Spectroelectrochemical sensing based on attenuated total internal reflectance stripping voltammetry. 1. Determination of lead and cadmium

The electrodeposition and subsequent stripping of lead and cadmium on an indium tin oxide (ITO) optically transparent electrode (OTE) were monitored by attenuated total internal reflectance. Light passing through the ITO-OTE is attenuated proportionally to the concentration of metal ion and deposition time. The wavelength dependence of the optical responses of deposited Pb and Cd was determined; optimal performance based on maximum sensor absorbance was at 750 nm for Pb and at 400 nm for Cd. Calibration curves were obtained over a range of 5 x 10(-8) to 5 x 10(-5) M and 1 x 10(-9) to 1 x 10(-5) M for Pb and Cd, respectively, using change in absorbance that accompanied deposition and subsequent stripping of the electrodeposited metal from the ITO.     

Spectroelectrochemical sensing based on attenuated total internal reflectance stripping voltammetry. 3. Determination of cadmium and copper

The optical and electrochemical properties of metallic films on ITO surfaces resulting from deposition of copper and cadmium were monitored by stripping voltammetry-attenuated internal reflectance spectroscopy. The voltammetric or optical responses of both metals were examined with respect to solution conditions such as pH and presence of dissolved oxygen. The morphologies of these films were also examined using environmental scanning electron microscopy, and the microscopic electrodeposition patterns were found to influence the optical response. The wavelength dependence of the optical response of deposited copper was determined and compared with calculations; optimal performance was at 400 nm for copper. A linear calibration curve was obtained over a range of 1 x 10(-7)-1 x 10(-4) M for copper and compared with that of cadmium. The simultaneous determination of cadmium and copper was demonstrated, and the mechanism of co-deposition is discussed.     

Determination of soil nitrate and water content using attenuated total reflectance spectroscopy

Direct determination of nitrate and soil moisture can significantly improve N-application management and thus reduce N-derived environmental pollution related to agriculture. Several studies have shown that Fourier transform infrared attenuated total reflectance (FT-IR/ATR) spectroscopy could be used to estimate the nitrate content of standardized soil pastes. Paste standardization appeared to be the main obstacle to in situ application of this approach, and the present study shows how FT-IR/ATR can be used to estimate both water content and nitrate concentration of field soil samples. Water content and nitrate concentration are determined sequentially using two subsamples of the initial soil sample. An a priori determined amount of highly concentrated nitrate solution is added to the first subsample and the ATR spectrum of this paste is used to estimate the sample water content. It is then possible to calculate the amount of water that should be added to the second subsample so that the resulting paste is very close to the ideal standard paste. Nitrate concentration, mg [N]/kg [dry soil], is estimated using the FT-IR/ATR spectrum of this second paste. Results are presented for a laboratory experiment with four agricultural soils, as well as for a field trial with a calcareous soil. For water content, the determination errors range from 0.01 to 0.02 g [water]/g [dry soil]. For nitrate concentration, the errors for three of the soils range from 5.9 to 8.4 mg [N]/kg [dry soil], while for the fourth, calcareous clay soil, the determination error is 13.6 mg [N]/kg [dry soil]. The determination errors obtained for the field trial are similar to the ones obtained for a similar soil under laboratory conditions, which shows the potential usefulness of the approach for improving N-application management and reducing environmental pollution.     

New spectrometric methods based on attenuated total internal reflection

New methods of spectrometry of attenuated total internal reflection are considered. By means of these methods, it is possible to obtain reflection spectra and the indices of refraction and absorption of highly refracting and strongly absorbing media. __________ Translated from Izmeritel’naya Tekhnika, No. 4, pp. 27–30, April, 2007.     

Determination of trace iodine in food and biological samples by cathodic stripping voltammetry.

This paper describes a sensitive and selective method for the determination of iodine in food and biological samples. The method involves treatment of samples by combustion in an oxygen flask and determination of iodide by cathodic stripping voltammetry of the solid phase formed with the quaternary ammonium salt Zephiramine as the ionic associating agent; Br- is used as the complexing agent in the preconcentration process. We have studied the effect of concentration of Zephiramine, Br-, I-, and some other elements presented, deposition potential, preelectrolysis time, and scan rate, on the stripping curve shape and maximum stripping current. Determinations of trace iodine in table salt, laver, and eggs were demonstrated as practical examples.     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 16. Sensing by fluorescence

A fluorescence spectroelectrochemical sensor capable of detecting very low concentrations of metal complexes is described. The sensor is based on a novel spectroelectrochemical sensor that incorporates multiple internal reflection spectroscopy at an optically transparent electrode (OTE) coated with a selective film to enhance detection limits by preconcentrating the analyte at the OTE surface. Nafion was used as the selective cation exchange film for detecting Ru(bpy)(3)(2+), the model analyte, which fluoresces at 605 nm when excited with a 441.6-nm HeCd laser. The unoptimized linear dynamic range of the sensor for Ru(bpy)(3)(2+) is between 1 x 10(-)(11) and 1 x 10(-)(7) M with a calculated 2 x 10(-)(13) M detection limit. The sensor employs extremely thin films ( approximately 12 nm) without significantly sacrificing its sensitivity. The sensor response is demonstrated with varying film thicknesses. A state-of-the-art flow cell design allows variable cell volumes as low as approximately 4 microL. Fluorescence of the sample can be controlled by electromodulation between 0.7 and 1.3 V. Sensor operation is not reversible for the chosen model film (Nafion) and sample (Ru(bpy)(3)(2+)) but it can be regenerated with ethanol for multiple uses.     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 2. Demonstration of selectivity in the presence of direct interferences

Three modes of selectivity based on charge-selective partitioning, electrolysis potential, and spectral absorption wavelength were demonstrated simultaneously in a new type of spectroelectrochemical sensor. Operation and performance of the three modes of selectivity for detection of analytes in the presence of direct interferences were investigated using binary mixture systems. These binary mixtures consisted of Fe(CN)(6)(3-) and Ru(bpy)(3)(2+) and of Fe(CN)(6)(4-) and Ru(CN)(6)(4)(-) in aqueous solutions. Results on the Fe(CN)(6)(3-)/Ru(bpy)(3)(2+) binary mixture showed that an anion-exchange coating consisting of PDMDAAC-SiO(2) [where PDMDAAC is poly(dimethyldiallylammonium chloride)] and a cation-exchange coating consisting of Nafion-SiO(2) can trap and preconcentrate analytes with charge selection. At the same time, such coatings exclude interferences carrying the same type of charge as that of the exchange sites in the sensor coating. Using the Fe(CN)(6)(4-)/Ru(CN)(6)(4-) binary mixture, the Fe(CN)(6)(4-) component can be selectively detected by restricting the modulation potential cycled to a range specific to the redox-active Fe(CN)(6)(4-) component and simultaneously monitoring the optical response at the overlapping wavelength of 420 nm. It was also shown that, when the wavelength for optical monitoring was chosen as 500 nm, which is specific to the Ru(CN)(6)(4-) component, interference from the Fe(CN)(6)(4-) component for spectroelectrochemical detection of Ru(CN)(6)(4-) was significantly suppressed, even though the cyclic modulation potential encompassed the redox range for the Fe(CN)(6)(4-) component.     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 6. Sensing with a mediator

The use of a mediator to detect a nonabsorbing analyte during spectroelectrochemical modulation is demonstrated. The charge-selective composite film of Nafion-SiO2 was used to entrap the mediator, Ru(bipy)3(2+). The change in deltaA as detected by attenuated total reflection was then observed upon addition of the analyte, ascorbate. The effects of scan rate, concentration of mediator, film thickness, and analyte charge were studied to achieve optimal sensor conditions. The model sensor exhibited a linear range from 0.26 to 2.0 mM (R2 = 0.998).     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 9. Incorporation of planar waveguide technology

Incorporation of planar waveguide technology into a spectroelectrochemical sensor is described. In this sensor design, a potassium ion-exchanged BK7 glass waveguide was over-coated with a thin film of indium tin oxide (ITO) that served as an optically transparent electrode. A chemically selective film was spin-coated on top of the ITO film. The sensor supported five optical modes at 442 nm and three at 633 nm. Investigations on the impact of the ITO film on the optical properties of the waveguide and on the spectroelectrochemical performance of the sensor are reported. Sensing was based on the change in attenuation of light propagated through the waveguide resulting from an optically absorbing analyte. By applying either a triangular or square wave excitation potential waveform, electromodulation of the optical signal has been demonstrated with Fe(CN)6(3-/4-) as a model electroactive couple that partitions into a PDMDAAC-SiO2 film [where PDMDAAC = poly(dimethyldiallylammonium chloride)] and absorbs at 442 nm.     

Determination of the feasibility of using attenuated total reflectance Fourier transform–infrared spectroscopy to evaluate thermal ageing of enamel-coated magnet wire

A study was initiated to determine the feasibility of employing attenuated total reflectance Fourier transform–infrared spectroscopy (ATR/FT–IR) to detect changes resulting from thermal ageing in the enamel of copper magnet wire. Polyamideimide (SX-81002) was cured on a zinc selenide (ZnSe) internal reflection element (IRE) coated with a thin film of metallic copper. The coated IRE was inserted in a Circle cell housed in a heating jacket and maintained at 250°C on the optical bench of an infrared spectrometer to simulate thermal ageing of enamel-coated magnet wire. Evaluation of the infrared spectra in the fingerprint region suggested that the polymer experienced chemical degradation within a 23 day period of thermal ageing. Through comparisons with controls containing no copper coatings, and ageing studies carried out at 28°C, it was determined that ageing at elevated temperature caused more pronounced chemical changes in the polymer than did exposure to the copper. These results indicate that ATR/FT–IR may be a useful tool to detect enamel fatigue after a short period of thermal ageing.     

Characterization and quantitation of aprepitant drug substance polymorphs by attenuated total reflectance fourier transform infrared spectroscopy

In this study, we report the use of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR) for the identification and quantitation of two polymorphs of Aprepitant, a substance P antagonist for chemotherapy-induced emesis. Mixtures of the polymorph pair were prepared by weight and ATR-FT-IR spectra of the powdered samples were obtained over the wavelength range of 700-1500 cm(-1). Significant spectral differences between the two polymorphs at 1140 cm(-1) show that ATR-FT-IR can provide definitive identification of the polymorphs. To investigate the feasibility of ATR-FT-IR for quantitation of polymorphic forms of Aprepitant, a calibration plot was constructed with known mixtures of the two polymorphs by plotting the peak ratio of the second derivative of absorbance spectra against the weight percent of form II in the polymorphic mixture. Using this novel approach, 3 wt % of one crystal form could be detected in mixtures of the two polymorphs. The accuracy of ATR-FT-IR in determining polymorph purity of the drug substance was tested by comparing the results with those obtained by X-ray powder diffractometry (XRPD). Indeed, polymorphic purity results obtained by ATR-FT-IR were found to be in good agreement with the predictions made by XRPD and compared favorably with actual values in the known mixtures. The present study clearly demonstrates the potential of ATR-FT-IR as a quick, easy, and inexpensive alternative to XRPD for the determination of polymorphic identity and purity of solid drug substances. The technique is ideally suited for polymorph analysis, because it is precise, accurate, and requires minimal sample preparation.     

Ultramicroband array electrode. 1. Analysis of mercury in contaminated soils and flue gas exposed samples using a gold-plated iridium portable system by anodic stripping voltammetry

The applicability of a gold-plated iridium Nano-Band array ultramicroelectrode (6 microm by 0.2 microm, 64-microm interspacing, 100 electrode bands) in the analysis of mercury using a portable system is demonstrated by anodic stripping voltammetry in real-life samples. Optimized measurement parameters, 0.1 M HCl electrolyte, plating potential of 0 mV, and staircase scan mode were identified. The dynamic linear range is 10-180 ppb at 5-s deposition time with 1.5 microC of gold plated. The experimental detection limit for Hg2+ in 0.1 M HCl was 0.5 ppb at a deposition time of 4 min and a scan rate of 10 V/s. Real-life samples, such as flue gas exposed samples from flue gas simulators could be analyzed within 5 min using the method of standard additions. We identified a field-portable extraction procedure for soil samples using 1:1 concentrated HNO3/30% H2O2 mixture, compatible with ASV and the iridium electrode. The detection limit for soils is 1 ppm. The results obtained using ASV are in good agreement with reference values using cold vapor atomic absorption for the sample matrixes studied here. To our knowledge, this is the first mercury application using a reusable iridium array ultramicroelectrode. The portable potentiostat is less than 500 g, and together with the portable digestion method, makes the Nano-Band Explorer system field applicable.     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 20. Detection of metal ions in different oxidation states

Spectroelectrochemical sensing a metal in two different oxidation states, both of which are weakly absorbing in the visible wavelength range, was demonstrated with ferrous and ferric ion. The sensor consisted of an indium tin oxide optically transparent electrode (ITO OTE) coated with a thin film of Nafion preloaded with the ligand 2,2'-bipyridine (bipy). Fe2+ in the sample partitioned into the film where it reacted with bipy to form Fe(bipy)3(2+), which absorbs strongly at 520 nm. The change in absorbance (DeltaA) at 520 nm associated with the accumulation of Fe(bipy)3(2+) complex in the film was measured by attenuated total reflectance spectroscopy and was proportional to the concentration of Fe2+ in the sample. Iron in the Fe3+ form can also be determined, but it has a more complex coordination chemistry involving formation of [Fe2(bipy)4O(H2O)2]4+ as well as Fe(bipy)3(3+) in the film. Fe3+ was detected indirectly by reducing the nonabsorbing Fe3+-bipy complexes that accumulated in the film to absorbing Fe(bipy)3(2+) and monitoring DeltaA at 520 nm. The effects of film thickness and ligand concentration in the film on sensor sensitivity and response time for Fe2+ were evaluated. Detection limits of 0.6 x 10(-6) M for Fe2+ and 2 x 10(-6) M for Fe3+ were obtained with 300 nm thick films after 30 min of exposure to a quiescent sample. Careful manipulation of the potential applied with simultaneous optical detection enables Fe2+ to be distinguished from Fe3+, which is the first step in developing a sensor for speciating the two oxidation states in a mixture.     

Dynamics and performance of fast linear scan anodic stripping voltammetry of cd, pb, and cu using in situ-generated ultrathin mercury films

The dynamics of fast linear scan (LS) ASV for the simultaneous detection of Cd, Pb, and Cu was investigated at various scan rates (0.5-10 V/s) and at different metal ion concentrations (50-800 nM) utilizing ultrathin mercury films (9 nm) at a conventional size (d(0) = 1 mm) electrode. Results of the investigation show that when the thin films were utilized, diffusion of metals through the mercury film was not the rate-limiting step of the stripping process at moderate to fast scan rates (0.5-10 V/s). A fairly linear relationship between the peak height and scan rate was observed at scan rates (0.5-10 V/s) beyond the upper limit of the theoretical model for the behavior of LS-ASV. In addition, peak width at half-height (b(1/2)) as low as 33 mV was achieved at 0.5 V/s. The behavior of LS-ASV in terms of peak width at these scan rates is thus different from what the theoretical model of LS-ASV would have predicted. For the ultrathin mercury films, at least two additional factors, kinetics and concentration, have significant effects on practical LS-ASV. Experimental results show that the stripping process of Cu was primarily kinetic-controlled for fast scans, while those for Cd and Pb were dependent on both scan rates and concentrations. The ultrathin mercury film resulted in a significant enhancement of the ratio of signal-to-baseline slope (i(p)/Δi(b), a ratio used to measure the effectiveness of discrimination of the peak signal against the steep sloping baseline in LS-ASV) for Cd and Pb stripping peaks, but only a slight enhancement for Cu stripping peaks. The optimal performance of LS-ASV in terms of sensitivity, peak width, and enhancement of the i(p)/Δi(b) ratio for the three metals was achieved at 2 V/s. Because of the high reproducibility of the background currents of the stable in situ MTFs, background subtraction was carried out at 2 V/s with little hysteresis. This feature, combined with the enhancement of the i(p)/Δi(b) ratio at the fast scan rate of 2 V/s, allowed for the detection of sub-ppb levels of Cd, Pb, and Cu at a deposition time of 2 min.     

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 5. Simulation of sensor response for different excitation potential waveforms

The simulation of the optical response in spectroelectrochemical sensing has been investigated. The sensor consists of a sensing film coated on an optically transparent electrode (OTE). The mode of detection is attenuated total reflection. Only species that partition into the sensing film, undergo electrochemistry at the potentials applied to the OTE, and have changes in their absorbance at the wavelength of light propagated within the glass substrate of the OTE can be sensed. A fundamental question arises regarding the excitation potential waveforms employed to initiate the electrochemical changes observed. Historically, selection has been based solely upon the effectiveness of the waveform to quickly electrolyze any analyte observable by the optical detection method employed. In this report, additional requirements by which the waveform should be selected for use in a remote sensing configuration are discussed. The effectiveness of explicit finite difference simulation as a tool for investigating the applicability of three different excitation potential waveforms (square, triangle, sinusoid) is demonstrated. The simulated response is compared to experimental results obtained from a prototype sensing platform consisting of an indium tin oxide OTE coated with a cation-selective, sol-gel-derived Nafion composite film designed for the detection of a model analyte, tris(2,2'-bipyridyl)ruthenium(II) chloride. Using a diffusion coefficient determined from experimental data (5.8 x 10(-11) cm2 s for 5 x 10(-6) M Ru(bipy)3(2+)), the simulator program was able to accurately predict the magnitude of the absorbance change for each potential waveform (0.497 for square, 0.403 for triangular, and 0.421 for sinusoid), but underestimated the number of cycles required to approach steady state. The simulator program predicted 2 (square), 3 (triangle), and 5 cycles (sinusoid), while 5 (square), 15 (triangle), and 10 (sinusoid) cycles were observed experimentally.     

Development of a method for total inorganic arsenic analysis using anodic stripping voltammetry and a Au-coated, diamond thin-film electrode

We demonstrate that a Au-coated, boron-doped, diamond thin-film electrode provides a sensitive, reproducible, and stable response for total inorganic arsenic (As(III) and As(V)) using differential pulse anodic stripping voltammetry (DPASV). As is preconcentrated with Au on the diamond surface during the deposition step and detected oxidatively during the stripping step. Au deposition was uniform over the electrode surface with a nominal particle size of 23 +/- 5 nm and a particle density of 109 cm-2. The electrode and method were used to measure the As(III) concentration in standard and river water samples. The detection figures of merit were compared with those obtained using conventional Au-coated glassy carbon and Au foil electrodes. The method was also used to determine the As(V) concentration in standard solutions after first being chemically reduced to As(III) with Na2SO3, followed by the normal DPASV determination of As(III). Sharp and symmetric stripping peaks were generally observed for the Au-coated diamond electrode. LODs were 0.005 ppb (S/N = 3) for As(III) and 0.08 ppb (S/N = 3) for As(V) in standard solutions. An As(III) concentration of 0.6 ppb was found in local river water. The relative standard deviation of the As stripping peak current for river water was 1.5% for 10 consecutive measurements and was less than 9.1% over a 10-h period. Excellent electrode response stability was observed even in the presence of up to 5 ppm of added humic acid. In summary, the Au-coated diamond electrode exhibited better performance for total inorganic As analysis than did Au-coated glassy carbon or Au foil electrodes. Clearly, the substrate on which the Au is supported influences the detection figures of merit.     

Direct analysis of oxidizing agents in aqueous solution with attenuated total reflectance mid-infrared spectroscopy and diamond-like carbon protected waveguides

A novel approach for the direct detection of oxidizing agents in aqueous solution is presented using diamond-like carbon (DLC) protected waveguides in combination with attenuated total reflectance (ATR) mid-infrared spectroscopy. Pulsed laser deposition was applied to produce high-quality DLC thin films on ZnSe ATR crystals with thicknesses of a few 100 nm. Scanning electron microscopy and X-ray photoelectron spectroscopy has been used to investigate the surface properties of the DLC films including the sp(3)/sp(2) hybridization ratio of the carbon bonds. Beside excellent adhesion of the DLC coatings to ZnSe crystals, these films show high chemical stability against strongly oxidizing agents. IR microscopy was utilized to compare differences in the chemical surface modification of bare and protected ATR waveguides when exposed to hydrogen peroxide, peracetic acid, and peroxydisulfuric acid. The feasibility of DLC protected waveguides for real-time concentration monitoring of these oxidizing agents was demonstrated by measuring calibration sets in a concentration range of 0.2-10%. Additionally, principal component regression has been applied to analyze multicomponent mixtures of hydrogen peroxide, acetic acid, and peracetic acid in aqueous solution. Due to high chemical stability and accurate monitoring capabilities, DLC protected waveguides represent a novel approach for directly detecting oxidizing agents in aqueous solution with promising potential for industrial process analysis.