Loop valve

Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

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 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... 

Table 2. Variance of Solute Bands Resulting from Two Internal Loop Valves...

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. 

In 1993, Jorgenson s group improved upon then earlier reverse phase HPLC-CZE system. Instead of the six-port valve, they used an eight-port electrically actuated valve that utilized two 10-p.L loops. While the effluent from the HPLC column filled one loop, the contents of the other loop were injected onto the CZE capillary. The entii e effluent from the HPLC column was collected and sampled by CZE, making this too a comprehensive technique, this time with enhanced resolving power. Having the two-loop valve made it possible to overlap the CZE runs. The total CZE run time was 15 s, with peaks occurring between 7.5 and 14.8 s. In order to save separation space, an injection was made into the CZE capillary every 7.5s,... 

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. 

Bergstrom et al. [63] used HPLC for determination of penicillamine in body fluids. Proteins were precipitated from plasma and hemolyzed blood with trichloroacetic acid and metaphosphoric acid, respectively, and, after centrifugation, the supernatant solution was injected into the HPLC system via a 20-pL loop valve. Urine samples were directly injected after dilution with 0.4 M citric acid. Two columns (5 cm x 0.41 cm and 30 cm x 0.41 cm) packed with Zipax SCX (30 pm) were used as the guard and analytical columns, respectively. The mobile phase (2.5 mL/min) was deoxygenated 0.03 M citric acid-0.01 M Na2HP04 buffer, and use was made of an electrochemical detector equipped with a three-electrode thin-layer cell. The method was selective and sensitive for mercapto-compounds. Recoveries of penicillamine averaged 101% from plasma and 107% from urine, with coefficients of variation equal to 3.68 and 4.25%, respectively. The limits of detection for penicillamine were 0.5 pm and 3 pm in plasma and in urine, respectively. This method is selective and sensitive for sulfhydryl compounds. 

A Waters Associates Prep LC System/500 was used for preparative SEC. Samples were injected using a 12 mL loop valve and the column set consisted of two 2.5-in diameter x 4-ft length columns with 80-lOoX and 700/2000/P Styragel packing. Operating conditions are shown below ... 

A liquid mobile phase is pumped under pressure through a stainless steel column containing particles of stationary phase with a diameter of 3-10 /jm. The analyte is loaded onto the head of the column via a loop valve and separation of a mixture occurs according to the relative lengths of time spent by its components in the stationary phase. It should be noted that all components in a mixture spend more or less the same time in the mobile phase in order to exit the column. Monitoring of the column effluent can be carried out with a variety of detectors. 

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. 

The alkylation reaction of isobutane with a mixture of C4 linear olefins was carried out in liquid phase at temperatures between 25 and 80°C, and at 30 Kg/cm, in a fixed-bed reactor. The space velocity was WHSV = 1 ft referred to the olefins. The isobutane is premixed with the olefins. The molar ratio used in this study was 15. The C4 olefins fraction contains 38% 1-butene, 22% trans-butene, 14% cis-2-butene and 26 % isobutene. In order to analyze the products coming out of the reactor, a ten-loop valve was used to collect the sample to be analyzed after the mn. Products are analyzed by GC, using a 100 m squalane column. Prior to the reaction, catalysts were pretreated in-situ, heating up to 250 C in an air stream. 

The solution was pumped through the system at a flow rate of 1 ml/min with a peristaltic pump. The sample (0.1-1 ml) was introduced with a three-way valve or a chromatographic sample loop valve. The height of the resulting temperature peak was used as a measure of the substrate concentration and was found to be linear with substrate concentration over a wide range. Typically it was 0.01 -100 mM, if not limited by the amount of enzyme or deficiency in any of the reactants. 

The calibration method most often used in ion chromatography is direct comparison of the peak area in an unknown sample with that of a solution with a known content of the same substance. This method requires the injection of constant volumes under constant chromatographic conditions. Errors in the sample delivery, however, are almost excluded upon application of a sample loop valve. A prerequisite is the existence of reference compounds for all sample components to be analyzed. In practice, several different standard solutions in the investigated concentration range are prepared and chromatographed [8], When the resulting peak area is plotted versus the concentration of the standards, one obtains a substance-specific calibration function. 

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. 

Figure 4.5 Loop valve. The hatched area is the rotor seal.

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