Showing papers in "Analyst in 1999"

Determination of total lipid using non-chlorinated solvents

Abstract: The restrictions on the use of chlorinated solvents under the Montreal Protocol makes it necessary to develop an alternative method to the Bligh and Dyer lipid extraction as currently applied to marine tissues. Several different solvent mixtures were systematically tested as a replacement for chloroform. The presence of a polar solvent is a prerequisite in order to obtain phase separation between the aqueous and organic phases, but too high a concentration of solvent in the aqueous phase prevents the more polar lipids from being extracted. A high content of water in the organic phase can result in co-extraction of non-lipids. Several combinations of solvents may be able to extract lipids, but for reasons of safety and toxicity, a propan-2-ol–cyclohexane–water (8 + 10 + 11 v/v/v) mixture has been proposed. The method is not sensitive to a wide range of sample-phase volume ratios provided that the solvent compositions remain constant. Application to plaice, mussel and herring samples showed results that were in agreement with the extraction following Bligh and Dyer using chloroform and methanol.

Theory of analyte extraction by selected porous polymer SPME fibres

Abstract: Extraction of analytes by the new porous polymer solid phase microextraction (SPME) fibres is based on adsorption rather than absorption. The equilibrium theory developed for the liquid poly(dimethylsiloxane) (PDMS) coating does not apply to these coatings. The paper presents theoretical description of the extraction process for adsorption-type fibres, including PDMS–DVB (divinyl benzene), Carbowax–DVB and Carbowax–TR (template resin). The model is based on Langmuir adsorption isotherm. Expressions describing the amount of analyte extracted by the fibre in two- and three-phase systems are presented and discussed. The effect of selected experimental variables is discussed. In general, there is a non-linear dependence between the amount of an analyte extracted by the fibre and its concentration in a sample. The dependence can be approximated by a straight line for low concentrations only. Matrix composition can significantly affect the amount extracted. Interferences co-extracted with the analyte of interest may reduce the amount extracted and the quasi-linear range of the response. Great care should be exercised therefore when performing quantitative analysis with porous polymer SPME fibres. The phenomena discussed are illustrated on an example of benzene and 4-methyl-2-pentanone extraction from water by PDMS–DVB and Carbowax–DVB fibres.

Dipicolinic acid (DPA) assay revisited and appraised for spore detection

Abstract: Delayed gate fluorescence detection of dipicolinic acid (DPA), a universal and specific component of bacterial spores, has been appraised for use in a rapid analytical method for the detection of low concentrations of bacterial spores. DPA was assayed by fluorimetric detection of its chelates with lanthanide metals. The influence of the choice and concentration of lanthanide and buffer ions on the fluorescence assay was studied as well as the effects of pH and temperature. The optimal system quantified the fluorescence of terbium monodipicolinate in a solution of 10 µM terbium chloride buffered with 1 M sodium acetate, pH 5.6 and had a detection limit of 2 nM DPA. This assay allowed the first real-time monitoring of the germination of bacterial spores by continuously quantifying exuded DPA. A detection limit of 104Bacillus subtilis spores ml–1 was reached, representing a substantial improvement over previous rapid tests.

Optimization of the solubilization, extraction and determination of inorganic arsenic [As(III) + As(V)] in seafood products by acid digestion, solvent extraction and hydride generation atomic absorption spectrometry

Abstract: A method for the selective quantitative determination of inorganic arsenic [As(III) + As(V)] in seafood was developed. In order to do so, various procedures for the solubilization and extraction of inorganic arsenic quoted in the literature were tested. None provided satisfactory recoveries for As(III) and As(V) in real samples. Consequently, a methodology was developed which included solubilization with HCl and subsequent extraction with chloroform. The arsenic was solubilized in 9 mol l–1 hydrochloric acid. After reduction by hydrobromic acid and hydrazine sulfate, the inorganic arsenic was extracted into chloroform, back-extracted into 1 mol l–1 HCl, dry-ashed, and quantified by hydride generation-atomic absorption spectrometry (HG-AAS). The analytical features of the method are as follows: detection limit, 3.07 ng g–1 As (fresh mass); precision (RSD), 4.0%; recovery, As(III) 99%, As(V) 96%. In the optimized conditions, other arsenic species—dimethylarsinic acid (DMA), arsenobetaine (AB), arsenocholine (AC) and tetramethylarsonium-ion (TMA+)—were not co-extracted. However, different percentages of minor species were extracted with chloroform: monomethylarsonic acid (MMA) 100%, and trimethylarsine oxide (TMAO) 3–10%. Real samples and reference materials of seafood (DORM-1, DORM-2, TORT-2, CRM-278 and SRM-1566a) were analyzed. The analysis of DORM-1 provided an inorganic arsenic value of 124 ± 4 ng g–1 As, dry mass (dm), which is very close to the value obtained by other authors using high performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) and ionic chromatography-hydride generation-atomic absorption spectrometry (IC-HG-AAS).

Interaction of a novel red-region fluorescent probe, Nile Blue, with DNA and its application to nucleic acids assay

Abstract: A novel fluorimetric method was developed for the rapid determination of DNA and RNA based on their quenching effect on the cationic red-region fluorescent dye Nile Blue (NB). In the investigation of the interaction of NB with DNA by steady-state polarization measurements, thermal denaturing study, determination of absorption and fluorescence characteristics, salt effect study and electrophoresis experiments, the results supported the suggestion that NB served as an intercalator to the stack base pairs of nucleic acids. Further evidence showed that the quenching could be ascribed to the static quenching mode. A binding constant of about 106 M–1 and a binding site size of about three base pairs were obtained by spectral methods. Under optimum conditions, the calibration curves for the determination of calf thymus DNA (CT DNA) and yeast RNA were linear over the ranges 3.0 ng mL–1–2.0 µg mL–1 and 27 ng mL–1–10 µg mL–1, respectively. The detection limits were 3.0 ng mL–1 for CT DNA and 27 ng mL–1 for RNA. The relative standard deviation (n = 6) was within 2.1% in the middle of the linear range. Interferences from some interesting co-existing substances in the determination of DNA were also examined.

Conductimetric sensor for atrazine detection based on molecularly imprinted polymer membranes

Abstract: Atrazine-sensitive conductimetric sensors were designed using molecularly imprinted polymer membranes. Membranes containing artificial recognition sites for atrazine were prepared by copolymerization of methacrylic acid and a cross-linker, tri(ethylene glycol) dimethacrylate, in the presence of atrazine as a template. In order to improve the flexibility and mechanical stability of the membranes, oligourethane acrylate was added to the mixture of monomers. The recognition sites complementary to atrazine were formed in the membranes after extraction of the template molecules with ethanol. Alternatively, reference polymer membranes were prepared with the same monomers but without the template. The responses of the membranes prepared with and without the template were monitored conductimetrically. The membranes prepared in the presence of atrazine showed significantly stronger responses to atrazine than to analogous compounds (triazine, simazine, prometryn). The response time was 6–15 min depending on the membrane thickness. The effect of the membrane composition and the porogen concentration on the magnitude of the conductimetric responses was also investigated. With the sensor designed here, the detection of the atrazine at concentrations down to 5 nmol dm–3 was demonstrated. During a 6 month period, the sensitivity of the molecularly imprinted membranes to atrazine was found to remain constant.

Screen-printed amperometric biosensors for the rapid measurement of L- and D-amino acids

Abstract: Screen-printed three-electrode amperometric sensors incorporating L- and/or D-amino acid oxidase for the general purpose measurement of L- or D-amino acids is described. The working electrode incorporates rhodinised carbon, to facilitate hydrogen peroxide oxidation at a decreased operating potential, and immobilised enzyme. The devices responded to all 20 common L-amino acids and all of the D-amino acids examined, the exceptions being L- and D-proline. Linear response profiles were observed for L-leucine, L-glycine and L-phenylalanine with limits of detection of 0.47, 0.15 and 0.20 mM respectively. The devices were reproducible and exhibited stability over a 56 d test period. The biosensor compares favourably with a standard photometric amino acid test and was used to monitor milk ageing effects. The assay is cheap, simple to perform and rapid, requiring only buffer–electrolyte and a small sample volume.

Analysis of tricyclic antidepressants using electrogenerated chemiluminescence

Abstract: A novel method has been investigated for the selective and sensitive determination of a range of tricyclic antidepressants including amitriptyline, doxepin, nortriptyline, promazine, chlorpromazine, imipramine, clomipramine, desipramine, protriptyline and trimipramine using electrogenerated chemiluminescence (ECL). The ECL mechanism is based on the reaction between tris(2,2′-bipyridyl)ruthenium(II) [Ru(bpy)3 2+] and the tertiary amino groups on the antidepressants. After selecting the best operating parameters calibration curves were obtained over three orders of magnitude for amitriptyline, doxepin, nortriptyline, promazine and chlorpromazine. Linear calibrations were used to obtain limits of detection in the range 0.09–0.24 µg ml–1 with relative standard deviations below 4% for five replicate samples. Rapid depression in the signal was observed with repeat analysis of imipramine, clomipramine, protriptyline, desipramine and trimipramine due to electrode fouling by the oxidation product of the reaction. Use of a lower concentration of the compound was found to alleviate the problem. Finally the concentration of doxepin was determined in a pharmaceutical preparation.

Determination of Cu, Fe, Mn, and Zn in blood fractions by SEC-HPLC-ICP-AES coupling

Abstract: The binding of Cu, Fe, Mn, and Zn to proteins in blood and in blood fractions was investigated, since their interactions in free radical metabolism in humans is of great interest. An HPLC-ICP-AES technique was developed allowing adequate separation of metalloproteins and of inorganic and organic metal species. For the separation of metalloproteins in erythrocytes and blood plasma a Merck Superformance Fractogel EMD BioSEC 650 (S) column was used. Size exclusion chromatography (SEC)-HPLC was hyphenated to ICP-AES both on-line and off-line for the detection of trace elements in the fractions resulting from HPLC separations. HPLC parameters, pH, temperature, flow rate and salt concentration were optimized for the protein separation and the optimal conditions were applied for the hyphenation to the ICP-AES detector. The separation column was calibrated with five standard proteins. For the element determination by ICP-AES a line selection with respect to the sensitivity was performed. Three different methods were used for the determination of trace elements in blood: direct determinations, on-line and off-line SEC-HPLC-ICP-AES measurements. For the optimizing experiments blood samples of one female subject were used. The direct determination by ICP-AES of the elements was performed in blood and blood fractions of ten different subjects to obtain the average concentration ranges. From the results the identification of the protein Cu/Zn superoxide dismutase in erythrocytes was possible. The LOD were 0.03 µg mL–1 for Cu, 0.026 µg mL–1 for Fe, 0.8 ng mL–1 for Mn, and 0.09 µg mL–1 for Zn in a synthetic blood matrix.

Novel calibration with correction for drift and non-linear response for continuous flow isotope ratio mass spectrometry applied to the determination of δ15N, total nitrogen, δ13C and total carbon in biological material†

Abstract: With automated analysis of a batch of samples using an elemental analyser isotope ratio mass spectrometer, the instrumental response is often non-linear (e.g., the isotope ratio varies with sample size) and is affected by changes over time (drift). Traditionally, drift and non-linearity effects are compensated for by including several reference samples in the batch with elemental masses, which closely match those of the samples. The novel calibration method presented here corrects for both drift and non-linearity, and thus allows for a wider range of sample masses in the batch, while accuracy and precision is maintained at the level attained by the traditionally accepted calibration procedure without increasing the number of reference samples.

Measurement uncertainty: Approaches to the evaluation of uncertainties associated with recovery†

Abstract: A number of approaches for evaluating recovery and its contribution to uncertainty budgets for analytical methods are considered in detail. The recovery, R, for a particular sample is considered as comprising three elements, m, Rs and Rrep. These relate to the recovery for the method; the effect of sample matrix and/or analyte concentration on recovery; and how well the behaviour of spiked samples represents that of test samples. The uncertainty associated with R, u(R), will have contributions from u(m), u(Rs) and u(Rrep). The evaluation of these components depends on the method scope and the availability, or otherwise, of representative certified reference materials. Procedures for evaluating these parameters are considered and illustrated with worked examples. Techniques discussed include the use of certified reference materials and spiking studies, and the use of extraction profiling to predict recoveries. All the approaches discussed evaluate the recovery and its uncertainty for the analytical method as a whole. It is concluded that this is a useful approach as it reduces the amount of experimental work required. In addition, most of the required data are frequently available from method validation studies.

Electrofunctional polymers: their role in the development of new analytical systems

Abstract: The overall analytical process involves a number of steps: sample collection, transportation and storage, analysis, data collection, processing and evaluation. Most of these essential steps commonly involve the use of polymeric materials in one form or another. Inert polymers are the usual materials of choice for sample collection, transportation and storage, and are indirectly involved in the form of engineering polymers in the other steps.

Elemental content of Vietnamese ricePart 1. Sampling, analysis and comparison with previous studies

Abstract: Various rice samples were collected in Vietnam from the Red River region, and some other parts of the country as well as from Yanco, NSW, Australia. The rice samples were analysed for 14 elements (P, K, Mg, Ca, Mn, Zn, Fe, Cu, Al, Na, Ni, As, Mo, and Cd) by ICP-AES, ICP-MS and FAAS. The analytical procedures were carefully controlled and validated using the rice flour certified material (NIES No10). In general, the results showed that Vietnamese rice has elemental concentration ranges similar to those of the Australian and other overseas rice. However, analyses for some elements are characteristic of some Vietnamese and Australian rice samples. Thus, Vietnamese rice shows somewhat higher levels of P, K and Mg than rice samples from other countries; Mn could be a marker for the Australian rice grown in the Yanco region, and the analysed Australian samples were also very low in Cd (<0.003 mg kg–1). An unusual distribution of Ni, present in higher concentrations in the white rice samples than in the brown ones, was observed for some Vietnamese rice samples.

A simple solvent selection method for accelerated solvent extraction of additives from polymers

Abstract: A simple solvent selection procedure for accelerated solvent extraction (ASE®) of polymers is described using Hildebrand solubility parameters. A series of extractions with a solvent with a solubility parameter several Hildebrand units (MPa1/2 ) different from the polymer are carried out at increasing temperatures until maximum extraction is reached. A second solvent with a solubility parameter close to that of the polymer is then added incrementally at the optimum temperature found from the initial experiments, until a maximum in the extraction rate is reached. This method is used to determine optimum conditions for ASE® of additives from ground polypropylene (PP), poly(vinyl chloride) (PVC) and nylon. Complete extractions were possible of Irganox 1010 (pentaerythrityl tetrakis (3,5-di-tert-butyl-4-hydroxyphenyl) propionate) from freeze ground PP in 18 min (propan-2-ol–cyclohexane (97.5∶2.5) at 140 °C), and dioctyl phthalate (DOP) from PVC in 13 min (hexane–ethyl acetate (60∶40) at 170 °C). ASE® extraction from PVC pellets was possible in reasonable time (1 h) without grinding or cutting the polymer. The results were not significantly different from those obtained by conventional solvent extraction methods. From nylon, 96% extraction of the dimer in 19 min (hexane–ethanol (60∶40) at 170 °C) was possible, compared to the total amount extracted by exhaustive ASE®. The behaviour of poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) was observed with different solvents at high temperature to determine likely optimum extraction conditions. These were ethyl acetate at 190 °C for PET and hexane–ethyl acetate (70∶30) at 150 °C for PMMA.

Determination of 17 trace elements in coal and ash reference materials by ICP-MS applied to milligram sample sizes

Abstract: This paper describes the evaluation of two digestion methods used to extract 17 elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb, Ba and Pb) from coal and coal ash, obtained as standard reference materials. An acid digestion method in open vessels using sulfuric, hydrofluoric, perchloric and nitric acid was compared with a sealed microwave digestion method using nitric acid only. The microwave method cannot break down silicates, which harbour many trace elements, but can extract As and Se quantitatively. Volatile elements such as As and Se might be lost during the open vessel digestion; therefore, the closed vessel is the method of choice. The effect of reducing the sample size from several hundred milligrams to amounts as small as 5 mg on the accuracy of determinations was investigated. No significant differences were observed so long as the total dissolved solids and the dilution factors of the final solutions remained constant.

Sono-cathodic stripping voltammetry of manganese at a polished boron-doped diamond electrode: application to the determination of manganese in instant tea

Abstract: Ultrasonically assisted cathodic stripping voltammetry at a boron-doped diamond electrode was developed for the detection of manganese. Differential-pulse voltammetry was used to give the analytical signal from a cathodic strip of electrodeposited MnO2; linearity was observed from 10–11 M to at least 3 × 10–7 M, with 10–1l M being the detection limit for a 2 min deposition. The procedure involves both ultrasonic–anodic deposition of MnO2 and ultrasonic–cathodic stripping. This novel analytical tool is robust, reproducible, mercury free, oxygen insensitive and highly specific towards manganese. The differential-pulse sono-cathodic stripping voltammetric technique was used to determine successfully the manganese content of two instant tea samples, giving excellent agreement with independent AAS analyses.

Studies of organic residues from ancient Egyptian mummies using high temperature-gas chromatography-mass spectrometry and sequential thermal desorption-gas chromatography-mass spectrometry and pyrolysis-gas chromatography-mass spectrometry

Abstract: The techniques of gas chromatography-mass spectrometry (GC-MS) and sequential thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) have been utilised to characterise the constituents of tissue-derived or applied organic material from two Pharaonic Egyptian mummies with a view to identifying embalming practices/substances. The results obtained using TD-GC-MS revealed a series of monocarboxylic acids with the C16:0, C18:1 and C18:0 components dominating in both mummies. The thermal desorption products related to cholesterol, i.e., cholesta-3,5,7-triene and cholesta-3,5-diene (only in Khnum Nakht), were detected in both mummies. Khnum Nakht also contained a number of straight chain alkyl amides (C16–C18) and an alkyl nitrile (C18). Other products included the 2,5-diketopiperazine derivative (DKP) of proline–glycine (pro–gly) which was a major component (7.9%) in Khnum Nakht but only a very minor component in Horemkenesi. Py-GC-MS of samples of both specimens yielded a series of alkene/alkane doublets (Horemkenesi C6–C18, Khnum Nakht C6-C24) which dominated their chromatograms. Series of methyl ketones in the C9–C19 chain length range were also present, with C5–C7 cyclic ketones occurring in Horemkenesi only. These ketones are indicative of covalent bond cleavage, probably of polymerised acyl lipids. Nitrogenous products included nitriles (C9–C18) which were significant in both samples, and amides which were only detected in Khnum Nakht. Also present amongst the pyrolysis products were three steroidal hydrocarbons, cholest-(?)-ene, cholesta-3,5,7-triene and cholesta-3,5-diene. High temperature-GC-MS of trimethylsilylated lipid extracts yielded similar monocarboxylic acids to that obtained using TD-GC-MS, while a series of α,ω-dicarboxylic acids and a number of mono- and di-hydroxy carboxylic acids not seen in the thermal desorption or pyrolysis GC-MS analyses were significant constituents in both mummy samples. Overall, the use of GC-MS and sequential TD-GC-MS and Py-GC-MS has demonstrated in both mummies the presence of a complex suite of lipids and proteinaceous components whose compositions indicates extensive alteration via oxidative and hydrolytic processes during long-term interment. None of the classical embalming resins was detected but an exogenous origin for at least a proportion of these components cannot be discounted since fats, oils and gelatin have been proposed as embalming agents in mummification. The combined approach of sequential TD- and Py-GC-MS has potential for application to the characterisation of embalming materials in mummies. Most importantly these techniques virtually eliminate any destruction of the mummified bodies thereby allowing the scope of investigations of ancient Egyptian funerary practices to be significantly extended.

Determination of total mercury in hydrocarbons and natural gas condensate by atomic fluorescence spectrometry

Abstract: A new and simple technique for the determination of total mercury in gas condensate was developed which eliminates the use of chemicals/additives and complicated digestion procedures. The determinations are carried out by vaporisation of the samples at 400 °C with adsorption of mercury species on a gold trap (Amasil) maintained at 200 °C. The trap is then heated at 900 °C to release metallic mercury, which is determined by atomic fluorescence spectrometry. The mercury recoveries from seven species, dimethylmercury (DMM), diethylmercury (DEM), diphenylmercury (DPM), methylmercury chloride (MMC), ethylmercury chloride (EMC), phenylmercury chloride (PMC) and mercury(II) chloride (MC) spiked individually into gas condensate were found to be in the range 80–100%. The mercury recoveries for mixtures of the seven species added in equal amounts to gas condensate were in the range 88–97%. For Conostan mercury standard added to the condensate, the recovery was 88%. The instrumental precision from 10 measurements of a toluene control was 4% RSD. For three mercury species. DEM, MC and EMC, added to condensate, the precision was between 2 and 5% RSD (n = 10). The limit of detection (3ςn–1 criterion) for the procedure was calculated to be 180 pg Hg in toluene and 270 pg in condensate. For three mercury species added to a condensate sample, the absolute detection limits were 270 pg Hg for DEM, 450 pg Hg for MC and 630 pg Hg for EMC. Total mercury measurements in five real condensate samples from two sites at different stages of production covered the range 7–50 ng ml–1 with uncertainties in the range 4–7% RSD. The total mercury concentration of two commercial heavy gas oil samples were found to be 22.2 ± 0.6 µg ml–1 with RSD 3% (n = 4) and 2.3 ± 0.1 µg ml–1 of mercury with RSD 3% (n = 7).

Amberlite XAD-2 functionalized with chromotropic acid: synthesis of a new polymer matrix and its applications in metal ion enrichment for their determination by flame atomic absorption spectrometry

Abstract: Chromotropic acid has been covalently linked with the benzene ring of Amberlite XAD-2, through an –NN– group, to design a new polymer matrix. Elemental analyses, thermogravimetric analyses and infrared spectra were used to characterize the resulting functionalized Amberlite XAD-2. It was used for the separation and preconcentration of CdII, CoII, CuII, NiII, FeIII and ZnII prior to their determination by flame atomic absorption spectrometry. The optimum pH values for quantitative sorption are 5.0–6.0, 4.0–4.5, 4.5–5.5, 4.0–5.0, 5.0–6.5 and 5.5–7.0 for CdII, CoII, CuII, FeIlI, NiII and ZnII, respectively. All these metal ions can be desorbed with 2 mol l–1 HCl or HNO3 (recovery, 96–100%). The sorption capacity of the resin is 9.35, 3.84, 8.50, 3.24, 6.07 and 9.65 mg g–1 of resin for Cd, Co, Cu, Ni, Fe and Zn, respectively. Tolerance limits for electrolytes NaCl, NaBr, NaNO3, Na2SO4 and Na3PO4 on the sorption of these metal ions are reported. NaNO3, Na2SO4 and Na3PO4 are tolerable up to a concentration level of a few millimoles in the sorption of 0.1 mg l–1 Cd. Similarly, NaNO3 with Cu (0.1 mg l–1) and Na3PO4 with Fe (0.1 mg l–1) interfere when their concentration exceeds a few millimoles. The enrichment of all of these metal ions is quantitative in the presence of 0.1 mol l–1 of MgII and CaII. The preconcentration factors are found to be 100, 150, 100, 120, 200 and 200 for Cd, Co, Cu, Fe, Ni and Zn (concentration level, 5–10 µg l–1), respectively, and their respective t1/2 values are 2.6, 4.3, 2.9, 5.8, 3.4 and 2.4 min. The simultaneous determination of Cd, Co, Cu, Fe, Ni and Zn is possible. The method was applied to the determination of these six metal ions in river water samples (RSD < 7.8%). Cobalt contents of pharmaceutical samples (vitamin tablets) were preconcentrated by the present chelating resin and estimated by flame atomic absorption spectrometry, with RSD < 1.1%.

Complexation study of diclofenac with β-cyclodextrin and spectrofluorimetric determination

Abstract: The inclusional complexation between the anti-inflammatory pharmaceutical diclofenac and β-cyclodextrin (β-CD) was studied by potentiometry, spectrophotometry and spectrofluorimetry, in both acid and neutral pH. Guest–host 1∶1 stoichiometries for the complexes in both media were determined, and their equilibrium constants were calculated. The values obtained from the different methods used are in very good agreement and are in the order of 103. From the analysis of the pKa value for diclofenac in both the absence and presence of β-CD (4.84 and 4.90 respectively), it was inferred that in the inclusion complex the carboxylic group is located outside the cavity. Further structural characterization of the inclusate was carried out by means of 1H NMR spectra and AM1 semiempirical calculations. Based on the obtained results, a spectrofluorimetric method for the determination of diclofenac in the presence of β-CD was developed in the range of 0–5 µg ml–1. Better limits of detection (0.03µg ml–1) and quantification (0.1 µg ml–1) were obtained in this latter case with respect to those obtained in the absence of β-CD. The method was satisfactorily applied to the quantification of diclofenac in pharmaceutical preparations.

Spectrofluorimetric determination of piroxicam in the presence and absence of β-cyclodextrin

Abstract: The complexation between β-cyclodextrin (β-CD) and piroxicam (PX) was investigated by both fluorescence and absorption spectrometry. A 1:2 guest:host stoichiometry for the complex was established, and its association constant was calculated by applying a non-linear regression method to the changes brought about by the presence of β-CD in both the fluorescence and absorbance spectra of PX. During the study of the influence of the pH on the fluorescence emission of the complex, an efficient enhancement of the signals at acidic pH was observed. This suggests that the protonated form of PX is included more effectively than the ionized form in the β-CD cavity. Based on the results obtained, spectrofluorimetric methods for the determination of PX were developed. The best limits of detection and quantification were obtained using β-CD at an acidic pH. The dynamic range in this latter case was 0.02–1 µg ml–1. The method was applied satisfactorily to the determination of piroxicam in a pharmaceutical preparation.

A study of L-cysteine adsorption on gold via electrochemical desorption and copper(II) ion complexation

Abstract: The adsorption of L-cysteine on gold electrodes was studied by the methods of electrochemical oxidative desorption and Cu2+ complexation. Comparisons with L-cystine adsorption, via the oxidative method, revealed lower saturation surface coverages for L-cystine (1.7 ± 0.17 nmol cm–2) than for L-cysteine (2.7 ± 0.11 nmol cm–2), perhaps due to an incomplete cleavage of the S–S bond on adsorption and consequent effects on molecular orientation. Electrochemistry of solution-phase redox probes at L-cysteine monolayer electrodes have shown a porous open monolayer structure amenable to solvent, electrolyte and probe molecule permeation. Both the L-cysteine- and the L-cystine-derived electrodes were capable of complexation of Cu2+ ions from solution which were then detectable by cyclic voltammetry. The binding constant of Cu2+ ions to the L-cysteine monolayer was estimated to be 2 × 105 L mol–1 while the molar ratios of L-cysteine to Cu2+ on L-cysteine- and L-cystine-derived films were 2∶1 and 2.5∶1, respectively. Detection of Cu2+ down to 10–7 M was possible with cyclic voltammetry, indicating the possible use of such simple films for trace metal analysis.

Anodic stripping voltammetry of copper at insonated glassy carbon-based electrodes: application to the determination of copper in beer

Abstract: The suitability of ultrasound-assisted anodic stripping voltammetry (sono-ASV) for the detection of total copper content in beer using both mercury thin film and glassy carbon electrodes has been investigated. An immersion horn probe is introduced into a small thermostatted conventional three electrode cell (20 cm3) opposite the working electrode: an ex situ mercury plated Nafion®-coated mercury film electrode or a bare glassy carbon electrode. Minimal sample pre-treatment is required which consists of acidification of the beer with dilute nitric acid and out-gassing with argon. After the deposition of copper (as the metal or its amalgam) on the electrode in the presence of ultrasound, a square wave scan is employed to get the analytical signal. In the absence of ultrasound, electrode passivation by organic species and lower rates of mass transport prevent the observation of any measurable signals. In situ cavitational cleaning of the electrode by insonation maintains the electrode activity. Total copper content levels in the range of 100 to 300 µg Cu L–1 were determined by sono-ASV using both electrode substrates and showed excellent agreement with values provided by an independent method. This highlights the validity of the sono-ASV method as a useful electroanalytical technique in hostile media.

Determination of ammonia and aliphatic amines in environmental aqueous samples utilizing pre-column derivatization to their phenylthioureas and high performance liquid chromatography

Abstract: Pre-column conversion of ammonia and a number of aliphatic amines into phenylthiourea or its derivatives by reaction with phenyl isothiocyanate, followed by HPLC, has been used for their determination in environmental waters. Optimum conversion was found when the reaction was carried out in sodium hydrogencarbonate–carbonate medium at 40°C for 15 min. Well separated peaks were obtained on a C18 column with an acetonitrile–water gradient (1 ml min–1) of 30% acetonitrile for an initial 5 min which was increased linearly to 100% over 15 min and then maintained isocratic for 5 min, the acetonitrile ratio finally being returned to 30% in 5 min. The derivatized analytes were subjected to off-line solid phase extraction on C18 sorbent. A linear calibration graph was obtained for 0.01–10 mg l–1 analytes with a correlation coefficient of 0.9954 for ammonia and in the range 0.9982–0.9996 for amines. The limit of detection for ammonia was 0.2 µg l–1 and for amines in the range 0.3–0.6 µg l–1. The method was applied to tap, underground, river and aquarium waters, the recovery being in the range 97–106% (RSD 1.8–4.5%). Many of the samples were found to contain more than the permissible limit of ammonia. Phenyl isothiocyanate is stable for long periods in aqueous medium over wide ranges of pH and temperature, and the resulting phenylthioureas have adequate retention on C18 sorbent and strong UV absorption, making this reagent suitable for the determination of amines in water.

Demonstration of isoelectric focusing on an etched quartz chip with UV absorption imaging detection

Abstract: The feasibility of isoelectric focusing (IEF) performed on-chip was demonstrated for the first time via absorption imaging detection. Microchannels on a quartz chip were fabricated using photolithography and a chemical etching process. The separation channels were 40 mm long, 100 µm wide and 10 µm deep, and were coated with linear polyacrylamide to reduce electroosmotic flow. A quartz chip cartridge for IEF was assembled in which two pieces of hollow fiber were glued to the two ends of the separation channel to isolate the electrolytes from the samples. Low molecular mass pI markers and myoglobin were selected as model samples which were mixed with 4% carrier ampholyte solution. Samples were injected into the channel via the connection capillary by pressure. A voltage of 3 kV was applied to perform IEF. The IEF current decreased from about 13.4 to 1.3 µA. The focused zones were monitored in real time by absorption imaging detection at 280 nm. The detection limit was about 0.3 µg ml–1 or 24 pg for pI marker 6.6, and 30 µg ml–1 or 2.4 ng for myoglobin with an optical pathlength of 10 µm. Good reproducibility and resolution were obtained for linear polyacrylamide coated channels. The total analysis time was less than 10 min. This imaged chip IEF provides a fast separation technique with quantitative ability and the potential for increasing throughput.

Guidelines for in-house validation of analytical methods for pesticide residues in food and animal feeds†‡

Abstract: Criteria are presented by which analytical methods may be judged to have been validated for the determination of pesticide residues. All stages of analysis are addressed, from initial preparation of samples to the production of results, but with a focus on simplicity and cost-effectiveness of the requirements. Criteria are provided for both quantitative and qualitative (screening) methods and they may be applied to single- or multi-residue methods.

Preconcentration of ultratrace rare earth elements in seawater with 8-hydroxyquinoline immobilized polyacrylonitrile hollow fiber membrane for determination by inductively coupled plasma mass spectrometry

Abstract: 8-Hydroxyquinoline immobilized on polyacrylonitrile hollow fiber membrane was synthesized and used for the preconcentration of rare earth elements (REEs) and the separation of the matrix in seawater prior to their determination by inductively coupled plasma mass spectrometry (ICP-MS). Both the capacity and chemical stability of the modified polyacrylonitrile hollow fiber membrane have been evaluated. The optimum experimental conditions, such as pH, sample flow rate, volume of eluents, were investigated. The concentration of REEs in seawater and the separation of the matrix components can be achieved with a concentration factor of at least 300 times. The recommended method has been applied for the concentration and determination of REEs in coastal seawater. Results indicated that the recovery of REEs ranged from 91% to 107%, and the relative standard deviations were found to be less than 5%. A comparison was made between the REE levels determined by this method and the literature reports, and a reasonable agreement was obtained.