Determination of nitrogen in solution by gas-phase molecular absorption spectrometry

Measurement of the molecular absorbance at 201 nm of the ammonia gas displaced from strongly alkaline solutions containing ammonium ions provides a rapid, sensitive and selective method for the determination of nitrogen. A simple apparatus based on an atomic absorption spectrometer is described. The method can be applied to the determination of the nitrogen content of soil and plant samples.     

An automatic gas-phase molecular absorption spectrometric system using a UV-LED photodiode based detector for determination of nitrite and total nitrate

An automatic gas-phase molecular absorption spectrometric (GPMAS) system was developed and applied to determine nitrite and total nitrate in water samples. The GPMAS system was coupled with a UV-light emitting diode photodiode (UV-LED-PD) based photometric detector, including a 255 nm UV-LED as the light source, a polyvinyl chloride (PVC) tube of 14 cm as the gas flow cell, and an integrated photodiode amplifier to measure the transmitted light intensity. The UV-LED-PD detector was compact, robust, simple and of low heat production, comparing with detectors used in other GPMAS works. For nitrite measurement, citric acid was used to acidify the sample, and ethanol to catalyze the quantitative formation of NO₂. The produced NO₂ was purged with air flow into the UV-LED-PD detector, and the gaseous absorbance value was measured. The total nitrate could be determined after being reduced to nitrite with a cadmium column. Limits of detection for nitrite and nitrate were 7 μmol/L and 12 μmol/L, respectively; and linear ranges of 0.021-5 mmol/L for nitrite and 0.036-4 mmol/L for nitrate were obtained. Related standard deviations were 1.81% and 1.08% for nitrite and nitrate, respectively, both at 2 mmol/L. The proposed method has been applied to determine nitrite and total nitrate in some environmental water samples.     

Flow-injection analysis-spectrophotometric determination of nitrite and nitrate in water samples by reaction with proflavin

A flow-injection manifold is proposed for determination of nitrite based on the reaction with 3,6-diamino acridine (proflavin sulfate) in hydrochloride acid medium. The assembly is adapted for nitrate determination by including a reductive column filled with copperized cadmium. The influence of foreign substances is also studied. The method gives a linear calibration graph over the range 0.06-4 mg 1(-1) nitrite, with an RSD <0.5%. The method was applied to nitrite and nitrate determinations in either waste water or coastal marine water samples.     

Development of a simple method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry

In this work, it was developed a method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry of NO produced by thermal decomposition of nitrate in a graphite furnace. The NO line at 215.360 nm was used for all analytical measurements and the signal obtained by integrated absorbance of three pixels. A volume of 20 μL of standard solution or groundwater sample was injected into graphite furnace and 5 μL of a 1% (m/v) Ca solution was co-injected as chemical modifier. The pyrolisis and vaporization temperatures established were of 150 and 1300 °C, respectively. Under these conditions, it was observed a difference of thermal stability among the two nitrogen species in the presence of hydrochloric acid co-injected. While that the nitrite signal was totally suppressed, nitrate signal remained nearly stable. This way, nitrogen can be quantified only as nitrate. The addition of hydrogen peroxide provided the oxidation of nitrite to nitrate, which allowed the total quantification of the species and nitrite obtained by difference. A volume of 5 μL of 0.3% (v/v) hydrochloric acid was co-injected for the elimination of nitrite, whereas that hydrogen peroxide in the concentration of 0.75% (v/v) was added to samples or standards for the oxidation of nitrite to nitrate. Analytical curve was established using standard solution of nitrate. The method described has limits of detection and quantification of 0.10 and 0.33 μg mL⁻¹ of nitrogen, respectively. The precision, estimated as relative standard deviation (RSD), was of 7.5 and 3.8% (n = 10) for groundwater samples containing nitrate–N concentrations of 1.9 and 15.2 μg mL⁻¹, respectively. The proposed method was applied to the analysis of 10 groundwater samples and the results were compared with those obtained by ion chromatography method. In all samples analyzed, the concentration of nitrite–N was always below of the limit of quantification of both the methods. The concentrations of nitrate–N varied from 0.58 to 15.5 μg mL⁻¹. No significant difference it was observed between the results obtained by both methods for nitrate–N, at the 95% confidence level.     

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.     

Sulphate determination after its reduction to hydrogen sulphide and volatile separation by molecular absorption spectrometry

A rapid spectrophotometric method for sulphate determination in a discontinuous mode is described. The method is based on sulphate reduction to hydrogen sulphide followed by its volatilization and absorption in an alkaline solution. The reduction is obtained when a sulphate sample is heated to 287 degrees C for 15 min, with a mixture of Fe(o)/KI and phosphoric acid. The resulting gas is swept by nitrogen flow into a 0.1 M sodium hydroxide solution and the absorbance of the sulphide ions is measured directly at 230 nm. The proposed method enabled us to determine 50-700 mug of total sulphate with a relative standard deviation of the order of 5%. The method has been applied for the determination of sulphates in liquid (mineral waters) and solid (gypsiferous soils) samples.     

Flow injection analysis of sulphite by gas-phase molecular absorption UV/VIS spectrophotometry

A flow injection gas-phase molecular absorption spectrophotometric method is described for the determination of sulphite in aqueous solution. The sulphite solution, 200 microl, is introduced into a stream of distilled water. The carrier stream containing a sulphite zone is reacted, in the first mixing coil, with a stream of sulphuric acid (1 M). The evolved sulphur dioxide is purged to the segments of nitrogen flow through the second mixing coil. The gaseous phase is separated from the liquid stream by the use of a purpose built gas-liquid separator and then is swept into a purpose built flow-through cell. The absorbance of the gaseous phase is measured at 200 nm using a UV/VIS spectrophotometer. Up to 440 microg of sulphite is determined. The limit of detection is 0.8 microg and the R.D.S. for the determination of 70 and 220 microg of sulphite are 1.02 and 0.76%, respectively. Up to 40 samples h(-1) can be analyzed. The effect of several anions and cations on the determination of sulphite was studied and the results showed that the method is relatively free from interferences. The proposed method was applied to the determination of sulphite in a synthetic sample, water sample and lemon juice.     

Application of free-radical chromogens to determination of nitrite and nitrate ions by EPR spectrometry

Various azines and substituted phenylene diamines are oxidized by the nitrite ion to give stable radicals in an autocatalytic reaction. This finding has now been developed into highly sensitive methods for nitrite determination by EPR spectrometry. Thus, when the reagent is phenothiazine, the detection limit for nitrite is 0.012 ppm, the relative standard deviation at 0.05 ppm is 1.9%, and the analytical range is 0-1.5 ppm. With N,N,N',N'-tetramethyl-p-phenylenediamine as the reagent, the corresponding values are 0.025 ppm, 2.6%, and 0-1.3 ppm. Nitrate can be determined after prior reduction to nitrite. A mixture of nitrite and nitrate ions can also be quantitatively analysed. The EPR methods were applied to the determination of the nitrite and nitrate contents of prepacked cooked ham and of soft-spreading cheese. The results agreed well with those obtained by ISO and AOAC standard methods for these samples. ca*|Author for correspondence.     

Flow injection analysis of nitrite by gas phase molecular absorption UV spectrophotometry

A flow injection method on the basis of gas phase molecular absorption is described for the determination of nitrite in the aqueous solution. 200 mul of nitrite solution is introduced into a carrier stream of distilled water. The carrier stream containing nitrite zone is reacted with a stream of hydrochloric acid (2 M). The stream is then segmented by O(2) gas. The produced gaseous products are purged into the O(2) segments, react with O(2) and are carried toward the gas-liquid separator. The gaseous phase is separated from the liquid stream by the use of home-made gas-liquid separator and then is swept into a home-made flow cell. The absorbance of gaseous phase is measured at 205 nm using a UV/VIS spectrophotometer. Under selected conditions, two linear ranges, up to 1000 mug ml(-1) and 1000-2000 mug ml(-1) of nitrite were obtained. The limit of detection was 7.5 mug ml(-1) NO(2)(-). The relative standard deviations of repeated measurements of 100 and 500 mug ml(-1) NO(2)(-) were 3.7 and 1.0%, respectively. Up to 30 samples h(-1) can be analyzed. Interferences in the proposed method were few and were readily overcome. The proposed method was successfully applied to the determination of nitrite in the spiked water samples, a number of meat products and urine.     

Controlled potential lodometric determination of nitrate after reduction to nitrite by coppered cadmium

A controlled-potential coulometric iodometric method previously developed for the accurate determination of small amounts of nitrite has been extended for the determination of nitrate after its reduction on a coppered cadmium reductor. The conditions for quantitative reduction have been investigated with respect to type of reductor and pH. Nitrate-nitrogen in the range 0.01-100 mug ml may be determined with high accuracy in less than 10 min, including the reduction step. The method has been applied with good results to a large variety of samples such as meat products, juices and waste waters.     

On the optimum conditions for the reduction of nitrate to nitrite by cadmium

The variables of direct importance in the reduction of nitrate to nitrite by a metallic reductant such as cadmium used in a reductor column are discussed with special reference to the determination of nitrate as nitrite in very dilute solutions, e.g., natural waters. As a result of these considerations the effect of flow-rate (expressed as bed-volumes min ), pH, temperature, chloride concentration and various types of reductor cadmium on the yield of nitrite is investigated. The effect of dissolved oxygen in the sample solution on pH and cadmium concentration in the reduced solution is demonstrated. At constant pH a maximum yield of nitrite is obtained at a certain flow-rate, which is explained as the result of a rapid formation and simultaneously proceeding slow reduction of nitrite. With increasing pH this maximum is shifted to lower flow-rates, and grows broader whilst the yield at maximum approaches 100%; at pH 9.5 a yield of 99.9 +/- 0.1% is obtained. The temperature has little effect on the reduction rate in the interval 20-30 degrees but at 10 degrees the reduction is noticeably slower. Chloride ions have a strongly retarding effect on the reduction rate but the yield at maximum is not affected. Electrolytically precipitated cadmium, filings of pure cadmium or amalgamated pure cadmium all give practically the same yield at maximum though some differences in reduction rate are observed. Impure cadmium or copper-cadmium and silver-cadmium, owing to the formation of galvanic cells with higher reducing power, give a high reduction rate, which also applies to nitrite, causing a poorer yield at maximum. The practical consequences of the results are thoroughly discussed.     

Simultaneous determination of sulphide and sulphite by gas-phase molecular absorption spectrometry. Comparative study of different calculation methods

A method for the simultaneous determination of sulphide and sulphite is described, which involves continuous H(2)S and SO(2) generation, preconcentration in a liquid nitrogen trap and measurement of the molecular absorption spectra of volatiles in the gas phase in 190-220 nm range. Under the recommended conditions (sample flow: 50 ml/min and concentrated sulphuric acid flow: 12 ml/min; generation time: 4 min) linear response ranges from 0.05 mug/ml for S(2-) and 0.20 mug/ml SO(2-)(3) are obtained with detection limits of 0.05 and 0.20 mug/ml respectively. Synthetic mixtures of the two components have been solved and a comparative study of different calculation methods has been made. In conclusion, multiwavelength methods offer better precision and accuracy.     

Flow-injection determination of nitrite and nitrate with biamperometric detection at two platinum wire electrodes

The flow-injection determination of nitrite is based on oxidation of iodide by nitrite. The triiodide formed is detected amperometrically in a flow-through cell with two teflonized graphite or platinum wire electrodes polarized with a voltage of 100 mV. More sensitive and faster response was observed with the platinum wire electrodes. The same detector is used for determination of nitrate after reduction to nitrite in a reductor column containing copperized cadmium. Detection limits under optimized conditions are 6 μg l^?1 for both nitrite- and nitrate-nitrogen. Effects of oxygen and interfering metal ions are discussed.     

Ultra-rapid analysis of nitrate and nitrite by capillary electrophoresis

Rapid analysis of nitrate and nitrite by capillary electrophoresis (CE) has been limited by the ions' very similar electrophoretic mobilities. With a pKa of 3.15, the mobility of nitrite can be selectively reduced using a low pH buffer in CE. A much shorter capillary can be used and separation voltages can be increased. With this method, nitrate and nitrite are separated in just over 10 s. This is roughly 20 times faster than current separation methods. Direct UV detection at 214 nm was employed and offered sub microM detection limits. Total analysis time (pre-rinse, injection, and separation) was less than 1 min, making this method ideal for high-throughput analysis.     

Automated simultaneous determination of nitrate and nitrite by pre-valve reduction of nitrate in a flow-injection system

Nitrate is reduced to nitrite by using the pre-valve in-valve reduction technique prior to the sampling system. One loop of a two-position sampling valve is replaced by a copperised cadmium column. Nitrite from the samples as well as nitrite formed in the reduction procedure is sampled by a second valve and introduced into the flow system. The two sampling valves are synchronised in such a way that two peaks are obtained, one corresponding to the nitrate plus nitrite and the other to the nitrite only. The method is suitable for the simultaneous determination of nitrate and nitrite at a sampling rate of up to 72 determinations per hour with coefficients of variation better than 1.96% for nitrate and 0.83% for nitrite.     

Flow-injection chemical vapor-generating procedure for the determination of Au by atomic absorption spectrometry

Volatile Au species in an acidified medium were generated at room temperature by reduction with NaBH₄ in acidified aqueous medium using a flow-injection chemical vapor-generation atomic absorption spectrometric (FI–CVG–AAS) system in the presence of micro amounts of sodium diethyldithiocarbamate (DDTC). Precision of 2.0% RSD (n = 11, 2.0 mg L^–1 level) was obtained at a sample throughput of 180 h^–1. A detection limit of 24 ng mL^–1 (3σ) was obtained with 300 μL sample solution. The method was used for the determination of gold in ore sample digests, and the results obtained agreed well with those obtained by flame AAS.     

Flow-injection chemical vapor-generating procedure for the determination of Au by atomic absorption spectrometry

Volatile Au species in an acidified medium were generated at room temperature by reduction with NaBH4 in acidified aqueous medium using a flow-injection chemical vapor-generation atomic absorption spectrometric (FI-CVG-AAS) system in the presence of micro amounts of sodium diethyldithiocarbamate (DDTC). Precision of 2.0% RSD (n = 11, 2.0 mg L(-1) level) was obtained at a sample throughput of 180 h(-1). A detection limit of 24 ng mL(-1) (3sigma) was obtained with 300 microL sample solution. The method was used for the determination of gold in ore sample digests, and the results obtained agreed well with those obtained by flame AAS.     

Simultaneous spectrophotometric determination of nitrite and nitrate by flow-injection analysis

An automatic direct spectrophotometric method for the simultaneous determination of nitrite and nitrate by flow-injection analysis has been developed. Nitrite reacts with 3-nitroaniline in the presence of hydrochloric acid (0.96-1.8 M HCl or pH 0.5-0.7) to form a diazonium cation, which is subsequently coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride to form a stable purple azo dye, the absorbance of which is measured at 535 nm. Nitrate is reduced on-line to nitrite in a copper-coated cadmium column which is then treated with azo dye reagent and the absorbance due to the sum of nitrite and nitrate is measured; nitrate is determined from the difference in absorbance values. A copper column incorporated into the reaction manifold before the copperised cadmium column not only improves the long-term accuracy, but also extends the life time of the copperised cadmium column. Various analytical parameters, such as effect of acidity (pH), flow rate, sample size, dispersion coefficient, time, temperature, reagent concentration and interfering species, were studied. The calibration graphs were rectilinear for 0.1-3.5 mug ml(-1) of NO(3) and 10 ng ml(-1)-2.2mug ml(-1) of NO(2). The method is successfully applied to some food samples (meat, flour and cheese), environmental waters (inland and surface), beer and soil samples. Up to 30 samples can be analysed per hour with a relative precision of approximately 0.1-2%.     

Simultaneous determination of nitrate and nitrite by flow injection analysis

An automatic method for the simultaneous determination of nitrate and nitrite by flow injection analysis is described. Nitrate is reduced to nitrite with a copperized cadmium column. Nitrite is diazotized and coupled with N-(l-naphthyl)ethylenediammonium dichloride. The merging zones approach is used to minimize reagent consumption. The injector system is arranged so that two peaks are obtained, one corresponding to nitrite and the other to nitrite plus nitrate. A sampling rate of about 90 samples per hour is possible; the precision is better than 0.5% for nitrite in the range 0.1–0.5 mg l^t and 1.5% for nitrate in the range 1.0–5.0 mg l^t