Analytica Chimica Acta (v.439, #2)

Detection of single influential points in OLS regression model building by Milan Meloun; Jiřı́ Militký (169-191).
Identifying outliers and high-leverage points is a fundamental step in the least-squares regression model building process. Various influence measures based on different motivational arguments, and designed to measure the influence of observations on different aspects of various regression results, are elucidated and critiqued here. On the basis of a statistical analysis of the residuals (classical, normalized, standardized, jackknife, predicted and recursive) and diagonal elements of a projection matrix, diagnostic plots for influential points indication are formed. Regression diagnostics do not require a knowledge of an alternative hypothesis for testing, or the fulfillment of the other assumptions of classical statistical tests. In the interactive, PC-assisted diagnosis of data, models and estimation methods, the examination of data quality involves the detection of influential points, outliers and high-leverages, which cause many problems in regression analysis. This paper provides a basic survey of the influence statistics of single cases combining exploratory analysis of all variables. The graphical aids to the identification of outliers and high-leverage points are combined with graphs for the identification of influence type based on the likelihood distance. All these graphically oriented techniques are suitable for the rapid estimation of influential points, but are generally incapable of solving problems with masking and swamping. The powerful procedure for the computation of influential points characteristics has been written in Matlab 5.3 and is available from authors.
Keywords: Influential observations; Regression diagnostics; Outliers; High-leverages; Diagnostic plot; Influence measures;

Four methods are compared for quantifying the effect of measurement errors on the uncertainty in bilinear model predictions. These methods amount to (1) evaluating an approximate expression for prediction variance, (2) bootstrapping residuals left after fitting the data matrices using a singular value decomposition, (3) adding noise from an appropriate distribution to the original data, and (4) jack-knifing rows and columns of the data matrices. The comparison is carried out for liquid chromatography/ultraviolet data obtained from Malinowski and the models are constructed using the generalized rank annihilation method. It is found that the first three methods give highly consistent results, whereas the jack-knife yields uncertainty estimates that have no clear interpretation.
Keywords: Bilinear calibration; Error estimation; Monte Carlo simulation; Resampling; Bootstrap; Jack-knife;

Trace analysis of platinum in biological samples: a comparison between sector field ICP-MS and adsorptive cathodic stripping voltammetry following different digestion procedures by Sonja Zimmermann; Christoph M. Menzel; Zsolt Berner; Jörg-Detlef Eckhardt; Doris Stüben; Friedrich Alt; Jürgen Messerschmidt; Horst Taraschewski; Bernd Sures (203-209).
A study was carried out to analyse Pt trace levels in fish liver and mussel soft tissue, using two different analytical techniques. Sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) combined with microwave digestion was compared with adsorptive cathodic stripping voltammetry (ACSV) following high pressure ashing (HPA). A microwave digestion procedure for animal tissue was developed, using HNO3 and H2O2. The procedural detection limit of Pt with SF-ICP-MS in combination with this digestion procedure was 2.4 ng/l in the solution. The subsequent detection limit considering 300 mg liver tissue (wet weight) was found to be 200 pg/g Pt. In comparison, the detection power of ACSV after HPA digestion was about 20 times more sensitive. The higher procedural detection limit of the ICP-MS-based method is due to the high dilution necessary to reduce acid concentration. Nevertheless, the results of both analytical methods differed between 16 and 30% for Pt concentrations below 1 ng/g, and were almost identical (2–10% deviation) above this value. The Pt levels in the animal tissues analysed ranged between 0.1 and 2.3 ng/g.
Keywords: Pt; Sector field ICP-MS; Adsorptive cathodic stripping voltammetry; High pressure ashing; Microwave digestion; Animal tissue;

The effects of several variables on the As, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Pb, Se and Zn ultrasonic bath induced-acid leaching from lyophilised seafood samples have been studied by experimental designs. The determinations were carried out by electrothermal atomic absorption spectrometry (As, Cd, Co, Cr, Mn, Pb and Se), flame atomic absorption spectrometry (Ca, Cu, Fe, Mg and Zn) and by cold vapour-atomic absorption spectrometry (Hg). A Plackett–Burman experimental design was used to select the most significant variables that affect to the acid leaching process, while central composite designs were used to found optimum values for the variables involved in the acid leaching. The optimum conditions found for each variable and each element were tabulated. Accurate results were obtained by measuring DORM-1, DOLT-1 and TORT-1 certified reference materials, and the procedure was finally applied to mussel, clam and tuna samples.
Keywords: Ultrasound induced acid leaching; Experimental designs; Trace elements; Seafood products; Atomic absorption spectrometry;

The flow injection hydride generation technique together with atomic absorption spectrometry was used for the determination of arsenic, antimony and selenium in the iron-based water treatment chemical FeClSO4. Thiourea, l-cysteine and potassium iodide–ascorbic acid were used as masking agents to diminish the interference caused by the very high iron concentrations in the samples. These reagents act also as prereductants for As(V) and Sb(V). Thiourea and l-cysteine did not prevent the signal depression caused by such high iron content, but potassium iodide–ascorbic acid eliminated iron interference well even up to 2500 mg Fe l−1. The limits of detection (LODS) in aqueous solutions containing no iron were 0.037 μg l−1, 0.121 g l−1 and 0.131 μg l−1 for As, Sb and Se, respectively. The linear dynamic range was 0–10 μg l−1 for As and 0–30 μg l−1 for Sb and Se. The precision relative standard deviation was expressed as 2.6% for As, 4.4% for Sb and 2.9% for Se. The precision determinations were done on the FeClSO4 matrix at the level 0.5–0.8 μg l−1 for the elements to be analyzed. The accuracies of the methods were tested by using two standard reference materials (SRM 361, LA Steel and SRM 2074, river sediment). The concentrations obtained for As, Sb and Se were very close to the certified values.
Keywords: Atomic absorption spectrometry; Hydride generation; Flow injection; Arsenic; Antimony; Selenium; Iron interference; Water treatment chemical;

In this paper, poly(para-aminobenzoic acid (p-ABA)) (P-pABA) electrochemically modified glassy carbon electrode (GCE) was prepared. The electrochemical behavior of monoamine transmitters and their metabolites at this P-pABA modified electrode have been studied by cyclic voltammetry (CV). The results indicated that the modified electrode exhibited efficiently electrocatalytic oxidation for neurotransmitters and their metabolites with relatively high sensitivity, stability, and long-life. Coupled with high-performance liquid chromatography (HPLC), the P-pABA modified electrode was utilized for electrochemical detection (ECD) of these compounds. The linear ranges were over three orders of magnitude and the detection limits were 5.0×10−9  mol l−1 for norepinephrine (NE) and epinephrine (E), 2.0×10−8  mol l−1 for dopamine (DA), 3,4-dihydroxyphenyl acetic acid (DOPAC), serotonin (5-HT), 5-hydroxylindole acetic acid (5-HIAA) and homovanillic acid (HVA), respectively. The method has been successfully applied to assess the contents of monoamines and metabolites in rat brain microdialysates. To study the physiological function of nitric oxide (NO) on striatal DA release, a continual perfusion of 3.0×10−5  mol l−1 sodium nitroprusside (SNP) was performed into rat striatum, which increased striatal DA release with 1.5-fold to the basal level.
Keywords: P-pABA modified electrode (ME); Monoamines; HPLC–ECD; Microdialysis; Effect of NO; Rat corpus striatum;

Separation of rare earth elements by high performance liquid chromatography using a covalent modified silica gel column by R. Garcia-Valls; A. Hrdlicka; J. Perutka; J. Havel; N.V. Deorkar; L.L. Tavlarides; M. Muñoz; M. Valiente (247-253).
A new modified silica gel material with chemically bound phosphonic acid was tested as lanthanide ions adsorption and their basic characteristics (thermodynamics, stability and physical description) studied. This inorganic chemically active beads (ICABs) are suitable for the solid phase extraction (SPE) and separation of the rare earth elements (REE). The HPLC procedure, using the 10 μm diameter ICAB with chemically immobilized phosphonic acid [alpha]-hydroxyisobutyric acid (HIBA) as the eluent and the post-column derivatization using Arsenazo III as the reagent, was worked out and found efficient to perform a good separation of REE.
Keywords: HPLC; Rare earths; Modified silica; Separation;

The Zorbax BP-SIL silica gel acted as a cation exchanger under strongly acidic conditions and was successfully applied to ion chromatography with conductimetric detection (IC-CD) for the simultaneous separation of common mono- and divalent cations (Li+, Na+, NH4 +, K+, Mg2+, and Ca2+). Therefore, the main cause of the cation-exchange characteristics of the Zorbax BP-SIL silica gel was investigated for the preparation of a newly advanced silica-based cation-exchange stationary phase for IC-CD using strongly acidic eluents. As a result, it was found that the main cause could be attributed to zirconium present as an impurity in the silica matrix. The preparation of silica gel modified with zirconium (Zr-silica) was carried out by the reaction of the silanol group on the surface of silica gel with zirconium butoxide (Zr(OCH2CH2CH2CH3)4) and the Zr-silica prepared was applied to IC-CD for these mono- and divalent cations. Highly sensitive indirect-conductimetric detection and excellently simultaneous separation for these mono- and divalent cations were achieved in 12 min on the Zr-silica column (150  mm×4.6  mm i.d.) using a 5 mM tartaric acid containing 10 mM 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane) as eluent.
Keywords: Ion chromatography with conductimetric detection; Acidic eluent; Unmodified silica gel; Zirconium-modified silica gel; Cations;

Sulfate-selective electrodes based on hydrotalcites by Marco Morigi; Erika Scavetta; Mario Berrettoni; Marco Giorgetti; Domenica Tonelli (265-272).
An ion-selective electrode (ISE) for sulfate based on a dispersion of hydrotalcite particles into a polymeric membrane using coated-wire (CW) configuration is described. Membranes, based both on poly(vinyl chloride) (PVC) containing different plasticizers and poly(dimethylsiloxane) (PDMS) were studied. The best performance in terms of slope (−29.6±0.8 mV per decade) and response time (<1 min) was displayed by the PDMS based membrane whereas the linearity range (4.0×10−5 to 4.0×10−2  M) was practically the same for all the developed ISEs. The interference of hydrophobic inorganic anions was very low in comparison with the conventional ion-exchanger based electrodes, as shown by the selectivity coefficients determined by the matched potential method (MPM). The selectivity was also good towards the most common univalent and divalent anions and improved in the presence of 2-[4-(2-Hydroxyethyl)-1-piperarazinyl]ethanesulfonic acid (HEPES). The potentiometric response was practically constant in the pH range 4–7. A successful application of sulfate determination in commercial mineral waters, using direct potentiometry, is presented.
Keywords: Ion-selective electrodes; Sulfate determination; Hydrotalcites; Mineral water;

Theoretical and experimental studies of metallated phenanthroline derivatives as carriers for the optimization of the nitrate sensor by George E Andredakis; Elizabeth A Moschou; Katherine Matthaiou; George E Froudakis; Nikolas A Chaniotakis (273-280).
Experimental and theoretical investigations were carried out on a series of metallated phenanthroline derivatives in order to elucidate the role of both the metal center and the ligand employed, to fully optimize the analytical characteristics of the nitrate ISE. The organic ligands of the ionophores used are based on the phenanthroline moiety, metallated with metals in the 2+ oxidation state. A series of sensors were constructed based on these ionophores in combination with different plasticizers and ionic additives. The potentiometric data combined with the information obtained from ab initio theoretical calculations of the electron density and the electronegativity of the metals were used to optimize the overall analytical characteristics of the sensor towards the anion of nitrate. It is concluded that there is a direct coordination of the metal center with the nitrate and other basic anions, since both the electronegativity of the transition metal and the acidity of the phenanthroline derivative affect both the binding constant and the selectivity of the ionophore towards the nitrate anion. Finally, Ni(II) 5-phenyl-1,10-phenanthroline dissolved in the plasticizer tris(2-ethylhexyl) phosphate (TEHP) and adsorbed onto a porous carbon transducer produces a nitrate sensor with very good selectivity and exceptional signal stability.
Keywords: Nitrate; Electrode; Anion; Phenanthroline;

Carbon fibre microelectrodes modified with rhodium for the electrocatalytic determination of hydrazine by J.M Pingarrón; I Ortiz Hernández; A González-Cortés; P Yáñez-Sedeño (281-290).
The electroanalytical capabilities of cylindrical carbon fibre microelectrodes (CFMEs) modified with electrodeposited rhodium for their use as amperometric microsensors in the detection of hydrazine under flow-injection conditions are discussed. Metallized CFMEs were prepared potentiostatically at −0.80 V in an acetate buffer solution of pH 4.5 containing 100 mg l−1 Rh(III), and characterised by cyclic voltammetry. Rh-CFMEs showed electrocatalytic ability towards the oxidation of hydrazine and some other organic compounds. A good stability of the metallised microelectrodes was found when they were used for the amperometric detection of hydrazine under flowing conditions, with no need to regenerate the electrode surface. Moreover, the signal-to-noise ratio was remarkably better at the Rh-CFME than at a Rh-modified glassy carbon electrode of conventional size. A limit of detection of 6.2×10−7  mol l−1 hydrazine (approximately 20 μg l−1) was obtained. The effect of the presence of potential interferents was studied and the flow-injection method was applied to the determination of hydrazine in boiler water samples spiked with 0.64 mg l−1 analyte.
Keywords: Rh-metallised carbon fibre microelectrodes; Hydrazine; Flow injection; Amperometric detection;

The voltammetric behavior of Vitamin C (l-ascorbic acid) at carbon paste electrodes and electrodes modified with aza macrocycles have been studied. The use of zinc complexes, formed in situ at the electrode surface reduced the overpotential for the oxidation of ascorbic acid by about 200 mV and enhanced the peak currents. Linearity was observed over the range of 0.6–500 μg/cm3 with a detection limit of 0.1 μg/cm3 by differential pulse voltammetry. The modified electrode was used for the determination of Vitamin C in multivitamin–multimineral pharmaceutical preparations, fruit juices and wine samples.
Keywords: Vitamin C; Chemically modified electrodes; Macrocyclic compounds; Voltammetry; Electrocatalysis;

Flow injection analysis of the insecticide imidacloprid in water samples with photochemically induced fluorescence detection by J.L Vı́lchez; M.C Valencia; A Navalón; B Molinero-Morales; L.F Capitán-Vallvey (299-305).
A flow injection analysis (FIA) system, combined with photochemical induced fluorescence (PIF) detection is developed for the sensitive and rapid determination of imidacloprid. It is based on the conversion of imidacloprid into the fluorophore 1-(6-chloro-3-pyridyl-methyl)-2-(hydroxyimino)-3,4-didehydroimidalozolidene. In an aqueous medium, this compound shows native fluorescence with an excitation maximum at 334 nm and an emission maximum at 377 nm. The linear concentration range of application was 1.0–60.0 ng ml−1 of imidacloprid, with a relative standard deviation of 2.1% (for a level of 10 ng ml−1) and a detection limit of 0.3 ng ml−1. The method was applied to check whether imidacloprid was present above this limit in waters from Cuenca and Granada (Spain). It was validated applying a recovery test (Student’s t-test). Recovery levels of the method reached around 100% in all cases.
Keywords: Flow injection analysis (FIA); Photochemically induced fluorescence (PIF); Imidacloprid; Water analysis;

A new fluorescent reagent, 2-hydroxy-1-naphthaldehydene-8-aminoquinoline (HNAAQ), was synthesized. The fluorescent reaction of this reagent with molybdenum was studied. Based on this chelation, a highly sensitive spectrofluorimetric method was developed for determination of trace amounts of molybdenum in a water–ethanol (4:1 v/v) medium at pH 9.2. Under these conditions, the Mo-HNAAQ complex has excitation and emission maxima at 402 and 450 nm, respectively. The linear range of the method is from 0 to 30 μg l−1 and the 3σ detection limit is 83 ng l−1 of molybdenum. Interference of other ions was studied. It is necessary to remove the interfering cations through cation exchange. The procedure can easily be carried out and affords good precision and accuracy. This method has been successfully applied to the determination of molybdenum in pig liver and mussels.
Keywords: Molybdenum; 2-Hydroxy-1-naphthaldehydene-8-aminoquinoline; Spectrofluorimetry; Determination;

Acidity in methanol–water by Fernando Rived; Immaculada Canals; Elisabeth Bosch; Martı́ Rosés (315-333).
The different factors influencing acidity of solutes in methanol–water (acidity of the solute, basicity and dielectric constant of the solvent, and specific solute–solvent interactions) are discussed. General thermodynamic models and preferential solvation models are used to establish linear relationships between the acidity pK values of a family of compounds in any methanol–water mixture and the pK values of the compounds in water. The parameters (slope and intercept) of the relationships are related with solvent composition for the most common families of compounds (phenols, carboxylic acids, amines and pyridine derivatives). These equations allow accurate calculation of the pK values of any member of these families at any methanol–water mixture from the pK value of the compound in water, or conversely estimation of the pK value in water from pK values measured in methanol–water mixtures. Some examples of prediction, including chromatographic retention of acid–base compounds, are presented.
Keywords: Acidity; Methanol–water; Preferential solvation; pK;

AUTHOR INDEX (335-337).