Simplifying and improving the extraction of nitrate from freshwater for stable isotope analyses.

Determining the isotopic composition of nitrate (NO(3)(-)) in water can prove useful to identify NO(3)(-) sources and to understand its dynamics in aquatic systems. Among the procedures available, the 'ion-exchange resin method' involves extracting NO(3)(-) from freshwater and converting it into solid silver nitrate (AgNO(3)), which is then analysed for (15)N/(14)N and (18)O/(16)O ratios. This study describes a simplified methodology where water was not pre-treated to remove dissolved organic carbon (DOC) or barium cations (added to precipitate O-bearing contaminants), which suited samples with high NO(3)(-) (≥ 00 µM or 25 mg L(-1) NO(3)(-)) and low DOC (typically < 17 µM of C or 5 mg L(-1) C) levels. % N analysis revealed that a few AgNO(3) samples were of low purity (compared with expected % N of 8.2), highlighting the necessity to introduce quality control/quality assurance procedures for silver nitrate prepared from field water samples. Recommendations are then made to monitor % N together with % O (expected at 28.6, i.e. 3.5 fold % N) in AgNO(3) in order to better assess the type and gravity of the contamination as well as to identify potentially unreliable data.

High-yield production of graphene by liquid-phase exfoliation of graphite.

Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.