Valve-based modulators

When using modulating valves for steam, heat emitter output must be based on the steam pressure downstream of the valve, which often has a high-pressure drop across it, even when fully open. 

Valve-based modulators rely on flow switching mechanisms, and traditionally discrete (sub)sampling of effluent has been performed for subsequent separation on... 

Depending on the modulator used, various connecting setups can be adopted to link the D to the D. In most heated modulators, a piece of uncoated capillary tubing follows a piece of thick film capillary column to ensure phase termination and decrease the retention of analytes before creating the band transferred into the D column. For cryogenic modulators, modulation can take place directly on the D capillary column. For valve-based modulation, the valve sample loop acts as a connection between the two dimensions. In other cases, the modulation process can simply take place at the beginning of the D column. In the early... 

The several different modulators that are commercially available can be classified in two main categories thermal modulators and valve-based modulators. Thermal modulators are the most frequently used and in turn can be broken down into two categories those whose principle involves a temperature increase and, inversely, cryogenic modulators. 

The majority of commercial developments which relate to the automation of GC and HPLC pay little attention to sample preparation. There are few examples where pretreatment is not required. A fully automated system was developed by Stockwell and Sawyer [23] for the determination of the ethanol content of tinctures and essences to estimate the tax payable on them. An instrument was designed and patented which coupled the sample pre-treatment modules, based on conventional AutoAnalyzer modules, to a GC incorporating data-processing facihties. A unique sample-injection interface is used to transfer samples from the manifold onto the GC column. The pretreated samples are directed to the interface vessel hy a simple hi directional valve. An ahquot (of the order of 1 ml) can then he injected on to the GC column through the capillary tube using a time-over pressure system. 

Modifier Addition System. In the case where a continuous supply of modifier was desired, the base system was upgraded to perform continuous modifier addition with the addition of a second syringe pump module and a valving package (Isco SFE System 2200). The valving package consisted of two check-valves, a mixing tee, and the hardware necessary to install them in the system. Both pump modules were... 

The p-jump unit produced by Hi-Tech Limited (PJ-55 pressure-jump) is based on a design by Davis and Gutfreund (1976) and is shown in Fig. 4.7, with a schematic representation in Fig. 4.8. A mechanical pressure release valve permits observation after 100 /us. There is no upper limit to observation time. Changes in turbidity, light absorption, and fluorescence emission can be measured in the range of 200-850 nm. The PJ-55 is thermostated by circulating water from an external circulator through the base of the module. The temperature in the cell is continuously monitored with a thermocouple probe. A hydraulic pump assembly is used to build up a pressure of up to 40.4 MPa. A mechanical valve release causes the pressure build-up to be applied to the solution in the observation cell. The instrument has a dead time of 100 /us. A fast response UV/fluorescence... 

Design of spring-loaded pressure-relief valves is supported by a well-established mechanical engineering science base. With the use of established techniques, together with modern developments in zone and field modeling, there is potential for incremental improvements to be made to flow structure and stability or modulation of relief valves. However, for leading manufacturers supplying the mass market, the performance vs. cost balance must surely be close to its optimum. 

In summary, the versatility of a mixing chamber, the multiple tasks it can be used for and the distinct role it can have in the flow manifold are the driving forces for more advanced methodologies using mixing chambers, not only as a component of a flow system but also as a complete small-scale laboratory. Based on an active chamber module, flow batch [107] and lab-on-valve [108] systems can be included in this category, as they can perform diverse and relatively complex functions, as discussed in 2.6 and 5.3.3. 

---