Sample valves, construction

As a consequence of the high pressures that must be tolerated, LC sample valves are usually made from stainless steel. The exception to the use of stainless steel will arise in biochemical applications where the materials of construction may need to be bio-compatible. In such cases the valves may be made from titanium or some other appropriate bio-compatible material. 

Sampling Samples of sulfur dioxide may be safely withdrawn from a tank or from transfer lines, either of which should be equipped with a 3/8-in. nozzle and valve. Samples should be placed in sample cylinders constructed of 316 stainless steel, designed to withstand 1000 psig and equipped with 316 stainless-steel needle valves on both ends. 

Apart from its mechanical strength, pressure resistance and chemical inertness, high demands are put on the geometric construction of the flow lines in such valves, as discussed below. Usually all parts of sampling valves are made of 316 stainless steel. Carpenter 30 alloy or ceramic materials of extreme hardness such as an alumina ceramic. The sealing parts consist in most instances of PTFE or PTFE-filled materials such as Rulon. These materials commonly guarantee high mechanical and chemical resistance. 

It must be emphasized that if the sample contains labile material of biological origin, then all parts of the chromatographic system that can come in contact with the sample must be constructed of a biocompatible material. In practice, this means that every part between the sample valve and the detector should be made from titanium or some other appropriate material such as PEEK, or where there is little or no pressure. Teflon. 

Based on their construction, sampling valves can be divided into rotary, membrane, and piston types. The switching of gas tracks in rotary valves occurs through... 

Modern high-speed GC systems are able to separate some light hydrocarbons within a second [117, 118]. This approach requires special sampling valves, narrow-bore columns (diameter = 0.005 cm), and detectors with a fast response. However, in EGA these fast separations are not necessary. Taking samples on-line from the reactor and separating them within 1 minute is frequently satisfactory in order to better understand degradation kinetics. The apparatus for this approach can be constructed of commercially available parts. 

Without an autosampler, an interface can be constructed from an eight-port, two-position sample valve with sample and by-pass loops of appropriate volumes, an injection device that delivers aliquots of the column effluent isolated in the sample loop into the graphite furnace, and electronic control and switching circuitry that allows all steps in the analysis to occur in the correct sequence [55, 56]. All the components of the interface are commercially available. The injection device must... 

Large-scale models, to a scale of at least 1 30, are normally made for major projects. These models are used for piping design and to decide the detailed arrangement of small items of equipment, such as valves, instruments and sample points. Piping isometric diagrams are taken from the finished models. The models are also useful on the construction site, and for operator training. Proprietary kits of parts are available for the construction of plant models. 

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