Highly sensitive semi-quantitative field test for the determination of chromium (VI) in aqueous samples

A method for the semi-quantitative colorimetric determination of chromium(VI) at sub μg/L levels after sorptive preconcentration is presented. The method is based on the retention of the reaction product (preformed in liquid phase) between Cr(VI) and diphenylcarbazide on membrane embedded cation exchange material. The color intensity of the membrane can be correlated to Cr(VI) concentrations in the range 0.05–50 μg/L (i.e. almost three orders of magnitude lower than the conventional spectrophotometric procedure) with a detection limit of about 10 ng/L (using 50 mL of sample volume). Due to the visual inspection mode and comparative color detection the precision is only 30–80% rsd which, however, is regarded as sufficient for screening purposes. Analysis of real samples including different kinds of waters and extracts of soil and filter collected airborne particulate matter demonstrated the applicability of the method for fast and species selective screening. Recovery experiments generally gave reasonably good results, yet also revealed the risk of the conversion of chromium species during sample pretreatment procedures due to redox reactions. Received: 1 December 1997 / Revised: 23 February 1998 / Accepted: 10 March 1998     

Spectrophotometric determination of total phenolics by solvent extraction and sorbent extraction optosensing using flow injection methodology

Flow injection analysis (FIA) procedures for the Spectrophotometric determination of phenol involving in-line concentration by solvent and sorbent extraction have been developed. The imine product formed in the reaction between phenol and 4-aminoantipyrine (4-AAP) is either extracted into chloroform (solvent extraction) or is temporarily retained on C₁₈-modified silica material contained in a microcolumn (sorbent extraction). In the latter case two variants of detection have been used namely the Spectrophotometric measurement of the methanolic eluent containing the concentrated dye and at-column optosensing of the retained reaction product followed by methanol elution to maintain reversibility of the process. In the solvent extraction procedure a 10-fold increase of sensitivity compared to the common FIA method without extraction is achieved but no corresponding improvement in detectability is observed. Under optimized conditions the detection limit amounts to 8 μg l⁻¹. Using sorbent extraction methodology, high concentration factors can be obtained when large sample volumes are used. The only limitation in getting correspondingly lower detection limits are an increasingly high and variable blank value with sampling volume. The detection limits obtained for measurement of the absorbance in the eluent and on-column optosensing are 11 μg l⁻¹ and 0.4 μg l⁻¹, respectively. A study of the extractability of various phenol derivates using both solvent and sorbent extraction revealed lower relative response rates compared to the FIA method without extraction. Phenolics that possess an additional acidic group are present in ionized form at the high pH of the assay and are not extractable at all.     

Enhancement of sensors selectivity by gas-diffusion separation : Part 1. Flow-injection potentiometric determination of cyanide with a metallic silver-wire electrode

The potentiometric response of a metallic silver-wire electrode in the presence of silver ion complexing agents is theoretically derived on the basis of the Nernst equation. The cyanide response is shown to be in good agreement with the theory. The analytical utility of this inherently non-selective sensor is demonstrated by its application in gas-diffusion flow-injection analysis. By making use of a membrane barrier that prevents other than gaseous species from passing through, the almost specific determination of cyanide becomes feasible. Gaseous interferents (i.e., hydrogen sulphide, sulphur dioxide and nitrogen oxides) are chemically converted prior to entering the gas-diffusion unit. The apparent selectivity coefficients thus obtained are significantly better than those reported for common cyanide-selective electrodes.     

Coupled liquid chromatography-gas chromatography for the rapid analysis of gamma-oryzanol in rice lipids

An approach based on on-line coupled liquid chromatography-gas chromatography (LC-GC) was developed for the rapid analysis of gamma-oryzanol in rice. Total lipids were extracted from rice and subjected to LC-GC without any prior purification. gamma-Oryzanol was pre-separated by HPLC from rice lipids and transferred on-line to GC analysis in order to separate its major constituents. 24-methylenecycloartanyl ferulate, cycloartenyl ferulate, campesteryl ferulate, beta-sitosteryl ferulate and campestanyl ferulate. The identities of the compounds were confirmed by off-line GC-MS analysis. Total gamma-oryzanol content could be quantified by HPLC-UV detection and the distribution of gamma-oryzanol constituents could be determined by on-line coupled GC analysis. The proposed methodology paves the way for high-throughput investigations providing information on natural variations in gamma-oryzanol content and its composition in different rice varieties.     

The fluoride ion-selective electrode in flow injection analysis: Part 1. General Methodology

A simple flow-injection system for determination of traces of fluoride by means of the fluoride-selective electrode is presented. A comparison of several flow-cell arrangements confirmed the advantages of a well-jet design. Systematic investigations of the parameters affecting response times (i.e., polishing procedure, flow rate, carrier composition) established the optimal experimental conditions for measurements down to 1 μg l^?1 fluoride. Calibration plots in the lower μg l^?1 range were neither Nernstein nor linear, but good precision (0.5–5%) was obtained even when the potential differences for concentration steps of one decade were as small as 3 mV.     

Serial potentiometric and conductometric detection in fast non-suppressed ion chromatography applied to the analysis of filter collected airborne particulates

Non-suppressed ion chromatography (NSIC) has been optimized to permit the determination of chloride, nitrate and sulfate in the low mg/l concentration range within 3 min. Using conductometric detection the extraneous (positive) injection peak was found to adversely affect the early eluting chloride signal when samples with high amounts of total cations are analyzed. The serial arrangement of a downstream potentiometric detector with a chloride ion selective electrode, which does not respond to other alterations of the composition of the eluent than the change in chloride concentrations, is shown to be suitable for interference-free, sensitive and reliable chloride evaluation. Application of the proposed system to the determination of chloride in extracts of filter collected airborne particulates revealed (for those samples that could be evaluated by both detection methods) high precision and no systematic deviations between conductometric and potentiometric detection.Dedicated to Professor Dr. Peter Brätter on the occasion of his 60th birthday     

Determination of water traces in various organic solvents using Karl Fischer method under FIA conditions

Flow injection methods utilising the Karl Fischer (KF) reaction with spectrophotometric and potentiometric detection are described for the determination of the trace water content in various organic solvents. Optimisation of the methods resulted in an accessible (linear) working range of 0.01-0.2% water for many solvents studied with a typical precision of 1-2% R.S.D. Only 50 mul of organic solvent was injected and the sampling frequency was about 120 samples per h. Since the slopes of the calibration curves were different for different solvents appropriate calibration was required. Problems associated with spectrophotometric detection and caused by refractive index changes were pointed out and a nested-loop configuration was proposed to overcome this kind of interference. The potentiometric method with a novel flow-through detector cell was shown to surpass the performance of spectrophotometric detection in any respect. The characteristics of the procedures developed made them well applicable for on-line monitoring of technical solvent distillations in an industrial plant.     

Modification of nonionic adsorbent with eriochrome blue-black R for selective nickel(II) preconcentration in conventional and flow-injection atomic-absorption spectrometry

Chemical modification of nonionic sorbent Amberlite XAD-2 or anion exchanger Amberlyst A-26 with Eriochrome Blue-Black R (EBBR) produces a chelating resin of satisfactory chemical stability and resistance towards mineral acids. Retention of 10 metal ions has been examined for both resins. EBBR loaded XAD-2 was utilized for nickel(II) preconcentration in atomic-absorption spectrometry. In optimal conditions at a preconcentration time not exceeding 1 hr, nickel(II) can be determined at the 0.1 mug/l. level in flow measurements. Retention of metal ions on chelating resin is a convenient method of preconcentration and elimination of matrix interferences.     

Studies on the elimination of sulfide interference in the potentiometric determination of chloride using ion selective electrodes in a flow injection system

Detailed investigations have been reported on the application of a common procedure using bromate in nitric acid for the removal of the interference of sulfide in the direct potentiometric determination of chloride using silver-based ion selective electrodes in a flow injection system. It is shown that this procedure is not very efficient and carries a high risk of chloride underestimation. With increasing levels of sulfide the oxidation of chloride to elemental chlorine becomes progressively significant and under certain circumstances virtually no chloride is left irrespective of its initial concentration. Hydrogen peroxide in alkaline medium is proposed as a suitable alternative for the efficient removal of sulfide without any adverse effects on the potentiometric method.     

Vergleich von Potentiometrischer Stripping-Analyse und Anodischer Stripping-Voltammetrie in der Flie?injektionsanalyse. St?rungen bei der Bleibestimmung

Summary The interferences caused by inorganic ions, organic anions and Triton X-100 in the determination of lead by flow injection potentiometric and voltammetric stripping analysis at a mercury film electrode were investigated. The experiments were performed in the concentration range of 1–10 mg/l Pb²⁺. For both methods linear calibration plots were obtained in the presence of an excess of nitrate, chloride, perchlorate and sulphate. Iodide strongly interferes due to mercury complexation.Metal ion interference caused by formation of irreversible amalgams (as in the case of codeposition of Ni, Co, Fe and Cr) is avoided by proper choice of deposition potential. No effect of water soluble reduced species, i.e. Fe(II) and Cr(II), on the stripping signals was observed. The presence of acetate, citrate and tartrate does not limit the determination of lead by potentiometric stripping analysis (PSA) but has a marked effect on anodic stripping voltammetric (ASV) signals. No influence of Triton X-100 on the determination of lead by PSA was found. In ASV 10^–3% Triton X-100 diminished the peak current by 15%.

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Herrn Prof. Dr. Rolf Neeb aus Anlaß seines 60. Geburtstags gewidmet