Ian D. McKelvie

Determination of carbon, phosphorus, nitrogen and silicon species in waters

Abstract This review examines the analytical chemistry of the nutrient elements carbon, phosphorus, nitrogen and silicon in environmental waters. The speciation of these elements is discussed and the terminology used for classification is described. The analytical approach is considered in general terms, with particular regard to sample collection and preservation, sample treatment and methods of analysis. A critical appraisal of the analytical methods available for each of the elements (and their speciation) is provided together with the relevant analytical figures of merit.

Sampling, sample treatment and quality assurance issues for the determination of phosphorus species in natural waters and soils

Phosphorus is an important macronutrient and the accurate determination of phosphorus species in environmental matrices such as natural waters and soils is essential for understanding the biogeochemical cycling of the element, studying its role in ecosystem health and monitoring compliance with legislation. This paper provides a critical review of sample collection, storage and treatment procedures for the determination of phosphorus species in environmental matrices. Issues such as phosphorus speciation, the molybdenum blue method, digestion procedures for organic phosphorus species, choice of model compounds for analytical studies, quality assurance and the availability of environmental CRMs for phosphate are also discussed in detail.

Potential contribution of lysed bacterial cells to phosphorus solubilisation in two rewetted Australian pasture soils

Soil drying renders considerable amounts of phosphorus soluble upon rewetting, which may be partly derived from lysed microbial cells. Using direct bacterial cell counting in water and tetra-sodium pyrophosphate extracts of two Australian pasture soils, we found that almost all extractable cells were lysed following the rewetting of dry soils. The amounts of phosphorus in the lysed cells corresponded closely to the increases in water-extractable phosphorus following soil drying, suggesting that bacterial cell lysis is a major source of the released phosphorus.

Characterisation and quantification of organic phosphorus and organic nitrogen components in aquatic systems: A Review

This review provides a critical assessment of knowledge regarding the determination of organic phosphorus (OP) and organic nitrogen (ON) in aquatic systems, with an emphasis on biogeochemical considerations and analytical challenges. A general background on organic phosphorus and organic nitrogen precedes a discussion of sample collection, extraction, treatment/conditioning and preconcentration of organic phosphorus/nitrogen from sediments, including suspended particulate matter, and waters, including sediment porewaters. This is followed by sections on the determination of organic phosphorus/nitrogen components. Key techniques covered for organic phosphorus components are molecular spectrometry, atomic spectrometry and enzymatic methods. For nitrogen the focus is on the measurement of total organic nitrogen concentrations by carbon hydrogen nitrogen analysis and high temperature combustion, and organic nitrogen components by gas chromatography, high-performance liquid chromatography, gel electrophoresis, mass spectrometry, nuclear magnetic resonance spectrometry, X-ray techniques and enzymatic methods. Finally future trends and needs are discussed and recommendations made.

Adsorption of natural organic matter onto goethite

The factors influencing the association of two natural organic matter (NOM) samples (from Redwater Creek in Australia and Suwannee River in USA) with goethite were studied. Sorption occurred very rapidly (minutes) and at very low NOM concentrations (about 0.5 mg C 1−1), implying that mineral surfaces in the natural aquatic environment would not remain uncoated by NOM for long. The sorption behaviour of the two NOM samples followed the Langmuir adsorption equation. The maximum sorption density for the unfractionated Redwater Creek NOM was 1.1 μmol C mg−1 of goethite. The hydrophobic fractions (both acidic and neutral) separated from the Redwater Creek sample had much greater sorption affinities for the goethite surface than did the hydrophilic fraction. Calculation of the surface crowding indicates that a monolayer of sorbed NOM exists over the goethite surface. Sorption was influenced by solution pH and calcium ion concentration. Maximum sorption density occurred at around pH 3–4, and decreased at higher pH. The sorption density of NOM also appears to be influenced by the presence of non-ionic hydrophobic solutes. This component could shield the electrostatic repulsive forces between neighbouring adsorbed anionic NOM molecules on the goethite surface, thus leading to an increase in the maximum amount of NOM adsorbed. Calcium ion concentration increased sorption of NOM up to calcium concentrations of around 1 mM but higher calcium concentrations resulted in reduced sorption. Sorption of the hydrophobic acid fraction was most affected by calcium concentration. The main effect of NOM sorption on the surface characteristics of goethite is alteration of the surface charge. The isoelectric point of the NOM-coated goethite is shifted markedly to lower pH, compared to that of uncoated goethite. Thus, at the normal pH of natural waters, NOM-coated goethite has a net negative charge which would significantly influence the types of interaction occurring at the surface of the NOM-coated particles in natural aquatic systems. The model advanced for suspended particulate matter (SPM) in aquatic systems, namely a mineral core with coatings of hydrous metal oxides and NOM, appears to explain adequately many aspects of the behaviour of natural SPM.

Microfluidic paper-based analytical device for the determination of nitrite and nitrate.

A low-cost disposable colorimetric microfluidic paper-based analytical device (μPAD) was developed for the determination of nitrite and nitrate. Nitrite is determined directly by the Griess reaction while nitrate is first reduced to nitrite in a hydrophilic channel of the μPAD with immobilized zinc microparticles. This μPAD is fabricated by a simple and inexpensive inkjet printing method. Under optimal conditions, the limits of detection and quantification for nitrite are 1.0 and 7.8 μM, respectively, while the corresponding values for nitrate are 19 and 48 μM, respectively. The repeatability, expressed as relative standard deviation (RSD), is less than 2.9% and 5.6% (n ≤ 8) for the determination of nitrite and nitrate, respectively. This μPAD was successfully applied to the determination of nitrate and nitrite in both synthetic and natural water samples. It is user and environmentally friendly and suitable for on-site measurement of the analytes mentioned above in environmental and drinking waters.

A compact flow injection analysis system for surface mapping of phosphate in marine waters

The design, construction and validation of a compact, portable flow injection analysis (FIA) instrument for underway analysis of phosphate in marine waters is described. This portable system employs gas pressure for reagent propulsion and computer controlled miniature solenoid valves for precise injection of multiple reagents into a flowing stream of filtered sample. A multi-reflection flow cell with a solid state LED photometer is used to detect filterable reactive phosphate (0.2 mum) as phosphomolybdenum blue. All the components are computer controlled using software developed using the Labviewtrade mark graphical programming language. The system has the capacity for sample throughput of up to 380 phosphate analyses per hour, but in the mode described here was operated at 225 analyses per hour. Under these conditions, the system exhibited a detection limit of 0.15 muM, reproducibility of 1.95 % RSD (n=9) and a linear response (r(2)=0.9992) when calibrated in the field with standards in the range 0.81-3.23 muM. The system was evaluated for the mapping of phosphate concentrations in Port Phillip Bay, south eastern Australia, and during the course of a 150 km cruise, 542 analyses were performed automatically. In general, good agreement was observed between analyses obtained using the portable FIA system and those obtained from manual sampling and laboratory analysis.

Elimination of the Schlieren effect in the determination of reactive phosphorus in estuarine waters by flow-injection analysis

Abstract Conventional flow-injection manifolds with sample injection for the determination of reactive phosphorus in estuarine waters are limited by the Schlieren or refractive index (RI) effect which can cause major errors in quantification. A simple flow-injection (FI) manifold which obviates this RI error in reactive phosphorus measurement is reported. It involves the injection of acidic molybdophosphate reagent into a carrier stream of sodium chloride solution of similar refractive index, which is then sequentially merged with a sample (the salinity of which may vary widely from sample to sample) and a reductant. Despite the occurrence of sizeable blank signals, reactive phosphorus has been successfully measured in samples with salinities ranging from 0 to 34%. using calibration standards prepared in deionised water, with a detection limit of 6 μg l −1 P.

Spectrophotometric determination of dissolved organic phosphorus in natural waters using in-line photo-oxidation and flow injection

A rapid in-line photochemical digestion for use with flow injection (Fl) to determine the dissolved organic phosphorus (DOP) in natural waters is described. The Fl system includes a simple ultraviolet (UV) photoreactor and uses the tin(II) chloride reduction of phosphomolybdate for the spectrophotometric determination of the dissolved reactive phosphorus (DRP) produced by the photo-oxidation. Samples in the range 0.1–4 and < 0.1 mg l–1 of P can be analysed at rates of 72 and 50 samples h–1, respectively. The effective detection limit of the technique was 0.01 mg l–1. Comparison of the results for DOP obtained using the proposed method were in good agreement with those obtained using standard methods. The DRP could also be determined using the same Fl manifold but without UV irradiation.

Multi-reflection photometric flow cell for use in flow injection analysis of estuarine waters

A multi-reflection flow cell suitable for flow analysis is described. Light from an LED is directed through an optical fibre into a silver coated capillary through a sidewall aperture, and emerges through a similar aperture 10 mm along the capillary after undergoing an estimated 19 reflections. This process provides a sensitivity enhancement of approximately 2.5 compared with a conventional z-cell of the same nominal path length. This enhancement is due to both the increased optical path length achieved by multiple reflection of the light beam through the sample, and minimization of physical dispersion by the use of a short, small internal diameter capillary as the flow cell. The optical design of this flow cell also minimizes the Schlieren effect. Optical and hydrodynamic characteristics of this multi-reflection cell have been evaluated using a series of bromothymol blue dye studies. Application of the flow cell to the determination of reactive phosphorus in estuarine waters with wide variation in salinity and refractive index is also described.