Conductometric determination of ammonium by a multisyringe flow injection system applying gas diffusion

A multisyringe flow injection system (MSFIA) coupled to a gas-diffusion cell has been developed for the conductometric determination of ammonium in different water samples. Operation strategies, membrane, reagent concentrations, and flow rates have been studied to optimize the sensitivity of detection and to fit the required working range. The proposed MSFIA system has been compared with former FIA and SIA systems using gas diffusion. The system was applied to the determination of ammonium in water samples of different matrices in order to evaluate its performance. These samples were coastal waters, pond waters, and compost aqueous extracts. Good recoveries of 102?±?13% were obtained and no significant differences with the reference methods were found. The system can be used for a wide concentration range of ammonia, from 0.075 to 360?mg?L^?1, without sample dilution and with a precision better than 2% of RSD. The throughput of the method was 32 injections per hour.     

Determination of trace anions in concentrated weak acids by ion chromatography

Ion-exclusion chromatography (ICE) followed by ion chromatography (IC) was used for the determination of trace anionic contaminants in concentrated weak acids. The ICE separation was used as a pretreatment step to isolate the contaminant anions of strong acids from the excess of matrix ions. Then a fraction containing the analyte ions was separated using IC with suppressed conductivity detection. Microbore–ion-exchange columns were chosen to address the increased purity requirements for use of these concentrated acids in semiconductor applications. The chromatographic conditions were optimized for determining trace chloride, sulfate, phosphate, and nitrate in concentrated 24.5% (v/v) hydrofluoric acid; trace chloride, sulfate, and nitrate in concentrated 85% (w/w) phosphoric acid and trace chloride and sulfate in concentrated 0.7% (v/v) glycolic acid. Method detection limits for the anions of interest were below 100 μg/l.     

Determination of monovalent inorganic anions in high-ionic-strength samples by electrostatic ion chromatography with suppressed conductometric detection

A new ion chromatographic (IC) system has been established by using micelles of 3-(N,N-dimethylmyristylammonio)propanesulfonate (Zwittergent 3-14) loaded onto a reversed-phase packed column as the separation column with an electronic rotary switching valve packed-bed suppressor for conductometric detection of inorganic anions. An aqueous H3BO3-Na2B4O7 solution has been demonstrated to be the most desirable eluent for this IC system. The relationship between retention time and the concentration of the borate eluent was determined for a series of model anionic analytes and this relationship was found to be opposite to that exhibited in a conventional anion-exchange IC system. The rapid elution and complete separation of monovalent inorganic anions were obtained by initially using a high-concentration borate solution as the eluent for a short-period, and then switching to a lower-concentration borate eluent to complete the separation. Detection limits for nitrite, bromide, nitrate, and chlorate were 0.85, 0.88, 0.95 and 4.8 microM, respectively, when a 7.0 mM Na2B4O7 eluent was used. Moreover, the ability to directly detect these monovalent anions in samples containing high concentrations of sulfate and/or chloride ions provided a major advantage of this approach.     

Determination of total nitrogen in food by flow injection analysis (FIA) with a potentiometric differential detection system

A flow injection set-up based on potentiometric detection and gas diffusion device for the determination of total nitrogen in food is described. The detection system consisted of two ammonium-sensitive electrodes placed sequentially and each alternately operating as reference electrode. Tubular electrodes without an inner reference solution were prepared with a PVC membrane composed of nonactin in Tris (2-ethylhexyl) phosphate and potassium tetrakis (4-chlorophenyl) borate to reduce the membrane resistance. The food sample digests were inserted into the system, and the ammonium present was converted into ammonia gas. The gas diffused through a gas-permeable membrane to a buffer acceptor stream with a pH that ensured transformation to the ammonium cation, which was potentiometrically detected. Good agreement between FIA results and those provided by the reference procedure was obtained, with relative deviation errors below 5%. Using the proposed system, low reagent consumption is possible, a sampling rate of about 30 samples/h was achieved, as well as a good reproducibility for consecutive injections of the same sample (variation coefficient < 2%).     

Sequential flow injection analysis of ammonium and nitrate using gas phase molecular absorption spectrometry

A flow injection method on the basis of gas phase molecular absorption is described for the sequential determination of ammonium and nitrate. Two hundred microliters of sample solution is injected into the flow line. For ammonium determination, the sample zone is directed to a line in which reacts with NaOH (13 M) and produces ammonia. But for nitrate determination, the sample zone is passed through the on-line copperized zinc (Zn/Cu) reduction column and produces ammonium ion and in the follows ammonia. The produced ammonia in both cases is purged into the stream of N₂ carrier gas. The gaseous phase is separated from the liquid phase using a gas-liquid separator and then is swept into a flow through cell, which has been positioned in the cell compartment of an UV-Vis spectrophotometer. The absorbance of the gaseous phase is measured at 194 nm. Under selected conditions for sequential analysis of ammonium and nitrate, linear relations were found between the peak heights of absorption signals and concentrations of ammonium (10-650 μg ml⁻¹) and nitrate (20-800 μg ml⁻¹). The limit of detections for ammonium and nitrate analysis were 8 and 10 μg ml⁻¹, respectively. The relative standard deviations of repeated measurements of 50 μg ml⁻¹ of ammonium and nitrate were 2.0, 2.9%, respectively. Maximum sampling rate was about 40 samples/h. The method was applied to the determination of ammonium in pharmaceutical products and the sequential determination of ammonium and nitrate in spiked water samples.     

Determination of total nitrogen in food by flow injection analysis (FIA) with a potentiometric differential detection system

A flow injection set-up based on potentiometric detection and gas diffusion device for the determination of total nitrogen in food is described. The detection system consisted of two ammonium-sensitive electrodes placed sequentially and each alternately operating as reference electrode. Tubular electrodes without an inner reference solution were prepared with a PVC membrane composed of nonactin in Tris (2-ethylhexyl) phosphate and potassium tetrakis (4-chlorophenyl) borate to reduce the membrane resistance. The food sample digests were inserted into the system, and the ammonium present was converted into ammonia gas. The gas diffused through a gas-permeable membrane to a buffer acceptor stream with a pH that ensured transformation to the ammonium cation, which was potentiometrically detected. Good agreement between FIA results and those provided by the reference procedure was obtained, with relative deviation errors below 5%. Using the proposed system, low reagent consumption is possible, a sampling rate of about 30 samples/h was achieved, as well as a good reproducibility for consecutive injections of the same sample (variation coefficient < 2%). Received: 8 October 1998 / Revised: 7 January 1999 / Accepted: 12 January 1999     

Capillary zone electrophoretic determination of ionic impurities in silicone products used for electronic applications

Ionic contamination has a very negative impact on the lifetime of semiconductors and electronic devices. The ionic purity of the polymeric materials that remain in contact with active chip surfaces is regarded as a key factor to long term reliability. Currently, ion chromatography (IC) is utilised to measure and routinely control cationic and anionic contaminants (in the μg/l range) of silicone coatings and encapsulants used for electronic applications. However, this technique is expensive, time consuming and requires intensive maintenance. Capillary zone electrophoresis (CZE) has been identified as an excellent alternative to IC for the determination of extractable ionic impurities in silicone products. Main advantages of CZE for routine industrial operations are short analysis time, high sensitivity, simpler operation than IC and reduced maintenance cost.     

Sensitivity improvement of ammonia determination based on flow-injection indophenol spectrophotometry with manganese(II) ion as a catalyst and analysis of exhaust gas of thermal power plant.

The sensitivity improvement of a flow-injection spectrophotometric method for the determination of ammonia was examined based on an indophenol blue coloration reaction with salicylate and hypochlorite in the presence of manganese(II) as a reaction promotion catalyst. The optimal conditions for achieving higher sensitivity of ammonia determination were examined using a three-line flow system. The limit of detection corresponding to a signal-to-noise ratio (S/N) of 3 was 0.005 mg l(-1) (approximately equal to 5 ppb) of NH4+. A calibration graph was linear in the range from 5 ppb to 1,000 ppb of ammonium ion. The relative standard deviations (n = 9) for 50 ppb and 100 ppb of ammonium ion were 6.4% and 2.2%, respectively. The proposed method was applied to the determination of ammonia in the exhaust gas of a thermal power plant. Prior to the FIA determination, ammonia in the exhaust gas was absorbed into a boric acid solution; the absorption solution was then analyzed by the proposed FIA.     

Determination of ammonium in Kjeldahl digests by gas-diffusion flow-injection analysis with a bulk acoustic wave-impedance sensor

A novel flow-injection analysis (FIA) system has been developed for the rapid and direct determination of ammonium in Kjeldahl digests. The method is based on diffusion of ammonia across a PTFE gas-permeable membrane from an alkaline (NaOH/EDTA) stream into a stream of diluted boric acid. The trapped ammonium in the acceptor is determined on line by a bulk acoustic wave (BAW)-impedance sensor and the signal is proportional to the ammonium concentration present in the digests. The proposed system exhibits a favorable frequency response to 5.0 x 10(-6)-4.0 x 10(-3) mol l(-1) ammonium with a detection limit of 1.0 x 10(-6) mol l(-1), and the precision was better than 1% (RSD) for 0.025-1.0 mM ammonium at a through-put of 45-50 samples h(-1). Results obtained for nitrogen determination in amino acids and for proteins determination in blood products are in good agreement with those obtained by the conventional distillation/titration method, respectively. The effects of composition of acceptor stream, cell constant of conductivity electrode, sample volume, flow rates and potential interferents on the FIA signals were discussed in detail.     

Analysis of water extracts from airborne dust samples by capillary isotachophoresis

Application of capillary isotachophoresis (CITP) for the analysis of water extracts of the dust samples collected in different periods in air-filtration devices in Prague car traffic tunnels and in Parisian metro station is presented. The extracts were analyzed in cationic mode with a leading electrolyte (LE) of 10 mM KOH, 25 mM acetic acid, pH 4.4, and a terminating electrolyte (TE) of 10 mM β-alanine, adjusted to pH 4.4 with acetic acid, and in anionic mode with LE 10 mM HCl, 20 mM histidine, pH 5.8 and TE 10 mM 2-(N-morpholino)ethanesulphonic acid, pH 3.7. Extracted amounts of UV-absorbing substances, including pollen allergens and organic pollutants, the number of the found components and concentrations of some inorganic ions (e.g. Cl⁻, K⁺, Na⁺, Ca²⁺) in the dust samples were determined. It was found that the extracted amounts of anionic components and their number were much higher than those of cationic components. Significant differences have been found in the analyses of the extracts of different origin. Much more material and more components were present in the extracts of samples from the pollen-rich period than from the pollen-free period, especially in anionic CITP mode.     

Adjusting the selectivity of inorganic anion separation by capillary electrophoresis

The separation of the principal inorganic anions (bromide, carbonate, chlorate, chloride, fluoride, nitrate, nitrite, sulfate, phosphate) has been achieved using a capillary electrophoresis system with indirect UV detection at 260 nm. Several types of cationic surfactants (quaternary ammonium, phosphonium or methonium) were tested as electroosmotic flow modifiers and added to a chromatebased buffer prepared from potassium dichromate. The influence of many physicochemical parameters such as nature and concentration of cationic surfactant, buffer pH, dichromate concentration buffer, voltage and temperature upon the migration time of an analyte anion, peak efficiency, asymmetry factor, and finally resolution has been investigated. A linear relationship between the corrected area and the anion concentration in the 2.5–50 ppm range was obtained, thus allowing the quantitative analysis of anions in mineral water. Finally, by increasing the hydrodynamic injection time, the separation of inorganic anions at a low concentration level of 50 ng/ml was achieved without any loss of resolution.     

Application of ion chromatography for the investigation of the anionic and cationic composition in high-purity water production

A suitable and sensitive ion chromatographic measuring system for determining the main components at nanogram to milligram per liter levels in water samples from the electrodeionization process is presented. A modified Dionex system offers the possibility for the determination of anions and cations in the samples at ng/L, μg/L and mg/L levels. The ng/L level of anions and cations in 20–130 mL high-purity water can be analyzed immediately after preconcentration on appropriate exchange columns. The mg/L level samples are successfully determined by use of an auto-sampler. The quantification of each ion is achieved using the suppressor technique and a conductivity detector. Samples are taken from 5 steps of the electrodeionization process and stored in pre-cleaned FEP (fluorinated ethylene propylene) at 7?°C in darkness prior to the determination of chloride, nitrate, sulfate, carbonate, sodium, ammonium, potassium, calcium and magnesium. Eluents, ultrapure water and samples for the determination of carbonate were passed through special glass containers and flushed with helium gas to avoid the effect of atmospheric carbon dioxide. Results of the investigation of the cationic and anionic composition in water samples within the electrodeionization process are presented and discussed.     

Rapid automated determination of constants of solubility product and critical micelle concentrations by the flow-rate gradient technique

Several straightforward, rapid and automated methods for the determination of the constant of solubility product of various inorganic salts and hydroxides and the critical micelle concentrations of both anionic and cationic surfactants are reported. The underlying methodology relies on the establishment of flow-rate gradients in completely continuous flow systems including a conductometric or potentiometric detector. The results obtained are quite consistent with those reported in the chemical literature.     

Systematic examination of the signal area precision of a single quadrupole enhanced low mass option (ELMO) MSQ [corrected] mass spectrometer

To examine the precision of the signal area response of an enhanced low mass option (ELMO) MSQmass spectrometer, operated in the negative electrospray ionization (ESI) mode, extended tests were performed, using flow injection analysis mass spectrometry (FIA-MS). Analytes were nitrate, nitrite, malonic acid, and D,L-mandelic acid. Composition and concentration of injected samples, application of an ASRS anion suppressor and of the cone wash unit, methanol addition to the FIA flow medium, and the voltage bias of the hexapole transfer lens were test variables. Individual test cycles comprised up to 90 injections, processed within 20 h. With a few exceptions the signal response tended to decline over time leading to a loss of more than 80% of the initial signal area in extreme cases. A hexapole radio-frequency (RF) voltage bias of -0.3 V led to an overall low detector response and to high losses of sensitivity over time. Other correlations between the insufficient signal reproducibility and FIA-MS operating conditions could not be established. The test scheme gave hints how to localize the cause of the mass spectrometer malfunction. The repetition of the test scheme after remedying the detected electronic default demonstrated that relative standard deviations less than 5% can be achieved for a sequence of 30 injections if methanol is added to the FIA flow medium and if a suppressor is used. Based on these findings a recommendation is formulated to supplement current test schemes for instrument performance verification by a detector response precision criterion.     

Determination of ammonium in wastewaters by capillary electrophoresis on a column-coupling chip with conductivity detection

Analytical potentialities of a chip-based CE in determination of ammonium in wastewaters were investigated. CZE with the electric field and/or ITP sample stacking was performed on a column-coupling (CC) chip with integrated conductivity detectors. Acetate background electrolytes (pH ～3) including 18-crown-6-ether (18-crown-6) and tartaric acid were developed to reach rapid (in 7-8 min) CZE and ITP-CZE resolutions of ammonium from other cations (sodium, potassium, calcium and magnesium) present in wastewater samples. Under preferred working conditions (suppressed hydrodynamic flow (HDF) and EOF on the column-coupling chip), both the employed methods did provide very good repeatabilities of the migration (RSD of 0.2-0.8% for the migration time) and quantitative (RSD of 0.3-4.9% for the peak area) parameters in the model and wastewater samples. Using a 900-nL sample injection volume, LOD for ammonium were obtained at 20 and 40 μg/L concentrations in CZE and ITP-CZE separations, respectively. Very good agreements of the CZE and ITP-CZE determinations of ammonium in six untreated wastewater samples (only filtration and dilution) with the results obtained by a reference spectrometric method indicate a very good accuracy of both the CE methods presented.     

Analysis of ions in polar ice core samples by use of large injection volumes in ion chromatography

In the present study a non-suppressed ion chromatography system with conductivity detection was tested in terms of sampling effects, the effects on the ion separation efficiency and analysis detection limits to find optimum conditions for the determination of chloride, nitrate, sulfate, sodium, ammonium, potassium, calcium and magnesium ions in polar ice core samples.     

Highly Efficient Electrochemical Nitrate Reduction to Ammonia in Strong Acid Conditions with Fe2M-Trinuclear-Cluster Metal–Organic Frameworks

Nitrate-containing industrial wastewater poses a serious threat to the global food security and public health safety. As compared to the traditional microbial denitrification, electrocatalytic nitrate reduction shows better sustainability with ultrahigh energy efficiency and the production of high-value ammonia (NH₃). However, nitrate-containing wastewater from most industrial processes, such as mining, metallurgy, and petrochemical engineering, is generally acidic, which contradicts the typical neutral/alkaline working conditions for both denitrifying bacteria and the state-of-the-art inorganic electrocatalysts, leading to the demand for pre-neutralization and the problematic hydrogen evaluation reaction (HER) competition and catalyst dissolution. Here, we report a series of Fe₂M (M=Fe, Co, Ni, Zn) trinuclear cluster metal–organic frameworks (MOFs) that enable the highly efficient electrocatalytic nitrate reduction to ammonium under strong acidic conditions with excellent stability. In pH=1 electrolyte, the Fe₂Co-MOF demonstrates the NH₃ yield rate of 20653.5 μg h⁻¹ mg⁻¹_site with 90.55 % NH₃-Faradaic efficiency (FE), 98.5 % NH₃-selectivity and up to 75 hr of electrocatalytic stability. Additionally, successful nitrate reduction in high-acidic conditions directly produce the ammonium sulfate as nitrogen fertilizer, avoiding the subsequent aqueous ammonia extraction and preventing the ammonia spillage loss. This series of cluster-based MOF structures provide new insights into the design principles of high-performance nitrate reduction catalysts under environmentally-relevant wastewater conditions.     

Coated-wire organic ion-selective electrodes in titrations based on ion-pair formation: Part 2. Determination of ionic surfactants

Both cationic and anionic surfactants can be determined titrimetrically using a coated-wire electrode with a PVC membrane plasticized with 2-nitrophenyl 2-ethylhexyl ether. Cationic surfactants and other organic compounds of cationic character are titrated with sodium tetraphenylborate; for the titration of anionic compounds, cetylpyridinium bromide or 1-(ethoxycarbonyl)pentadecyl-trimethylammonium bromide (Septonex) is recommended. Characteristics of titration curves together with statistical evaluation of results are described for determinations of 15 specimens, involving pure substances as well as technical samples.     

Flow-injection determination of ammonia in kjeldahl digests by gas diffusion and conductometry

A flow injection/conductometric method is proposed for determing ammonia in solutions obtained from Kjeldahl digestion. The method is based on diffusion of ammonia through a PTFE membrane from an alkaline (NaOH/EDTA) medium to a deionized water stream. The change in conductance of the deionized water stream is proportional to the ammonia concentration present in the digest. The effects of flow parameters, temperature and potential interferences are reported. Approximately 100 samples can be injected per hour; the precision is about 1%. Results for total nitrogen in vegetable tissues, animal feeds and fertilizers are in good agreement with those obtained by the usual distillation/titration method.