Simultaneous determination of bromide, nitrite and nitrate ions in seawater by capillary zone electrophoresis using artificial seawater as the carrier solution

We describe capillary zone electrophoresis (CZE) for the simultaneous determination of bromide, nitrite and nitrate ions in seawater. Artificial seawater was adopted as the carrier solution to eliminate the interference of high concentrations of salts in seawater. The artificial seawater was free from bromide ion to enable the determination of bromide ion in a sample solution. For the purpose of reversing the electroosmotic flow (EOF), 3 mM cetyltrimethylammonium chloride (CTAC) was added to the carrier solution. A 100 microm ID (inside diameter) capillary was used to extend the optical path length. The limits of detection (LODs) for bromide, nitrite, and nitrate ions were 0.46, 0.072, and 0.042 mg/L (as nitrogen), respectively. The LODs were obtained at a signal to noise ratio (S/N) of 3. The values of the relative standard deviation (RSD) of peak area for these ions were 1.1, 1.5, and 0.97%. The RSDs of migration time for these ions were 0.61, 0.69, and 0.66%. Artificial seawater samples containing various concentrations of bromide, nitrite, and nitrate ions were analyzed by the method. The error was less than +/-12% even if the concentration ratio of bromide ion to nitrite or nitrate ion was 20-240. The proposed method was applied to the determination of bromide, nitrite, and nitrate ions in seawater samples taken from the surface and the seabed. These ions in other environmental waters such as river water and rainwater samples were also determined by ion chromatography (IC) as well as this method.     

Improvement of capillary zone electrophoresis sensitivity with artificial seawater as the background electrolyte utilizing transient isotachophoresis for the determination of nitrate and nitrate ions in seawater

We describe capillary zone electrophoresis (CZE) with transient isotachophoresis (ITP) for the determination of low concentrations of nitrite and nitrate ions in seawater. Bromide-free artificial seawater was adopted as background electrolyte (BGE) to eliminate the interference of high concentrations of salts in seawater. To reverse the electroosmotic flow (EOF), 3 mM cetyltrimethylammonium chloride (CTAC) was added to the BGE. High concentrations of chlorate were added to sample solutions as the terminating ion to generate the ITP process before the CZE separation. In general, the stacking effect increased with increasing amounts of chlorate injected into the capillary. The limits of detection (LODs) for nitrite and nitrate were 0.063 and 0.033 mg/L when the chlorate concentration was 600 and 200 mM, respectively; these were half of those obtained by CZE without the transient ITP. The LODs were obtained at a signal to noise ratio (S/N) of 3. The relative standard deviations (RSD, n = 10) of the peak areas for these ions were 3.2 and 2.9%. The RSDs of peak heights for these ions were 1.6 and 2.1%. The RSDs of migration times for these ions were 0.67 and 0.46%.     

Two-dimensional nano-liquid chromatography-mass spectrometry system for applications in proteomics

Transient isotachophoresis-capillary zone electrophoresis with artificial seawater as the background electrolyte (BGE) was improved to further lower the limit of detection (LOD) for determination of nitrite and nitrate in seawater. By lowering the pH of BGE, the difference between effective mobility of nitrite and that of nitrate increased, thereby permitting increased sample volumes to be tolerated and their LOD values to decrease. Artificial seawater with pH adjusted to 3.0 using phosphate buffer was adopted as the BGE. To reverse electroosmotic flow (EOF), a capillary was flushed with 0.1 mM dilauryldimethylammonium bromide for 3 min before the capillary was filled with the BGE. Limits of detection (LODs) for nitrite and nitrate were 2.7 and 3.0 microg/l (as nitrogen), respectively. The LODs were obtained at a signal-to-noise ratio of 3. Values of the relative standard deviation (RSD) of peak area for these ions were 2.0 and 0.75%, respectively, when nitrite concentration was 0.05 mg/l and that of nitrate was 0.5 mg/l. The RSDs of peak height were 4.4 and 2.3%. The RSD values of migration time for these ions were 0.19 and 0.17%. The proposed method was applied to determination of nitrite and nitrate in a proposed certified reference material for nutrients in seawater, MOOS-1, distributed by the National Research Council of Canada. Results agreed with the assigned tolerance interval. This method was also applied to determination of these ions in seawater collected around Osaka Bay. Results nearly agreed with those obtained by a conventional spectrophotometric method.     

Optimum conditions for effective use of the terminating ion in transient isotachophoresis for capillary zone electrophoretic determination of nitrite and nitrate in seawater,with artificial seawater as background electrolyte

We have examined transient isotachophoresis (ITP) conditions, e.g. the nature of the terminating ion, its concentration, and the injection procedure, to improve the limit of detection (LOD) for determination of nitrite and nitrate in seawater by capillary zone electrophoresis (CZE). Artificial seawater containing 3.0 mmol L(-1) cetyltrimethylammonium chloride (CTAC) was used as background electrolyte (BGE). After sample injection 600 mmol L(-1) acetate was separately injected into the capillary as the terminating ion for transient ITP. The LOD for nitrite and nitrate, obtained at a signal-to-noise ratio (S/N) of 3, were 15 and 7.0 microg L(-1) (as nitrogen), respectively. Relative standard deviations (RSD) of peak area for nitrite and nitrate were 7.3 and 0.8%, respectively, and the RSD of peak height were 5.7 and 1.2%, respectively, when the concentrations of nitrite and nitrate were 0.05 and 0.25 mg L(-1). The RSD of migration time for these ions was 0.2%. The proposed method was applied to the determination of nitrite and nitrate in seawater samples. The results for nitrite were nearly in agreement with those obtained by naphthylethylenediamine spectrophotometric analysis (SPA; correlation coefficient 0.9041).     

Capillary zone electrophoretic determination of iodide in seawater using transient isotachophoresis with artificial seawater as the background electrolyte

We describe an application of capillary zone electrophoresis (CZE) with transient isotachophoresis (ITP) as the on-line concentration procedure for the determination of iodide in seawater. The effective mobility of iodide was decreased by the addition of 10 mM cetyltrimethylammonium chloride (CTAC) to an artificial seawater background electrolyte (BGE) so that transient ITP functioned and iodide was separated from other coexisting anions such as bromide, nitrite, and nitrate in seawater samples. After sample injection, 600 mM acetate was separately injected into the capillary as the terminating ion to generate transient ITP. The limit of detection (LOD) for iodide was 3.0 microg/L. The LOD was obtained at a signal-to-noise ratio (S/N) of 3. The values of the relative standard deviation (RSD) of peak area, peak height, and migration time for iodide were 2.9, 2.1, and 0.6%. The proposed method was applied to the determination of iodide in seawater collected around the Osaka Bay. The results obtained by use of the calibration graph were agreed with those obtained by the addition of the standard solutions for iodide.     

Determination of phosphate in seawater by CZE with on-line transient ITP

We developed CZE with indirect UV detection for the determination of phosphate in seawater using transient ITP as an on-line concentration procedure. The following optimum conditions were established: BGE, 5 mM 2,6-pyridinedicarboxylic acid (PDC) containing 0.01% hydroxypropylmethylcellulose (HPMC) adjusted to pH 3.5; detection wavelength, 200 nm; vacuum injection period of sample, 3 s (45 nL); terminating ion solution, 500 mM MES adjusted to pH 4.0; vacuum injection period of the terminating ion solution, 30 s (450 nL); applied voltage, 30 kV with the sample inlet side as the cathode. The LOD for phosphate was 16 microg/L (PO(3-)(4) -P) at S/N of 3. The respective values of the RSD of the peak area, peak height, and migration time for phosphate were 2.6, 2.3, and 0.34%. The proposed method was applied to the determination of phosphate in a seawater certified reference material for nutrients, MOOS-1, distributed by the National Research Council of Canada (NRC). The results were very similar to certified values. The method was also applied to the determination of phosphate in coastal seawaters. The results agreed with those obtained using a conventional spectrophotometric method.     

Onboard determination of submicromolar nitrate in seawater by anion-exchange chromatography with lithium chloride eluent.

Ion-exchange chromatography using a high-capacity anion exchanger with UV detection was applied to the determination of nitrate in seawater. Major ions in seawater samples did not affect the peak shape and the retention time of the nitrate when an alkaline metal cation-chloride solution was used as an eluent at high concentrations (0.5-2 mol/l). At a wavelength of 220 nm, the peak of bromide was very small because of low absorption, while its separation from the nitrate peak was good at high concentrations. Among the eluents tested, lithium chloride gave the best separation of nitrate from bromide. It was estimated that the lithium ion had the least potential for ion-pair formation with nitrate, and its retention time was prolonged compared with the retention times when using other cations; with bromide and nitrite, such an effect was not observed. The results of shipboard seawater nitrate determination by our method in the South Pacific Ocean and Antarctic Sea showed good agreement with those by the conventional photometric method using continuous flow.     

Determination of trace inorganic anions in seawater samples by ion chromatography using silica columns modified with cetyltrimethylammonium ion

Conventional silica columns dynamically modified with cetyltrimethylammonium ions were evaluated for the determination of UV-absorbing bromide, nitrate, and nitrite in seawater samples. Cetyltrimethylammonium, which is a quaternary ammonium ion, was dynamically introduced onto silica surfaces. The first layer of the modifier was introduced by electrostatic interaction, whereas the second layer was introduced by hydrophobic interaction. The latter layer worked as the anion-exchange sites. The modified conventional silica columns could be used for separation of inorganic anions. Separation of authentic mixture of five anions was achieved within 17 min. The addition of 0.1 mM cetyltrimethylammonium ion to the eluent improved the repeatability of the retention time. Seawater samples could be directly injected onto the prepared conventional silica columns, and bromide, nitrate, and nitrite levels were determined to be 69, 0.13, and 0.016 ppm, respectively.     

Ion chromatography for determination of nitrite and nitrate in seawater using monolithic ODS columns

A fast and highly sensitive ion chromatographic method using monolithic ODS columns was developed for the determination of nitrite (NO2-) and nitrate (NO3-) in seawater. Two monolithic ODS columns (50 mm x 4.6 mm i.d. + 100 mm x 4.6 mm i.d.) connected in series were coated and equilibrated with 5 mM cetyltrimethylammonium chloride (CTAC) aqueous solution. The column efficiency with 0.5 M NaCl as the mobile phase did not decrease in spite of the increase in flow rate of the mobile phase. Thus, good chromatograms were obtained within 3 minutes for NO2- and NO3 in artificial seawater without interferences by coexisting ions. The detection limit (S/N = 3) with UV detection at 225 nm was 0.8 and 1.6 microg/L for NO2- and NO3-, respectively. The characteristics of the monolithic CTA(+)-coated ODS columns were discussed. The present method was successfully applied to the fast and sensitive determination of NO2- and NO3- in real seawater samples.     

Determination of magnesium and calcium ions in seawater by capillary zone electrophoresis

Capillary zone electrophoresis is proposed for the determination of magnesium and calcium ions in seawater. A carrier solution containing EDTA was adopted for the complexation of these ions and the effect of sodium chloride concentration in the sample solutions on the results was examined. It was found that magnesium and calcium ions could be determined without any pretreatment by injecting 100-fold diluted seawater samples. Linear calibration graphs were obtained for standard solutions containing up to 10.0 mg/l of calcium ion when both peak area and peak height were used. On the other hand, a linear calibration graph was obtained for standard solutions containing up to 20.0 mg/l of magnesium ion when the peak area was used, while a curved one was obtained when the peak height was used. Relative standard deviations were 0.8 and 1.2% when a standard solution containing 5.0 mg/l of magnesium and 8.0 mg/l of calcium ions was analysed 8 times using the peak area. Limits of detection for magnesium and calcium ions were 0.13 and 0.26 mg/l, respectively. The proposed method was applied to the determination of magnesium and calcium ions in surface and bottom seawater samples.     

Chemometric approach to the optimisation of LC-FL and GC-MS methods for the determination of nitrite and nitrate in some biological,food and environmental samples

Gas chromatography–mass spectrometry (GC-MS) method and a liquid chromatography–fluorescence (LC-FL) detection method using experimental design and optimisation approach were improved for the quantitative determination of nitrite and nitrate in biological, food and environmental samples. The obtained recoveries of nitrite and nitrate ions from samples based on both GC-MS and LC-FL results ranged from 98.5% to 98.9% for nitrite and 97.9% to 98.4% for nitrate. The precision of these methods, as indicated by the relative standard deviations (RSDs), was within the range from 2.4% to 3.6% for nitrite and 2.5% to 3.8% for nitrate, respectively. The limits of detection of nitrite and nitrate ions from samples based on GC-MS and LC-FL results ranged from 0.01 to 0.14 ng L^?1 for nitrite and 0.02 to 0.71 ng L^?1 for nitrate, respectively. The optimised isolation procedure by central composite design was successfully applied to real samples. The results revealed that the proposed procedure combined with GC-MS and LC-FL techniques is more sensitive, reliable and selective compared to the other methods available for the precise determination of trace levels of nitrite and nitrate in biological, food and environmental samples.     

Simultaneous determination of nitrite and nitrate in human plasma by on-capillary preconcentration with field-amplified sample stacking

A simple method for the determination of nitrite and nitrate in human plasma has been developed using CZE with minimal sample preparation. Field‐amplified sample stacking (FASS) was used to achieve submicromolar detection by dilution of the plasma sample with deionized water. In CZE, the separation of nitrite and nitrate was achieved within 10 min without adding EOF modifier. The optimal condition was achieved with 50 mM phosphate buffer at pH 9.3. The ninefold diluted plasma samples were injected hydrodynamically for 40 s into a 60 cm×75 μm id uncoated fused‐silica capillary. The separation voltage was 20 kV (negative potential) and UV detection was performed at 214 nm. The linearity curves for nitrite and nitrate were obtained by the standard addition method. The estimated LODs for nitrite and nitrate in ninefold diluted plasma sample were 0.05 and 0.07 μM, respectively. The LODs for nitrite and nitrate in original plasma samples were 0.45 and 0.63 μM. The intra‐ and inter‐day precisions for both analytes were <2.6% and the recovery ranged between 92.3 and 113.3%. It was found that nitrite was more stable than nitrate in the plasma after the sample preparation. This proposed method was applied to a number of human plasma samples and the measured nitrite and nitrate concentrations in human plasma were consistent with the literature ranges.     

Determination of low level nitrite and nitrate in biological, food and environmental samples by gas chromatography-mass spectrometry and liquid chromatography with fluorescence detection

Gas chromatography-mass spectrometry (GC-MS) and liquid chromatography with fluorescence detection (LC-FL) methods have been proposed for the determination of low level nitrite and nitrate in biological, food and environmental samples. The methods include derivatization of aqueous nitrite with 2,3-diaminonaphthalene (DAN), enzymatic reduction of nitrate to nitrite, extraction with toluene and chromatographic analyses of highly fluorescent 2,3-naphthotriazole (NAT) derivative of nitrite by using GC-MS in selected-ion-monitoring (SIM) mode and LC-FL. Nitrite and nitrate ions in solid samples were extracted with 0.5 M aqueous NaOH by sonication. The recoveries of nitrite and nitrate ions based on GC-MS and LC-FL results were 98.40% and 98.10% and the precision of these methods, as indicated by the relative standard deviations (RSDs) were 1.00% for nitrite and 1.20% for nitrate, respectively. The limits of detection of the GC-MS in SIM mode and LC-FL methods based on S/N = 3 were 0.02 and 0.29 pg/ml for nitrite and 0.03 and 0.30 pg/ml for nitrate, respectively.     

A new method of analysis of peroxydisulfate using ion chromatography and its application to the simultaneous determination of peroxydisulfate and other common inorganic ions in a peroxydisulfate matrix

A new method for the determination of peroxydisulfate using ion chromatography has been developed. Elution of peroxydisulfate was effected by isocratic elution using 200 mM NaOH at 40°C. A modification of the method using gradient elution was able to simultaneously determine other common inorganic ions (nitrate, nitrite, sulfate and chloride) down to significantly low concentrations in a peroxydisulfate matrix. The relative standard deviations (RSD) were in the range of 0.5-5%, for peak areas and <0.2% for peak retention times. The recoveries were between 95% and 120% for a concentration range of about 0.5-42 ppm. The limit of detection for peroxydisulfate ion was 0.2 ppm and for the other ions were ≤2×10(-2) ppm. The calibration curves were linear with slope and intercepts close to 1 and 0, respectively.     

Dermal nitrate: an old marker of firearm discharge revisited with capillary electrophoresis

The present work describes a capillary electrophoretic method for nitrite and nitrate determination to be used as a screening tool for investigating the residues of firearm discharge. The use of capillary electrophoresis allowed the rapid determination of nitrite and nitrate, which are major inorganic components of gunshot residues, offering a quantitative and selective alternative to the traditional paraffin test (dermal nitrate test). The method is simpler, cheaper, and faster than the modern approaches to gunshot residue analysis based on the determination of barium, lead and antimony by using flameless absorption spectrometry, inductively coupled plasma-mass spectrometry (ICP-MS), or scanning electron microscopy. The analysis was carried out in a bare fused-silica capillary (75 microm inner diameter) with a 100 mM borate buffer (pH 9.24). The detection was by UV absorption at 214 nm. Separation took place under reversed voltage of 15 kV. Bromide was used as the internal standard. Sensitivity was about 1 mM for both nitrite and nitrate. Reproducibility (intraday and day-to-day) was also good with relative standard deviations (RSDs) < 1.0% for relative migration times and < 4.5% for peak areas in both standard solutions and real matrix. Hair and skin samples from a victim shot in the head were successfully analyzed for the presence of nitrite and nitrate.     

Determination of nitrate in seawater by capillary zone electrophoresis with chloride-induced sample self-stacking

A capillary zone electrophoresis (CZE) method was established to determine low concentration nitrate which was online preconcentrated with chloride-induced leading-type sample self-stacking for seawater samples. The sample self-stacking was based on transient isotachophoresis in which chloride served as leading ion, and dihydrogenphosphate in the background electrolyte (0.1 M phosphate) as the terminating one. Due to the small mobility difference between nitrate and chloride, the isotachophoresis time was so long that nitrate could not separate from the rear sharp boundary between chloride and the background electrolyte (BGE) when it migrated to the detection window. A zwitterionic surfactant, 3-(N,N-dimethyldodecylammonio)propane sulfonate was added to the BGE to enlarge the mobility difference for its selective interaction with anions. Thus, a highly conductive sample could be injected in a large volume with about fourfold sensitivity enhancement compared to that of field amplification sample stacking in which nitrate was dissolved in pure water. The relative standard deviations (n=5) of migration time, peak area, peak height were 0.1, 3.0, 1.5%, respectively. The limit of detection (S/N=3) for nitrate was 35 microg/l in seawater samples with relatively low concentration BGE (0.1 M sodium phosphate, pH 6.2). The overall procedure consisting of online preconcentration and separation was as simple as routine CZE except for a slightly longer sample injection time (3-4 min).     

Rapid and direct determination of iodide in seawater by electrostatic ion chromatography

An electrostatic ion chromatographic (IC) method for rapid and direct determination of iodide in seawater is reported. Separation was achieved using a reversed-phase ODS packed column (250x4.6 mm I.D.) modified by coating with Zwittergent-3-14 micelles, with an eluent comprising an aqueous solution containing 0.2 mM NaClO4 and 0.3 mM Zwittergent-3-14 and using UV detection at 210 nm. Samples prepared by dissolving NaIO3, NaNO2, NaBr, NaBrO3, NaNO3, NaI, and NaSCN in artificial or real seawaters were analyzed using this IC system. Nitrite, iodate, bromide, bromate, and nitrate showed very little or no retention, while iodide and thiocyanate were well separated, being eluted within 6 and 16 min, respectively. The detection limit for iodide obtained by injecting 400 microL of sample was 0.011 microM (S/N = 3), and the precision values obtained by analyzing samples containing 0.1 or 0.3 microM iodide in real seawater samples were 2.3% RSD and 1.2% RSD, respectively. Direct determination of iodide in real seawater samples was possible using this proposed IC system.     

Simultaneous determination of iodide and iodate in seawater by transient isotachophoresis-capillary zone electrophoresis with artificial seawater as the background electrolyte

We developed capillary zone electrophoresis with transient isotachophoresis (ITP) as an on-line concentration procedure for simultaneous determination of iodide and iodate in seawater. The effective mobility of iodide was decreased by addition of 20 mM cetyltrimethylammonium chloride to an artificial seawater background electrolyte so that transient ITP functioned for both iodide and iodate. Limits of detection for iodide and iodate were 4.0 and 5.0 microg/l (as iodine) at a signal-to-noise ratio of 3. Values of the relative standard deviation of peak area, peak height, and migration times for iodide and iodate were 2.9, 1.3, 1.0 and 2.3, 2.1, 1.0%, respectively. The proposed method was applied to simultaneous determination of iodide and iodate in seawater collected at a pond at our university.     

凝固漂浮有机液滴-分散液液微萃取-高效液相色谱-串联质谱法测定海水中苯并三唑类紫外线过滤剂

建立了海水中7种苯并三唑类紫外线过滤剂的凝固漂浮有机液滴-分散液液微萃取-高效液相色谱-串联质谱分析方法。优化后的萃取实验条件：20 μL十二醇为萃取溶剂,400 μL甲醇为分散溶剂,NaCl质量分数为8%,pH小于6,涡旋振荡时间2 min。目标化合物经Hypersil GOLD色谱柱（150 mm×2.1 mm,5 μm）结合甲醇-水梯度洗脱分离后,用正离子多反应监测模式进行质谱分析。在较宽的线性范围内,7种化合物的线性相关系数（r²）均大于0.99;基质加标回收率为68.3%~127.5%,相对标准偏差为0.9%~15.2%,方法的检出限为0.001~0.090 μg/L,定量限为0.003~0.300 μg/L。将建立的方法用于大连8个海滨浴场海水中苯并三唑类紫外线过滤剂的测定,部分浴场海水有不同程度的检出。该方法简便、快速、环境友好、灵敏度高,可用于海水中苯并三唑类紫外线过滤剂的分析检测。     

Quantification of nitrite and nitrate in seawater by triethyloxonium tetrafluoroborate derivatization-headspace SPME GC-MS

Triethyloxonium tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography-mass spectrometry (GC-MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO₂). Isotopically enriched nitrite [¹⁵N] and nitrate [¹⁵N] are employed as internal standards and for quantification purposes. HS-GC-MS provided instrumental detection limits of 0.07 μM NO₂⁻ and 2 μM NO₃⁻. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.