection (FID) or mass spectrometry... 

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. 

High pressure valves are most often used for sampling in LC, although syringe injection similar to that used in GC was formerly employed. A typical valve has six ports and two positions, one for loading and one for... 

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