Gravity flow injection

Hydrodynamic injection can be performed in three different ways. (1) In pressure injection, a precisely controlled external pressure is used to force a controlled amount of sample into the capillary. (2) In vacuum injection, a vacuum is applied to the buffer reservoir at the detector end of the capillary for a controlled period of time at a regulated reduced pressure. (3) For gravity flow injection, the sample vial with one end of the capillary is elevated to a certain height above the other end of the capillary for a given period of time. Gravity forces a sample plug into the capillary. 

The injection well was cased to a depth of about 1495 m (4900 ft) and extended into dolomite to a total depth of 1617 m (5300 ft). Injection began in the early 1960s and averaged around 340 L/min (90 gal/min). The natural fluid level was 60 m (200 ft) below the wellhead, and wastes were injected using gravity flow that is, the pressure head of the well when filled to the surface with fluid was sufficient to inject fluids without pumping under pressure.181... 

The continuous-stream flow-injection system (Figure 2) consisted of a gravity-feed electrolyte reservoir, a sample injection valve (Rheodyne, Model 50) fitted with a 30 /xL-sample loop, and a flow-through electrochemical detector cell. The channel diameter of the Teflon tubing for the stream was 0.8 mm. The tubing length from injector to detector was 10 cm. 

The introduction of the samples onto the capillary column can be carried out by either displacement techniques or electrokinetic migration. Three methods of displacement or hydrostatic injection are available a) direct injection, or pressure b) gravity flow, or siphoning and c) suction. The electrokinetic injection method arose from findings that electroosmosis act like a pump (80). Both methods have advantages and disadvantages. For example, a bias has been reported in electrokinetically injected... 

Fig. 4 Optimized SdFFF fractogram of ES Cells. Representative fractogram of ES cell suspensions after SdFFF elution. Elution conditions Flow injection of 100 pL of ES suspension flow rate, 0.6 mL/min (sterile PBS, pH 7.4) and external multigravitational field, 40 (O.lg spectrophotometric detection at 254 nm). Fractions were collected as follows PFl, 3 min 40 sec/4 min 15 sec PF2, 4 min 20 sec/4 min 50 sec PF3, 5 min 0 sec/5 min 50 sec. ER corresponds to the end of channel rotation. In this case, the mean externally applied field strength was equal to zero gravity thus RP, a residual signal, corresponds to the release peak of reversible cell accumulation wall sticking. (View this art in color at www.dekker.com.)...

A schematic layout of a typical GPC unit is shown in Fig. 4.24. Since the elution rates by gravity flow through a vertical column, as in conventional chromatography, will be slow and nonreproducible, a mechanical pump is usually employed to force the sample and the elution solvent through the column at pressures of up to 1000 to 4000 psi and at a rate of 2 to 3 ml/min. The sample is injected into the column entry from a graduated hypodermic syringe (typically, 0.5 to 3 ml of a 0.005 to 0.1% solution of the polymer) by means of a mechanical inlet device. The pulse of the polymer solution injected into the column entry becomes diluted and attenuated as the different species are separated on the column packings. 

J.C. Andrade, M. Ferreira, N. Baccan, A constant-flow gravity liquid-feed system for use in flow injection analysis, Quim. Nova 9 (1986) 123. 

Picric acid Waters, pharmaceutical formulations Ethyl acetate UV-Vis 1.06 mg L-1 Sequential injection system independent extraction and detection units phase separation by gravity flow reversal for improving extraction ion association with MPVIL [478]... 

Cai, Z., Chen, H., Chen, B., and Huang, C., A gravity driven micro flow injection wetting film extraction system on a polycarbonate chip, Talanta, 68, 895-901, 2006. 

The sample can be introduced into the capillary by several methods. The simplest approach is to remove the end of the capillary from the anode buffer reservoir and place it in the sample vial that has been elevated slightly above the level of the cathode buffer container. Gravity flow for several seconds will move some of the sample in the separation capillary. Another approach is to place the anode end of the capillary into the sample vial and apply pressure to the analyte solution. The next method involves placing the anode end of the capillary in the sample vial and applying a vacuum to the cathodic side of the capillary to draw solution into the tube. The previous three means of sample introduction are referred to as hydrodynamic modes of injection. The last method involves placing the anodic end of the capillary in the sample vial and applying alow voltage for several seconds. This approach is referred to as electrokinetic injection. 

For many years, flow injection analysis has been used for automated spectrophotometric determination of anionics. Although UV absorbance at 224 nm has been reported for FLA of LAS (68), FIA is normally based upon the more robust methylene blue visible spectrophotometric method (69-71). Other cationic dyes may be used in place of methylene blue (72,73). The methylene blue method requires a two-phase system, a feature which is a continuing target of optimization experiments. Most commercial systems rely on gravity... 

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electro osmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electro osmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

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