Sample external loop

There are two types of sample valve commonly used in LC, the internal loop valve and the external loop valve. In order to improve the seating and eliminate leaks, the valve faces are sometimes made from appropriate ceramics. The internal loop valves are largely used with small bore columns, that is to say columns having internal diameters of less than 1.5 mm. The external loop valves are used for larger diameter columns up to semi-preparative columns. 

The external loop valve is the more commonly used sampling system and offers a wider choice of readily adjustable sample sizes. A modified form of the external loop sample valve has become very popular for quantitative LC analysis, a diagram of which is shown in Figure 3. 

The basic difference between this type of valve and the normal external loop sample valve is the incorporation of an extra port at the front of the valve. This port allows the injection of a sample by a syringe directly into the front of the sample loop. Position (A) shows the load position. Injection in the front port causes the sample to flow into the sample loop. The tip of the needle passes through the rotor seal and, on... 

Figure 3. The Modified External Loop Sample Valve...

Although the problems associated with septum injectors can be eliminated by using stop-flow septumless injection, currently the most widely used devices in commercial chromatographs are the microvolume sampling valves (Fig. 8.3) which enable samples to be introduced reproducibly into pressurised columns without significant interruption of the mobile phase flow. The sample is loaded at atmospheric pressure into an external loop in the valve and introduced into the mobile phase by an appropriate rotation of the valve. The volume of sample introduced, ranging from 2 piL to over 100 /iL, may be varied by changing... 

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. 

Where peak dispersion has not been constrained to very small volumes the external sample loop injector can be used and the external loop sample system, which employs six ports, is depicted in figure 15. In the external loop sample valve, three slots are cut in the rotor so that any adjacent pair of ports can be connected. In the loading position shown on the left, the mobile phase supply is connected by a rotor slot to port (4) and the column to port (5) thus allowing mobile phase to flow directly through the column. In this position the sample loop is connected to ports (3) and (6). Sample flows from a syringe into port (1) through the rotor slot to the sample loop at port (6). At the same... 

Figure 30.4, displays the operation of a sample loop in two different modes i.e., (a) sampling mode and (b) injection mode. Here, the sample is loaded at atmospheric pressure into an external loop in the microvolumesampling valve, and subsequently injected into the mobile-phase by a suitable rotation to the valve. However, the volume of sample introduced usually ranges between 2 p/ to over 100 p/ but can be varied either by altering the volume of the sample loop or by employing specific variable-volume sample valves. 

The most common method of sample introduction in HPLC is via a rotary valve, e.g. a Rheodyne valve. A schematic diagram of a rotary valve is shown in Fig. 32.17. In the load position, the sample is introduced via a syringe to fill an external loop of volume 5, 10 or 20 L. While this occurs, the mobile phase passes through the valve to the column. In the inject position, the valve is rotated so that the mobile phase is diverted through the... 

Figure 4.28. The FI A split-loop injection technique, the concept of which is that the (external) loop of an injection valve is split into two sections a and 6), which in the LOAD position are filled with either sample (5) and reagent (/ ), respectively (a), or with a mixture of both (b). In the first variant, the dimensions of sections a and b determine the volumes, which, upon turning of the valve to position INJECT (90° turn of the figure), are propelled into the FI A system by carrier solution (C) in the latter variant the injected volume is determined by the size of section b while the premixed ratio of sample and reagent is a function of the two pumping rates Qr and Qs.

With in vivo microdialysis (Delgado et al., 1972 Ungerstedt, 1984), the extracellular fluid sample is ftirther qualified by a small sack or loop of semipermeable dialysis membrane (Fig. 51B). Artifidal CSF is slowly circulated into and out of the dialysis r on at flow rates of 1 -5 il/min, so that recovery of small molecules across the membrane from the extracellular space is effective. In essence the probe acts as an artificial blood vessel which may be sampled externally as a function of time during basal behavioral periods, drug administration (via the probe or elsewhere), external stimuli, etc. 

The external loop sample system, which employs six ports, is depicted in figure 7.6. 

The external loop sample valve has three slots cut in the rotor so that any adjacent pair of ports can be connected. In the loading position, shown on... 

There are two types of sample valve in general use in analytical LC the internal loop valve and the external loop valve. The two types of sample valve are shown diagrammatically in Figure 3. 

The two types of sample injection valve are the external sample loop and the internal sample loop. External sample loop valves are typically used for injecting vapor samples. The volume of sample injected can be as large as desired and as small as the physical distance between ports and tubing diameter permit. External loop sizes typically range from 2 pi up to 10 ml. 

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... 

The instrument is shown diagrammatically in Figure 10.1, and consists essentially of a sample tube, forming an external loop, connected to a GC. This loop can be isolated and the sample heated to the required temperature. After an initial heating period the volatile constituents liberated from the sample are flushed on to the chromatographic column by a... 

H = External loop made in part of 18 gauge stainless-steel capillary tubing J = Sample split heater... 

---