Analytical and Bioanalytical Chemistry (v.371, #3)

What is it that analytical chemists do? by Ursula E. Spichiger-Keller (283-284).

The aim of this review is to describe the state-of-the-art in the analysis of A- and B-trichothecene mycotoxins in cereals and to support knowledge and experience exchange between laboratories in the field of Fusarium mycotoxin analysis. Current screening tests and quantitative methods for the most prevalent type-A and -B trichothecenes, HT-2 and T-2-toxin, and deoxynivalenol (DON) are reviewed. This includes the extraction and clean-up procedures and chromatographic methods (TLC, HPLC, GC) applied and the immunochemical methods, especially enzyme-linked immunosorbent assay (ELISA), employed for the determination of these mycotoxins. Results from recent intercomparison studies of the determination of DON are also discussed. Experience gained during these intercomparisons clearly shows the need for further improvement in the determination of trichothecenes, to obtain more accurate and comparable results. This also indicates there is a strong need for the development of further certified reference materials (CRM) which would enable comparison of measurement results between different European laboratories for several A- and B-trichothecenes. For both A- and B-trichothecenes there is still a lack of simple and reliable screening methods enabling the rapid detection of these mycotoxins at low cost.

Matching reference materials with AOAC International methods of analysis by Wayne R. Wolf; Jonathan DeVries; William Ikins (300-306).
Proper implementation and use of validated analytical methodology with use of appropriate reference materials (RM) is a preferred means of helping to ensure equivalent analytical method performance in diverse laboratories. Choice of an appropriate RM that not only matches the analyte and matrix of the required determination, but also has been demonstrated to be within the applicability of a specific analytical method, are key factors. In response to numerous requests since its founding in 1993, the Technical Division on Reference Materials (TDRM), AOAC International is implementing a program for recognizing the matching of specific reference materials to specific AOAC methods of analysis. This recognition is accomplished by means of a thorough peer-reviewed selection system, under the auspices of the AOAC official methods board and the executive committee of the TDRM. Potential RM/method matching (RM/MM) proposals will be submitted to an RM/MM committee. After technical review of the suitability of the proposed RM by the RM/MM committee, acceptable matches are recommended for review by the current AOAC process responsible for review and recognition of new methods and modifications to existing AOAC methods of analysis. Several trial matches have been used to develop and test this system. The end product of this effort will ultimately be made available as either a stand-alone document, a section of the AOAC Official Methods of Analysis, or a site within the AOAC web site listing recognized matches.

ArtTAX – a new mobile spectrometer for energy-dispersive micro X-ray fluorescence spectrometry on art and archaeological objects by H. Bronk; S. Röhrs; A. Bjeoumikhov; N. Langhoff; J. Schmalz; R. Wedell; H.-E. Gorny; A. Herold; U. Waldschläger (307-316).
A newly developed spectrometer for energy-dispersive micro X-ray fluorescence spectrometry has been designed for the demands of archaeometry. ArtTAX combines the advantages of non-destructive and sensitive multi-elemental analysis at sub-mm resolution with the possibility of working outside the laboratory. The spectrometer consists of an air-cooled, low-power molybdenum tube, new generation polycapillary X-ray optics, a silicon drift detector without the need for liquid-nitrogen cooling, a CCD camera, and three light diodes for sample positioning. The motor-driven measurement head is fixed on a x,y,z-flexible tripod support which can be assembled and dismantled within minutes. The spot size of the primary X-ray beam was determined to be 94 µm for the Cu(Kα) energy, the detection limits are in a range of a few tens of µg g–1 for the medium energy-range in glass. Additional open helium purging in the excitation and detection paths enables the determination of elements down to sodium, thus avoiding vacuum conditions or a size-limiting sample chamber. A selection of qualitative and quantitative results on pigment, metal, glass, and enamel analyses are presented to show the potential of ArtTAX in the field of art and archaeology.

Nested genetic algorithm for resolving overlapping spectra by Xiu Zhang; Jian Zheng; Hong Gao; Yun Zeng (317-322).
A nested genetic algorithm, including a genetic parameter level and a genetic implemented level, has been proposed and applied for resolving simulated overlapping spectra. Parameters of genetic algorithms (GA) were optimized by use of the genetic parameter level. The number of overlapping peaks was, moreover, detected simultaneously. Parameters of individual peaks in multiplets were computed by use of the genetic implemented level. It is obvious that the parameters of GA can be optimized and the number of overlapping peaks can be detected by itself. The optimization results are less affected than in traditional curve-fitting by the initial values of the parameters of the overlapping bands. Consequently, the nested genetic algorithm is superior to the curve-fitting technique for resolution of overlapping peaks.

Unsupervised pattern-recognition methods and Kohonen neural networks have been applied to the classification of rapeseed and soybean oil samples according to their type and quality by use of chemical and physical properties (density, refractive index, saponification value, and iodine and acid numbers) and thermal properties (thermal decomposition temperatures) as variables. A multilayer feed-forward (MLF) neural network (NN) has been used to select the most important variables for accurate classification of edible oils. To accomplish this task different neural networks architectures trained by back propagation of error method, using chemical, physical, and thermal properties as inputs, were employed. The network with the best performance and the smallest root mean squared (RMS) error was chosen. The results of MLF network sensitivity analysis enabled the identification of key properties, which were again used as variables in principal components analysis (PCA), cluster analysis (CA), and in Kohonen self-organizing feature maps (SOFM) to prove their reliability.

Resolution of the essential constituents of Ramulus cinnamomi by an evolving chemometric approach by Cheng-Jian Xu; Yi-Zeng Liang; You-Qun Song; Ji-Shan Li (331-336).
The analysis of complex mixtures, such as those of traditional Chinese medicine (TCM), is difficult by conventional methods of analysis. Chemometric methods provide a new way to solve such problems. Subwindow factor analysis (SFA) paired with the evolving window orthogonal projection (EWOP) method, has been used as a new evolving approach to the resolution of volatile components of Ramulus cinnamomi (RC). Compared with conventional chromatographic analysis, the chromatographic separation conditions necessary are greatly mitigated in our approach, yet the accuracy of qualitative and quantitative results is improved, because the measured data matrix has been resolved into chromatograms and mass spectra of the chemical components. Our method is, moreover, friendly to use and easy to program. Experimental results show the efficiency and convenience of the proposed approach. Forty-seven of the seventy-eight separated constituents in an essential oil, accounting for 89.55% of the sample, were identified by mass spectroscopy (MS).

9,10-Phenanthrenequinone (PQ) supported on graphite powder by adsorption was dispersed in propyltrimethoxysilane-derived gels to yield a conductive composite which was used as electrode material to fabricate a PQ-modified carbon ceramic electrode. In this configuration, PQ acts as a catalyst, graphite powder guarantees conductivity by percolation, the silicate provides a rigid porous backbone, and the propyl groups endow hydrophobicity and thus limit the wetting region of the modified electrode. Square-wave voltammetry was exploited to investigate the pH-dependent electrochemical behavior of the composite electrode and an almost Nernstian response was obtained from pH 0.42 to 6.84. Because the chemically modified electrode can electrocatalyze the reduction of iodate in acidic aqueous solution (pH 2.45), it was used as an amperometric sensor for the determination of iodate in table salt. The advantages of the electrode are that it can be polished in the event of surface fouling, it is simple to prepare, has excellent chemical and mechanical stability, and the reproducibility of surface-renewal is good.

The influence of a surface oxide on V(100) on the sticking of atomic hydrogen was studied. The oxide was produced by dosing with oxygen at different surface temperatures and characterized by AES and XPS. Thermal desorption spectroscopy was used to determine the sticking coefficient of atomic hydrogen. The kinetics of oxidation and the stability of the formed oxide have been studied in detail. The highest oxidation rate was obtained at a surface temperature of 473 K, producing a V2O3 over-layer. Although the sticking coefficient for atomic hydrogen decreases from 0.14 on the oxide free V(100) surface to 0.015 on the V2O3 surface, the sticking coefficient for atomic hydrogen was still several orders of magnitude higher than that for molecular hydrogen. This is relevant in respect of the use of the hydrogen–vanadium system for energy storage.

The analysis of seized nuclear material aims at identifying the origin of the material. Determination of the n(18O)/n(16O) ratio for the uranium oxide adds another characteristic property to the pattern which enables location of the production area of the material. A method has been developed for n(18O)/n(16O) ratio measurement which uses thermal ionization mass spectrometric (TIMS) analysis of the 238UO+ species. It has been shown that uranium oxides of different geographic origin have significantly different n(18O)/n(16O) ratios, whereas different samples of the same origin have constant oxygen isotopes ratios.

The pre-concentration of mercury(II) and methylmercury by adsorption of their dithiophosphoric acid diacyl ester (DDTP) chelates on a C18 column, then detection with cold-vapor atomic-absorption spectrometry was investigated. Conditions such as sample pH, reductant and chelating agent flow and concentration, and eluent and carrier gas flow were optimized. Optimization was performed by use of evolutionary operation with a proper factorial design. At a sample flow of 5.3 mL min–1 and a loading time of 4.5 min, column adsorption efficiency ranged from 88 to 93% for both species. Detection limits down to 10 ng L–1 were obtained at a sample throughput of 12 h–1. There was good agreement between found and certified values in the analysis of certified reference materials after their microwave-assisted mineralization with HNO3 and H2O2.

A method has been developed for the determination of chromium in presence of V(V), Mo(VI), and Fe(III). The effects of interferences were evaluated by using the apparent content curves method, and their separation was performed by solid-phase extraction with an anionic exchanger. The sample-treatment conditions and the influence of the sample conductivity were studied. Tolerance limits were established, and the proposed procedure was used to determine chromium in certified samples and for speciation of chromium in waste water. Our results were always in agreement with the theoretical content.

A simple and rapid flow-injection (FI) method is reported for the simultaneous spectrophotometric determination of Fe(II) and Fe(III) in pharmaceutical products. The method is based on the reaction of Fe(II) with 2,2′-dipyridyl-2-pyridylhydrazone (DPPH) in acidic medium to form a water-soluble reddish complex (λ max=535 nm). Fe(III) reacts with DPPH under flow conditions only after its on-line reduction by ascorbic acid (AsA). Both analytes were determined in the same run via a double-injection valve, which enabled the simultaneous injection of two sample volumes in the same carrier stream ("single-line double-injection" approach). The two well-defined peaks produced corresponded to total iron [Fe(II)+Fe(III)] and Fe(II). Speciation of the analytes in their mixtures was achieved by multiple regression analysis. The calibration curves obtained were linear over the ranges 0–30 and 0–50 mg L–1 for Fe(II) and Fe(III), respectively, and the precision [s r=1.0% for Fe(II) and 1.5% for Fe(III)] was satisfactory. The method proved to be selective and adequately sensitive (c L=0.25 and 0.17 mg L–1 for Fe(III) and Fe(II), respectively, in mixtures). Application of the method to the analysis of pharmaceutical samples resulted in excellent accuracy; the percent mean recoveries were in the range 99.0–102.0% for both Fe(II) and Fe(III) and the mean relative error was e r=1.0%.

Unknown fatty acid ethoxylate samples have been transesterified in supercritical methanol, using a loop made of a stainless-steel tubing as the reactor vessel. The initiator acids, now present as the corresponding methyl esters, were determined by gas chromatography–mass spectrometry (GC–MS). Quantitative transesterification is achieved by heating a solution of the ethoxylate sample in methanol at 280 °C for 10 min under pressure. The influence of reaction time, temperature, and presence of a Lewis acid catalyst has been investigated. The method of transesterification was optimized in respect of low cost, short reaction time, and availability to laboratories with standard analytical equipment.

A sequential injection analysis (SIA) system is proposed for the determination of zinc in pharmaceutical samples. The method is based on the spectrophotometric detection of zinc using xylenol orange as a colour reagent at 568 nm. The system can be used to monitor zinc at a frequency of 30 samples h–1 with an average recovery of ±98.5% and a relative standard deviation of less than 1%. A linear relationship between peak height and zinc concentration is obtained between 10 and 60 mg L–1. The results obtained are in good agreement with those obtained by use of a standard method.

The amino acid composition of L-lysine fermentation juices from potatoes and cane molasses from a green biorefinery has been determined by gas chromatography–mass spectrometry. N-Methyl-N-tert(butyldimethylsilyl)trifluoroacetamide (MTBSTFA) was used as derivatization reagent to prepare the t-butyldimethylsilyl derivatives of the amino acids present in the juices. The amino acids in these derivatives were identified from both their EI and CI mass spectra and their retention times in the gas chromatogram, and they were quantified employing the GC response signals relative to cycloleucine as internal standard.

For selective dissolution of Mn oxides as components of soil (easily reducible oxides) 0.05 mol L–1 and 0.1 mol L–1 NH2OH·HCl acidified to pH 2 and for dissolution of Fe oxides (crystalline form of the oxides) 0.25 mol L–1 NH2OH·HCl in 25% acetic acid, 0.2 mol L–1 oxalate buffer and 0.1 mol L–1 ascorbic acid in oxalate buffer were tested. Dissolved elements were determined by ICP–MS and ICP–OES. The studies indicate that the use of 0.05 mol L–1 NH2OH·HCl in nitric acid solution (pH 2) and 0.1 mol L–1 ascorbic acid in 0.2 mol L–1 oxalate buffer led to selective leaching of trace metals bound by Mn and Fe oxide phases in soil. Comparison of different extraction schemes indicates that the trace elements investigated are mainly bound to minerals consisting of crystalline Fe oxides and insoluble minerals (under the extraction conditions used). The studies also indicate that Mn oxides and organic matter retain a major mobile fraction of the elements investigated in the soil.

The usefulness of coprecipitation with lanthanum phosphate for separation and preconcentration of some heavy metals has been investigated. Although lanthanum phosphate coprecipitates iron(III) and lead quantitatively at pH 2.3, iron(II) can barely be collected at this pH. This coprecipitation technique was applicable to the separation and preconcentration of iron(III) before inductively coupled plasma atomic-emission spectrometric (ICP–AES) determination; the recoveries of iron(III) and iron(II) from spiked water samples were 103–105% and 0.2–0.7%, respectively. The coprecipitation was also useful for separation of 20 µg lead from 100 mL of an aqueous solution that also contained 1–100 mg iron. Coprecipitation of iron was substantially suppressed by addition of ascorbic acid, which enabled recovery of 97–103% of lead added to the solution, bringing the recovery to within 1.6–5.0% of the relative standard deviations. Lanthanum phosphate can also coprecipitate cadmium and indium quantitatively, although chromium(III), cobalt, and nickel and large amounts of sodium, potassium, magnesium, and calcium are barely coprecipitated at pH≈3.

A method is proposed for the determination of low concentrations of Ti (< 2 µg g–1) in polypropylene in presence of Al and Mg by ETAAS after elimination of the polymer matrix and Si. The optimum instrumental conditions for the determination of Ti are presented. Interferences of Al and Mg in the determination of Ti and some necessary precautions providing better accuracy and precision for the determination of Ti by ETAAS are discussed. A simple procedure for sample preparation is proposed. The detection limit of the method is 12 ng g–1. The method developed has been applied to real polypropylene samples.

An on-line sequential injection system has been developed for spectrophotometric determination of chloride in drinking mineral, natural, and ground waters. Samples containing different concentrations of chloride were analyzed. The analysis is based on detection of the red iron(III) thiocyanate complex. The complex was monitored spectrophotometrically at 480 nm using de-ionized water as the carrier stream at a flow rate of 3.21 mL min–1. The method was found to be linear within the range 0–50 mg L–1 chloride; the detection limit was 3.01 mg L–1. The fully automated method can be used to analyze 37 samples per hour with a relative standard deviation (RSD) better than 2.50%.

Clavulanate-selective electrodes – application to pharmaceutical formulations by Adriana M. Pimenta; Cristina M. Couto; Alberto N. Araújo; Conceição M. Montenegro (400-403).
The construction and assessment of a clavulanate anion-selective electrode and its application to the analysis of pharmaceutical formulations by direct potentiometry are described. The electrode, prepared without inner reference solution, was fabricated by use of a PVC membrane, with bis(triphenylphosphoranylidene)ammonium clavulanate as ion-exchanger dissolved in 2-nitrophenyl octyl ether as intermediate solvent and p-t-octylphenol as additive. The response of the electrode was linearly dependent on concentration within the range 2.4×10–3–1×10–1 mol dm–3 (ionic strength adjusted to 0.1 mol dm–3); the slope of the calibration plot was –59.4±0.9 mV  decade–1 and the reproducibility ±0.6 mV day–1. The response time was less than 20 s. Relative errors were <3.5% when results from analysis by direct potentiometry were compared with those from the reference method.