Capillary restrictors

Capillary restrictor tube Capillary restrictor tube ... 

Importantly, the operating efficiency of a chromatograph is directly dependent on the maintenance of a highly constant carrier gas-flow-rate. Carrier gas passes from the tank through a toggle value, a flow meter, a few feet of metal capillary restrictors, and a 0-4 m pressure gauze. The flow rate could be adjusted by means of a needle value mounted on the base of the flow meter and is controlled by the capillary restrictors. On the downstream side of the pressure regulator, a tee (T) may split the flow and direct it to the sample and the reference side of the detector. 

The bulk of the SFE experiments performed to date were executed with systems typically consisting of a syringe or reciprocating pump, a high-pressure containing sample vessel comprised of HPLC column hardware, and a fused silica capillary restrictor. Extraction vessel temperatures of 40-80°C were usually accomplished using a converted oven or with the use of a thermostatted tube heater (2,3). Instrument manufacturers now offer a variety of commercially available SFE systems that vary in design, operation, features, ease of operation, and limitations. 

In addition, solute focusing is possible by maintaining a low initial temperature (e.g. 40 °C) for a long period of time (8-12 min ) to allow the mixture of decompressed carbon dioxide, helium gas and the solutes to focus on the GC column. The optimization of the GC inlet temperature can also lead to increased solute focusing. After supercritical fluid analysis, the SF fluid effluent is decompressed through a heated capillary restrictor from a packed column (4.6 mm i.d.) directly into a hot GC split vaporization injector. 

Huber et al. [43] used a commercial capillary electrophoresis system connected to a LC pump to achieve the gradient elution. His approach is shown in Fig. 2.14. Two LC pumps were connected to one end of a capillary restrictor through a static mixing T-piece. The other end of the restriction capillary was connected to an injection valve. This valve was connected to a poly (ether ether keton) (PEEK) cross (17.2 pL) which... 

In supercritical fluid chromatography (SFC), a supercritical fluid, usually carbon dioxide, is used. An SFC column resembles a GC column or a packed LC column. The operating temperature is lower than that in GC, which makes it more suitable for the analysis of thermally labile compounds. Another advantage is that water is not present and this is particularly valuable for the analysis of phosphatic antioxidants. In addition, high molecular mass antioxidants can be analyzed with SFC. SFC and FTIR have been used for the analysis of light stabilizers (UV absorbers) and antioxidants [43]. The eluted compounds were deposited on a KBr window after they had passed through a capillary restrictor. The hmit of detection was aroimd 100 ng. 

We were anxious to evaluate the use of supercritical carbon dioxide (scCOj) to remediate an urban soil. A two-stage approach was prompted by our efiforts to make the process continuous. SCCO2 extraction of particulate media is inherently a batch process. Pressure within the extractor is maintained with a capillary restrictor (in our case a 50 pm diameter silica tube) that is prone to fouling. The non-polar organic Section of an aqueous (surfactant) extract can be removed continuously (either on site or off site) with scCOj in a counter current liquid-liquid like process. The heavy metal contaminants remain with the water fraction for subsequent treatment. The organics fraction is the subject of this short review on SCCO2 processing. 

One of the major advantages of SFC is its compatibility with both GC and HPLC detectors. GC flame detectors, such as the flame ionization detector (FID) [11,12], nitrogen thermionic detector [12,13], and flame photometric detector [14] have all been interfaced with SFC systems using a capillary restrictor which, while maintaining supercritical conditions in the column, also effectively decompresses the fluid to ambient pressure just before it enters the flame tip [10,15]. HPLC detectors such as ultraviolet and fluorescence detectors are employed when pure organic mobile phases or modified mobile phases are used. With these detectors, analytes are detected spectroscopically in a flow-through cell prior to decompression [16]. 

Chester etal. Sucrose polyesters Capillary column 10 m x 65 jum DB-1 Carbon dioxide FID Use of robot-pulled capillary restrictor pressure program... 

In a needle valve restrictor, the flow rate is controlled by changing the valve position. This can be performed during the extraction to avoid blockage. The drawbacks with needle valves are virtually the same as for capillary restrictors. The preferred choice is electrically controlled restrictors, which maintains a back-pressure or flow rate set by the operator. These devices are not very often subject to blockage, which makes them highly reliable and often form part of commercial SFE instrument. Two types of restrictor designs are shown in Figure 2. 

Detector Di monitors the output from the tandem column ensemble. Detector D2 monitors a small fraction of the effluent from Ca- Capillary restrictor R controls the amount of the Ca effluent directed to D2. This second detector is optional, but it is useful for method development. 

A far better gas-flow purge system is shown in Figure 11.15. Here the rotameter is upstream of a fixed, capillary restrictor that has been designed to provide a specific, known flow for a particular application. Further, the purge joins the impulse line close to the process rather an close to the transmitter. 

An even better system, particularly for specific gravity applications, is shown in Figure 11.16. Here the fixed capillary restrictor is used in conjunction with a purge-flow regulator. In addition to being more reproducible, this system does not require critical flow. 

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