Time-based sampling

The flow analyser is well suited for handling aqueous samples, which are directly inserted into the manifold by taking advantage of loop-based or time-based sample introduction. Due to the portability, ruggedness, low power requirement and low maintenance requirement inherent to the unsegmented flow systems, specific approaches for in situ sample collection and handling have been proposed. 

Since for time based sampling it is more convenient to express the sample and concentrate vtriumes in terms of time and flow-rates. i.e., V, = q/, and V, = Q,i being the sample introduction rate and introduction time in seconds, equation 1.10 may be exfnessed as... 

Decreased sampling frequencies due to the requirement of a carrier loading stage. Disability in performing time-based sampling in various preconcentration systems (see Chapter 4). 

Other combinations are of course feasible, such as using time-based sampling to introduce larger sample volumes or using dropping segmeniors to achieve more reproducible segmentations. This will depend on the individual objectives of the application. 

A typical manifold for FI liquid-liquid extraction with flame AA spectrometric detection, using time- based sampling, which is suitable for preconcentration and general use. 

With a defined loading period, the amount of analyte retained on the column, reflected in the analytical response of the eluate, always reaches a maximum with an increase in sample loading rate in time-based sampling, beyond which an increase in sample volume leads to decrease in signal due to insufficient contact time. Although complete retention of the analyte is not a prerequisite for FI procedures, systems with excessively low P values are more vulnerable to interferences. The optimized sample loading rate should therefore be based on an overall consideration for sensitivity and... 

Although applications for column separation or preconcentration systems coupled to chemiluminescence determinations are few, published reports show no particular difficulties in such applications, except for the requirement of an adjustment of the chemical conditions of the eluate to suit the chemiluminescence reaction. Interferences due to refractive index effects are not likely to occur, owing to the often used spiral shape of the chemiluminescence flow-cell and to the fact that light emission is measured perpendicular to the direction of the flow. Therefore, column washing is usually not as important as for spectrophotometric applications, so that time-based sample loading manifolds such as those used for flame AAS may be used to advantage for improving the concentration, efficiencies. 

Fig. 7a FI manifold of gas-diffusion preconcentration system using multifunctional valve with time-based sampling (valve in injection position). Experimental conditions are for the preconcentration of ammonia. S. sample C, carrier (water) R], reagent for generation of volatile species (base) R2, acceptor reagent stream (buffered acid-base indicator) V.

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... 

Flow difiiision analysis (FDA), a novel method of valveless sample introduction, is a combination of membrane separation and time based sampling. It increases the flexibility of analytical readout and minimizes the amount of sample volume Less mechanical components are required and thus its transformation into the pTAS-scale should be favorable compared to other flow analysis techniques. 

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