Analytical and Bioanalytical Chemistry (v.330, #6)

Based on the accepted definition of “accuracy” in analytical chemistry the distinction between accuracy and trueness is discussed and the superiority of trueness to accuracy in the hierarchy of terms is stressed.the generation of a true value by primary calibration applying complete synthesis or reconstitution of the sample portion and then establishing secondary reference samples, deliberately adapted to the analytical problem. This is distinguished fromthe formalised application of these reference samples, in order to transfer the trueness from the reference to the analytical sample. The use of pure elements and an appropriate weighing system are required for the generation of true results. The process of reconstitution is computer controlled and comprises the choice of the contents of synthetic reference samples, the display of the nominal weights of the pure elements, the calculation of new real contents of synthetic reference samples (according to the actual weights of the pure elements) and finally a linear regression analysis. The hardware consists of various instrumental systems, an ultra microbalance Sartorius MP 8-1 and a micro PDP 11/73 computer running under the real time multiuser/multitask operating system RSX 11 M.The balance is connected via a free programmable V 24 interface to the computer, allowing a data transfer without additional devices or programs.To demonstrate the efficiency of the proposed method the case of Certified Reference Sample Columbite IGS 33 is discussed in detail.

Multicomponent analysis often suffers from matrix effects. In the present paper several methods are employed to cope with those effects and to keep experimental expense as low as possible and hence to reduce analysis time. Based on the determination of Na, K, and Ca by flame atomic emission spectroscopy experimental design in combination with multivariate calibration (matrix methods) has been tested for suitability in multicomponent analysis. Good results are achieved by statistically designed experiments. The experimental domain should be kept small to allow for linear models. Comparable results are obtained using the P method. As expected the K approach (Beer model) yields the worst results.

A new type of flow injection titration method where a well-stirred mixing chamber is placed in the middle of the injection loop of a 6-way valve has been developed. With this method expensive or unstable reagents can be used for the titration. Moreover, it is potentially available for slow titration reactions. Its fundamental characteristics have been investigated using acid-base titration reaction with phenolphthaleine as the indicator. The sampling rate was 60–120 samples per hour and its reproducibility was also high (3%).

A new rapid FIA method for determination of halide and pseudohalide ions without Chromatographic columns was developed. The detection is performed electrochemically, mostly amperometrically. It is also possible to select these anions. In combination with special methods for standardizing (calibration with increase and decrease of the concentration of the measuring ions) very good agreement with potentiometric determinations of chloride ions in water was achieved. The relative standard deviation was mostly 1.5%.

A new rapid FIA method for the determination of sulfate-, sulfite-, nitrate- and nitrite-ions without Chromatographic columns was developed. The working electrode is a Pb/PbSO4 electrode for the determination of sulfate, a Pb electrode for sulfite, an Sn-electrode for nitrate and an Sb electrode for nitrite. The Pb/PbSO4 electrode is produced by chemically passivating in sulphuric acid/hydrogen peroxide mixture. The detection is performed electrochemically, either potentiometrically or amperometrically. In combination with special methods for standardizing (calibration with increasing and decreasing concentration of the measuring ions) a good agreement of the results for these ions in water with gravimetric and volumetric methods was obtained. The relative standard deviations were mostly 1.5% (sulfate, sulfite) and 2.5% (nitrate, nitrite).

The molecular absorption spectrometric method using the yellow phosphovanadomolybdate complex,adopted as ISO-standard for determination of phosphorus in steel, has been optimized.High and varying blank values often reported in literature were found to be due to silicon interference. The silicon is released from the glass beaker during fuming with perchloric acid and the problem is avoided by either using teflon beakers or by adding hydrofluoric and boric acid after fuming.Interferences from elements present in the metal (arsenic, chromium, silicon and the refractory elements) were quantified and procedures are described for masking these interferences in order to make the method applicable to high-alloy steels, tool steels, nickel-base alloys and Zircaloys. Arsenic, hafnium, niobium, tantalum and tungsten cause the most severe interferences. Chromium, titanium and zirconium were found to have weak interferences whereas molybdenum and silicon did not interfere at all despite large amounts of precipitated oxides.Results reported by other authors that many old certified reference materials of low-and high-alloy steels have too high certified phosphorus values were confirmed with the described method as well as with the ICP-OES technique. However, the lower values were only found for alloys without Hf, Nb, Ta or W. Alloys containing these elements were often found to have higher phosphorus values, which reflects interferences not completely removed in the previous analyses of these alloys.

A method is described for the determination of zinc in gallium arsenide by ETAAS with platform atomization and matrix modification. The sample is decomposed with nitric and hydrochloric acids, ortho-phosphoric acid is added as a matrix modifier and the diluted solution is injected into the furnace. The optical correction of the nonspecific absorbance is readily achieved by means of a conventional deuterium lamp. Platform atomization improves the peak repeatability by a factor often in comparison with wall atomization. For a 40 mg sample, with reduced argon flow rate in the atomization step, the detection limit is 0.08 μg Zn g−1 (4×1015 atoms cm−3). Results obtained by analysing Zn-doped GaAs samples are presented. The relative standard deviation of the overall procedure is 4–8%.

A method for antimony determination is described comprising hydride generation and transport into a flow-cell placed in a UV-visible molecular absorption spectrophotometer with diode-array detection. Nitrogen is employed as carrier gas. The measurement is performed at 198 nm. The calibration curve is linear from 3 to 440 μg/ml of antimony. The method was successfully applied to the analysis of PVC.

Microtrace determination in high-purity seleniumMikrospurenbestimmung in hochreinem Selen by E. Ivanova; N. Vracheva; I. Havezov; N. Jordanov (516-517).
A simple and rapid method is developed for the determination of traces of As, Sb and Sn in selenium of semiconductor grade purity. The limit of detection is 0.05 ppm, the relative standard deviation betwen 3 and 9%. The procedure may be applied to the determination of all trace elements which remain in solution during the reduction of selenium with hydrazine hydrate (As, Sb, Sn, Co, Cd, Mn, etc.).

Ion-exchange on Chelex-100, donnan dialysis and the method of ion-retardation are applied to characterize the bonding strengths of the minerals Ca, Mg and Zn in milk and extracts of meat and flour. Another ion-exchange procedure and direkt potentiometry with an Ca-selective electrode are applicate for the quantification of weak bindings. By use of the above mentioned methods it is possible to calculate element species stabilities, which represent characteristic values. Both methods give comparable results for milk, extracts of meat and spinach, grape-juice, red and white wine. Detailed examinations of binding conditions in milk proved citrate to the strength binding compound for Ca and Mg. Model-examinations with citrate, caseine and phosphate agree well with calculations and measurements in milk. Furthermore differences in the binding of Ca and Mg in raw and cream spinach are examined. The above mentioned methods are suitable to characterize and quantify the binding of elements in foodstuffs. The results give informations about chemical changes due to technological processes e.g., in regard to storage life.

2.6 Lebensmittel (564-566).