FURTHER DEVELOPMENTS OF FIA

In the early 1990s, a variant of FIA was introduced, that is, SIA (Ruzicka and Marshall, 1990). Termed the second generation of FIA, it was at the end of that decade sup- plemented by the third generation of FIA (Ruzicka, 2000), also named the lab-on-valve. Both these approaches have, in their own right, proven to entail a number of specific advantages. Thus, for instance, miniaturization of the manifolds, which, in turn, drastically reduces the consumption of sample and reagent solutions, and hence leads to generation of minute amounts of waste. Or allowing complex sample manipulations to be facili- tated in simple fashions. Or readily permitting the integration of sequential unit operations. Therefore, these two analytical concepts deserve detailed, individual description and discussion. Hence, in the present section the principles of each method will be illustrated, whereas

Figure 9 (a) Simple stopped-flow FIA-manifold. When the sample S is injected, the electronic timer T is activated by a microswitch on the injection valve. The time from injection to stopping of the pumping (delay time) and the length of the stop period can be both preset and controlled by a computer. (b) The principle of the stopped-flow FIA method as demonstrated by injecting a dyed sample zone into a colorless carrier stream and recording the absorbance by means of a flow-through cell: (A) Continuous pumping; (B) 9 s pumping, 14 s stop period, and continuous pumping again; (C) the dashed line indicates the curve that would have been registered if a zero- or pseudo- zero-order chemical reaction had taken place within the flow cell during the 14 s stop interval, that is, the slope would then be directly proportional to the concentration of analyte. [From Ruzicka and Hansen (1988a), courtesy John Wiley and Sons.]

selected applications, showing their potentials and versati- A. Sequential Injection Analysis

While most FIA procedures employ continuous, uni- directional pumping of carrier and reagent streams, SIA is based on using programable, bi-directional discontinu- ous flow as precisely coordinated and controlled by a com- puter. A sketch of a typical SIA-manifold is reproduced in Fig. 10.

The core of the system is a multiposition selection valve (here shown as a six-port valve), furnished with a central communication channel (CC) that can be made to address each of the peripheral ports (1 – 6), and a central communication line (CL) which, via a holding coil (HC), is connected to a syringe pump. By directing the central communication channel to the individual ports, well- defined sample and reagent zones are initially aspirated time-based sequentially into the holding coil where they are stacked one after the other. Afterwards, the selection valve is switched to the outlet port (here position 5), and the segments are propelled forward towards the detector, being dispersed on their way and thereby partial mixing with each other, and hence promoting chemical reaction, the result of which is monitored by the detector. Notable advantages of SIA are in particular that it allows the exact metering of even small volumetric volumes (of the order of a few tenths of microliters or less), and that it, thanks to the use of a syringe pump, readily and reproducibly permits flow reversals. Besides, it is extre- mely economical as in terms of consumption of sample and reagents, and hence in waste generation. And since

all manipulations are computer controlled, it is easy and simple to reprogram the system from one application to another one. However, it is generally difficult to accommo- date (stack) more than two reagents along with the sample, although additional reagents might be added further down- stream, that is, by making an FIA/SIA-hybrid. And due to the use of a syringe pump, SIA has a somewhat limited operating capacity, although this in practice is rarely a constricting factor.

B. Lab-on-Valve

SIA. However, here an integrated microconduit is placed on top of the selection valve. The microconduit, which normally is fabricated by Perspex, is potentially designed to incorporate and handle all the necessary unit operations required for a given assay, that is, it acts as a small labora- tory, hence the name lab-on-valve.

Thus, it may contain facilities such as mixing points for the analyte and reagents; appropriate column reactors packed, for instance, with immobilized enzymes or small beads furnished with active groups such as ion-exchangers, which in themselves might be manipulated within the LOV in exactly the same manner as liquids; and even detection facilities. For optical assays (e.g., UV–Vis or fluorometry), this can readily be achieved by use of optical fibres, the ends of which, furthermore, can be used to define the optical path length to yield optimal measurement conditions. Thus, one of the fibres is used to direct the light from a power source into the LOV, whereas the other one serves to guide the transmitted light to an appropriate detection

Figure 10 SIA system, as based on using a selection valve (SV), the central communication channel (CC) of which randomly can address each of the six individual ports. Initially, sample and reagent are by means of the syringe pump, and via the central communi- cation line CL, aspirated into a holding coil and stacked there as individual zones. Thereafter the zones are through port 5 and the reaction coil forwarded to the detector D, which monitors the product formed as the result of the dispersion of the zones into each other during the transport.

The LOV (Fig. 11) encompasses many of the features of

device [Fig. 12 (Wu et al., 2001)]. For other detectors, lity, are given in Sections IV and V.

such as AAS or ICPMS, it is, of course, necessary to make use of external detection devices, as shown in Fig. 11. In SIA and LOV all flow programing is computer-controlled, which implies that it is readily possible, via random access to reagents and appropriate manipulations, to devise different assay protocols in the microsystems.

IV. SELECTED APPLICATIONS OF FIA/SIA