Wolfhard Wegscheider

Multifactor model for the optimization of selectivity in reversed-phase chromatography

Abstract Three variables characterizing the mobile phase composition, pH, elution strength and ionic strength, have been studied in order to construct a three-dimensional semiempirical model for predicting retention times of dibasic substances. The solutes treated quantitatively include dibasic acids and bases, an amino acid and two dipeptides. Experimental effort was minimized by arranging them as 6 × 3 × 2 factorial design and deriving the coefficients of the model with a variable projection algorithm that separates linear from non-linear parameters. The coefficients are then used to predict capacity factors, k′, and relative retentions, α, for all solute pairs in a computerized grid search. Within the limits of the model, it is an easy task to reduce the grid size to calculate all combinations of 25 pH, 20 elution strength and 10 ionic strength values. The predicted optimal selectivity was verified experimentally and the experimental retention data found to be in good agreement with the computed retention times.

Surfaces and materials of electrothermal atomic absorption spectrometry — more than a merely morphological study

ZusammenfassungEs wird ein Überblick über morphologische Studien an Oberflächen der elektrothermischen AAS mit Hilfe der Rasterelektronenmikroskopie gegeben. Prinzipielle Möglichkeiten werden diskutiert wie Störungen, die mit dem Material oder der Oberfläche der Atomisierungseinrichtung zusammenhängen, verringert oder beseitigt werden können. An zahlreichen Materialien, die entweder schon verbreitet Anwendung finden, oder sich noch in der Entwicklungsphase befinden, wurden Oberflächen von neuen und korrodierten Teilen studiert. Zu den untersuchten Materialien gehörenRohre aus polykristallinem Elektrographit mit Pyrographitbeschichtung oder Metallsalzimprägnierung,Plattformen aus massivem Pyrographit,Rohre aus massivem Pyrographit,Rohre aus glasartigem Kohlenstoff,Rohre aus Wolfram,Rohre und Plattformen mit Refraktärmetallcarbidbeschichtung durch physikalische Abscheidung aus der Dampfphase. Neue Gesichtspunkte zur Bildung von Graphitknollen in Graphitöfen werden dargelegt. Die Bedeutung von Dauerversuchen mit Rohren für die elektrothermische AAS für die Routine wird erläutert.SummaryAn overview of morphological studies on evaporation surfaces in electrothermal AAS by scanning electron microscopy is presented. Principal possibilities for the reduction or prevention of interferences are discussed as far as they are related to the material and surface of the atomizer. Unused and corroded surfaces were studied for a wide range of materials currently in general use or still in a developmental stage:polycrystalline electrographite tubes coated with pyrolytic graphite or impregnated with metal salt solutions,total pyrolytic graphite platforms in combination with pyrolytic graphite coated tubes,total pyrolytic graphite tubes,glassy carbon tubes,tungsten tube atomizers,polycrystalline electrographite tubes and total pyrolytic graphite platforms coated with refractory metal carbides by physical vapor deposition. New aspects on graphite nodule formation in graphite furnaces are presented. The significance of tube endurance experiments for routine electrothermal AAS is elucidated.

Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry—I: Polycrystalline electrographite tubes with and without pyrographite coating

Abstract Morphological studies on graphite tube surfaces by scanning electron microscopy are presented for unused and corroded tubes from various endurance experiments in electrothermal AAS. Tubes always break in their center as this is the hottest part; tube thinning occurs predominantly at the outside. The inner surface of tubes coated with pyrographite is usually only slightly corroded, unless highly aggressive reagents are applied. Deposition of carbon in thin layers as well as in nodules is observed in the hot central zone.

Relationships between chromatographic response functions and performance characteristics

Abstract Numerous objective functions used with formal strategies of experimental optimization are reviewed. It is shown that several of them give no meaningful indication of the quality of separation unless various other chromatographic parameters, e.g., number of plates and asymmetry, are specified. Another group of chromatographic response functions is characterized by the need for operator-defined weighting factors that not only establish the exact location of the optimal conditions but also limit the usefulness of the optimization in general. Performance characteristics, like accuracy, precision and separation time, can be related to numerical values of the response functions. Important chromatographic situations for the separation of two and ten components are simulated. It is concluded that only functions derived from Kaisers peak separation number are directly dependent on performance characteristics.

Single- and multi-channel detection for generalized quantitative analysis in cases of unresolved chromatographic peaks

Abstract Computerized quantification of components under overlapping chromatographic peaks is done by calibration of chromatograms against component mixtures. For conventional (single-channel) detectors, the limitations of earlier methods based on ordinary multiple regression, can be circumvented by data reduction with the aid of principal component analysis with the partial least-squares approach. Simulation studies show that the method can be applied even when there is severe peak overlap, unstable baseline, noisy chromatograms or non-linear detector response. Advantages in the quantification of fused peaks by means of multichannel detectors are outlined. Present limitations on the quantitative evaluation of several overlapping component peaks from a single spectro-chromatogram by means of the partial least-squares method combined with multiple regression on the pure component spectra, are discussed with respect to practical high-performance liquid chromatography.

Statistical investigations of interferences in graphite furnace atomic absorption spectroscopy@@@Statistische Untersuchungen ber Strungen bei der flammenlosen Atomabsorptionsspektralphotometrie: II. Cadmium@@@II. Cadmium

ZusammenfassungInterferenzen, die bei der flammenlosen Bestimmung von Blei in der HGA 72 auftreten, wurden bei HCl, HNO3, H2SO4, H3PO4, HClO4, HF und einigen Kationen studiert. In gewissen Fällen reicht ein nur 1000 facher Überschuß des Störions, um Abweichungen vom Signal des Bleistandards hervorzurufen. Da das Auftreten von Interferenzen auch oft matrixabhängig ist, können Matrixvariationen zur Unterdrückung von Interferenzen eingesetzt werden. Die Optimierung des Temperaturprogrammes wird erfolgreich zur Beeinflussung des Zufallsfehlers und/ oder systematischer Fehler herangezogen. In der Graphitrohrküvette reichte die Stabilität der Bleiverbindung aus, um beim Veraschungsschritt die Temperatur bis auf 900° C (20 s) zu steigern.SummaryInterferences in the flameless determination of lead using the HGA 72 were studied for HCl, HNO3, H2SO4, H3PO4, HClO4, HF and for a number of cations. In certain instances interferents in just 1000-fold concentration over lead are shown to cause deviations in the lead signal. Many of the interferences are matrix-dependent themselves and can be reduced by matrix variations. Optimization of the thermal pretreatment can be used effectively to reduce random error and/or systematic errors. The thermal stability of lead compounds in the graphite furnace was sufficient for charring procedures up to 900° C for 20 s.

Preconcentration of trace elements on oxine cellulose

SummaryPreconcentration of Trace Elements on Oxine Cellulose 8-Hydroxychinoline, covalently bound to filter cellulose (oxine cellulose), was used for preconcentrating trace metal ions from complex matrices and for matrix separation, respectively. Mn, Fe, Co, Ni, Cu, Zn and Cd traces were preconcentrated on columns filled with oxine cellulose and analysed by AAS and ICP after being eluted with dilute HNO3. At a given pH-value, alkali and alkaline earth metal ions can be quantitatively separated as matrix elements; high concentration factors are obtained and the eluates can be measured without interference. The usefulness of the method is shown by control analyses of EPA- and NBS-SRMs.

Determination of trace elements in organic material by inductively coupled plasma-mass spectrometry

SummaryFor purposes of trace element analysis of organic materials a combined analytical procedure is tested. It consists of a pressure decomposition by nitric acid at 220° C in quartz vessels. The measurements are performed in the diluted digests by inductively coupled plasma/mass spectrometry. Analytical figures of merit are given.

Development of EDXRF techniques for the quantitative multielement analysis of urban dust samples

SummaryThe investigation and monitoring of suspended particulate matter in air require accurate and sensitive multielement analysis of large numbers of samples. In the work described, airborne particles and settled urban dusts were examined by energy dispersive X-ray fluorescence spectrometry with secondary-target excitation. The practical requirements of trace-element determinations in dusts are reasonably satisfied by two techniques. Deposition of fine particles (under 3.5μm) on Nucleopore membrane filters provides thin-film samples, for which absorption and particle-size effects are usually negligible. Calibration is achieved by sputtering of certified NBS standard glasses onto membrane filters. In the destructive “fusion and direct solidification” technique, sample inhomogeneities and particle-size effects are eliminated by fusing the sample with lithium borate in a Pt/Au alloy crucible. La2O3 or WO3, added as heavy absorbers, can reduce the interelement-effects of the thick specimens to a certain extent. A newly developed computer program, based on a modified mathematical expression for the intensity, compensates for absorption and secondary fluorescence effects. It is demonstrated that for some trace elements the background can be easily predicted as a linear function of the incomplete charge-collection of the Si(Li) detector. An intermethod comparison with atomic-absorption results is presented; the results are verified with the NBS Fly Ash standard reference material.ZusammenfassungDie Untersuchung und Überwachung luftgetragener, feiner Stäube erfordert eine hinreichend genaue und empfindliche Multielement-Analysen-methode, die eine schnelle Bestimmung von Serienproben ermöglicht.Es wird über die Untersuchung luftgetragener Staubteilchen und sedimentierter Stadtstäube mittels energiedispersiver Röntgenfluoreszenzspektrometrie mit Sekundärtargetanregung berichtet. Den praktischen Anforderungen einer Spurenbestimmung in umweltrelevanten Staubmatrices wird dabei durch folgende 2 Methoden entsprochen.Feine luftgetragene Staubteilchen unter 3,5μm werden auf Nucleopore Membranfilter abgeschieden und direkt mittels EDXRF zerstörungsfrei gemessen. In diesen quasi unendlich dünnen Schichten sind Matrixeffekte und Teilchengrößeneffekte praktisch zu vernachlässigen. Die Eichung erfolgt mittels dünner Schichten, die durch „Sputtering“ von NBS-Gläsern mittels Argon-Ionenstrom hergestellt werden.Werden die Proben mit Li2B4O7 in einem Pt/Au-Tiegel zu homogenen glasartigen Scheibchen geschmolzen, so lassen sich alle Inhomogenitätsund Teilchengrößen-Effekte, die eine Analyse stören, vermeiden. La2O3 oder WO3, den Proben als „schwerer Absorber“ zugesetzt, reduziert die Interelementeffekte der praktisch unendlich dicken Proben. Es wird über ein neu entwickeltes Computerprogramm zur Datenreduktion berichtet, das nach der theoretischen und empirischen Methode für Absorptionsund Interelementeffekte korrigiert. Es wird gezeigt, daß für einige Spurenelemente der Untergrund eine lineare Funktion der „incomplete charge collection“ des Si-(Li-)Detektors ist und auf empirischem Wege einfach berechnet werden kann.Abschließend wird ein Methodenvergleich mit der AAS präsentiert; die Ergebnisse werden mit dem Standardreferenzmaterial NBS Fly Ash verifiziert.

A novel method for non-linear least-squares analysis of energy-dispersive X-ray spectra

Abstract A new energy response function, consisting of the Gaussian function, a function describing the asymmetry on the low energy side of the peak, a step function and a background function is developed for the quantitative description of EDXRF-spectra. It is suitable for computerized deconvolution with non-linear least-squares algorithms; the procedure is demonstrated using both the Marquardt algorithm and an algorithm that treats linear and non-linear parameters separately. The assessment of accuracy of the new method is performed on single element spectra with high signal to background ratios and by comparison of results obtained by an independent approach.