Injection systems internal loop valve

To minimize the effect of sample volume on dispersion, and ensure that there was minimum dispersion from the valve and valve connections, a 0.2 jxl Valeo internal loop valve was employed to place the sample on the column. In addition, the sample valve was used with an intra-column injection system [2] that ensured the sample was placed in the center of the column about 5 mm below the packing surface. TMs device also determined that, even if the packing settled, there would be no void above the point of injection and the effective length of the column was not changed. All connecting tubes were 0.007 in. I.D. and their lengths were kept to a minimum (Le., <5 cm). The detector employed was the Perkin Elmer LC-85B fitted with a cell 1.4 pi volume and was used in conjunction with an electronic amplifier that had an effective time constant of 24 millisec. The columns employed had internal diameters of 8-9 mm to ensure the peak volumes were very large compared to any dispersion introduced by extra-column dispersion. As a result, the extra-column dispersion was maintained at a level of less than 2% of the peak volume for all measurements and, in most instances, was maintained at a level of less than 1%. 

Valve 2 is an internal loop valve used for the injection of liquid standards it may be disconnected when the system is used for gaseous halocarbons only. Valve 2 can also serve as a device for introducing internal standards into the sample stream which is especially useful for quantifying late peaks in the chromatogram. A suitable internal standard is bromotri-chloromethane (CBrCb) dissolved in methanol. 

C14W) that has an internal loop volume of 60 nL. The mobile phase then enters the colunm at the end of which is a UV detector (Jasco UV-1S70). The pressure in the system is determined by a back pressure regulator (Jasco BP1S80-81) which is located downstream of the UV detector. The column and the injection valve are housed in a temperature controlled water bath. Upstream and downstream pressures are measured using pressure transducers (Trafag 8891). 

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. 

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. 

Extra column dispersion, as would result from large injection volumes > 1 pi and dead volumes introduced by frits and fittings must be reduced to obtain optimum performance. A variety of injection systems have been reported, including those of split injection, internal loop-rotary valve, microfeeder and pneumatic microsyringes [130]. Considerable advances have been made in quantitative reproducibility. Dead volumes have been minimised by locating the columns directly into the injection and detector systems. 

High-pressure gradient pumping system and a Rheodyne Model 7725 injection valve equipped with a 20-pL internal loop. 

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... 

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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