Search

Colloidal quantum dots offer broad tuning of semiconductor band structure via the quantum size effect. In this paper, we present a detailed investigation on the influence of the thickness of colloidal lead sulfide (PbS) nanocrystals (active layer) to the photovoltaic performance of colloidal quantum dot solar cells. The PbS nanocrystals (QDs) were synthesized in a non-coordinating solvent, 1-octadecene, using oleic acid (OA) as the ligand. It was found that the device with 50 nm of thickness of active layer showed a high Efficiency (η) of 0.667 under simulated Air Mass 1.5 Global (AM 1.5G) irradiation (100 mW/cm2) compared to the device with low thickness of active layer.

Fabrication and electrochemical characterization of a sensor for the determination of epinephrine (EP), uric acid (UA) and folic acid (FA) is described. The sensor was prepared using carbon paste electrode (CPE) modified with 3,4-dihydroxybenzaldehyde-2,4-dinitrophenylhydrazone (DDP) and carbon nanotubes (CNTs), which makes the modified electrode highly sensitive for the electrochemical detection of these compounds. Cyclic voltammetry (CV) at various scan rates was used to probe the fabrication and characterization of the modified electrode. In order to characterize these new modified electrode, the electroanalytical response was evaluated for EP performing cyclic voltammetry, differential pulse voltammetry and chronoamperometry experiments. Under the optimum pH of 7.0, the oxidation of EP occurs at a potential about 215 mV less positive than that of the unmodified CPE. Differential pulse voltammetry (DPV) of EP at the modified electrode exhibited two linear dynamic ranges with a detection limit (3σ) of 70 nM. DPV was used for simultaneous determination of EP, UA and FA at the modified electrode, and quantification of EP in some real samples by the standard addition method.

The acidity constants of TAR in different water–organic solvent binary mixtures at 25 °C have been determined spectrophotometrically. DATa ANalysis (DATAN) program was used for calculations of the acidity constants and pure spectra of all formed species and their concentration distribution diagram in applied pH interval. Results show that there are a fairly inverse relationship between acidity constants and the mole fraction of the organic solvent in the binary mixtures. Effects of solvent composition on the p K a s and pure spectrum of each component are also discussed.

A highly sensitive method to determine of indium is proposed by adsorption stripping differential pulse cathodic voltammetry (AdSDPCV) method. The complex of indium ions with xylenol orange is analyzed based on the adsorption collection onto a hanging mercury drop electrode (HMDE). After accumulation of the complex at −0.20 V vs. Ag/AgCl reference electrode, the potential is scanned in a negative direction from −0.40 to −0.75 V with the differential pulse method. Then, the reduction peak current of In(III)–XO complex is measured. The influence of chemical and instrumental variables was studied by factorial design analysis. Under optimum conditions and accumulation time of 60 s, linear dynamic range was 0.1–10 ng/ml (8.7 × 10−10 to 8.7 × 10−8 M) with a limit of detection of 0.03 ng/ml (2.6 × 10−10 M); at accumulation time of 5 min, linear dynamic range was 0.04–10 ng/ml (3.4 × 10−10 to 8.7 × 10−8 M) with a limit of detection of 0.013 ng/ml (1.1 × 10−10 M). The applicability of the method to analys...

A carbon paste electrode, modified with 2, 2′-[1,7-hepthandiylbis(nitriloethylidyne)]-bis-hydroquinone and TiO2 nanoparticles, was used for the simultaneous determination of dopamine (DA), uric acid (UA), and l-cysteine. The study was carried out by using cyclic voltammetry, chronoamperometry, and square wave voltammetry (SWV) techniques. Some kinetic parameters such as the electron transfer coefficient (α) and heterogeneous rate constant (ks) were also determined for the DA oxidation. A dynamic range of 8.0–1400 μM, with the detection limit of 8.4 × 10−7 M for DA, was obtained using SWV (pH = 7.0). The prepared electrode was successfully applied for the determination of DA, UA, and l-cysteine in real samples.

The anodic oxidation of hydroxylamine has been studied on a glassy carbon electrode by electrocatalytic effect of Alizarine red S (ARS) as a homogenous mediator in 0.1 M the phosphate buffer (pH 6), using the diagnostic techniques of cyclic voltammetry, linear sweep voltammetry and chronoamperometry. Anodic oxidation of hydroxylamine by cyclic voltammetry showed that the catalytic current of the system depends on the hydroxylamine concentration. The magnitude of the peak current for ARS increased sharply in the presence of hydroxylamine and was proportional to its concentration. Hydroxylamine was detected over a linear concentration range of 10–800 μM with low detection limits, which was 7.2 μM with linear sweep voltammetric method. The diffusion coefficient of hydroxylamine was also estimated using chronoamperometry (D = 2.16 × 10−6 cm2 s−1). The chronoamperometric studies were also used to determine the catalytic rate constant for catalytic reaction of ARS with hydroxylamine (k = 1.86 × 103 M−1 s−1). The experimental results showed that the peak current varied in the presence of different metal cations but these effects were omitted in the presence of EDTA, while almost all of the anions had no effect on the peak current of hydroxylamine. This new voltammetric method of hydroxylamine determination is very rapid, reproducible, selective and sensitive, and can be used for the real sample analysis. The importance of the technique is because of its ability to electrocatalytically determine the hydroxylamine with ARS as homogenous electrocatalyst, while it does not need to be prepared as for the modified electrodes. The independency of the system from the interferences and its ability of easily removing the effect of most cationic interferences are the important features of the technique.

Chemically modified electrodes based on zeolite-containing graphite pastes were constructed and evaluated as sensor electrodes for the voltammetric determination of trace cobalt in solution. Zeolite molecular sieves with pore sizes of 3, 4, and 5 A were all suitable for chemical deposition and subsequent voltammetric quantitation of trace Co(II). In order to cover a large range of parameters, the investigations were carried out using three zeolites (A, X, and Y), selected for their different cation-exchange capacities and ion-sieving properties. The analytical scheme, “preconcentration/voltammetric detection” for the determination of cobalt(II) species using zeolite-modified carbon paste electrodes (ZMCPE) was chosen. Compared to the corresponding unmodified electrodes, improvements in sensitivity were observed when using ZMCPEs due to analyte accumulation at the electrode’s surface by ion exchange in zeolite particles. The detection limit obtained for Co(II) was 3 ppm following 15 min of chemical deposition. A good correlation was found between the results obtained by ion exchange voltammetry and conventional ion-exchange characterization.

Cyclic voltammetry, chronoamperometry and rotating disk electrode voltammetry were used to investigate the electrochemical behavior of cysteine at glassy carbon electrode modified with quinizarine. The modified electrode showed high electrocatalytic activity toward cysteine; the current was enhanced significantly relative to the situation prevailing when a bare glassy carbon electrode was used. The electrocatalytic process was highly dependent on the pH of the supporting electrolyte. The kinetics parameters of this process were calculated, the apparent electron transfer rate constant k s and α (charge transfer coefficient between glassy carbon electrode and quinizarine) were 4.44 s −1 and 0.66, respectively. The experimental parameters were optimized and the mechanism of the catalytic process was discussed. The differential pulse voltammetry response of the modified glassy carbon electrode was linear against the concentration of cysteine in the range of 8.0 × 10 −7 to 1.0 × 10 −5 M. The limit of detection was found to be 1.6 × 10 −7 M.

A new carbon-paste electrode, based on natural zeolite (clinoptiolite) modified by the Fe2 + ion as carrier, was developed. The electrode exhibits a linear response to monohydrogen arsenate over a wide concentration range (5.0 × 10−8–1.0 × 10−3 M) with a Nernstian slope of −28.3 ± 1.0 mV per decade (r 2 = 0.9998). The limit of detection was 3.0 × 10−8 M. It has a fast response time 5–10 s and can be used for more than 2 months. Selectivity coefficients determined by the fixed interference method indicate that a good discriminating ability towards monohydrogen arsenate ion in comparison with other anions. The monohydrogen arsenate-selective electrode could be used in the pH range of 7.0–11.0. This chemically modified carbon-paste electrode was successfully used for the determination of arsenic in water and wastewater samples.

A coated-wire ion selective electrode based on zinc(II) acetylacetonate as ionophore exhibited linear response with Nernstian slope of -59.6 ± 1.0 mV/decade over a wide concentration range (1.0´10-5-1.0´10-1 mol L-1). This electrode has 5.0´10-6 mol L-1 detection limit. The proposed electrodes has a response time between 5-10 s to achieve a 95% steady potential for salicylate concentrations. The electrode was suitable for use in aqueous solutions in a wide pH range of 2.7 to 7.8. The electrode life time was tested over a period of two months to investigate its stability. No significant change in the performance of the electrode was observed during this period, but after two months, a gradual decrease ocurred in the slope. The proposed electrode showed high selectivity for salicylate over a number of common inorganic and organic anions. The electrode was successfully applied to the determination of salicylate in pharmaceutical samples. Um eletrodo na forma de fio modificado com acetilacetonato de zinco(II) como agente ionóforo, mostrou um comportamento de íon-seletivo exibindo uma resposta linear Nernstiana com uma inclinação igual a -59.6 ± 1.0 mV/década para uma ampla faixa de concentração (1.0 ´ 10-5-1.0 ´ 10-1 mol L-1). O eletrodo apresentou um limite de detecção igual a 5.0 ´ 10-6 mol L-1 e tempo de resposta entre 5 a 10 s para atingir um potencial estável de 95% em diferentes concentrações de salicilato. O eletrodo funciona adequadamente em soluções aquosas em uma faixa de pH de 2,7 a 7,8. A vida útil foi testada por dois meses a fim de investigar sua estabilidade e, durante esse período, não se observou alteração significativa. Contudo, após dois meses, a curva apresentou uma queda gradual. O eletrodo proposto apresentou uma alta seletividade ao salicilato em relação a anions inorgânicos e orgânicos comuns e foi aplicado com sucesso na determinação de salicilato em amostras de fármacos.

A highly selective poly(vinyl chloride) (PVC) membrane electrode based on an N,N'-ethylene-bis(4-methyl-salicylidineiminato) nickel(II) [Ni(EBMSI)] complex as a carrier for a thiocyanate-selective electrode is reported. The influences of the membrane composition, pH and possible interfering anions were investigated based on the response properties of the electrode. The electrode exhibited a good Nernstian slope of -58.9 +/- 0.7 mV decade(-1), over a wide pH range of 3.5 - 8.5 and a linear range of 1.0 x 10(-6) - 1.0 x 10(-1) M for thiocyanate. The detection limit of electrode was 3.1 x 10(-7) M SCN(-). The selectivity coefficients determined by a fixed interference method (FIM) indicate that a good discriminating ability towards the SCN- ion compared to other anions. The proposed sensor had a fast response time of about 5 - 15 s and could be used for at least 3 months without any considerable divergence in the potential. It was applied as an indicator electrode in the titration of thiocyanate with Ag+ and in the potentiometric determination of thiocyanate in saliva and urine samples.

A new sulfate-selective electrode based on the complex N,N'-bis(2-amino-1-oxo-phenelenyl)phenylenediamine copper(II) (CuL) as the membrane carrier was developed. The electrode exhibited a good Nernstian slope of -29.5 +/- 0.5 mV/decade and a linear range of 1.0 x 10(-7) - 1.0 x 10(-1) M for sulfate. The limit of detection was 1.0 x 10(-8) M. It has a fast response time of 10 s and can be used for more than three months. The selective coefficients were determined by the fixed interference method (FIM). The electrode could be used in the pH range 3.5 - 8.0. It was employed as an indicator electrode for direct determination of sulfate in pharmacy and cement samples.

A stable modified glassy carbon electrode with oracet blue (OB) has been fabricated and its electrochemical behavior was investigated by cyclic voltammetry. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited films were examined. The charge transfer coefficient, α , and the charge transfer rate constant, k s , for electron transfer between OB and glassy carbon electrode were calculated as 0.56 and 45.98 ± 0.78 s −1 , respectively. The modified electrode shows electrocatalytic activity toward dopamine (DA) oxidation in phosphate buffer solution (pH 8.0) with a diminution of the overpotential of about 220 mV and an increase in peak current. The kinetic parameters such as the electron transfer coefficient, α , the heterogeneous rate constant, k ′, and the standard heterogeneous rate constant, k 0 , for the oxidation of DA at the OBMGCE surface were determined using various electrochemical methods. The OBMGCE was applied to the electrocatalytic oxidation of dopamine (DA), ascorbic acid (AA) and uric acid (UA), and resolved the overlapping of the anodic peaks DA, AA and UA into three well-defined voltammetric peaks in differential pulse voltammetry (DPV). This modified electrode was quite effective not only to detect DA, AA and UA, but also in simultaneous determination of these species in a mixture. Also, the modified electrode exhibits the stable and sensitive responses for individual solutions of DA, AA and UA. The calibration curves for DA, AA and UA were linear for a wide range of concentrations of each species.

A new method is developed for the catalytic oxidation of uric acid at graphite–zeolite-modified electrode doped with iron (III) (Fe 3+ Y/ZCME). Iron (III) exchanged in zeolite Y act as catalyst to oxidize uric acid. First, the electrochemical behavior of iron (III) incorporated in the zeolite Y-modified electrode was studied. The results illustrate that diffusion controls the ferric/ferrous redox process at the Fe 3+ Y/ZCME. Then, the behavior of electrocatalytic oxidation reaction for uric acid has researched. The charge transfer coefficient ( α ) for uric acid was calculated as 0.51. The electrode was employed to study electrocatalytic oxidation of uric acid, using cyclic voltammetry, rotating disk electrode and chronoamperometry as diagnostic techniques. The rate constant for the catalytic reaction ( k ) was also determined by above methods. The diffusion coefficient of uric acid was 1.54 × 10 −6 cm 2 s −1 . A linear calibration graph is obtained over the uric acid concentration range of 0.1–10.0 mM. Finally, it has been shown that using the Fe 3+ Y/ZCME, UA can be estimated by amperometry with high sensitivity and good limit of detection by high level of stability of electrode.

A new oxalate-selective electrode based on the complex 2,2′-[1,4-butandiyle bis(nitrilo propylidine)]bis-1-naphtholato copper(II) (CuL) as the membrane carrier was developed. The electrode exhibited a good Nernstian slope of −29.2 ± 0.6 mV/decade (mean value ± standard deviation, n = 5) and a linear range of 5.0 × 10−8 to 1.0 × 10−1 M for oxalate. The limit of detection was 5.0 × 10−8 M. This electrode represents a fast response time (i.e. 10–15 s) and could be used for more than 3 months. The selectivity coefficients were determined by the fixed interference method (FIM) and could be used in the pH range of 2.0–7.0. It was employed as an indicator electrode for the determination of oxalate in water samples.

A highly selective poly(vinyl chloride) (PVC) membrane electrode based on butane -2,3-dione bis(salicylhydrazonato) zinc(II) [Zn (BDSH)] complex as carrier for thiocyanate-selective electrode is reported. The influence of membrane composition, pH and possible interfering anions were investigated on the response properties of the electrode. The sensor responds to thiocyanate in linear range from 1.0x10(-6) to 1.0x10(-1)M with a slope -56.5+/-1.1mVdecade(-1), over a wide pH range of 3.5-8.5. The limit of detection of the electrode was 7.0x10(-7)M SCN(-). Selectivity coefficients determined with fixed interference method (FIM) indicate a good discriminating ability towards SCN(-) ion in comparison to other anions. The proposed sensor has a fast response time of about 5-15s and can be used for at least 3 months without any considerable divergence in potential. It was applied as indicator electrode in titration of thiocyanate with Ag(+) and in potentiometric determination of thiocyanate in saliva and urine samples.

The oxidation of norepinephrine (NE) on a preactivated glassy carbon electrode leads to the formation of a deposited layer of about 4.2 × 10−10 mol cm−2 at the surface of the electrode. The electron transfer rate constant, ks, and charge transfer coefficient, α, for electron transfer between the electrode and immobilized NE film were calculated as 44 s−1 and 0.46, respectively. The NE-modified glassy carbon electrode exhibited good electrocatalytic properties towards ascorbic acid (AA) oxidation in phosphate buffer (pH 7.0) with an overpotential of about 475 mV lower than that of the bare electrode. The electrocatalytic response was evaluated by cyclic voltammetry, chronoamperometry, amperometry and rotating disk voltammetry. The overall number of electrons involved in the catalytic oxidation of AA and the number of electrons involved in the rate-determining step are 2 and 1, respectively. The rate constant for the catalytic oxidation of AA was evaluated by RDE voltammetry and an average value of kh was found to be 8.42 × 103 M−1 s−1. Amperometric determination of AA in stirred solution exhibits a linear range of 2.0–1300.0 μM (correlation coefficient 0.9999) and a detection limit of 0.076 μM. The precision of amperometry was found to be 1.9% for replicate determination of a 49.0 μM solution of AA (n = 6). In differential pulse voltammetric measurements, the NE-modified glassy carbon electrode can separate the AA and uric acid (UA) signals. Ascorbic acid oxidizes at more negative potential than UA. Also, the simultaneous determination of UA and AA is achieved at the NE-modified electrode.

Sol–gel electrode and coated-wire ion-selective electrode (CWISE) based on Schiff base as a neutral carrier, was successfully developed for detection of Pb(II) in aqueous solution. The sol–gel and CWISE exhibited linear response with Nernstian slope of 29.1 and 28.9 mV/decade, respectively, within the concentration ranges of 5.0 × 10 −6 to 1.0 × 10 −1 mol L −1 lead ion for sol–gel and coated. The sol–gel and CWISE has shown detection limits of 3.0 × 10 −6 and 5.0 × 10 −6 M, respectively. The electrodes exhibited good selectivities over a number of alkali, alkaline earth, transition and heavy metal ions. Those sensors yielded steady potential within 10–20 s at linear dynamic range. The electrodes are suitable for use in aqueous solutions in a wide pH range of 4.0–8.2. Applications of those electrodes for determination of lead in real samples and as indicator electrode for potentiometric titration of Pb 2+ ion using K 2 CrO 4 are reported.

A potentiometric ion-selective electrode based on new compound, as a carrier, has been successfully developed for detection of perchlorate anion in aqueous solution. Within the perchlorate ion concentration range 1.0×10−6 to 1.0 mol L−1 the electrode had a linear response with a Nernstian slope of 60.6±1.0 mV per decade . The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration plot was 8.0×10−7 mol L−1. The proposed electrode has fairly a good discriminating ability towards ClO 4 − ion in comparison to other anions. The sensor has a response time of ≤10 s and can be used for at least 2 months without substantial divergence in potential. It was successfully applied to direct determination of perchlorate in urine and water.

The electrochemical study of a tetrabromo- p -benzoquinone modified carbon paste electrode (TBQ-MCPE), as well as its efficiency for electrocatalytic oxidation of ascorbic acid, dopamine and uric acid, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. Three linear segments were found with slope values of −58.4 mV/pH, −28.1 mV/pH and 0.0 mV/pH in the pH range 2.0–7.1, pH 7.1–9.0 and pH 9.0–11.0, respectively. The apparent charge transfer rate constant, k s , and transfer coefficient, α , for electron transfer between TBQ and CPE were calculated as 3.79 ± 0.10 s −1 and 0.55, respectively. The electrode was also employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry and differential pulse voltammetry as diagnostic techniques. It has been found that the oxidation of AA at the surface of TBQ-MCPE occurs at a potential of about 430 mV less positive than that of an unmodified CPE. The diffusion coefficient of AA was also estimated using chronoamperometry. The kinetic parameters such as the electron transfer coefficient, α , and heterogeneous rate constant, k h ′ , for oxidation of AA at the TBQ-MCPE surface was determined using cyclic voltammetry. Differential pulse voltammetry (DPV) exhibits two linear dynamic ranges and a detection limit of 0.62 μM for AA. In DPV, the TBQ-MCPE could separate the oxidation peak potentials of AA, DA and UA present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This modified electrode was quite effective not only to detect AA, DA and UA, but also in simultaneous determination of each component concentration in the mixture.

The construction, performance, and applications of nitrate-selective electrode based on complex of bis(2-hydroxyacetophenone)ethylenediimine vanadyl (IV) as carrier, in plasticized poly(vinylchloride) (PVC) matrix are described. The electrode exhibited linear response with Nernstian slope of −58.5±1.0 mV per decade within the concentration range 5.0×10−6 to 1.0 M nitrate ion. The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration graph, was 1.0×10−6 M. The response time of electrode was

A highly selective poly(vinyl chloride) (PVC) membrane electrode based on 1,8-dibenzyl-1,3,6,8,10,13-hexaazacyclotetradecane–Ni(II) as a membrane carrier with unique selectivity toward thiocyanate is reported. The influence of membrane composition, pH and foreign anions were investigated. The sensor exhibits a Nernstian response for thiocyanate over a wide concentration range of 3.3×10 −6 to 0.10 M, with a slope 58.4±0.3 mV per decade. The limit of detection is 3.0×10 −6 M SCN − . The sensor has a response time of − ion in comparison to other anions. It was successfully applied to direct determination of thiocyanate in urine and saliva and it was also used as an indicator electrode in titration of thiocyanate with Ag + ions.

A new salicylate-selective electrode based on the complex of (2-[(E)-2-(4-nitrophenyl)hydrazono]-1-phenyl-2-(2-quinolyl)-1-ethanone) Cu(II) as the membrane carrier was developed. The electrode exhibited a good Nernstian slope of � 59.6 ± 1.0 mV/decade and a linear range of 1.0 · 10 � 6 to 1.0 M for salicylate. The limit of detection was 5.0 · 10 � 7 M. The electrode had a fast response time of 10 s and can be used for more than 3 months. The selective coefficients were determined by the fixed interference method and could be used in the pH range of 4.0 to 10.5. The electrode was employed as an indicator electrode for direct determination of salicylate in pharmaceutical and biological samples. � 2005 Published by Elsevier Inc.

A sol-gel electrode and a coated wire ion-selective poly(vinyl chloride) membrane, based on thiosemicarbazone as a neutral carrier, were successfully developed for the detection of Cu (II) in aqueous solutions. The sol-gel electrode and coated electrode exhibited linear response with Nernstian slopes of 29.2 and 28.1 mV per decade respectively, within the copper ion concentration ranges 1.0 x 10(-5) - 1.0 x 10(-1) M and 6.0 x 10(-6) - 1.0 x 10(-1) M for coated and sol-gel sensors. The coated and sol-gel electrodes show detection limits of 3.0 x 10(-6) and 6.0 x 10(-6) M respectively. The electrodes exhibited good selectivities for a number of alkali, alkaline earth, transition and heavy metal ions. The proposed electrodes have response times ranging from 10-50 s to achieve a 95% steady potential for Cu2+ concentration. The electrodes are suitable for use in aqueous solutions over a wide pH range (4-7.5). Applications of these electrodes for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu2+ ion using EDTA, are reported. The lifetimes of the electrodes were tested over a period of six months to investigate their stability. No significant change in the performance of the sol-gel electrode was observed over this period, but after two months the coated wire copper-selective electrode exhibited a gradual decrease in the slope. The selectivity of the sol-gel electrode was found to be better than that of the coated wire copper-selective electrode. Based on these results, a novel sol-gel copper-selective electrode is proposed for the determination of copper, and applied to real sample assays.

A new salicylate-selective electrode based on the complex N,N'-1,4-butylene bis(3-methyl salicylidene iminato) copper(II) as the membrane carrier was developed. The electrode exhibits a good Nernstian slope of 59.1 1.0 mV/decade and a linear range of -1.0 M for salicylate. The limit of detection was M. It has a fast response time 10 s and can be used for more than three months. The selective coefficients were determined by the fixed interference method (FIM) and could be used in the pH range 4.5-10.5. It was employed as an indicator electrode for direct determination of salicylate in pharmaceutical samples.

A new solvent polymeric membrane electrode, based on N,N'-bis(salicylidene)-2,2-dimethylpropane-1,3diamine Ni(II) as the ionophore, was designed. The oxalate-selective electrode has the dynamic range between 1.0 × 10 −6 M and 1.0 × 10 −1 M with a Nernstian slope of −28.7 ± 1.0 mV per decade. The detection limit was 6.3 × 10 −7 M. The proposed electrode revealed good selectivities for oxalate over a variety of other anions and could be used in a pH range of 2.0-7.8. The electrode can be used for at least two months without any considerable divergence in potential. The designed electrode was applied as an indicator electrode in the potentiometric determination of oxalate in real samples.