Sample valve injection principle

Principles and Characteristics Continuous-flow (or dynamic) FAB/FTB [102] and frit FAB/F1B [103] offer a means of introducing samples in solution into a continuous flow of solvent which terminates at the modified FAB/FIB probe tip, and they extend the applicability of FAB. Samples are injected through a conventional HPLC injection valve, or solutions are simply drawn in by the high vacuum in the ionisation source of the mass spectrometer. These very similar techniques are particularly amenable to coupling with HPLC columns, and ionisation of the sample is unchanged with respect to conventional FAB and FIB/LSIMS. 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Autosamplers take this same loop and valve principle and automate the filling and handle-turning sequence. The major differences between models on the market are in the way they get sample into the loop and the method of cleaning between injections. Most autoinjectors use a carousel loaded with sample valves to hold samples until their turn for injection occurs. Sample vials are usually capped with a screw cap fitted with a septum, although some recent autosamplers replace the carousel with microtiter plates having 96-364 wells containing the samples for use with robotic workstations. Conical vials are available for limited samples and 1-jUL injections are possible with some... 

In all the previous examples, the sample zone was injected into the microconduit channel from an external sample valve, yet ultimately this function, of course, ought to be integrated into the microconduit. Miniaturization of a rotary valve is one possibility (see below), while another is the use of the hydrodynamic injection principle ([338 cf. also Section 5.1.3], which involves a combination of hydrodynamic and hydrostatic forces to aspirate, meter, and inject the sample solution in the form of a well defined plug into the carrier stream. 

So far, two principles of sampling have been applied in high-performance LC (HPLC) syringe and valve injection. Both types, conmercially available or custom-made, are discussed in detail below. 

A detailed description of IC is given in reference 1 however, the basic principles of the method can best be described by an example. Figure 1 schematically represents both an anion and a cation IC analysis. In both cases, the instrumentation involves a pumping system, an eluent, an injection valve, an ion-exchange separator column, an ion-exchange suppressor column and a conductivity cell. The sample is first injected into the flow system then the well known reaction equilibrium shown in Figure 1 results in the separation of sample anions or cations on the separator column (2). 

Fig. 20. Schematic design of linking a chromatograph on-line to bioprocesses. In principle, the design is almost identical to an FIA system. This is why FIA is often characterized as chromatography without a column. However, degassing of the sample is essential, in particular, when no internal standard is added (as in this sketch). In addition, the technical designs of injection valves differ and the injector to a gas chromatograph is heated to 200 or 250°C which means it needs, therefore, a special construction...

Sample Injection. In principle, the samples can be injected in two ways with a syringe through a septum, or through a multiport valve at the channel inlet. The septum material must be inert to the solvent, but usually cannot withstand more than 30 punctures before replacement is necessary. Furthermore, the septum variant of injection complicates the entire FFF operation and is rarely used anymore. Sample injection by multiport valves does not suffer from these complications. 

Fig. 5.5. (A) Scheme of a flow digestion system and the principle of pressure equilibration A pressure reactor, B heating zone, C cooling zone, D digestion coil, E cooling device, F connection for gas supply, G restrictor tube, H collector vial, I temperature sensor, J high-pressure pump, K injection valve, L sample loop, M sample, N and O peristaltic pumps. (Reproduced with permission of the American Chemical Society.) (B) Manifold for dynamic microwave-assisted extraction I solvent, 2 pump, 3 microwave oven, 4 extraction chamber, 5 temperature set-point controller, 6 thermocouple, 7 fluorescence detector, 8 recording device, 9 restrictor, 10 extractor. (Reproduced with permission of Elsevier.)...

The principle behind the FIA technique (reversed mode) has been exploited to develop a completely automated instrument [44] (Fig. 15.15) in which the sample is continuously pumped along the system and in which different injection valves [45] or a single injection valve aided by a selecting valve [46] are used to introduce the reagents required for the determination of each analyte in the appropriate sequence and at the required time intervals. The reacting plug is driven to the fiow-ceii of a photometric detector, and the... 

This type of sample introduction is by far the most popular. Despite the numerous types of injection valves available, virtually all of them operate on more or less the same principle. 

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