Solvent chambers

The piston is driven in and out of a solvent chamber by an eccentric cam or gear. On the forward stroke, the inlet check valve closes, the outlet valve opens, and mobile phase is pumped to the column on the return stroke the outlet valve closes and the chamber is refilled. Unlike syringe pumps, reciprocating pumps have an unlimited capacity, and their internal volume can be made very small, from 10-100 p. The flow rate can be varied by changing the length of stroke of the piston or the speed of the motor. Access to the valves and seals is usually fairly straightforward. 

Mobile phase is displaced from a chamber by using a variable speed stepper motor to turn a screw which drives a piston. The chamber has a volume of 200-500 cm3. The flow is pulseless, and can be varied by changing the motor speed. The mobile phase capacity is limited to the volume of the solvent chamber. Although this is fairly large, so that many chromatograms can be run before the chamber has to be refilled, a lot of solvent is wasted in flushing out the pump when a change is required. 

Reciprocating Pump Figure 30.2 represents the schematic diagram of a typical reciprocating pump along with its various essential components. The piston is moved in and out of a solvent chamber by an eccentric cam or gear. The forward-stroke closes the inlet-check value while the outlet valve opens and the respective mobile phase is duly pumped into the column. Consequently, the retum-stroke-closes the outlet valve and it refills the chamber. 

The procedure of paper and thin-layer chromatography. A Application of the sample. B Setting plate in solvent chamber. C Movement of solvent by capillary action. D Detection of separated components and calculation of Rf. 

The solvent chamber should have a capacity of at least 500 mL for analytical applications, but larger reservoirs are required for preparative work. In order to avoid bubbles in the column and detector, the solvent must be degassed. Several methods may be used to remove unwanted gases, including refluxing, filtration through a vacuum filter, ultrasonic vibration, and purging with an inert gas. The solvent should also be filtered to remove particulate matter that would be drawn into the pump and column. 

Figure D1.1.2 Goldfisch lipid extraction unit (Courtesy of Labcorico Corporation, Kansas City, MO). A single beaker is used as the solvent chamber. Samples are placed between a boiling solvent and a cold surface. The solvent vaporizes, condenses on the cold surface, and washes down through the samples into the boiling solvent below.

Another potential application of perfluorocarbons is their use as bulking agents where the volume of conventional solvent is reduced by replacement with a perfluorocarbon. Although the halex reaction is a successful industrial process, there are problems recovering the toxic dipolar aprotic solvents. Chambers [80] has shown that, on a preparative scale, up to 75% of the sulfolane can be replaced in the halex reaction by an equivalent volume of perfluorohydrophenanthrene (b. pt. = 215°C). On cooling the reaction mixture, it is a simple matter to separate off the fluorous solvent at the end of the reaction for recycling. 

RESOLVE Rapid expansion of supercritical solutions into a liquid solvent chamber that can contain surfactant that act to impede particle growth... 

The apparatus consists of a 10 cm diameter glass tube about 75 cm long, held vertically on a ring stand by a chain clamp. The solvent chamber is made from 25 mm glass tubing with one end closed and flattened. The antsiphon bar and the paper holder bar are made from 8 mm glass rod. 

Fig. 1. Diagram of a pneumatic amplifier pump, a = gas pressure controller, b, c = valves, d = gas piston, e = liquid piston, f = piston seal, g = solvent Chamber, h = column check valve, i = to column, j = reservoir check valve, k = reservoir.

In a simple diaphragm (or membrane) reciprocating pump, the piston moves in an oil chamber which is separated from the solvent chamber by a thin diaphragm... 

Fig. 7. Diagram of a simple diaphragm reciprocating pump, a = motor, b = drive mechanism, c = plunger piston, d = piston seal, e = low-pressure hydraulic chamber, f = safety device, g = high-pressure hydraulic chamber, h = diaphragm, i = solvent chamber, j = column check valve, k = to column, 1 = reservoir check valve, m = reservoir.

The outlet flow-rate of one pump head depends on the length of travel of the piston and its cycle frequency, and also on the back-pressure. Indeed, solvent delivery occurs during the forward piston stroke only when the column check valve opens, that is, when the pressure in the solvent chamber becomes equal to the back-pressure. As the pressure in the solvent chamber is atmospheric at the end of the backward stroke and because of the compressibility of liquids (about 10 /atm for most chromatographic solvents) and the elasticity of the chamber, part of the forward stroke of the piston serves to compress the liquid in the solvent chamber before delivery against the back-pressure. Hence the outlet fTow--rate decreases nearly linearly when the outlet back-pressure increases. The importance of this phenomenon increases with increasing chamber volume and liquid... 

A safety valve limits the oil pressure to 50 atm, so that with a 9 1 amplification ratio the maximum solvent pressure is about 450 atm. The intensifiers have a double check valve assembly, the balls and seals of which are made of Kfil-F and ruby, respectively. The piston seal is made of graphite-impregnated PTFE. The solvent chambers have a capacity of 2.5 ml. Thus, with four intensifiers, the maximal flow-rate of 10 ml/min is obtained at the maximal frequency of 1 cycle/min. 

Figure 1. Schematic of the equipment used to dialyze dilute solutions against a mixed solvent solution chamber, C, solvent chamber, C dialysis membrane, M supporting glass frits, F ground glass surface, S teflon-coated magnetic stirrer, T solvent inlet, I outlet, O, teflon tubing. A fresh solvent reservoir, R and solvent waste, W. The chambers hold approximately 20 mL each.

At the beginning of an osmotic experiment, the difference in heights A/z observed after filling both chambers of the osmometer does not correspond to the osmotic pressure at equilibrium. The equilibrium pressure is only observed after solvent molecules permeate the membrane. If A/z is greater than the equilibrium osmotic pressure, the solvent molecules permeate from the solution chamber into the solvent chamber, and in the reverse direction if A/z is smaller than the equilibrium osmotic pressure. The time taken to reach equilibrium increases with the amount of solvent that must be displaced, i.e., increases with the diameter of the capillaries. Since, experimentally, problems such as dirt in the capillaries, etc., limit the size of capillary that one can go down to, and since the membranes must be tight (semipermeability), the establishment of osmotic equilibrium can take days or weeks. Other problems such as poor solvent drainage in the capillaries, adsorption of solute on the membrane, partial permeation of solute through the membrane, etc., can interfere with the attainment of a true osmotic equilibrium. The absence or presence and allowance for these complications must be individually established. 

The time required to reach equilibrium is much reduced through the use of novel technology in commercially available automatic membrane osmometers. If, for example, the capillary height in the solution chamber increases because solvent permeates from the solvent chamber, this is immediately compensated by the application, via a servomechanism, of a pressure on the solution chamber, such that the capillary heights above solvent and solution remain the same. Since this method involves the transport of only very small amounts of liquid, equilibrium is reached after only 10-30 min. 

Samples are spotted on the plate, which is developed in a solvent chamber as described for paper chromatography. The solvents used for TLC are the same ones used in column chromatography on the same stationary phase. 

During the brief period of its application the TLC of alkaloids has become such a general method that many authors consider it no longer necessary to quote in their publication the adsorbent, whether loose or adhering, the solvent, chamber saturation etc. 

However, the problem of the permeation of species of low molar mass through the membrane is a real one, especially for samples under the molar mass threshold of 5 X 10 to 10" g-mor diffusion toward the solvent chamber can indeed... 

Turn on the Bridge switch and turn the T-AT switch to T". Approximately zero the meter with the T potentiometer and observe the drift of the needle. If the solvent chamber is at equilibrium, the needle should not drift more than 1 to 2 mm during one complete heating cycle a steady drift to the right indicates that the chamber is still warming up if T is stable, switch the selector to the AT position. 

---