Sample application valves

Online coupling SPE to either LC or GC is easily performed. In the simplest method, a precolumn is placed in the sample loop position of a six-port switching valve. After conditioning, sample application, and cleaning via a low-cost pump, the precolumn is coupled to an analytical column by switching the valve into the inject position. The solutes of interest are eluted directly from the piecolumn to the analytical column by an appropriate mobile phase. The sequence can be fully automated (Fig. 28). It is also a simple matter to enhance the gap between two solutes in elution from a precolumn (70). 

The calibration method most often used in ion chromatography is direct comparison of the peak area in an unknown sample with that of a solution with a known content of the same substance. This method requires the injection of constant volumes under constant chromatographic conditions. Errors in the sample delivery, however, are almost excluded upon application of a sample loop valve. A prerequisite is the existence of reference compounds for all sample components to be analyzed. In practice, several different standard solutions in the investigated concentration range are prepared and chromatographed [8], When the resulting peak area is plotted versus the concentration of the standards, one obtains a substance-specific calibration function. 

The Sample Application Unit. In the case of radio-HPLC, the more useful system is the injector valve with a sample loop of desired volume, allowing safe filling of the Tc sample. 

At lower pressure (<7-10.5 MPa) syringe injection of the sample through a membrane is possible, but mostly loop injection valves are used for sample application. These valves may be operated at a pressure up to 42-49 MPa. 

A third type of application of this double sample injection valve utilizes a packed reactor, placed in the reagent loop (cf. Fig. 5.10d). This con-... 

The most common applications for ball valves in refineries and chemical plants at present are for loading valves on both tank car and truck racks, utility air service connections, utility water service connections, gas bleed-off valves, sample connection valves, low pressure steam service connections, fire systems, etc. 

Valve injection. Valve injection of the sample is now the preferred and accepted technique. Sample application is rapid, the solvent flow from the pump does not have to be stopped and these systems are easy to use, readily adapted for automated injection and can operate at pressures up to 6000psi (41.4MPa) with reproducibility >0.2%. Six-port valves are commonly used, either fitted with an internal or an external sample loop and are an integral component of an HPLC system. 

On column. One on-column sample application method involves stop-flow techniques. In this instance, the pump is stopped and isolated by a three-way valve from the column, the sample is then loaded via a syringe through an injection port which does not contain a septum. The pump is then restarted, the flow restored by switching the valve, and the sample is rapidly flushed onto the column. There is no apparent loss in efficiency but inaccuracies in retention measurement occur due to the finite time required for flow to be established and therefore this technique is redundant. 

Figure 5.6. Arrangement for automated coupled-column switching in liquid chromatography. Position (I) is for sample application and fractionation and position (II) for transfer of extracted analytes and subsequent separation. P = pump, AS = autosampler, ASV = automated switching valve, PC = precolumn, AC = analytical column, D = detector and W = waste. (From ref. [41] Elsevier).

The use of probes in the sampling operation is imperative. Without the use of a probe in the line, an accurate sample cannot be taken. A sample probe should be in the center one-third of the pipeline and equipped with a full open ball or gate valve. The placement of the sample probe is important in all sampling applications. Probes must be kept away from piping elbows, tees, manifolds, reduced port valves and orifice plates. 

Fig. 2 Diagram of the twin-chamber injector with a needle in position of sample application. 1 - quartz sinter, 2 - steel core, 3 - magnet, 4 - quartz tube, 5, 6 - lower and upper housing, 7, 10, 12 - ball valve, 8 - injecting cylinder, 9 - solvent evaporating chamber, 11 - analyte evaporating chamber, 13, 14 -electric heater, 15, 16, 17 - radiator, 18, 25, 26, 32 - needle valve, 19, 27, 28, 31 - solenoid valve, 20, 23 - carrier gas inlet, 21 - chromatographic column port, 24 - six-way valve, 29, 30, 33 - auxiliary gas inlet, 34 - connector pipe for sample introduction, 35 - micro-syringe needle channel, 36 - membrane, 37 - auxiliary gas outlet.

The controller is capable of controlling all aspects of the experisental procedure. The flow rates of the pumps are controlled very precisely the sample is applied automatically to the coluam by the V-7 valve which is also under the control of the controller the coluam can be easily re-equilibrated for the next sample application. Use of the V-7 valve allows the sample to be pumped onto the column at the same flow rate as used during the chromatography. 

X 40 cm plate prepared in this way is shown in Figure 10, as seen from above. After sample application, the plate is placed in the instrument in such a way that, on closing the chamber, the mobile phase inlet is located exactly in the inlet channel, and the mobile phase outlet fits tightly into the outlet channel. The water cushion is then pressurized. Before the separation is started, with the optimized mobile phase, the mobile phase inlet valve is closed and the eluent pump is switched on in order to establish the appropriate solvent pressure. This procedure ensures rapid distribution of the mobile phase throughout the inlet channel, and, therefore, linear migration of the mobile phase, when the separation is subsequently started by opening the inlet valve. 

In this mode of operation, the split valve is open throughout (Figure 2.70). In classical sample application, this mode of injection is suitable for concentrated solutions, through which the column loading can be adapted to its capacity and... 

The adoption of these valves stem from the fact they are septumless and hence suffer from none of the previously discussed disadvantages of septa, that they operate at pressures of up to 30—40 MPa (4350 —5800 p.s.i.) and may be automated. The author is convinced that the problems of manual sample application in analytical HPLC have thus been solved by the valve manufacturers. 

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

As a consequence of the high pressures that must be tolerated, LC sample valves are usually made from stainless steel. The exception to the use of stainless steel will arise in biochemical applications where the materials of construction may need to be bio-compatible. In such cases the valves may be made from titanium or some other appropriate bio-compatible material. 

The technique of column switching can increase the versatility of the liquid chromatograph significantly. Examples of the use of column switching will be given in the chapter on applications but, as the technique employs valves similar to those used for sample injection the... 

The sampling of a suction lysimeter is initiated by applying a vacuum (approximately 40-50 cm of mercury) through the vacuum/pressure line with a hand pump or electric pump. The valve on the sampling line must be closed. A constant vacuum may be maintained on the lysimeter using an electric pump. The time required before collecting a sample from a lysimeter will depend on the method of vacuum application, the moisture content of the soil, and the soil type. 

The 1/16" x 0.02" i.d. transfer line also functioned as a sample dilution device in other applications, a stainless steel column packed with glass beads has been found to be useful for dilution. This simple dynamic dilution technique has been used extensively in flow injection analysis.3 A refractive index detector is typically used to measure the sample transfer time. As shown in Figure 4, approximately 5 minutes is required to transfer the sample plug to the Rheodyne valve. As the apex of the sample band passes though the Rheodyne valve, the valve is activated and 1 pi injected onto the liquid chromatographic column. The sample transfer time was checked periodically over 1 year of operation and found to be stable. 

The equipment used in this application included two Waters M-45 pumps, a Waters 481 UV detector with microbore cell, an air-actuated Rheodyne 7413 injection valve with a 1-pl injection loop, an air-actuated Valeo four-port sampling valve (A2CI4UW2) with no groove in the injection entry ports, an air-actuated Valeo three-port switching valve (AC3W), and a Digital Equipment LSI-11/23 microcomputer. The LC system was located in a purged cabinet suitable for use in Class I, division 2 areas. The cabinet was in a heated room about 40 feet from the reactor column. The two Valeo valves were mounted next to the reactor column, while the microcomputer was located in the control room. 

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