Valve, sample internal loop

Sample valve There are two types of sample valves the internal loop valve and the external loop valve. The former are for use with small bore columns (columns having internal diameters of less than 1.5 mm) and the latter are used for larger diameter columns up to semipreparative columns. Sample volumes generally range from 0.5 to 5 pi. 

D = Clear plasticised PVC tubing (1.5 m x 6.4 mm bore), packed with copper sulfate crystals, CUSO4.5H2O, > 44 mesh E = Katharometer F = Pneumatic sample valve G = Internal loop... 

The sample volume of the internal loop valve is contained in the connecting slot of the valve rotor and can range in capacity from 0.1 pi to about 0.5 pi. The dispersion that... 

The peaks shown were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of... 

There are basically two types of LC sample valve, those with an internal loop and those with an external loop. Valves with an internal loop are normally designed to deliver sample volumes of less than one microliters. Valves with external loops can deliver sample volumes ranging from a few microliters to several milliliters or more. In general, LC sample valves must be able to sustain pressures up to 10,000 p.s.i., although they are likely to operate on a continuous basis, at pressures of 3,000 p.s.i. or less. 

As already stated, the valve system can take two basic forms, the internal loop sampling valve and the external loop sampling valve. A diagram of the internal loop valve which utilizes only four ports is shown in figure 14. 

The sample is loaded at atmospheric pressure into an external or internal loop, or groove in the valve core and introduced into the mobile phase stream by a short rotation of the valve. The volume of sample injected is normally V2u ied by changing Ute volume of the sample loop or by partially filling a sample loop with a fraction of its nominal volume. External sample loops have volumes from about 5 microliters up to about 1 ml. 

For smaller samples, four port valves with an internal loop can be used. In these, the sample loop is an engraved slot in the body of the valve. With both types, the volume of sample that has to be used to flush out and fill the loop is about ten times the loop volume. Fig. 2.2g shows the operating principle of each type. 

Two types of injectors are frequently employed. For packed column SFC, a standard six port rotary valve with an external sample loop of 1-10 pL has proven to be quite reliable. For capillary column SFC, a similar rotary valve with an internal "loop of 0.2 to 0.5 pL is typically employed. Frequently the rotor is pneumatically actuated in a very rapid fashion to allow only a small fraction of sample to be introduced ("time-split ) this is done to avoid column overload. Alternatively, the flow from the injector is split off in the same fashion as in GC. A disadvantage of the latter mode is the potential for sample discrimination. 

Good reproducibility has been reported for capillary supercritical fluid chromatography using a direct injection method without a split restrictor. This method (Fig. 1.2(b)) utilises a rapidly rotating internal-loop injector (Valeo Inst. Switzerland) which remains in-line with the column for only a short period of time. This then gives a reproducible method of injecting a small fraction of the loop into the column. For this method to be reproducible the valve must be able to switch very rapidly to put a small slug of sample into the column. To attain this a method called timed-split injection was developed (Lee Scientific). For timed split to operate it is essential that helium is used to... 

The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in figure 9. These dispersion curves were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of the same length and a flow rate of 2 ml/min was employed. The peaks were recorded on a high speed... 

Because the column inner diameter is small, the amount of stationary phase is also very small and, as a consequence, the amount of sample that can be introduced into the column without overloading is very small (typically a few nanoliters). In most cases, the preferred sample introduction system consists of an injection valve containing an internal loop smaller than 0.1 ijuh. 

Samples are usually injected through a high-pressure injection valve fitted with a small internal loop. 

Figure 4.5 shows clearly how a loop valve operates. The loop is filled with sample solution and the internal channelled rotor seal is then turned to bring the loop within the eluent flux. These valves are known as six-port valves as they have six connections. 

Loops are available in sizes ranging from 5 to 2000 )Xl. They are easily attached to the valve and quickly changed. Smaller amounts of sample, as required for 3 )Xm and microcolumns, are best injected using a special valve with an internal loop of capacity between 0.5 and 5 tl. 

Figure 2. The modified Valeo 0.5 p 1 internal loop sampling valve for split injection in narrow-bore packed column HPLC (Reproduced with permission from Ref. 12, copyright Elsevier Sci. Publ. Co.)...

The disadvantage of internal loops is that they are fixed and can only be altered by changing the rotor. This has led to the development of external interchangeable loop valves of the type marketed by Valeo and Rheodyne. Here dead volumes are minimised and the loop can readily be exchanged for other desired sample sizes, e.g. 5 pi, 10 pi, 20 pi and 50 pi. Microsample injection valves capable of delivering 0.5-5 pi are also available. Operation of the valve is similar to those with internal loops, i.e. a load position and an inject position (Figure 6.16). 

As mentioned earlier, external sample loops or internal bores or cavities are used to store the sample. Valves with internal sample storage have the disadvantage that only a fixed volume of sample can be injected, while valves with external sample loops do not have this disadvantage as the sample loop is interchangeable. On the other hand, however, the minimal injection volume is usually smaller with the internal cavity or bore storage. 

The sample is loaded at atmospheric pressure into an external or internal loop, or groove in the valve core and introduced into the mobile phase stream by a short rotation of the valve. The volume of sample injected is normally varied by changing the volume of the sample loop or by partially filling a sample loop with a fraction of its nominal volume. External sample loops have volumes from about 5 p.1 up to about 5 ml, although typical injection volumes for conventional diameter columns are 10-50 xl. Injections from 1 p,l to about 40 nl require micro-injection valves equipped with replaceable internal loops [7,32-34]. Injection volumes less than about 40 nl are performed by positioning a split vent between the injector and the column. Typical injection volumes that preserve column efficiency for packed columns of different internal diameters are summarized in Table 5.1. For packed capillary columns with internal diameters < 0.2 mm direct injection will usually require the use of a split vent to minimize volume overload unless on-column focusing is possible. Injection volumes about 5 times larger than those indicated in Table 5.1 are sometimes used to increase sample detectability but with some decrease in the column separation power. 

The solutes chosen were (S) and (R) 4-benzyl-2-oxazolidinone. The apparatus consisted of an LC pump, a 1 pi internal loop sample valve and the column was situated in a thermostatically controlled oven. The oven could be operated at temperatures between 5°C and 85°C with a precision of 0.2°C. The mobile phase was passed through a preheater, consisting of 1 m length of coiled stainless steel tube, 0.010 in. ID., situated in the thermostat, between the sample valve and the column. The column outlet was connected directly to a UV detector operating at 220 nm. The eluent from the detector was passed to a 50 ml Grade A burette and the retention volumes were measured directly in ml of mobile phase. The burette was read with an accuracy of 0.02 ml. The output of the detector was displayed on a digital meter and an electronic recorder/integrator. 

The gas sample to be injected is isolated in a sample loop of defined volume at a known temperature and pressure (usually ambient). Subsequent rotation of the valve body causes the carrier gas to sweep the entire contents of the sample loop onto the column. External sample loops with defined volumes in the range 0.5 pi to 5 ml are commonly used. Valves with internal sample loops are available for handling small sample volumes of 0.2-5 pi. Multiport valves with two or more sample loops can be used for... 

The internal loop valve has only four ports and the sample volume is contained in the connecting slot of the valve rotor. As a consequence, this type of valve is used only for relatively small sample volumes. Internal sample loop valves deliver samples ranging from 0.1 to 0.5 pi in volume. The operation of the valve... 

For sample introduction, six-way injection valves are commonly used. The reconstituted sample extract is loaded in an external or internal loop (e.g., 20 pL loop), and introduced into the mobile phase by rotation of the valve. The external loop (>5 pL) is interchangeable in order to adjust the injection volume. For injection volumes below 5 pL, internal loops should be used. The typical injection volume is 1-20 pL onto the column. To... 

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