Multiple peak recordings in flow injection analysis

The quality and level of the information provided by conventional flow-injection systems can readily be enhanced by simple modifications. The different possibilities available for obtaining several peaks per injected sample are reviewed. Among the topics are simultaneous and sequential multidetection, multicomponent determinations (including speciation), reaction-rate methods, and automatic amplification of conventional analyte concentration ranges.     

Examination of the temporal effect in a flow injection analysis system using multi-channel absorbance detection

In addition to the Poiseuille effect, a so-called temporal effect was proposed recently to elucidate the commonly observed tailing peak signals of flow injection analysis (FIA). A multi-channel absorbance detector was used in this study to obtain the FIA peaks on both the spatial and temporal coordinates. The temporal effect was analyzed by comparison of the profiles between the experimental and the corresponding Gaussian peaks, and by comparison of asymmetry factors between the spatial and the temporal peaks. The temporal effect appeared to be the major factor under flow rates ranging from 0.5 to 8 ml min⁻¹. This was despite the presence of a spatially frontal peak observed in the FIA tubing, which was found to result in a tailing peak on the temporal coordinate due to this discussed cause. In addition, the temporal effect became greater as the flow rate increased.     

Parcel model for peak shapes in chromatography numerical verification of the temporal distortion effect to peak asymmetry

The traditional plate concept has been reassessed and improved to a parcel matrix model, which can be used to imitate the chromatographic behavior of a hypothetic column on a computer worksheet. Under programmed conditions, various peak shapes (nearly Gaussian, and with prolonged or fronting tails) are generated. The peak tailing has been separated into two major fractions: spatial and temporal. The former fraction is caused by the retention nature of a column, whereas the latter is induced by the observer's relative position and the changing of the zone broadening rate. The temporal distortion effect can be identified qualitatively and quantitatively through a normalized peak-overlapping process. In general, a chromatographic peak may carry a prolonged (or normal type) tail under linear isotherms, while both prolonged and fronting tails will appear under non-linear conditions. The temporal distortion is proved to be significant, and may be regarded as the major cause of peak asymmetry in most cases. This is in contrast to the conclusions of many previous studies. The model is also eligible to simulate chromatographic peaks for various injection sizes.     

Commutation in flow injection analysis

Recent advances in flow injection analysis, including the introduction of zone-sampling and zone-trapping processes and the incorporation of ion exchangers, have involved electronically operated injector/commutators. Other techniques based on manual commutation, such as manifold modifications and intermittent and alternating streams, have been reported. A comprehensive review of these techniques is given. The use of commutation for difference injection procedures (loop-based, time-based, hydrodynamic, sequential and nested injection) is emphasized. An alternative stopped-flow approach without stopping the pump is suggested.     

Time-based flow injection analysis

Time-based flow-injection systems are novel measurement systems which use induced dispersion of the sample in the system and measurement of time to evaluate the concentration of the analyte in the sample. There are two types of time-based system. In the standard systems, assays are based on the chemistries of the classical intensity assays; in the pseudo-titration systems, the assays are based on classical titration chemistries. These systems are discussed in terms of basic theory, classification of the systems and their advantages.     

A new approach to the integrated calibration in flow injection analysis

A new proposal how to perform the analytical procedure according to the integrated calibration method is presented. An original flow injection system has been designed for this purpose. When using only a single standard solution, the measurement information gathered during a single analytical course permits construction of four calibration graphs and calculation of as many as four independent estimations of the analyte concentration in the sample examined. As the calibration method applied integrates the set of standards method and the standard addition method, the analytical estimations may be obtained in both the interpolative and the extrapolative manner and the final result can be effectively verified in terms of accuracy. The system developed was experimentally tested on the example of spectrophotometric determination of chromium. It has been proved to be capable of saving time and reagents as well as providing reliable analytical results. Owing to the instrumental simplicity and analytical efficiency the system is expected to be useful for routine analysis.     

Zone-sampling processes in flow injection analysis

In flow injection analysis, it is possible to select a small portion of a dispersed zone and inject it into another carrier stream. The potentialities of such zone-sampling processes are discussed, and the main factors involved are investigated by spectrophotometric measurements. The usefulness of zone sampling in routine work is demonstrated in the atomic absorption spectrometry of potassium in plant digests. The simple system, which provides automatic dilution, allows analysis of 120 samples per hour, although the required sample dispersion factor was 0.0076; the results agreed with those obtained by conventional a.a.s. after hundredfold manual sample dilution.     

Confluent streams in flow injection analysis

The addition of a confluent stream increases the mean length of the sample zone and simultaneously decreases the involved concentrations. Simple equations describing these effects under ideal mixing conditions are proposed. The effects of the confluent stream on the overall sample dispersion may be more or less compensated, depending on the decrease in the post-confluence dispersion. When this compensation is almost total, a paradoxical situation occurs, in which the recorded peak height becomes practically unaffected by variations in the flow rate of the merging stream. In this situation, the peak width approaches a limiting value which is independent of the injected volume and of the flow rate of the confluent stream. Limiting values for the peak width and for the mean length of the sample zone are postulated. When the post-confluence dispersion is negligible, the effects of the confluence appear in the recorded peak. The confluent stream addition reduces the overlap between successive sample zones. The site of the confluence may be an important dispersion factor. A saturation index is proposed and practical implications are discussed.     

Application of flow injection analysis to water pollution studies

The application of flow injection analysis and reverse flow injection analysis under field conditions could completely change our present techniques of water pollution monitoring.     

An estimation of the column efficiency made by analyzing tailing peak profiles

It has been shown previously that most columns are not radially homogeneous but exhibit radial distributions of the mobile phase flow velocity and the local efficiency. Both distributions are best approximated by fourth-order polynomial, with the velocity in the column center being maximum for most packed columns and minimum for monolithic columns. These distributions may be an important source of tailing of elution peaks. The numerical calculation of elution peaks shows how peak tailing is related to the characteristics of these two distributions. An approach is proposed that permits estimations of the true efficiency and of the degree of column radial heterogeneity by inversing this calculation and using the tailing profiles of the elution peaks that are experimentally measured. This method was applied in two concrete cases of tailing peak profiles that had been previously reported and were analyzed by applying this new inverse approach. The results obtained prove its validity and demonstrate that this numerical method is effective for deriving the true column efficiency from experimental tailing profiles.     

Annealing of silica to reduce the concentration of isolated silanols and peak tailing in reverse phase liquid chromatography

Non-porous, colloidal silica particles were annealed at three different temperatures, 800, 900 and 1050 °C. The adsorption of lysozyme, a probe of surface roughness, was consistent with progressively reduced surface roughness as temperature increased. The heat treated silica particles were rehydroxylated and then used to pack UHPLC columns. The cationic protein lysozyme was used to probe silanol activity, which exhibited progressively less tailing as the annealing temperature increased. FTIR spectroscopy confirmed that the abundance of isolated silanols on the surface was reduced by annealing at 900 °C or 1050 °C. FTIR also revealed that there was markedly increased hydrogen bonding of the isolated silanols to neighbors after rehydroxylation. These results combine to support the hypothesis that (a) isolated silanols on silica cause tailing in RP-LC and (b) nonplanar topography gives rise to isolated silanols.     

Seeking high-sensitivity flow injection analysis

The use of flow-injection systems for biotechnology assays is attrctive but the reduced sensitivity of such systems over their manual descrete-sample counterparts has limited their use. Model systems are studied to develop the concepts needed for the design of high-sensitivity flow-injection systems for the determination of macromolecules. Both open- and packed-tube systems were examined. Initial hydrodynamic studies were done with dye; then selected designs were tested for protein determinations based on a bromocresol green assay. The packed-bed systems provided better mixing, but dispersion was about the same as that of the open-tube systems and back-pressures were much higher. This work indicates that combinations of narrow (i.d. ? 0.5 mm) and short (? 10 cm) open tubes, small-volume flow cells (8 μl) and slow flow rates (? 0.05 ml min^-1) give better sensitivity.     

Novel approaches to analysis by flow injection gradient titration

Two novel procedures for flow injection gradient titration with the use of a single stock standard solution are proposed. In the multi-point single-line (MP-SL) method the calibration graph is constructed on the basis of a set of standard solutions, which are generated in a standard reservoir and subsequently injected into the titrant. According to the single-point multi-line (SP-ML) procedure the standard solution and a sample are injected into the titrant stream from four loops of different capacities, hence four calibration graphs are able to be constructed and the analytical result is calculated on the basis of a generalized slope of these graphs. Both approaches have been tested on the example of spectrophotometric acid–base titration of hydrochloric and acetic acids with using bromothymol blue and phenolphthalein as indicators, respectively, and sodium hydroxide as a titrant. Under optimized experimental conditions the analytical results of precision less than 1.8 and 2.5% (RSD) and of accuracy less than 3.0 and 5.4% (relative error (RE)) were obtained for MP-SL and SP-ML procedures, respectively, in ranges of 0.0031–0.0631 mol L⁻¹ for samples of hydrochloric acid and of 0.1680–1.7600 mol L⁻¹ for samples of acetic acid. The feasibility of both methods was illustrated by applying them to the total acidity determination in vinegar samples with precision lower than 0.5 and 2.9% (RSD) for MP-SL and SP-ML procedures, respectively.     

A Simple injection valve for flow injection analysis

A novel sample introduction valve, based on a modified glass syringe, is described for flow injection analysis. Sample volumes of 1.4 μl or more can be injected manually or automatically, by driving with a 12-V solenoid. The valve has advantages of freedom from maintenance and long lifetime.     

New injection valves and their application in flow injection analysis

Summary Two variable volume injection valves, equipped with bypass loops, have been developed, primarily to circumvent problems with matrix effects in high sensitivity FIA.The application of a dual channel valve for simultaneous analysis, zone sampling and merging zones techniques is discussed.Examples for on-line dilutions up to several thousand-fold and the direct determination of calcium in Kjeldahl digests using the zone sampling approach and a study of the interference of ascorbic acid on the Griess reaction for nitrite are presented. The application of chasing zones techniques by coupling a simple single channel valve in a new way is proposed.

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Neue Injektionsventile und ihre Anwendung in der Fließinjektionsanalyse

     

Laminar-flow bolus shapes in flow injection analysis

Bolus shapes for injected samples have been calculated for times of interest in experiments on flow injection analysis. The effect of both system and molecular parameters on the shapes and the dispersion of samples is considered. The implications for merging-zone experiments are discussed briefly.     

Enzyme reactors in unsegmented flow injection analysis

The design of immobilized-enzyme reactors for use in flow injection analysis is discussed. The reactors should be optimized for a short residence time and a very high (> 99.9%) conversion of substrate to products. Selection of carrier and immobilization method is important in order to increase the amount of active enzyme per unit volume. The effeciency of the reactor can be increased by decreasing the particle size in packed-bed reactors and the radius of open tubular reactors. The maximum inherent rate constant that can be obtained under optimal conditions is estimated for a number of enzymes of analytical interest; it is shown that with high rate constants and small particle diamters, residence times less than seconds can be obtained. Some applications of immobilized-enzyme reactors in flow systems are reviewed.