Rotary sampling valves

The most important source of error in analyses by the method just described is usually the uncertainty in the v olunic of sample occasionally, the rate of injection is also a factor Samples arc usually small ( I pL), and the uncerlaintics associated with injection of a reproducible volume of this size with a microsyringe.may amount to several percent relative. The situation is even worse in G(. where the sample must be injected into a heated sample port here, evaporation from the needle tip may lead to large variations in the volume injected, lirrors in sample volume can be reduced to perhaps 1% to 2% relative by means of autosaniplers or a rotary sample valve such as that described in Chapter 27. 

FIGURE 27-5 A rotary sample valve valve position (a) is for fiiling the sample loop ACH position (b) is lor introduction of sample into column. 

FIGURE 3 A rotary sample valve, (a) Valve position for filling sample loop and (b) for introduction of sample into the column. 

It should be stressed that only those surfaces that actually come in contact with the sample need to be bio-compatible and the major parts of the valve can still be manufactured from stainless steel. The actual structure of the valve varies a little from one manufacturer to another but all are modifications of the basic sample valve shown in figure 13. The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal. 

Figure 9 Schematic diagram of typical six-port rotary injection valve at (a) filling and (b) emptying position. (1) Sample-loop inlet, (2) carrier inlet, (3) outlet to the flow cell, (4) sample-loop outlet, (5) outlet to waste, and (6) sample inlet.

GAS SAMPLE VALVE. With all the problems associated with syringe injection of gas samples, it is not surprising that a more accurate way of injecting gas samples has been found. This system makes use of a gas sampling valve. There are a number of these valves on the market using either rotary or push-pull actuation and interchangeable volumes are standard. A schematic for a rotary valve is shown in Figure 4.15. In the load position the volume of the valve is connected to the in and out load ports. 

Figure 4.15. Flow schematic of a typical rotary gas sample valve.

Fig. 10.13. Venturi tube (1 feed, 2 metering pump, 3 nozzle, 4 rotary gear pump, 5 pump, 6 heat exchanger, 7 funnel, 8 sample valve, 9 valve).

Sampling valve A rotary valve used to inject small portions of a sample onto a chromatographic column usually used in conjunction with a sampling loop. 

Low pressure rotary injection valves are commonly used, e.g., those from Rheodyne , typically fabricated from PTFE, with six ports and an external sampling loop (Fig. 6.9). Rotary valves with more ports, e.g., 8—12, have also been used. 

It is interesting to note that in the pioneering application of this approach [13], loop-based sample injection was accomplished using a channel drilled into the rotor of a rotary injection valve, which acted as the volume-selecting element instead of an external sampling loop. 

Figure 4.37. Schematic diagram of a flow-injection serial dynamic dialysis system in which the acceptor part of a dialysis unit constitutes the sample loop of a rotary injection valve (RV). RP, reagent pump SP, sample pump RCl and RC2, reaction coils and W, waste. (From Ref. 1268 by permission of Elsevier Scientific Publishing Co.)...

In all the previous examples, the sample zone was injected into the microconduit channel from an external sample valve, yet ultimately this function, of course, ought to be integrated into the microconduit. Miniaturization of a rotary valve is one possibility (see below), while another is the use of the hydrodynamic injection principle ([338 cf. also Section 5.1.3], which involves a combination of hydrodynamic and hydrostatic forces to aspirate, meter, and inject the sample solution in the form of a well defined plug into the carrier stream. 

Higher pressures are required for pressure-assisted separations in packed columns used for capillary electrochromatography [235,376,464-466]. These columns require a high-pressure pump to provide the supplementary mobile phase flow. Conventional rotary injection valves and autosamplers can be used for sample introduction with this arrangement if a special inlet tee housing the electrode and split line with a restrictor is installed [422]. Microcolumn pumps are also useful for conditioning columns before initial use in capillary electrochromatography (section 8.4.2). There are no com-... 

The sample valve is a rotary valve system that interposes the sample, contained in a sample loop, in line with the carrier gas flow, so that it is swept by the carrier gas onto the column. In GC, sample valves are almost exclusively used for gas samples and are not necessary for the majority of chiral chromatography analyses. Liquid samples or solutions are placed on the column by means of a injection syringe and silicone septum. There are two types of syringe injection systems, those that are used for packed columns, and those that are used for capillary columns. Today about 80% of all GC analyses are carried out using capillary columns, but for the sake of completion, the syringe system that is used with packed columns will also be described. 

FIGURE 18.8 Venturi tube (1, feed 2, metering pump 3, nozzle 4, rotary gear pump 5, pump 6, heat exchanger 7, funnel 8, sample valve 9, valve). 

Based on their construction, sampling valves can be divided into rotary, membrane, and piston types. The switching of gas tracks in rotary valves occurs through... 

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