Peter S. Ellis

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.

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.

Flow injection analysis as a tool for enhancing oceanographic nutrient measurements—A review

Macronutrient elements (C, N and P) and micronutrient elements (Fe, Co, Cu, Zn and Mn) are widely measured in their various physico-chemical forms in open ocean, shelf sea, coastal and estuarine waters. These measurements help to elucidate the biogeochemical cycling of these elements in marine waters and highlight the ecological and socio-economic importance of the oceans. Due to the dynamic nature of marine waters in terms of chemical, biological and physical processes, it is advantageous to make these measurements in situ and in this regard flow injection analysis (FIA) provides a suitable shipboard platform. This review, therefore, discusses the role of FIA in the determination of macro- and micro-nutrient elements, with an emphasis on manifold design and detection strategies for the reliable shipboard determination of specific nutrient species. The application of various FIA manifolds to oceanographic nutrient determinations is discussed, with an emphasis on sensitivity, selectivity, high throughput analysis and suitability for underway analysis and depth profiles. Strategies for enhancing sensitivity and minimizing matrix effects, e.g. refractive index (schlieren) effects and the important role of uncertainty budgets in underpinning method validation and data quality are discussed in some detail.

Spectrophotometric Determination of Ammonia in Estuarine Waters by Hybrid Reagent‐Injection Gas‐Diffusion Flow Analysis

Abstract A flow‐injection gas‐diffusion technique is described for the online determination of ammonia in estuarine waters covering a salinity range of S=0 to 36. The flow analysis system, which is a hybrid of reagent injection and conventional sample‐injection flow systems, avoids the need for a rotary injection valve. Whereas gas‐diffusion techniques have been widely applied in conventional sample‐injection flow analysis, reagent‐injection flow analysis involving gas diffusion has been little used because it is susceptible to interference from dissolved gaseous species such as carbon dioxide coexisting with ammonia in the sample. This source of interference has been overcome by online adjustment of sample to pH 8.4 prior to the injection of the base that initiates gas diffusion of ammonia. The pore sizes of hydrophobic membranes used in gas diffusion were characterized by a bubble‐point test prior to use in the flow analysis system. These showed wide variation in pore size, and grading and careful selection was necessary in order to obtain reliable gas diffusion measurements of ammonia. The proposed flow‐injection system can be operated in a continuous flow mode, at a sample throughput of 135 measurements hr−1 with a typical limit of detection (LOD) of 9 µg N L−1, or in stopped‐flow mode at 60 measurements hr−1 with a LOD of 3 µg N L−1. The technique was validated using water samples containing a wide range of dissolved carbon dioxide concentrations, salinity, and pH. Excellent agreement (r=0.999) was observed between results obtained using the reagent‐injection system and an approved reference method. The authors were invited to contribute this paper to a special issue of the journal entitled “Spectroscopy and Automation”. This special issue was organized by Miguel de la Guardia, Professor of Analytical Chemistry at Valencia University, Spain.

A compact portable flow analysis system for the rapid determination of total phosphorus in estuarine and marine waters.

The development and evaluation of a portable flow analysis system for the in situ determination of total phosphorus is described. The system has been designed with rapid underway monitoring in mind. The system employs an ultra-violet photo-reactor and thermal heating for peroxodisulfate digestion of total phosphorus to orthophosphate, followed by spectrophotometric detection with a multi-reflective flow cell and low-power light emitting diode using the molybdenum blue method. Reagents are stored under gas pressure and delivered using software controlled miniature solenoid valves. The fully automated system has a throughput of 115 measurements per hour, a detection limit of 1 microg PL(-1), and gives a linear response over the calibration range of 0-200 microg PL(-1) (r(2)=0.9998), with a precision of 4.6% RSD at 100 microg PL(-1) (n=10). Field validation of the instrument and method was performed in Port Philip and Western Port Bays in Victoria, SE Australia, where 2499 analyses were performed over a 25 h period, over a cruise path of 285 km. Good agreement was observed between determinations of samples taken manually and analysed in the laboratory and those measured in situ with the flow analysis system.

Determination of dissolved reactive phosphorus (DRP) and dissolved organic phosphorus (DOP) in natural waters by the use of rapid sequenced reagent injection flow analysis.

A FI system for rapid sequential determination of dissolved reactive and organic phosphate is described. It utilizes on-line UV photo-oxidation for digestion of dissolved organic phosphate (DOP) with detection of the phosphate produced as phosphomolybdenum blue at 690 nm after reduction of phosphomolybdate with tin(II) chloride. Two injections are performed in the analysis of each sample: the first of sample solution alone enables DRP determination, while the second is of sample plus alkaline peroxydisulfate solutions, which under the photo-oxidation conditions used converts DOP to DRP. The DOP content is evaluated from the difference of the two injections. The digestion efficiency for DOP, evaluated using a range of model organic P compounds of varying stability was greater than 97%. Calibrations were linear over the range of 0.01-6.00 mg Pl(-1) for both DRP and DRP+DOP graphs, with a detection limit (3s) of 0.01 mg Pl(-1) for both species. Relative standard deviations were 0.3% (n=11, 0.50 mg Pl(-1)) for the DRP determination and 1.0% (n=11, 0.50 mg Pl(-1)) for the DRP+DOP determination. Injection throughput of 22 h(-1) was achieved. The proposed system was validated by analysis of the certified reference materials and comparison with the previous flow injection system. Additional advantage of this system is that it requires the use of only small amounts of the oxidant, with the result that nuisance gas bubble formation is correspondingly minimized.

Flow analysis methods for the direct ultra-violet spectrophotometric measurement of nitrate and total nitrogen in freshwaters

Second derivative ultra-violet spectrophotometric methods are described for the measurement of nitrate and total nitrogen in freshwaters using flow analysis techniques. A simple flow system consisting of a peristaltic pump and a single-reflection flow-through cell was used for the measurement of nitrate. Quantification of total nitrogen using alkaline peroxodisulfate photo-digestion was achieved by incorporating an ultra-violet photo-reactor, a hollow-fibre filter and a debubbler into the flow system. The nitrate system featured a limit of detection of 0.04 mg N L(-1), 0.4%RSD (1 mg N L(-1) as nitrate, n=10), a coefficient of determination (R(2)) of 0.9995 over the calibration range 0.0-2.0 mg N L(-1), and a data acquisition time of 1.5s per spectrum. The total nitrogen system featured a limit of detection of 0.05 mg N L(-1), 1%RSD (1 mg N L(-1) as ammonium chloride, n=10), a coefficient of determination of 0.9989 over the calibration range 0.0-2.0 mg N L(-1), and a throughput of 5 sample h(-1) measured in triplicate. Digestions of five model nitrogen compounds returned recoveries of >88%. Determinations carried out using the developed systems show a high degree of agreement with data obtained using reference methods. These methods require no colorimetric reagents and eliminate the requirement for a toxic cadmium reduction column. The overlap of chloride and nitrate spectra in seawater is not eliminated entirely by the use of second derivative spectrophotometry, and consequently the methods are restricted to the analysis of freshwaters.

Spectrophotometric determination of iodate in iodised salt by flow injection analysis

A simple and sensitive flow injection (FI) method for the determination of iodate is proposed. The method is based on the reaction of iodate with hydroxylamine in acidic solution. Sulfanilamide is diazotised by the nascent nitrite and the diazonium ion produced is then coupled with N-(1-naphthyl)ethyenediamine in hydrochloric acid medium to form an azo dye which is measured spectrophotometrically. The calibration graph for iodate is linear in the range of 0.1-30mgL-1 with a correlation coefficient of 0.9992. The limit of detection and relative standard deviation are 0.02mgL-1 and 1.2% (5mgL-1, n=8). The method has been applied to the determination of iodate in table salts and accuracy was assessed through recovery experiments and independent analysis by a conventional titrimetric method.

A versatile total internal reflection photometric detection cell for flow analysis.

A total internal reflection (TIR) flow-through cell that is highly tolerant of schlieren effects, has limited hydrodynamic dispersion and does not trap gas bubbles, and which is suitable for sensitive photometric measurements in flow analysis, is described. Light from an optical fibre is introduced into a short length of quartz capillary through the sidewall at an incident angle of ca. 53 degrees. Under this condition, incident light undergoes total internal reflection from the external air-quartz interface and is propagated by successive reflections from the external walls through the aqueous liquid core of the cell. Detection of the transmitted beam is enabled by intentionally introducing an optical coupling medium at a predetermined distance along the capillary wall, which allows the internally reflected light to be captured by a second optical fibre connected to a charge-couple device detector. This configuration embodies a number of the desirable features of a liquid core waveguide cell (i.e. total internal reflection), a multi-reflection (MR) flow cell (i.e. minimum susceptibility to schlieren effects, low hydrodynamic dispersion and little tendency to trap bubbles), and a conventional Z-cell (wide dynamic range). When employed with a flow injection system, a limit of detection of 2.0 microg PL(-1) was achieved for the determination of reactive phosphate using the TIR cell, compared with LOD values of 3.8 microg PL(-1) and 4.9 microg PL(-1) obtained using the MR and Z-cells with same manifold. The combined advantages of schlieren-tolerance and lack of bubble entrapment of the MR cell with the higher S/N ratio and wider dynamic range of a conventional Z-cell, make the TIR cell eminently useful for photometric measurements of samples with widely differing refractive indices.

Underway determination of dissolved inorganic carbon in estuarine waters by gas-diffusion flow analysis with C4D detection

The development and evaluation of a gas diffusion flow analysis system for the underway determination of dissolved inorganic carbon in marine and estuarine waters is described. Carbon dioxide produced when sample is injected into an acidic donor stream, diffuses through an efficient hollow fibre microporous membrane into an acceptor stream of ultrapure water, where the resultant changes in electrical conductivity are detected using a contactless capacitively coupled conductivity detector (C4D). The optimal parameters for the construction and operation of the C4D system are reported. Under field operational conditions, the flow analysis method had a linear calibration range for DIC of 0.2–10 mM, a limit of detection of 0.12 mM, repeatability of 0.46% RSD (n = 9 at 6 mM), a sample throughput of 90 h−1 and excellent correlation with comparative analyses (R2 = 0.9951, n = 16). The system was used to perform >250 determinations of DIC measurements underway during a short cruise on the Yarra River estuary, which demonstrated that DIC was conservative over much of the salinity gradient, with the exception of the low salinity region which exhibited the effects of respiratory CO2 and other DIC inputs. The results illustrate the advantages of the use of rapid flow analysis techniques for chemical mapping in transient environments like estuaries.