Wolfgang Frenzel

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.

Mercury films on a glassy carbon support: attributes and problems

Abstract The main problems associated with the preparation and application of mercury film electrodes on a glassy carbon support (MFGCE) are outlined. The role of the GC support material and in particular of the surface condition (microstructure and polishing procedure) for deposition of homogeneous mercury films and the proper performance of the resulting MFGCE in stripping voltammetric measurements is exploited. Highly polished electrodes without surface defects were found to be unsuitable because elemental mercury does not adhere to the GC surface. Instead, defined roughening of the GC support is proposed and is shown to yield mechanically stable films with excellent electrochemical characteristics. It is further shown that properly prepared MFGCEs may quickly lose their performance characteristics owing to fouling. The main reasons can be attributed to mechanical deterioration, formation of surface layers of insoluble mercury(I) compounds, adsorption of polymeric organic compounds and oversaturation with themercury film by elements that form irreversible amalgams or intermetallics. The respective interferences are manifested in decreased sensitivity, increased background currents, loss of hydrogen overvoltage, impairment of peak resolution of adjacent signals or even totally erratic behaviour. The advantages of flow-through stripping voltammetry in minimizing or even avoiding these kinds of interferences are emphasized.

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

Abstract 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.

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.

Spectrophotmetric determination of phenolic compounds by flow-injection analysis

Abstract Flow-injection procedures for the spectrophotometric determination of phenol by two different colour reactions have been developed. The common 4-aminoatnipyrine (4-AAP) reaction and the oxidative coupling of phenol with 3-methyl-2-benzothiazoline hydrazone (MBTH) are both readily applicable and permit the determination of phenol at a sampling rate of 60 h −1 and a relative standard deviation better than 3%. The sensitivity of the BMTH method is about four times higher that of the 4-AAP reaction, with detection limits of 12 and 30 μg/l phenol, respectively. A study of the response of the two methods to phenol derivatives revealed particular merits and limitations. The 4-AAP reaction prouducts have similar absorbance maxima but the reactivity is comparatively low. MBTH reacts with products differ singificantly, a problem is the appropriate choise of the measuring wavelength.

Enhanced performance of ion-selective electrodes in flow injection analysis: non-nernstian response, indirect determination, differential detection and modified reverse flow injection analysis

Some general aspects concerning the measuring device, cell design, flow manifold and sensor requirements for the incorporation of ion-selective electrodes (ISEs) in flow injection systems are outlined. The main aim of this paper is to demonstrate how the control of sample dispersion and timing in flow injection analysis (FIA) can be used to attain unique potentialities. The detection limits of ISEs can be decreased, even below the intrinsic solubility of the sensor membrane. The integration of sample processing steps within the manifold improves the reliability of the electrode for reaction products which are unstable or volatile. The series differential detection method is shown to increase the sensitivity (slope of the calibration graph) and to simplify data evaluation. The indirect determination of trace amounts of aluminium with a fluoride ISE can be performed with a simple experimental set-up and high sample throughput. Finally, the modified reverse FIA technique is shown to be a novel method for process analysis, continuous monitoring purposes and high precision calibration.

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.

Fluoride ion-selective electrode in flow injection analysis : Part 2. Interference Studies

Abstract The influence of the sample composition on the response characteristics of the fluoride ion-selective electrode in flow injection analysis is described. Sample parameters such as ionic strength, viscosity and pH affect the response time of the electrode and cause transient signals when limiting values are exceeded. The respective limiting values depend on the total ionic-strength adjustment buffer (TISAB) used and these interferences can be minimized by proper choice of the TISAB. The complex formation of fluoride by several elements in the presence of TISAB containing CDTA is discussed. Aluminium and magnesium were found to interfere when present at levels above 1 and 100 mg l −1 , respectively. The signal decrease in the presence of iron, calcium and silicon can be attributed to ionic strength effects rather than complexation. Provided that the ionic strength is taken into account and corrected for, no influence occurs even in the presence of 0.5, 2 and 5% of iron, calcium and silicon, respectively.

Flow-injection potentiometris stripping analysis — a new concept for east trace determinations

Abstract Flow-injection potentiometric stripping analysis is introduced as a highly versatile technique for micro and trace determinations of heavy metals. The simple instrumentation used, with extremely short residence times (below 1 s), permits a simple throughput of 200 h –1 . Up to four elements can be determined simultaneously at lower μgl − levels up to % levels. On-line sample manipulation (e.g., dilution and matrix modification is possible with one- and two-channel flow systems. The utility of f.i.p.s.a. is evaluted by comparing response, sensitivity, and practical aspects of four different flow-through cells. The method is successfully applied to the fast sequential measurements of zinc, lead and copper in tap water and to the direct determination of lad and cadmium in acid digests of biological samples without further treatment.

Flow-through sorptive preconcentration with direct optosensing at solid surfaces for trace-ion analysis

This article highlights the potential of the solid-phase optosensing scheme as a microanalytical technique for the monitoring of ionic species at the low ng/ml level. We describe the background of solid-phase optosensing, critically comparing the performance of its applications in batch and flowing stream systems. We discuss in detail the current state of the art of flow-through solid-phase extraction (SPE)-based sensors, which are regarded as being a mature technique, and we outline its intrinsic and unique analytical features. We give special emphasis to the criteria to be fulfilled by both bead and membrane sorbent materials if used in a reversible fashion. We also thoroughly address the potentialities and limitations of different optrode cell designs devoted to absorbance/reflectance detection. We present the concept of renewable-bead injection as a powerful scheme that can be exploited with a wide variety of reagent-based assays, including those relying on irreversible reactions. As a practical example to compare the analytical performance of the variety of approaches and cell configurations described in the bulk of the text, we selected on-column monitoring of ultratrace levels of nitrite, which involved the reversed-phase extraction of the derivative compound resulting from the Shinn reaction.