The determination of total dissolved free primary amines in seawater: Critical factors, optimized procedure and artefact correction

The paper reviews the methods described in the literature for the determination of total dissolved free primary amines (TDFPA) by fluorescence. A wide set of reaction conditions can be found, but they rely on few experiments for their validation. Among fluorogenic compounds, o-phthaldialdehyde (OPA) is more sensitive than fluorescamine and was thus examined here. However, the use of mercaptoethanol (ME) in the reaction (as an additional derivatization compound) is able to generate unreliable results, in particular when standardization relies on glycine. We suggest replacing ME with 3-mercaptopropionic acid (MPA) which induces more stable and comparable fluorescence among amine compounds. A systematic study was therefore undertaken to define reagent concentrations and pH effects on the reaction rates for a variety of primary amines with particular focus on amino acids. The reaction rate is increased by increasing OPA concentration and pH, but slowed by excess MPA. Ammonium interference is influenced by several factors, but spectral investigation showed that the choice of conditions can drastically reduce it. The magnitudes of natural and OPA-induced background fluorescence signals have been assessed in various mediums and it is shown that their contribution to the signal amounts to a large fraction, when not most, of the measured fluorescence. A segmented flow method is proposed with a protocol for adequate correction of biases.     

Impact of environmental factors on in situ determination of iron in seawater by flow injection analysis

A sensitive method for iron determination in seawater has been adapted on a submersible chemical analyser for in situ measurements. The technique is based on flow injection analysis (FIA) coupled with spectrophotometric detection. When direct injection of seawater was used, the detection limit was 1.6 nM, and the precision 7%, for a triplicate injection of a 4 nM standard. At low iron concentrations, on line preconcentration using a column filled with 8-hydroxyquinoline (8HQ) resin was used. The detection limit was 0.15 nM (time of preconcentration = 240 s), and the precision 6%, for a triplicate determination of a 1 nM standard, allowing the determination of Fe in most of the oceanic regimes, except the most depleted surface waters. The effect of temperature, pressure, salinity, copper, manganese, and iron speciation on the response of the analyser was investigated. The slope of the calibration curves followed a linear relation as a function of pressure (C_p = 2.8 × 10^{− 5}P + 3.4 × 10^{− 2} s nmol^{− 1}, R² = 0.997, for Θ = 13 °C) and an exponential relation as a function of temperature (C_Θ = 0.009e^0.103Θ, R² = 0.832, for P = 3 bar). No statistical difference at 95% confidence level was observed for samples of different salinities (S = 0, 20, 35). Only very high concentration of copper (1000 × [Fe]) produced a detectable interference. The chemical analyser was deployed in the coastal environment of the Bay of Brest to investigate the effect of iron speciation on the response of the analyser. Direct injection was used and seawater samples were acidified on line for 80 s. Dissolved iron (DFe, filtered seawater (0.4 μm), acidified and stored at pH 1.8) corresponded to 29 ± 4% of Fe_a (unfiltered seawater, acidified in line at pH 1.8 for 80 s). Most of Fe_a (71 ± 4%) was probably a fraction of total dissolvable iron (TDFe, unfiltered seawater, acidified and stored at pH 1.8).     

Determination of picomolar iron in seawater by double Mg(OH)2 precipitation isotope dilution high-resolution ICPMS

A low-blank pre-concentration procedure is described for the analysis of picomolar iron (Fe) in seawater by isotope dilution high-resolution inductively coupled plasma mass-spectrometry (HR-ICPMS). The procedure uses a two-step Mg(OH)₂ co-precipitation procedure to extract Fe from a 50 ml seawater sample into a 100 μl 4% nitric acid (HNO₃) solution followed by HR-ICPMS measurement. The high pre-concentration ratio ( 500:1) achieved by the procedure minimizes the Fe blank due to ICPMS instrumental Fe background and results in a detection limit of  2 pM and a precision of  4% at the 50 pM Fe level. The measurement of a low-Fe seawater sample spiked with gravimetric Fe standard shows that the method can clearly distinguish 0.01 nM Fe from 0.02 nM Fe in seawater with high accuracy. The method is demonstrated by the analysis of dissolved Fe in the equatorial Pacific Ocean.     

A simplified resorcinol method for direct spectrophotometric determination of nitrate in seawater

A direct, spectrophotometric method has been adapted for quantitative determination of nitrate concentrations in seawater. The method is based on nitration of resorcinol in acidified seawater, resulting in a color product. The absorption spectrum obtained for the reaction product shows a maximum absorption at 505 nm, with a molar absorptivity of 1.7 × 10⁴ L mol^− 1 cm^− 1. This method has a detection limit of 0.5 μM and is linear up to 400 μM for nitrate. The advantage of this method is that all reagents are in aqueous solutions without involving cadmium granules as a heterogeneous reactant, as in conventional methods, and therefore is simple to implement. Application of the resorcinol to seawater analysis demonstrated that the results obtained are in good agreement with the conventional approach involving the reduction of nitrate by cadmium followed by diazotization.     

A catalytic oxidation method for the determination of total nitrogen dissolved in seawater

A detailed examination of a high-temperature catalytic oxidation method for liquid samples in the analysis of total nitrogen dissolved in seawater is reported. The nitrogenous compounds in liquid samples are oxidized on a platinum catalyzer at 680°C under oxygen atmosphere and the generated NO₂ is absorbed into a chromogenic reagent, followed by a spectrophotometric determination. The results of this method are much higher than those of wet oxidation methods. Molecular size dependency of the results clearly indicates that the above discrepancy is caused by the low oxidation capacity of the wet oxidation method against high-polymer organic matter dissolved in seawater. The results revealed that the concentration of total nitrogen in seawater is nearly constant from surface to bottom, ranging from 30 to 40 μM 1^?1, which organic nitrogen concentration higher in the surface layer, and a rapid decrease with depth. An examination of molecular size distribution indicates that the concentration of high-polymer organic nitrogen decreases rapidly from surface to deeper layers, with molecular sizes ranging from 5 × 10³ to 3 × 10⁴. Because of the well-defined principle of the oxidation process, its reliability, ease of sample handling and of analytical procedure on board or in the land laboratory, the present method is much more suitable for the marine analytical chemistry of total and organic nitrogen than the other previous methods.     

ICP-OES标准加入法准确测定海水中的硼

硼作为海水中的重要组分,其准确测定对海洋环境生物地球化学过程研究具有重要意义。海水中硼的常用测定方法具有前处理步骤繁琐、耗时长、样品易被污染、灵敏度低等缺点,不适于大批量海水样品的准确测定。为剔除海水高盐基体效应及干扰,本文运用电感耦合等离子体发射光谱法(ICP-OES),将海水样品进行稀释,采用标准加入法建立工作曲线,对海水中硼含量进行准确测定。正交实验显示,ICP-OES最佳工作条件为射频功率1400 W,雾化气流量0.7 L/min,观测高度14 mm,进样泵速1.5 m L/min。标准加入法标准曲线相关系数大于0.999,相对标准偏差为0.76%~1.27%,加标回收率为94.6%~101.8%,方法检出限为1.073μg/L。实际的海水样品分析表明,该方法可消除海水基体干扰,减少测定误差,并且前处理操作简单快速,化学试剂用量少,回收率高,精密度好,灵敏度高,可用于海水及其它高盐样品中硼含量的准确测定。     

Automatic determination of dissolved organic carbon in seawater in the sub-ppm range

The improvement of a dissolved organic carbon (DOC) analysis technique is described. This continuous flow system, based on both persulfate and UV oxidations, allows the automatic determination of DOC in sea and estuarine waters. The use of FID detection gives a very high sensitivity, permitting the detection of small variations in carbon content. The efficiency of the method and its sensitivity are tested, together with the influence of initial pH and duration of nitrogen bubbling. The system can also be used to determine total or inorganic carbon.     

光谱法测定海水中溶解态无机氮的研究进展

溶解态无机氮(dissolved inorganic nitrogen,DIN)主要由亚硝酸盐-氮(NO^-₂-N)、硝酸盐-氮(NO^-₃-N)和铵氮(NH⁺₄-N)组成,它们在海洋的生物地球化学循环过程中起重要作用。但人类活动向海洋输入了大量无机氮,导致一系列环境问题。为了更好地开展海洋氮循环研究和环境污染管理,需对海水中的DIN进行测定。在众多分析方法中,光谱法因其通用性好、适用范围广、所需设备简单,成为测定海水DIN的首选。本文总结了近10年来基于光谱法测定海水DIN的研究进展,包括紫外分光光度法测定NO^-₃-N、萘乙二胺分光光度法测定NO^-₂-N和NO^-₃-N、次溴酸盐氧化-分光光度法测定NH⁺₄-N、靛酚蓝分光光度法测定NH⁺₄-N...     

Transformation dynamics and reactivity of dissolved and colloidal iron in coastal waters

We have investigated the chemical forms, reactivities and transformation kinetics of Fe(III) species present in coastal water with ion exchange and filtration methods. To simulate coastal water system, a mixture of ferric iron and fulvic acid was added to filtered seawater and incubated for a minute to a week. At each incubation time, the seawater sample was acidified with hydrochloric acid and then applied to anion exchange resin (AER) to separate negatively charged species (such as fulvic acid, its complexes with iron and iron oxyhydroxide coated with fulvic acid) from positively charged inorganic ferric iron (Fe(III)′). By monitoring the acid-induced Fe(III)′ over an hour, it was found that iron complexed by fulvic acid dissociated rapidly to a large extent (86–92% at pH 2), whereas amorphous ferric oxyhydroxide particles associated with fulvic acid (AFO-L) dissociated very slowly with the first-order dissociation rate constants ranging from 6.1 × 10^− 5 for pH 3 to 2.7 × 10^− 4 s^− 1 for pH 2. Therefore, a brief acidification followed by the AER treatment (acidification/AER method) was likely to be able to determine fulvic acid complexes and thus differentiate the complexes from the AFO-L particles (the dissolution of AFO-L was insignificant during the brief acidification). The acidification/AER method coupled with a simple filtration technique suggested that the iron–fulvic acid complexes exist in both the < 0.02 μm and 0.02–0.45 μm size fractions in our coastal water system. The truly dissolved iron (< 0.02 μm) was relatively long-lived with a life-time of 14 days, probably due to the complexation by strong ligands. Such an acid-labile iron may be an important source of bioavailable iron in coastal environments, as a significant relationship between the chemical lability and bioavailability of iron has been well recognised.     

Submarine groundwater discharge: A large, previously unrecognized source of dissolved iron to the South Atlantic Ocean

This paper reports the initial results of a study of groundwater and coastal waters of southern Brazil adjacent to a 240 km barrier spit separating the Patos Lagoon, the largest coastal lagoon in South America, from the South Atlantic Ocean. The objective of this research is to assess the chemical alteration of freshwater and freshwater–seawater mixtures advecting through coastal permeable sands, and the influence of the submarine discharge of these fluids (SGD) on the chemistry of coastal waters. Here we focus on dissolved iron in this system and use radium isotopic tracers to quantify SGD and cross-shelf fluxes. Iron concentrations in groundwaters vary between 0.6 and 180 μM. The influence of the submarine discharge of these fluids into the surf zone produces dissolved Fe concentrations as high as several micromolar in coastal surface waters. The offshore gradient of dissolved Fe, coupled with results for Ra isotopes, is used to quantify the SGD flux of dissolved Fe from this coastline. We estimate the SGD flux to be 2 × 10⁶ mol day^− 1 and the cross-shelf flux to be 3.2 × 10⁵ mol day^− 1. This latter flux is equal to about 10% of the soluble atmospheric Fe flux to the entire South Atlantic Ocean. We speculate on the importance of this previously unrecognized iron input to regional ocean production and on the potential significance of this source to understanding variations in glacial–interglacial ocean production.     

A preliminary methods comparison for measurement of dissolved organic nitrogen in seawater

Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods.This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses.This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology.     

Wet-oxidation and automated colorimetry for simultaneous determination of organic carbon, nitrogen and phosphorus dissolved in seawater

We developed a simple and reliable method which allows simultaneous determination of organic forms of carbon (DOC), nitrogen (DON) and phosphorus (DOP) dissolved in seawater. Conversion of dissolved organic matter (DOM) to inorganic products (carbon dioxide, nitrate+nitrite and soluble reactive phosphate) is performed by a persulfate wet-oxidation in low alkaline condition. After oxidation, the concentration of the inorganic products dissolved in the sample was measured automatically by colorimetry using a 3-channel Technicon AutoAnalyzer system. A number of pure organic compounds were tested in the concentration range encountered in coastal and open ocean, indicating a high efficiency of the digestion procedure. The recovery range is similar to that obtained by other wet-oxidation procedures and by high-temperature catalytic oxidation techniques. Direct comparisons with usual methods used for separate determination of DOC, DON and DOP indicated a high efficiency of the procedure. Reproducibility tests demonstrated a very good precision (around 5%) for lagoonal and coastal waters, while precision was sometimes around 10–25% in oligotrophic oceanic waters, especially for DOP where values approached limits of detection for measuring phosphate. This method is highly suitable for routine analysis and especially appropriate for shipboard work.     

Automatic colorimetric analyzer prototype for high frequency measurement of nutrients in seawater

This paper describes the characteristics of a prototype of a modular multiparametric analyzer (MicroMAC FAST MP3) for automatic monitoring of seawater and analytical methods for nutrients.The MicroMAC FAST reactor is an evolution of the basic LFA (Loop Flow Analysis) reactor. It has been conceived to assay ammonium, nitrate–nitrite and orthophosphate at low concentration in seawater samples. A sample analysis is 3–4 times faster than that obtainable with a standard LFA reactor. With respect to the previous analyzer a temperature control (30–52 °C) on the measurement cell has been added (only for modules NH₄ and PO₄), while the colorimeter and the related links for transporting the sample have been moved beyond the Loop and form a hydraulic-optical set almost completely independent from the main LFA. All the steps of a wet-chemical colorimetric analysis method are carried out in an analysis cycle sequentially. The hermetic closed Loop provides full protection against background interference, which is a basic requirement for stable trace analysis. At the start of a cycle the loop is washed and filled with sample. The sample color is measured for compensation. Small amounts of concentrated reagents are added and mixed with high intensity. This new technique allows the preparation of two products of reaction which can be introduced at intervals of 150 s in the measurement cell. The intensity of the color of the reaction product is measured on the colorimeter using a monochromatic light beam of specific wavelength. The statistical test shows that the results of automated and manual analyses agree for all the examined parameters. Precision of all three analyses is ≤ 4% RSD.Multiparametric online analyzer: it is possible to connect the analytical modules to a data logger with analogue and digital signals, in order to have online simultaneous analysis of the sample. A typical application is used during research at sea which vessel does not require an operator.     

Simultaneous determination of dissolved organic carbon and total dissolved nitrogen in seawater by high temperature catalytic oxidation: conditions for precise shipboard measurements

Appropriate conditions have been achieved for the accurate, rapid, and highly precise shipboard simultaneous determination of dissolved organic carbon and total dissolved nitrogen in seawater by high temperature catalytic oxidation. A nitrogen-specific Antek 705D chemiluminescence detector and a CO₂-specific LiCor Li6252 IRGA have been coupled in-series with a Shimadzu TOC-5000 organic carbon analyser. Precision of both simultaneous measurements is ≤1.5%, i.e. ±1 μmol C l⁻¹ and ±0.3 μmol N l⁻¹, respectively. Quality of analysis is not compromised by vibrations associated with ocean going research vessels.     

Management priorities for seawater desalination plants in a marine protected area: A multi-criteria analysis

The development of seawater desalination plants to increase water reliability in coastal areas poses a threat to the health of near shore marine ecosystems and may affect the effectiveness of marine protected areas (MPAs) that have been established to meet international conservation targets. This paper applies a multi-criteria analysis approach to quantify stakeholder groups’ priorities for seawater desalination plants that have been proposed in communities adjacent to a National Marine Sanctuary. All groups placed the highest importance on minimizing environmental impacts on protected areas and endangered species that could be affected by water intake and brine discharge emphasizing the need for integrated land and sea conservation. Minimizing socio-economic impacts on coastal communities was much less important. Stakeholders also weighted reducing pressure on water levels in rivers, streams, and aquifers as more important than increasing water for residential consumption, which may foster coastal growth rather than replacing water taken from other sources. The study further revealed differences in the importance of multiple management objectives among stakeholder groups, which highlights the need to elicit distinct priorities of all groups to understand concerns and potential conflicts of desalination with existing marine users. The analysis of consistency ratios revealed that around half of all surveyed stakeholders had high inconsistencies in their responses, which suggests either a lack of understanding of desalination, or reflects the complexity of establishing desalination plants in coastal areas adjacent to a marine protected area.     

A sensitive and rapid method for analysis of dissolved mono- and polysaccharides in seawater

A spectrophotometric method is described for the determination of dissolved mono- and polysaccharides in seawater. It is based upon the well known alkaline ferricyanide reaction, but uses the reagent 2,4,6-tripyridyl-s-triazine (TPTZ) to give a strongly colored complex with the reduced iron. The method has been tested on model carbohydrates and other compounds, and also on natural samples of coastal and oceanic waters. Total carbohydrate content of the natural samples ranged from 5.2 to 25.1 μmol glucose-Cl⁻¹. The coefficient of variation was typically below 6% for values near 17 μmol Cl⁻¹ and approximately 10% for values near 3.5 μmol Cl⁻¹.     

Quantum yield for the photochemical production of dissolved inorganic carbon in seawater

The direct photooxidation of coloured dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) may provide a significant sink for organic carbon in the ocean. To calculate the rate of this reaction on a global scale, it is essential to know its quantum yield, or photochemical efficiency. We have determined quantum yield spectra, φ(λ), (moles DIC/mole photons absorbed) for 14 samples of seawater from environments ranging from a turbid, eutrophic bay to the Gulf Stream. The spectra vary among locations, but can be represented quite well by three pooled spectra for zones defined by location and salinity: inshore φ(λ)=e^{−(6.66+0.0285(λ−290))}; coastal φ(λ)=e^{−(6.36+0.0140(λ−290))}; and open ocean φ(λ)=e^{−(5.53+0.00914(λ−290))}. Production efficiency increases offshore, which suggests that the most highly absorbing and quickly faded terrestrial chromophores are not those directly responsible for DIC photoproduction.     

A comparative study of three methods for the determination of iodate in seawater

A comparative study of three methods for the determination of iodate-iodine in seawater is described. In one method the iodate is determined polarographically while in the others the iodate is determined colorimetrically as iodonium ions. In one of the colorimetric methods each sample is pre-treated with excess iodine-water in an attempt to eliminate suspected interference from naturally occurring reducing agents. The tests were conducted on a selection of open-ocean and near-shore waters with iodate concentrations ranging from 0 to 60 μg 1^?1-I. The tests indicated that the polarographic method and the colorimetric method without iodine-water give the more reliable measurement of iodate concentration. Also, the method with iodine water was found to be in error especially at low iodate concentrations. Reducing agents, if present, were found not to interfere significantly.     

Effects of iron limitation on intracellular cadmium of cultured phytoplankton: Implications for surface dissolved cadmium to phosphate ratios

This study compares intracellular Cd content (Cd:C) of cultured marine phytoplankton grown under various Fe levels, with estimated particulate Cd:P ratios derived from regression slopes of Cd versus PO₄³⁻ relationships from a global dataset. A 66-fold difference in Cd:C ratios was observed among the seven species grown under identical Fe concentrations, with oceanic diatoms having the highest Cd quotas and prymesiophytes the lowest. Interestingly, all species significantly increased their Cd:C ratios under Fe-limitation (on average 2-fold). The global data set also showed that the mean estimated Cd:P ratio of surface water particulates in HNLC (high nutrient low chlorophyll) regions were approximately 2-fold higher than non-HNLC regions. A sequence of events are proposed to explain high Cd:P ratios in HNLC waters. First, the seasonal relief from Fe-limitation in HNLC regions leads to blooms of large chain forming diatoms with high intrinsic Cd:P ratios. These large blooms may, in theory, deplete surface water CO₂ and Zn concentrations, which ultimately, would result in increased Cd uptake. Eventually these blooms will run out of Fe, which has been shown to further increase intercellular Cd via growth biodilution and increased Cd uptake through non-specific Fe(II) transporters. Ultimately, Fe-limited diatoms with enhanced Cd quotas will sink out of surface waters leading to pronounced regional differences in Cd:P ratios between HNLC and non-HNLC waters in the global ocean.     

On the feasibility of some photochemical reactions of iodide in seawater

Aerated solutions of potassium iodide in de-ionised water, of between 5–20% (w/v), were exposed to ambient spring sunlight to estimate the rate of photochemical production of molecular iodine from iodide and oxygen in seawater. This rate cannot be measured directly as other reactions, for example the reduction of molecular iodine by organic matter, interfere. Also, a parallel photo-oxidation, initiated by organic matter in real seawater, may also occur. The experiments yield a half-life for iodide in tropical surface waters of about 29 months suggesting that the reaction is insignificant. At this rate it will not compete effectively against the reduction of molecular iodine by organic matter, and hence molecular iodine should not appear. The experiments also consider the photo-oxidation by nitrate, and iodate, a combination of nitrite and oxygen, and eliminate significant interference by chloride, bromide and the phosphate buffer. The rate of photo-oxidation with each of the first three oxidations is found to be first order with respect to oxidant concentration. The variation of photo-oxidation rate with pH is also studied, with a brief investigation without conventional oxidant, where electron cage complexes still promote photo-oxidation. The photochemical action spectrum for these reactions in sunlight is shown to extend between 300 and 425 nm. The photo-oxidation of iodide by iodate is more interesting to marine chemistry as the photo-reduction of iodate. Nevertheless, the reduction-rate is judged to be several orders too low to be significant in seawater. The mechanism of the reactions are discussed and lessons drawn on the stability of potassium iodide solutions used in iodate analysis. The KI actinometer is recommended to those studying other photochemical systems activated by UV-A light as it is linear and very simple and reliable.