Mobile phase injector

The major method is HPLC, operating as partition chromatography. This is what is commonly implied when the term HPLC is used. It is the mode we will discuss initially as we cover the design and function of the pumps, columns, stationary and mobile phases, injectors, and detectors. Recall the discussion in Section 11.5, where initially, the partitioning of analytes between nonpolar liquid mobile phases and silica particles with adsorbed water became known as normal-phase (NP) ... 

In the load position the sampling loop is isolated from the mobile phase and is open to the atmosphere. A syringe with a capacity several times that of the sampling loop is used to place the sample in the loop. Any extra sample beyond that needed to fill the sample loop exits through the waste line. After loading the sample, the injector is turned to the inject position. In this position the mobile phase is directed through the sampling loop, and the sample is swept onto the column. 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

Figure 12.22 SFC-GC analysis of aromatic fraction of a gasoline fuel, (a) SFC trace (b) GC ttace of the aromatic cut. SFC conditions four columns (4.6 mm i.d.) in series (silica, silver-loaded silica, cation-exchange silica, amino-silica) 50 °C 2850 psi CO2 mobile phase at 2.5 niL/min FID detection. GC conditions methyl silicone column (50 m X 0.2 mm i.d.) injector split ratio, 80 1 injector temperature, 250 °C earner gas helium temperature programmed, — 50 °C (8 min) to 320 °C at a rate of 5 °C/min FID detection. Reprinted from Journal of Liquid Chromatography, 5, P. A. Peaden and M. L. Lee, Supercritical fluid chromatography methods and principles , pp. 179-221, 1987, by courtesy of Marcel Dekker Inc.

Although the problems associated with septum injectors can be eliminated by using stop-flow septumless injection, currently the most widely used devices in commercial chromatographs are the microvolume sampling valves (Fig. 8.3) which enable samples to be introduced reproducibly into pressurised columns without significant interruption of the mobile phase flow. The sample is loaded at atmospheric pressure into an external loop in the valve and introduced into the mobile phase by an appropriate rotation of the valve. The volume of sample introduced, ranging from 2 piL to over 100 /iL, may be varied by changing... 

Where peak dispersion has not been constrained to very small volumes the external sample loop injector can be used and the external loop sample system, which employs six ports, is depicted in figure 15. In the external loop sample valve, three slots are cut in the rotor so that any adjacent pair of ports can be connected. In the loading position shown on the left, the mobile phase supply is connected by a rotor slot to port (4) and the column to port (5) thus allowing mobile phase to flow directly through the column. In this position the sample loop is connected to ports (3) and (6). Sample flows from a syringe into port (1) through the rotor slot to the sample loop at port (6). At the same... 

The second is associated with the irreproducibility of the mobile phase gradients that may be formed. The latter is overcome by forming the gradient at a conventional flow rate and then splitting prior to the injector. 

The injection device is also an important component in the LC system and has been discussed elsewhere (2,18). One type of injector is analogous to sample delivery in gas chromatography, namely syringe injection through a self-sealing septum. While this injection procedure can lead to good column efficiency, it generally is pressure limited, and the septum material can be attacked by the mobile phase solvent. 

FIGURE 6.9 Horizontal chamber for fully online HPTLC by Nyiredy 1 — evaporator, 2 — diode-array detector, 3 — quartz glass cover plate, 4 — septum, 5 — injector block, 6 — mobile phase, 7 — filter paper, 8 — Teflon chamber, 9 — chromatoplate. (Modified from Nyiredy, Sz., J. Planar Chromatogr. 15, 454-457, 2002.)... 

The basic SFC system comprises a mobile phase delivery system, an injector (as in HPLC), oven, restrictor, detector and a control/data system. In SFC the mobile phase is supplied to the LC pump where the pressure of the fluid is raised above the critical pressure. Pressure control is the primary variable in SFC. In SFC temperature is also important, but more as a supplementary parameter to pressure programming. Samples are introduced into the fluid stream via an LC injection valve and separated on a column placed in a GC oven thermostatted above the critical temperature of the mobile phase. A postcolumn restrictor ensures that the fluid is maintained above its critical pressure throughout the separation process. Detectors positioned either before or after the postcolumn restrictor monitor analytes eluting from the column. The key feature differentiating SFC from conventional techniques is the use of the significantly elevated pressure at the column outlet. This allows not only to use mobile phases that are either impossible or impractical under conventional LC and GC conditions but also to use more ordinary... 

We should therefore use small bore tubing in short lengths for making the injector-column and the detector-column connections, and the injector and detector must be designed so that their internal volume is as small as possible. Dead volume before the introduction of the sample should also be minimised, to facilitate rapid changes of mobile phase composition that may be required during gradient elution. 

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