H. Poppe

Dispersion in open tubes and tubes packed with large glass beads : The Single Bead String Reactor

Abstract A semi-quantitative description of the dispersion process in open tubes is presented for conditions prevailing in flow injection analysis. It is shown that uncoupling the two functions of the tube (i.e., mixing and creating a residence time) can be accomplished by making use of a simple mixing device followed by a tube packed with glass beads of relatively large diameter (up to 70% of the i.d. of the tube): this gives a single bead string reactor (s.b.s.r.). With a flow injection system consisting of such an s.b.s.r., longer residence times can be obtained without loss of peak height or loss of sampling frequency.

Transport phenomena in flow injection analysis without chemical reaction

Abstract Dispersion phenomena play a very important role in flow injection analysis. In this paper, physical transport phenomena in flow injection methods are discussed. Three different types of reactor — a straight tube, a helically coiled tube and a new single bead string reactor — are compared. Under similar flow conditions, the dispersion in the single bead string reactor is the lowest. The specific advantages of single bead string reactors are their very simple preparation and maintenance and the good reproducibility of the peaks. It is shown that in open capillary tubes (coiled or not) the Taylor dispersion equation is of very limited use, because the residence times are too short, and because secondary flow occurs in the case of coiled tubes.

Some theoretical aspects of flow injection analysis

Abstract The injection and detection methods in flow injection analysis (f.i.a.) are theoretically described. Theory is developed for a simple flow model based on laminar flow without diffusion. The results lead to the conclusion that exact specifications for injection and detection devices in experimental f.i.a. are necessary. The influence of the sample volume is described in more detail with a systems analysis model of f.i.a. in which the tanks-in-series model is used for transport of the fluid. The theoretical and experimental results are in good agreement, thus the theory developed for sample injection in f.i.a. appears to be valid.

Characterization and design of liquid phase flow-through detector systems

Abstract Characterization of liquid phase flow-through detection systems as used in column liquid chromatography and flow injection analysis is discussed. Linear range, selectivity, peak broadening and detection limit are the most important characteristics. Peak broadening is treated with the aid of the concepts of systems analysis. The total peak broadening effect is given as the sum of contributions from connecting tubes or reactors, measuring volume and time constants in electronics and transducers. The influence of noise and signal frequency content on the precision of analytical results is treated qualitatively. The detection limit of a flow-through detection system is defined, taking these effects into account qualitatively. These characteristics are related to the performance of the whole analytical system with regard to concentration detection limit, absolute detection limit and maximum sample frequency.

A possible approach to the optimization of flow injectin analysis

The theory of sample zone dispersion is well known for most cases of practical interest in flow injection analysis. This paper offers a theoretical analysis which allows for the optimal design of single-line flow systems. For various reactor types, a detailed analysis is provided in terms of physical constants, design parameters and constraints. It is shown that, within practical constraints and using a pressure drop of less than 1 bar, it is possible to operate flow systems at 100 samples per hour, with a residence time of 100 s and a reagent consumption of 8 μl for each determination. Further miniaturization of flow systems must rely on smaller detector volumes than those currently available, a situation not unlike that in liquid chromatography.

An electrochemical reactivation method for solid electrodes used in electrochemical detectors for high-performance liquid chromatography and flow injection analysis

Abstract An electrochemical reactivation method for solid electrodes used in electrochemical detectors is optimized for glassy carbon electrodes. Application of a voltage pulse train for 5 min is effective in restoring the response of the electrode after deactivation by organic compounds. Potassium hexacyanoferrate(II) and dl -synephrine were used to test the reactivation of electrodes; 2,6-dihydroxybenzoic acid and urine served to deactivate them. The effects of the amplitude, d.c. level, and frequency of the applied voltage pulse train, as well as the mode of termination, are discussed.

Flow versus batch detection of radioactivity in column liquid chromatography

Abstract For column liquid chromatography the precision or flow counting of radioactivity is compared with that of batch counting. Theoretical derivations are presented to show the decrease of precision in flow counting as a function of the sample count rate, the background count rate and the residence time in the detector cell. A long residence time in the detector cell enhances the precision of the results, but this time can only be increased at the expense of either chromatographic resolution or retention time. The compromises which have to be made are treated quantitatively. Experimental results are shown which support the theoretical conclusions. The measurements were carried out with a GM counter, a NaI(Tl) crystal and an organic solid scintillator. It is demonstrated that the selectivity of radiometric detection can be used to improve the accuracy of the analytical results.

The application of strongly reducing agents in flow injection analysis: Part 1. Chromium(II) and vanadium(II)

Abstract Many reagenst cannot easily be applied in quantitative analysis, because of their instability under atmospheric conditions. When such reagents are prepared in a flowing stream, their applicability is very promising; for example, in flow injection analysis, a reagent need be stable only for 20–30 s. The application of chromium(II) and vanadium(II) in flow injection analysis is described. Nitrate and nitrile can be determined in the concentration range 5 × 10 −5 −5 × 10 −3 . Calibration graphs show good linearity.

Behaviour of solid electrodes in normal and differential pulse voltammetric methods

In order to investigate the behaviour of solid electrodes in normal and differential pulse voltammetry, step functions have been applied to the electrochemical cell containing the electrodes to be tested, in the absence of electroactive species. The large residual current observed could be attributed to electrochemical reactions of the electrode material.

The performance of some liquid phase flow-through detectors

Miniaturization of flow-through analytical systems such as flow injection analysis and liquid chromatography is necessary to achieve higher rates of sample throughput and smaller absolute detection limits. During this miniaturization, however, it is essential to preserve the concentration detection limits of the devices used. Some principles for liquid phase flow-through detection, u.v.-visible absorption photometry, fluorimetry and electrochemical detection, are discussed from this point of view. It appears that, although technical difficulties will be large, there are good prospects of detecting concentrations of about 1 ng ml−1, while keeping the peak broaadening down to values of about 1 nl.