Mobile phase electronic pressure control

Piston rods in a conventional high performance instrument typically are driven by a cam. The cam smooths the delivery of the mobile phase. Dual pistons allow one cylinder to recharge while compression of the other maintains the operating pressure. More recently, electronic flow sensing has been used to continuously adjust and maintain control of the flow rate. Check valves ensure that flow is unidirectional, as well preventing a drop in pressure during the recharge cycle. 

Some altitude effects on the operation of chromatographic instruments are anticipated. To achieve reproducible retention times for identifying compounds, mobile-phase flows need to be controlled so that they are independent of ambient pressure. Detectors may also respond to changes in pressure. For example, the electron capture detector is a concentration-sensitive sensor and exhibits diminished signal as the pressure decreases. Other detectors, such as the flame ionization detector, respond to the mass of the sample and are insensitive to altitude as long as the mass flow is controlled. 

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. 

A column consisting of a deactivated silica-based stationary phase is used for the packed-column mode. A packed column allows larger volumes of sample solvent to be injected, thus improving sensitivity. Generally, the column dimensions are 1 x 100-250 mm and the particle size is 5 / m. Commercial SFC instruments are also available that will handle the classical 4.6 x 150-mm or 250-mm columns. With the introduction of electronically controlled variable restrictors to control the back pressure, the packed columns are becoming increasingly more popular. This feature allows the independent flow and pressure control of mobile phases, thus helping in rapid optimization of selectivities. Some of the commonly used packed columns are as follows ... 

A successful chromatographic analysis depends on the precise performance of the HPLC instmmentation, i.e., control of pressure, the composition of mobile phase, the performance of the analytical column, the detector, the injector or autosampler, and the electronic data handling system. 

Since its introduction in the 1960s, SFC has experienced several ups and downs in its development. Either a gas or a Uquid above its critical temperature and pressure is used as the mobile phase for SFC. In most cases, COj is used because of its favorable critical parameters (i.e., a critical temperature of 31 °C and a critical pressure of 7.3 MPa). Moreover, CO2 is cheap, nontoxic, and nonflammable. A high-pressure pump delivers the mobile phase through either a packed (pSFC) or capillary column (cSFC) to the detector. The mobile phase is maintained under supercritical or subcritical conditions via an electronic controlled variable restrictor that is positioned after detection (pSFC) or via a fixed restrictor positioned before a gas-phase detector (cSFC). The retention characteristics of the analytes are influenced by the properties of the stationary phase and by the polarity, selectivity, and density of the CO2 mobile phase. The density is controlled by variation of the temperature and pressure of the supercritical medium. Furthermore, the elution of very polar compounds under high densities can be achieved with a precolumn addition of polar modifiers such as methanol. Nowadays, pSFC formats use the same injector and column configurations as LC methods. Consequently, pSFC formats are considered to be more useful for routine operation than cSFC. The most remarkable... 

In HPLC the mobile phase velocity is, upto a limit, controllable electronically. This limit is established by the maximum pressure gradient that can be maintained across the colunm. For HPTLC, the mobile phase velocity is governed by the capillary forces which transport the solvent through the sorbent bed. This disadvantage is partially offset by the fact that the development process is different for the two techniques. In HPLC the development occurs by elution chromatography, that is each sample component must travel the complete length of the column bed and the total separation time is determined by the time required... 

Operation of HPLC pumps for delivering micro- to nanoliters per minute flow rates is based on the capability to make precise flow measurements and the feedback control through an electronically driven purge valve situated before the flow sensing area. Such an electronic flow control system may be placed after formation of the required mobile-phase composition (from two or more high-pressure pumps) or may be used for each specific pump. In the first case, the mobile-phase flow exceeding the threshold is purged as a waste, while in the second case the solvent may be recirculated back to the reservoir. The two setups are depicted in Fig. lOA and B. 

The driving force producing mobile phase flow in GC is simply the cylinder of compressed gas dehvered via a pressure regulator, although modern electronic flow controls introduce a useful level of sophistication (Hinshaw 2002b). However, HPLC pumps must provide a continuous constant flow of the eluent through the injector, colnmn and detector, and are a crucial component of any HPLC system (Kazakevich 1996). 

It is essential to have a pressure regulating valve which has a small internal volume and has the capability of controlling the pressure independently of mobile phase mass flow rate. In addition to these features, it is desirable to heat the effluent stream to prevent blockages by solid particles of carbon dioxide and precipitated solutes caused by the self cooling of the fluid as it expands after depressurisation. The back pressure regulation on the latest system is by means of an electronically controlled valve described by Saito et al. [9] and manufactured by the Jasco Corporation of Japan. This device has proved to be much more reliable than the manually operated valve and has a very small internal volume. 

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