Valveless injection

Normal IC separations rely on a high-pressure eluent pump, injection valve, column and detector. However, a novel method for speciation was developed by Gjerde and Wiederin [4], in which a low-pressure column, valveless injection method was employed. The system configuration is shown in Fig. 11.1. An example with this type of IC to perform a separation of Cr(III) and Cr(IV) is shown in Fig. 11.2. Nitric acid eluent is acceptable in this case because it does not oxidize Cr(III). The separation method is anion exchange, so Cr(III) is unretained and Cr(IV) is eluted by the nitrate... 

Initial experiments have demonstrated the feasibility of flow manifolds having a stacked configuration for chemical analysis systems A substantial reduction in system size may be accomplished when micromachined silicon elements are employed Implementation of the concept of merging zones of sample and reagents and the valveless injection scheme result in more efficient consumption of reagents In order to benefit fully from the possible advantages of miniaturization, a carefril design of the components as well as the analysis methods is required... 

Kuban P. and Karlberg B., On-line monitoring of kraft pulping liquors with a valveless flow injection-capillary electrophoresis system, Anal. Chim. Acta, 404, 19, 2000. 

Figure 5.19 — Flow-through biochemical sensor based on the twofold immobilization of the catalyst (urease) and reagent (an acid-base azo dye) in the sensing microzone for the determination of urea in kidney dialysate. (A) Sensing microzone held in a microcircuit. (B) Valveless flow injection manifold. P pumps T timer S sample W waste. For details, see text. (Reproduced from [57] with permission of Elsevier Science Publishers).

Urea in kidney dialysate can be determined by immobilizing urease (via silylation or with glutaraldehyde as binder) on commercially available acid-base cellulose pads the process has to be modified slightly in order not to alter the dye contained in the pads [57]. The stopped-flow technique assures the required sensitivity for the enzymatic reaction, which takes 30-60 s. Synchronization of the peristaltic pumps PI and P2 in the valveless impulse-response flow injection manifold depicted in Fig. 5.19.B by means of a timer enables kinetic measurements [62]. Following a comprehensive study of the effect of hydrodynamic and (bio)chemical variables, the sensor was optimized for monitoring urea in real biological samples. A similar system was used for the determination of penicillin by penicillinase-catalysed hydrolysis. The enzyme was immobilized on acid-base cellulose strips via bovine serum albumin similarly as in enzyme electrodes [63], even though the above-described procedure would have been equally effective. 

Flow switches using fluidics in small Reynolds number are fabricated. The principle of the flow switch is shown in Fig. 4 [21]. Mixing of the sample stream and carrier liquid is negligible when the contact area is small and the contact time within subsecond range. The width of the sample stream is controlled by two carrier flows. This structure can be applied for a valveless sample injection in FIA and for sorting of particles and living cells in flow cytometry. A flow switch having 5 outlets has also been obtained by this method. 

FIGURE 3.36 Schematic representation of a micromachined pre-focused 2x6 flow switch with six inlet ports and six outlet ports. The flow switch integrates two important microfluidic phenomena, including hydrodynamic focusing and valveless flow switching. The pre-focused samples can be injected into desired outlet ports precisely [263]. Reprinted with permission from the Institute of Physics Publishing. 

On-line SPE-ESI-MS Injection onto a short trapping column, washing away of the salts and/or SDS, and elution of the proteins into the ESI source [25-26], An integrated device for on-column enrichment and desalting via an exit vent was reported as well (valveless SPE-LC) [27],... 

According to Kentfield (1993), a pulse combustor is a combustion-driven device with self-aspirating feature, and this effect is achieved as a consequence of the internal unsteady flow events. In contrast, a pulsed combustor is a device with cyclic but nonresonant combustion as dictated by wave events. Pulsed combustors usually operate at a much lower than natural frequency, often controlled by an ignition, fuel injection, or a valve sequence. Therefore, valveless or flapper valve combustors fall into category of pulse combustors while mechanically driven valves (e.g., rotary valve) used to control either air or fuel inflow, flue gas outflow, or both should be categorized as pulsed combustors, unless the operation of a mechanical valve is controlled by resonant phenomena in a feedback mode. Such a design is known as a frequency-tunable pulse combustor. 

A novel method of valveless sample introduction in flow analysis systems called flow-diffusion analysis (FDA) is described Due to the reduction of mechanical parts, i e an injection valve is dispensable, the system becomes very simple and its transformation into the micro-scale should require less effort than the respective transfer of conventional flow-injection systems Additionally, due to the time based sampling, the method is more flexible in terms of sensitivity and linear range compared to established volume based methods... 

---