Gerhard Schulze

Different approaches to the determination of ammonium ions at low levels by flow injection analysis

Several flow-injection methods are examined critically for their suitability for determing submicromolar levels of ammonium ion on a routine basis. Nesslerization and the Berthelot reaction are not sufficiently sensitive. The gas-diffusion separation is more satisfactory. On the basis of a simple diffusion model, the effects of operating parameters on the ammonium concentration in the acceptor line of the gas-diffusion unit are discussed and experimentally verified. With the optimized gas-diffusion conditions, spectrophotometric detection with an acid-base indicator or the application of a liquid-membrane ammonium-sensitive electrode offers detection limits around 10 −8 mol 1-1. Carbon dioxide interference in the acid-base indicator method causes problems with baseline stability and long-term drift. A severe limitation on futher enhancement of sensitivity by in-valve preconcentration is the contamination levelof ammonium present in water and reagents. Effective preconcentation (20-fold in 1 min) is obtained by using an ion-exchange microcolumn placed in the injection loop of the valve.

Potentiometric stripping analysis and anodic stripping voltammetry with carbon fiber electrodes

Abstract High-modulus carbon fibers are used as working electrodes in differential-pulse stripping voltammetry and potentiometric stripping analysis. Different types of electrodes (particularly single-fiber and brush electrodes) are compared with regard to reproducibility, sensitivity, and practical aspects. Zinc, cadmium, and lead are determined in the 100 μg 1 −1 to 1 mg l −1 range and optimum experimental parameters are described. The cut single-fiber electrode has the best general characteristics.

Separation of mono-, di- and tributyltin compounds by isocratic ion-exchange liquid chromatography coupled with hydride-generation atomic absorption spectrometric determination

Abstract A method is described for the speciation determination of n-butyltin compounds. The three substances are separated from each other and from inorganic tin ions within 10 min by ion-exchange chromatography using a mixture of citric and oxalic acid in methanol as eluent. They are determined by atomic absorption spectrometry after on-line hydride generation and decomposition of the volatile hydrides in an electrically heated quartz T-tube. The limits of detection were 27, 40 and 31 nmol/l for tributyltin, dibutyltin and monobutyltin, respectively. Linear calibration graphs were obtained between 0.1 and 10 μmol l−1. The method was applied to spiked samples of sea water after enrichment by solid-phase extraction.

Simultaneous determination of iron(II)/iron(III) by sorbent extraction with flow-injection atomic absorption detection

Abstract Flow-injection atomic spectrometry with sorbent extraction preconcentration is applied to the simultaneous determination of iron(II) and iron(III). Upon injection of a sample containing iron in both oxidation states, iron(III) is directly carried to the flame atomic absorption spectrometer whereas the iron(II)-ferrozine complex formed in-line is temporarily retained on a C 18 -modified silica column placed between the injection valve and spectrometer and is subsequently eluted with methanol.

Spectrophotometric flow-through gas sensor for the determination of atmospheric nitrogen dioxide

A flow-through gas sensor has been developed which relies on membrane based analyte collection and trapping in a temporarily halted liquid absorber and in situ spectrophotometric detection of the reaction product formed in solution. Through a stop-and-go sequence of the absorber solution provision is made for real-time detection of the preconcentration taking place in the minute absorber solution and ready elution after signal evaluation. The determination of nitrogen dioxide at concentration levels relevant to measurement of urban atmospheres is used to demonstrate the principle and capabilities of an inherently universal applicable gas sensing device. The detection chemistry used in the present work is identical to that of common standard methods (i.e., reaction of NO 2 with mixed reagent containing sulfanilamide and N-(1-naphthyl)-ethylene-diamine) so that the same selectivity arises. The working range covered in this study is 50-250 μg/m 3 NO 2 with a time resolution of few seconds. Lower concentrations are accessible when longer preconcentration times are used. Reasonable precision (typically < 5% R.S.D.) and good long-term stability is given which makes frequent calibration unnecessary.

Simultaneous ion chromatographic determination of anions and cations by series conductivity and flame photometric detection

Abstract A method for the simultaneous determination of anions and cations using non-suppressed ion chromatography (NSIC) is proposed. Anions are separated on an ion-exchange column and determined with conductivity detection. In the same run the non-retained cations leaving the chromatographic system as a discrete sample zone are detected by a multi-element flame emission spectrometer which is coupled directly to the outlet. The principal analytical features of the proposed method are discussed and the experimental requirements of interfacing the two detection methods are outlined. System performance is evaluated for the high-speed NSIC of chloride, nitrate and sulphate and the multi-channel flame emission spectroscopic detection of sodium, potassium and calcium. The figures of merit are given and application to the analysis of tap, surface and mineral waters is presented.

Open thin-layer cell—A flow-through electrode for potentiometric stripping analysis

An electrochemical flow-through cell is described that is substantially different from all conventional cells. It has an open structure and is made from a piece of Plexiglass that contains the working and auxiliary electrodes. The flow channel is replaced by a strip of filter paper, which adheres to the cell body and covers the inlet hole and both electrodes. The carrier solution is therefore precisely directed along the filter strip, which dips into the waste reservoir containing the SCE.Potentiometric stripping analysis (PSA) was used to evaluate the performance characteristics of the cell in terms of flow-rate, injection volume and carrier composition. Linear calibration graphs were obtained in two concentration ranges (10–500 µg l–1 and 1–10 mg l–1) for Cd, Pb and Cu under slightly different experimental conditions. The practical advantages and the long-time response stability of the cell were demonstrated by the PSA determination of Zn in tap water. In the range 0.1–20 mg l–1 a sample throughput of 120 h–1 is obtainable with a reproducibility better than 5%.

Einsatz der Doppeldetektion zur Optimierung der Gasdiffusion in der Fließinjektionsanalyse — Bestimmung von Ammonium und Sulfid

SummaryUsing gas diffusion, ammonium and sulfide can be determined selectively by flow injection analysis. An estimation of the membrane transference is achieved by dual-detection. It permits the determination of the volatile component in both the donor and the acceptor stream. This is performed by use of a retardation loop in the case of sulfide. For ammonium, two gas diffusion units are coupled with each other. As a measure of the membrane transference the effectivity of transfer is defined, estimating 30–40% for hydrogen sulfide and ca. 7% for ammonia. An elevation of temperature to 80 °C increases the membrane transference by a factor of 1.2 and 1.8, respectively. Different membranes were tested.

Ion-exchange micro columns for on-line preconcentration of heavy metals I. A rapid voltammetric screening test of ion-exchange kinetics

SummaryA screening test for the investigation of the kinetical behaviour of ion-exchange materials is described. The test enables to decide within a short time which resin may be suitable for a special preconcentration problem. It is based on voltammetric measurements carried out in a single vessel. Using Zn, Cd, Pb and Cu, seventeen ion-exchange materials were checked. Experimental conditions including pH, quantity and particle size of the resins were studied. With regard to water analysis the influence of Na, Ca, Mg, Zn and Fe was investigated. For the determination of the four trace elements in drinking water eight ion exchangers proved to be recommendable at pH 8.

Vergleich von Potentiometrischer Stripping-Analyse und Anodischer Stripping-Voltammetrie in der Fließinjektionsanalyse. Störungen bei der Bleibestimmung

SummaryThe 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%.