Septum-type syringe injectors

This type of sample introduction can be subdivided into two classes (i) septum-type syringe injectors and (ii) septumless syringe injectors. 

In conclusion, septum-type injection is the easiest and cheapest method of sample introduction. It can be used without problems up to 100 atm for the Single-septum type and up to ZOO atm for the double-septum construction. The injection of large volumes at high pressures, however, is limited because of the force needed to press the plunger against high pressures. Disadvantages of 

The syringe is removed, the port is closed and the eluent stream is started again. An example of such a stopped flow injector, which is commercially available (Varian), is shown in Fig. 7. For precise retention time measurements, stopped flow injection is less preferable because of the time needed to pressurize the system, which might be of the order of 3-2D sec depending on the capacity of the pumping device. On the other hand, for routine analysis this is less important and stopped flow injection might be a very valuable, cheap and reliable sample introduction technique. 

In conclusion, generally syringe injection is very convenient with respect to the variation of the injection volume and to the excess of amount of sample 

In practice, the injection of larger volumes ( ID pi) is possible only at lower inlet pressures. Further, the precision of syringe injection is usually not batter than W. 

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Injector The sample, typically 5-200 )J,L, is placed in the carrier stream by injection. Although syringe injections through a rubber septum are used, a more common means of injection is the rotary, or loop, injector used in ITPLC and shown in Figure 12.28 of Chapter 12. This type of injector provides reproducible injection volumes and is easily adaptable to automation, a feature that is particularly important when high sampling rates are desired. 

The injection device is also an important component in the LC system and has been discussed elsewhere (2,18). One type of injector is analogous to sample delivery in gas chromatography, namely syringe injection through a self-sealing septum. While this injection procedure can lead to good column efficiency, it generally is pressure limited, and the septum material can be attacked by the mobile phase solvent. 

Direct vaporisation injection. For packed columns and megabore columns of 530 pm, which typically use a flow rate of 10 ml/min, direct vaporisation is a simple way to introduce the sample. All models of this type of injector are a variation of a simple assembly which uses a metal tube with a glass sleeve or insert. The glass insert is swept by the carrier gas and heated to the vaporisation temperature for the analytes undergoing chromatography. One end of the injector contains a septum made of silicone rubber that allows the syringe needle to pass through it into the system. The other end of the injector is connected to the head of the column (Fig. 2.4). The entire sample is injected into the column in a few seconds. 

A sample must be introduced onto the column in an efficient and reproducible manner. One of the most popular injectors is the syringe injector. The sample, in a microliter syringe, is injected through a neoprene/Teflon septum. This type of injection can be used at pressures up to 3000 psi. 

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