Enhancement of flow injection optosensing by sorbent extraction and reaction rate measurement

Abstract

The molybdenum blue reactlon for the determination of phosphate Is used as a model to Illustrate the extension of the use of hydrophoblc sorbents In flow Injection analyds for the preconcentratlon of an anlon and for on-cdumn detec#on, Le., optosenslng. Optosenslng provides for real-time monltorlng of the rate of slgnai change, dA /d T , so that the rate of color devebpment durlng the reductlon step of the analysis can be measured. The synerglstlc relationship between the rates of formation of the phosphate and sHlcate heteropdy complexes is examlned. A kinetic optosenslng method has been developed In which the difference In reduction rate for the heteropoly complexes allows simultaneous determlnatlon of phosphate, In the parts per bllllon range, and slllcate, in the parts per mllllon range. The system used an Inexpensive mlnlspec-20 spectrophotometer wtth a net path length through the sorbent of less than 2 mm. Partlal least-squares analyds was used to analyze the data, and predlctlon errors of approxlmately 10 % were obtained for both components. INTRODUCTIONEnhancement of the selectivity of instrumental methods by flow injection analysis (FIA) has been accomplished by various means, such as the use of solvent extraction ( 1 ) and gas diffusion techniques (2). These may be considered homogeneous techniques in that the chemistry occurs in the homogeneous media of the flowing stream itself. Additionally, sensitivity has been increased by on-line preconcentration using ion exchange (3). The latter technique may be viewed as heterogeneous since isolation of the analyte on a surface has occurred, creating a region of increased concentration within the flowing stream and may be considered similar to solute focusing (4). Only very recently, attention is being given to the concept of sorbent extraction in FIA, which has been used so far only for the preconcentration of metal complexes (5) using hydrophobic interactions on C-18. Further increases in sensitivity can be achieved by flow injection optosensing (6), which is based on the measurement of an analyte retained on the solid sorbent surface. Use of cation exchangers for this purpose has been described by Yoshimura and Waki (7) while Valcarcel and co-workers (8) used an anion exchanger in a detailed study of the iron(II1) thiocyanate complex. This paper deals, for the first time, with the use of sorbent extraction for the optosensing of anionic complexes and exploits in a novel way the differential kinetics of competing species.The present study of differential kinetics allows for the simultaneous determination of two analytes or the determination of one analyte in the presence of an interferent. Thus, while to date the use of sorbents has been driven by the "all or nothing" philosophy that focuses on steady-state chemistry, it is shown in this work how the horizon of applications for sorbent use in FIA can be greatly broadened with the incorporation of kinetic techniques.Previous work in this laboratory has focused on the enhancement of instrumental response at roo...

Trace and ultratrace analysis methods for the determination of phosphorus by flow-injection techniques

“…The MG method coupled with solvent extraction/FIA enabled to determine phosphate at several nanomolar levels of phosphorus, and was applied to river water analysis [76]: LOD was 0.1 g/l of phosphorus. The MG ion associate was enriched on-line on a solid phase, such as Shephadex (cyclodextrin derivatives) packed in a flow-through cell [77], molybdenum blue on polymer beads packed in a syringe [78] and styrene-divinylbenzene polymer packed in a short desk (1.6 mm) [79]. A signal improvement factors of 30-50 could be achieved.…”

Advances in Flow Analysis – Instrumentation and its Application in Environmental Analysis

“…been found to be enhanced by detecting product directly in the column by flow injection 'optosensing' [97]. The advantages of this technique are that lower detection limits can be reached since there is no dilution during an elution step, and kinetic information can be used to discriminate interferents.…”

Construção de uma cela de fluxo para medidas por espectrofotometria em fase sólida

“…Essas características são evidenciadas pelo acoplamento da EFS com sistemas de análise em fluxo (FI-EFS) 3 . Neste caso, um aumento de seletividade pode ser alcançado devido às diferenças na cinética de retenção do analito e das espécies concomitantes no suporte sólido 1,4,5 . Em procedimentos envolvendo EFS, o feixe incidente sofre tanto absorção quanto espalhamento de radiação 6 .…”

“…Entretanto, essa limitação vem sendo superada pelo emprego de lentes para a dispersão da radiação incidente, ou pelo posicionamento da cela próximo à entrada do sistema de detecção 3,6,7 . Para medidas em fluxo por espectrofotometria em fase sóli-da têm sido empregadas celas comerciais [8][9][10] ou especialmente construídas para FI-EFS 4,11 . Entretanto, celas de fluxo disponí-veis comercialmente apresentam limitações na quantidade de sólido que pode ser utilizada, podendo comprometer a sensibilidade e a faixa de resposta.…”