Sample valves, construction materials

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. 

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. 

The sample inlet system shall be constructed of materials that are inert and nonadsorptive with respect to the components in the sample. The preferred material of construction is stainless steel. Copper, brass, and other copperbearing alloys are unacceptable. The sample inlet system from the cylinder valve to the GC column inlet must be maintained at a temperature constant to 1 C. 

Other Start-up Shut down Relief system Power/service failure Corrosion/erosion Materials of construction T oxicity/asphyxia Maintenance (provision of isolation, etc.) Double valves (trapped liquids) Valve access Provision for testing instruments, trips, alarms Fire Static electricity Noise Thermal radiation Ionising radiation Sampling Spares Anything else ... 

Figure 2.18. Gradient dilution is based on selecting readouts at the tail of the dispersed sample zone (right-hand side of left curve), where elements of fluid can be located within which the sample material has been diluted by the disperson process. The selection may be mechanical, that is, the zone is selected via the delay time, captured by means of a second valve, and then reinjected into a second FIA system. Electronic dilution is achieved by means of a microprocessor, by selecting via the delay time a detector readout from which a calibration curve is constructed (middle curves). The peaks, recorded with four different concentrations of analyte (labeled 25%, 50%, 75%, and 100%) were sliced at different delay limes yielding readouts from which calibration curves of different slopes were recorded (right curves).

In both commercial and home-built FIA instruments, the injection valve is made of PVC and/or Teflon, the coils and connecting tubes of polyethylene, polypropylene, or Teflon, and the pump tubes of PVC. All these materials are fairly resistant to polar solvents, and as long as aqueous solutions are used throughout, no problems will be encountered. Yet if nonpolar solvents have to be handled, such as in solvent extraction, it is necessary to use pump tubes resistant to these solvents [e.g., Acidflex or Viton tubes made of black flurorplast rubber, or the recently introduced Marprene tubes (see Table 5.1)], Teflon coils, and to construct the system so that the aqueous samples are injected into an aqueous carrier stream (i.e., the Perspex or PV(T made parts of the injection port do not contact the organic solvent). For FIA procedures performed exclusively in non-aqueous media, compatible materials have to be chosen carefully and the injection valve must be made entirely of Teflon or a stainless-steel-Teflon combination, such as that used in chromatographic valves. 

It is often said that most process analyzer problems can be traced to the sample system. While some progress has been made in modularizing sample systems, generally the type of maintenance that must be performed has not changed. In many cases, steam-traced hnes have been replaced with electrically heated lines, which eliminate maintenance related to steam traps and steam supply. However, filters must still be replaced, valves must still be maintained, and materials of construction that corroded in the past will still corrode. 

The use of Nansen bottles has some disadvantages, one being based on their construction. The closing mechanism impairs flushing of the sampler due to dead volumes and turbulent mixing. Modern samplers of the Nansen type are usually made of plastic materials such as polycarbonate (PC) and poly (vinyl chloride) (PVC) instead of brass (Table 1-2). Such samplers are less robust, but lighter, they are non-corrosive, greasing of the valves becomes unnecessary and interference with sample composition is minimized. 

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