Injection systems external loop valve

The internal standard method is less often used in liquid chromatography than in gas chromatography because injection of repeatable volumes has been made easier by the use of precise and reliable injection systems (loop valves). More generally, gradually, the internal standard method is being abandoned. The external standard method is, nowadays, the most common method and the use of an internal standard seems to be restricted to very specific applications for example, when preliminary to the chromatographic analysis, the solute of interest must be extracted by means of a complex protocol. 

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.

Sample preparation, injection, calibration, and data collection, must be automated for process analysis. Methods used for flow injection analysis (FLA) are also useful for reliable sampling for process LC systems.1 Dynamic dilution is a technique that is used extensively in FIA.13 In this technique, sample from a loop or slot of a valve is diluted as it is transferred to a HPLC injection valve for analysis. As the diluted sample plug passes through the HPLC valve it is switched and the sample is injected onto the HPLC column for separation. The sample transfer time typically is determined with a refractive index detector and valve switching, which can be controlled by an integrator or computer. The transfer time is very reproducible. Calibration is typically done by external standardization using normalization by response factor. Internal standardization has also been used. To detect upsets or for process optimization, absolute numbers are not always needed. An alternative to... 

Fig. 9.6. Schematic representation of die BEST system (Brnker Biospin see also [21]). 1, Bottle with transport liquid 2, dilutor 402 single syringe (5mL) with 1100 iL tube 3, dilutor 402 3-way valve 4, sample loop (250-500 pL) 5, 6-way valve (standard version) loading sample 6, 6-way valve (standard version) injecting sample 7, injection port 8, XYZ needle 9, rack for sample vials 10, rack for recovering vials 11, rack for washing fluids and waste bottle (3 glass bottles) 12, external waste bottle 13, flow probe with inner lock container 14, inert gas pressure canister for drying process.

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. 

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