Development and application of a microfluidic in-situ analyzer for dissolved Fe and Mn in natural waters.

The redox sensitive trace metals iron and manganese are two important elements that help shape the biogeochemistry of aquatic systems and thus their measurement is important. Current laboratory methods are expensive, time consuming and cannot provide the spatial and temporal resolution needed to characterize these elements in natural waters. Here we describe the first autonomous analyzer capable of providing vertical profiles as well as routine in-situ determinations of dissolved Fe(II) and Mn in aquatic environments. The spectrophotometric sensor uses microfluidic methods (Lab-on-a-chip technology) and mixes reagents and samples using a novel in-cell diffusion process. Fe(II) and Mn can be measured with a frequency of up to 12 and 6 samples per hour respectively with limits of detection of 27nM for Fe(II), 2.1% precision (n=20), and 28nM for Mn, 2.4% precision (n=19). The device combines relatively low cost, low power usage, low reagent consumption, portability, and tolerance to pressures up to at least 170 bars, with high precision and accuracy. We present data from a successful demonstration of the sensor during a cruise to the Gotland and Landsort Deep Basins of the Baltic Sea.

Ice sheets as a significant source of highly reactive nanoparticulate iron to the oceans.

The Greenland and Antarctic Ice Sheets cover ~ 10% of global land surface, but are rarely considered as active components of the global iron cycle. The ocean waters around both ice sheets harbour highly productive coastal ecosystems, many of which are iron limited. Measurements of iron concentrations in subglacial runoff from a large Greenland Ice Sheet catchment reveal the potential for globally significant export of labile iron fractions to the near-coastal euphotic zone. We estimate that the flux of bioavailable iron associated with glacial runoff is 0.40-2.54 Tg per year in Greenland and 0.06-0.17 Tg per year in Antarctica. Iron fluxes are dominated by a highly reactive and potentially bioavailable nanoparticulate suspended sediment fraction, similar to that identified in Antarctic icebergs. Estimates of labile iron fluxes in meltwater are comparable with aeolian dust fluxes to the oceans surrounding Greenland and Antarctica, and are similarly expected to increase in a warming climate with enhanced melting.

The measurement of organically complexed FeII in natural waters using competitive ligand reverse titration.

Whilst there is increasing evidence for the presence of stabilized Fe(II) associated with organic matter in aquatic environments, the absence of a reliable method for determining Fe(II) speciation in solution has inhibited the study of this aspect of Fe biogeochemistry. A technique is described here for the determination of Fe(II) organic complexation in natural waters that is based on competitive ligand reverse titration and a model fit to experimental results, from which ligand concentration and a conditional stability constant can be obtained. Spectrophotometry was used to detect the Ferrozine (FZ) complex with reactive Fe(II), which in combination with a liquid waveguide capillary cell (LWCC) enabled high sensitivity and precision measurements of Fe(II) to be made. A series of samples was collected in the Itchen River in Southampton, UK to test the method at a wide range of salinities including river water. Levels of Fe(II) and total dissolved Fe were within previously reported values for this system. Fe(II) was found to occur organically complexed with values for K'(Fe(II)L) (conditional stability constant for Fe(II)-natural ligand complexes) of ≈8 at salinities between 0 and 21, whilst no measurable complexation was detected at a salinity of 31. This work demonstrates that spectrophotometry can be used in combination with ligand competition to investigate metal speciation in natural waters.

Lab-on-chip measurement of nitrate and nitrite for in situ analysis of natural waters.

Microfluidic technology permits the miniaturization of chemical analytical methods that are traditionally undertaken using benchtop equipment in the laboratory environment. When applied to environmental monitoring, these "lab-on-chip" systems could allow high-performance chemical analysis methods to be performed in situ over distributed sensor networks with large numbers of measurement nodes. Here we present the first of a new generation of microfluidic chemical analysis systems with sufficient analytical performance and robustness for deployment in natural waters. The system detects nitrate and nitrite (up to 350 µM, 21.7 mg/L as NO(3)(-)) with a limit of detection (LOD) of 0.025 µM for nitrate (0.0016 mg/L as NO(3)(-)) and 0.02 µM for nitrite (0.00092 mg/L as NO(2)(-)). This performance is suitable for almost all natural waters (apart from the oligotrophic open ocean), and the device was deployed in an estuarine environment (Southampton Water) to monitor nitrate+nitrite concentrations in waters of varying salinity. The system was able to track changes in the nitrate-salinity relationship of estuarine waters due to increased river flow after a period of high rainfall. Laboratory characterization and deployment data are presented, demonstrating the ability of the system to acquire data with high temporal resolution.

Nutrients in estuaries--an overview and the potential impacts of climate change.

The fate and cycling of macronutrients introduced into estuaries depend upon a range of interlinked processes. Hydrodynamics and morphology in combination with freshwater inflow control the freshwater flushing time, and the timescale for biogeochemical processes to operate that include microbial activity, particle-dissolved phase interactions, and benthic exchanges. In some systems atmospheric inputs and exchanges with coastal waters can also be important. Climate change will affect nutrient inputs and behaviour through modifications to temperature, wind patterns, the hydrological cycle, and sea level rise. Resulting impacts include: 1) inundation of freshwater systems 2) changes in stratification, flushing times and phytoplankton productivity 3) increased coastal storm activity 4) changes in species and ecosystem function. A combination of continuing high inputs of nutrients through human activity and climate change is anticipated to lead to enhanced eutrophication in the future. The most obvious impacts of increasing global temperature will be in sub-arctic systems where permafrost zones will be reduced in combination with enhanced inputs from glacial systems. Improved process understanding in several key areas including cycling of organic N and P, benthic exchanges, resuspension, impact of bio-irrigation, particle interactions, submarine groundwater discharges, and rates and magnitude of bacterially-driven recycling processes, is needed. Development of high frequency in situ nutrient analysis systems will provide data to improve predictive models that need to incorporate a wider variety of key factors, although the complexity of estuarine systems makes such modelling a challenge. However, overall a more holistic approach is needed to effectively understand, predict and manage the impact of macronutrients on estuaries.

A new methodology for precise cadmium isotope analyses of seawater.

Previous studies have revealed considerable Cd isotope fractionations in seawater, which can be used to study the marine cycling of this micronutrient element. The low Cd concentrations that are commonly encountered in nutrient-depleted surface seawater, however, pose a particular challenge for precise Cd stable isotope analyses. In this study, we have developed a new procedure for Cd isotope analyses of seawater, which is suitable for samples as large as 20 L and Cd concentrations as low as 1 pmol/L. The procedure involves the use of a (111)Cd-(113)Cd double spike, co-precipitation of Cd from seawater using Al(OH)(3), and subsequent Cd purification by column chromatography. To save time, seawater samples with higher Cd contents can be processed without co-precipitation. The Cd isotope analyses are carried out by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The performance of this technique was verified by analyzing multiple aliquots of a large seawater sample that was collected from the English Channel, the SAFe D1 seawater reference material, and several samples from the GEOTRACES Atlantic intercalibration exercise. The overall Cd yield of the procedure is consistently better than 85% and the methodology can routinely provide ε (114/110)Cd data with a precision of about ±0.5 ε (2sd, standard deviation) when at least 20-30 ng of natural Cd is available for analysis. However, even seawater samples with Cd contents of only 1-3 ng can be analyzed with a reproducibility of about ±3 to ±5 ε. A number of experiments were furthermore conducted to verify that the isotopic results are accurate to within the quoted uncertainty.

Interferences in the analysis of nanomolar concentrations of nitrate and phosphate in oceanic waters.

This paper reports on investigations into interferences with the measurements of nanomolar nitrate+nitrite and soluble reactive phosphate (SRP) in oceanic surface seawater using a segmented continuous flow autoanalyser (SCFA) interfaced with a liquid-waveguide capillary flow-cell (LWCC). The interferences of silicate and arsenate with the analysis of SRP, the effect of sample filtration on the measurement of nanomolar nitrate+nitrite and SRP concentrations, and the stability of samples during storage are described. The investigation into the effect of arsenate (concentrations up to 100 nM) on phosphate analysis (concentrations up to 50 nM) indicated that the arsenate interference scaled linearly with phosphate concentrations, resulting in an overestimation of SRP concentrations of 4.6+/-1.4% for an assumed arsenate concentration of 20 nM. The effect of added Si(OH)(4) was to increase SRP signals by up to 36+/-19 nM (at 100 microM Si(OH)(4)). However, at silicate concentrations below 1.5 microM , which are typically observed in oligotrophic surface ocean waters, the effect of silicate on the phosphate analysis was much smaller (< or = 0.78+/-0.15 nM change in SRP). Since arsenate and silicate interferences vary between analytical approaches used for nanomolar SRP analysis, it is important that the interferences are systematically assessed in any newly developed analytical system. Filtration of surface seawater samples resulted in a decrease in concentration of 1.7-2.7 nM (+/-0.5 nM) SRP, and a small decrease in nitrate concentrations which was within the precision of the method (+/-0.6 nM). A stability study indicated that storage of very low concentration nutrient samples in the dark at 4 degrees C for less than 24 h resulted in no statistically significant changes in nutrient concentrations. Freezing unfiltered surface seawater samples from an oligotrophic ocean region resulted in a small but significant increase in the SRP concentration from 12.0+/-1.3 nM (n=3) to 14.7+/-0.6 nM (n=3) (Student's t-test; p=0.021). The corresponding change in nitrate concentration was not significant (Student's t-test; p>0.05).

Southern Ocean deep-water carbon export enhanced by natural iron fertilization.

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north-south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol(-1) was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom(6). The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

Limitations to Accuracy in Extracting Characteristic Line Intensities From X-Ray Spectra.

The early development of quantitative electron probe microanalysis, first using crystal spectrometers, then energy dispersive x-ray spectrometers (EDXS), demonstrated that elements could be detected at 0.001 mass fraction level and major concentrations measured within 2 % relative uncertainty. However, during this period of extensive investigation and evaluation, EDXS detectors were not able to detect x rays below 1 keV and all quantitative analysis was performed using a set of reference standards measured on the instrument. Now that EDXS systems are often used without standards and are increasingly being used to analyse elements using lines well below 1 keV, accuracy can be considerably worse than is documented in standard textbooks. Spectrum processing techniques found most applicable to EDXS have now been integrated into total system solutions and can give excellent results on selected samples. However, the same techniques fail in some applications because of a variety of instrumental effects. Prediction of peak shape, width and position for every characteristic line and measurement of background intensity is complicated by variations in response from system to system and with changing count rate. However, with an understanding of the fundamental sources of error, even a total system can be tested like a "black box" in areas where it is most likely to fail and thus establish the degree of confidence that should apply in the intended application. This approach is particularly important when the microanalysis technique is applied at lower electron beam voltages where the extraction of line intensities is complicated by extreme peak overlap and higher background levels.

Improved X-ray Spectrum Simulation for Electron Microprobe Analysis.

The accurate calculation of characteristic peak intensity is essential for interpreting X-ray spectra in electron microprobe analysis. Conventionally, the measured intensity from a standard of known composition is used as a reference to simplify the calculation. However, if no such standard is available, then all factors influencing X-ray generation and X-ray detection efficiency must be included. If the intensity and energy distribution of the background radiation can also be calculated, the investigator can simulate an entire spectrum from an assumed composition, gaining powerful benefits in setting up an experiment and in confirming the results. The study presented here demonstrates a fast method of spectrum simulation, suitable for energy-dispersive spectroscopy (EDS), and assesses the accuracy using 309 spectra from samples of known composition. These include K, L, and M lines from elements of atomic number 6-92, excited by beam energies in the range of 5-30 keV. The RMS error between 360 measured and calculated peak intensities was found to be 7.1%. Central to the method is the use of the ratio of peak intensity/total background intensity, which allows spectra to be compared from instruments of differing collection efficiency, thereby easing the collection of data over a wide range of conditions.