Fluid Propulsion

A flow system comprises three units fluid propulsion, sample handling and detection and the components necessary to assemble and integrate these units, e.g., transmission lines and connectors. This chapter describes all of these units and components. 

An ideal propulsion system should ensure reproducible flow rates on a short-term (hours) and long-term (days) basis, multi-channel capability (at least four parallel pumping channels to provide system versatility), resistance to aggressive reagents and solvents, readily adjustable flow rates and low initial investment and running costs [7]. The maintenance of a consistent flow is essential to obtain good analytical reproducibility. Flow rates are typically in the 0.2—5.0 mL min-1 range so that the system operates under low pressure, normally lower than 10 psi [0.689 bar]. The number of channels used depends on the manifold complexity. 

There are various strategies for manipulating the flow during the course of an analytical cycle (see 3.1.1.3). Regarding temporal variations in flow rates, the most common schemes for handling the flowing samples exploit  

The specific temporal variation in flow rates adopted in a flow system imposes certain requirements on the propulsion system. Various devices for fluid propelling are therefore described below. 

Peristaltic devices with a variable drive speed can provide a wide range of flow rates, typically 0.2—5.0 mL min-1, in several channels, each with a different flow rate if necessary. Partial damping is inherent in these fluid propulsion devices due to tube elasticity. 

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Several dilferent fluid propulsion systems have been investigated, including electroosmosis (see Chapter 33),... 

Another alternative for designing multi-commuted flow systems operating in the pumping mode is to exploit syringes as fluid propulsion devices. This led to the proposal of multi-syringe flow injection analyser by Albertus et al. in 1999 [109] as an advanced means of managing multichannel flow analysis. 

In the air-segmented analytical procedure outlined in Fig. 2.4, suggested flow rates for the sample/wash, air and reagent streams are 1.6, 0.20 and 0.40 mL min-1, respectively. The available fluid propulsion device provides flow rates of 1.8, 0.20 and 0.20 mL min-1. Determine the correction factor for adapting the reagent concentrations. Do not consider any kinetic aspect or any deficiency in mixing conditions as relevant. 

Another alternative for fluid propulsion is the gas-pressurised reservoir (Fig. 6.6) which was exploited in the very first flow injection analysers [35,36]. The strategy offers mechanical simplicity due to the absence of any moving part, delivers a pulseless flow and does not require a power supply. The pressure involved is usually <1 bar therefore, use of... 

FIGURE 6.6 Schematic representation of fluid propulsion by a gas-pressurised reservoir. 1 = gas cylinder 2 = manometer 3 = gas transmission line 4 = stopper 5 = fluid... 

Another option for fluid propulsion in flow analysis relies on the osmotic and electro-osmotic processes [40,41]. These processes exploit the osmotic pressure created across a semi-permeable membrane separating a saturated salt solution from a lower salinity solution [42], Water from the more dilute solution diffuses into the more saline solution, squeezing a flexible but impermeable internal bag containing the fluid to be pumped, thus generating a flow. Temperature dependence and low flow rates limit the use of these propelling devices in flow analysis. 

Time-based introduction is better implemented automatically, i.e., without human intervention. In this mode, the sampler arm selects the sample, the carrier/wash and eventually the reagent solutions (Fig. 6.8) to be directed towards the manifold. The sampling time is the main parameter governing the volumes of these solutions introduced, which can also be modified by changing the aspiration rate in the fluid propulsion unit. This is usually an option in modem instmments. 

P.K. Dasgupta, S. Liu, Electroosmosis a reliable fluid propulsion system for flow injection analysis, Anal. Chem. 66 (1994) 1792. 

FIGURE 8.26 Block diagram of a typical expert flow system. Fluid propulsion, sample handling, detection = units of the flow analyser (Chapter 6) S, C, 2R = sample, carrier, reagents solutions (inlet represented by the empty arrow) arrows = computer/unit interactions. For details, see text. 

Looking into the past, the first microfluidic technology was developed in the early 1950s when efforts to dispense small amounts of liquids in the nanoliter and sub-nanoliter range were made, providing the basics of today s ink-jet technology [4]. In terms of fluid propulsion within microchannels with sub-millimeter cross sections, the year 1979 set a milestone when a miniaturized gas chromatograph (GC) was realized on a silicon (Si) wafer... 

For those willing to contemplate more complex systems, the use of MHD (or possibly EHD - electrohydrodynamics - see Jones, 1973) can be used for fluid propulsion, mixing and separations. Qian and Ban (2005) have tested an MHD stirrer, illustrated in Figure 3.16. When a potential difference (PD) is applied across one or more pairs of electrodes, the current that results interacts with the magnetic field to induce Lorentz forces and fluid motion. The alternating application of the PD results in chaotic advection and mixing, but the authors point out that the system needs perfecting. 

At lirst, microOuidic How channels and mixers were coupled with traditional macroscale fluid propulsion systems and valves. The downsizing of the fluid flow channels showed great promise, but the advantages of low reagent consumption and complete automation were not realized. However, in more recent developments, monolithic systems have been used in which the propulsion systems, mixers, flow channels, and valves are integrated into a single structure." ... 

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