I.7.
Flow based analytical techniques share one serious drawback; the flow systems continuously consume reagents and generate toxic waste. This problem can be mitigated by flow programming and by downscaling of the volume of the flow path, through which the sample is to be transported.
Efficiency and Scaling
Continuous flow, air segmented analyzers (CFA) can not be flow programmed, since gas segments are compressible, which makes stop/flow operations irreproducible. Compared with the early models of the AutoAnalyzer system, the flow path volume of the current instruments is smaller, as the inner diameter of the flow channel has been reduced from 2mm to 1 mm. Yet, further downscaling of CFA systems has reached its limit, since air segmentation is not feasible in very narrow conduits. Since length of CFA conduit through which sample has to travel has not been reduced, the typical volume of CFA system is several milliliters.
Flow injection systems were , since the beginning, designed with narrow bore (0.5mm I.D) and short flow channels, the typical volumes well below 1 milliliter. Therefore, combined sample and reagent volume are several hundred microliters, but again, due to continues pumping, the regent use and waste generation remains an issue that needs to be corrected. The FI flow programming, presented in Chapter 1, offers a feasible remedy.
Sequential injection, is without question, the most economical of all regent based flow techniques. It operates in a digital flow pattern, which means the solutions are pumped only when sample is being analyzed. (In a continuous flow systems the waste is generated regardless wheher samples are processed or not, such as due to inattention, or during startup and shut down periods). When SI is performed in LOV platform, the flow channel is automatically downscaled to accommodate only the volumes of sample and reagents, which are typically less than 100 microlites. Mixing is promoted by flow reversal and incubation by stopping the flow. The volume of flow channel trough which the sample is to travel is 200 microliters or less.
Scaling is a complex task, since its success depends on addressing factors which are not obvious components of the system to be redesigned. Downscaling of a flow analyzer could seem to be a straightforward, task as long as one would apply the principles of similarity, relevant to fluid mechanics. Starting with the simplest, geometric similarity, which is similarity of shape, and which operates with a scaling factor that is the ratio of any length in one system to the corresponding length in the other system might appear to be a correct approach. Yet any chemist would feel that there is something amiss, if one would try to downscale a flow ystem, designed to carry out analysis, in way depicted at the above illustration.. This brings us to the topic of microfluidics and Lab-on-chip technology.
