Elsevier

Analytica Chimica Acta

Volume 674, Issue 2, 3 August 2010, Pages 117-122
Analytica Chimica Acta

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

https://doi.org/10.1016/j.aca.2010.06.030Get rights and content

Abstract

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 μg P L−1, and gives a linear response over the calibration range of 0–200 μg P L−1 (r2 = 0.9998), with a precision of 4.6% RSD at 100 μg P L−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.

Introduction

In natural waters phosphorus is an essential macronutrient for plant and algal growth, and in fresh waters is often growth limiting [1]. Aquatic phosphorus can be functionally categorised into two macro groups; i.e. species that can pass through a 0.22 μm filter (filterable phosphorus) and those that cannot (particulate phosphorus). Further distinction can be made between inorganic and organic phosphorus [2]. Total phosphorus is the measurement of the sum of all phosphorus containing compounds within a body of water; i.e. phosphorus that exists in colloidal and particulate matter, phytoplankton, bacteria and in dissolved forms [3].

Excessive loading of nutrients, including phosphorus, can lead to eutrophication, which is a widespread cause of aquatic system degradation [4], [5]. As algae die they cause depletion of dissolved oxygen, both through oxidative consumption, and reduction of photosynthetic activity due to light obstruction from the surface algal films. Monitoring of nutrients is essential to gain a greater understanding of the underlying causes of eutrophication and to enable more effective waterway management. Of particular concern is the dissolved inorganic specie, orthophosphate, which is the most readily available form of phosphorus for algal production [1], and thus measurement of orthophosphate concentration can give insight into whether an aquatic system is at risk of undergoing deleterious processes associated with eutrophication. Similarly, total phosphorus is also of interest, as it represents the maximum potential amount of bioavailable phosphorus, and for this reason is often the parameter that is specified in discharge licenses.

Phosphorus distribution in surface waters can show a high degree of variability, and large numbers of analyses must be performed in order to obtain sufficient spatial and temporal data to fully understand phosphorus cycling within an aquatic system [6]. The cost and labour involved in manual sampling, followed by laboratory based analysis, is frequently a hindrance to achieving this goal. There is also the risk of sample degradation during off-site filtration, transport and storage. In situ measurement avoids many of these complications, and if rapid automated instruments are used for this purpose, there is the potential of obtaining intensive spatial and temporal data.

An ideal in situ monitoring system should have the following characteristics: light-weight, low power consumption, economic use of stable reagents, reliability and durability, simplicity and low cost. Analytical features such as accuracy, high sample throughput, sensitivity and precision are also required to produce quality data with the spatial and temporal resolution capable of capturing short-term events or mapping diffuse inputs. Flow injection analysis (FIA) techniques are capable of achieving all of these objectives if used for in situ, or at least onsite, operation. Traditional FIA methods involve the injection of a small volume of sample into a constantly flowing carrier stream which is later merged with reagents [7]. When coupled with spectrophotometric detection, flow analysis techniques can achieve excellent precision, part per billion sensitivity, and achieve throughputs of about 100 samples per hour. A similar technique, called reagent injection FIA, is the injection of small amounts of reagent into a constantly flowing stream of sample. This approach provides the advantage of minimising reagent consumption and waste production; and may also enhance sensitivity [7]. A reagent injection FIA instrument for the measurement of filterable reactive phosphorus via the molybdenum blue method has been previously developed and operated in the field by Lyddy-Meaney et al. [8].

While there has been extensive research into field instrumentation for the measurement of filterable reactive phosphorus [8], [9], [10], [11], there has been comparatively little effort toward the development of a portable system for the measurement of total phosphorus. Measurement of total phosphorus requires that particulate and filterable inorganic and organic species of phosphorus be mineralised to orthophosphate prior to detection using the molybdenum blue method [2]. Determining total phosphorus has all the complications of measuring molybdenum blue reactive phosphate, with the additional requirement of mineralisation of all phosphorus containing species within a heterogeneous, unfiltered sample to orthophosphate.

Upon exposure to ultra-violet light, a peroxodisulfate solution will generate hydroxyl and sulfate radicals, which are strong oxidising agents [12]. McKelvie et al. [13] have shown that using a low pressure mercury lamp reactor and a 40 g L−1 peroxodisulfate oxidising agent, complete mineralisation of organic phosphorus compounds can be achieved in as little as 20 s reactor residence time. In order to mineralise condensed phosphate species, assist in the breakdown of colloidal or particulate-bound phosphorus, and to avoid precipitation of magnesium or calcium phosphates [14], an acidification of the sample is required. Acidification may be achieved by either using an acidic peroxodisulfate oxidising medium [15], [16], [17], by allowing the peroxodisulfate to decompose thermally to generate acid [18], or by a use of an alkaline medium with an additional in-line acidification step [19]. However, many methods report incomplete recovery of condensed phosphates [15], [16], [19], unless very long delay times of the order of 15–60 min and high acid concentrations ca. 4 M H2SO4 are used, which affect the throughput and sensitivity of the determination because of suppression of phosphomolybdenum blue formation with increasing acid concentration [20]. Given that condensed phosphates hydrolyse to orthophosphate within a matter of hours in natural waters, many researchers have considered the necessary delay times and accompanying loss of sensitivity because of the increased acidity as an unacceptable compromise [21].

While numerous methods exist for the detection of orthophosphate in waters, the molybdenum blue method has been the most widely adopted for both laboratory and field use, due to its reliability, selectivity, speed and simplicity [3]. In aqueous and acidic conditions, molybdenum will form a yellow heteropoly acid complex with orthophosphate. While the molybdenum yellow complex can be detected photometrically, the complex may be reduced, typically by either tin(II) chloride or ascorbic acid, to form the charge-transfer complex phosphomolybdenum blue which has a higher molar absorptivity than the yellow complex. Stannous chloride offers a faster rate of reduction compared to ascorbic acid [3], [22], although it is prone to suppression with increasing chloride concentration [3]. Silicic acid may also form a yellow heteropoly acid with molybdate, but under reactions conditions of less than pH 1, the molybdenum blue method is highly selective for orthophosphate [23].

In this paper the development and field employment of a portable flow analysis system used for the determination of total phosphorus is described. The instrument comprises a UV photo-reactor, constructed from Teflon® tubing wound around a medium pressure mercury lamp, as well as a heating unit consisting of an electrically heated Teflon® tubing coil. Sample is merged with an acidic peroxodisulfate solution prior to passing through the photo-reactor and heater. Digested sample is filtered through a 0.22 μm in-line hollow-fibre filter before being pumped into a FIA system that employs miniature solenoid valves to deliver small amounts of gas pressurised reagents for the detection of orthophosphate by the molybdenum blue method. Phosphomolybdenum blue is then detected spectrophotometrically. Investigations into chloride suppression of the stannous chloride based molybdenum blue method and the difficulties encountered using of an alkaline oxidising medium in marine waters are also discussed. The flow analysis system was deployed aboard the SV Pelican 1, during the Two Bays program in January 2010 for the underway measurement of total phosphorus.

Section snippets

Experimental

All reagents were prepared from analytical grade chemicals and ultrapure water (MilliQ, Millipore, Model: Milli-Q Academic).

Optimisation of the digestion procedure

Preliminary investigations indicated that the ultra-violet irradiation of phytic acid, a recalcitrant organic phosphorus compound, in the presence of peroxodisulfate resulted in its rapid conversion to orthophosphate. Given that rapid sample throughput is a primary objective of an instrument designed for underway chemical mapping, the digestion procedure was optimized so that minimum residence time was required to achieve 100% mineralisation of total phosphorus.

The effect of the photo-reactor

Conclusion

The concept, development and evaluation of a portable flow analysis system for the determination of total phosphorus in situ have been described. This work indicates that flow analysis using a photo-reactor and thermal decomposition unit in-line, coupled with a reagent injection flow injection analyser was successful at rapidly and reliably determining total phosphorus in surface marine, estuarine and fresh waters. The instrument meets many of the criteria for unattended field-use such as total

Acknowledgements

This research was financially supported by an Australia Research Council Linkage Grant (LPO669359) and EPA Victoria. The assistance of Garry McKechnie, Natalie Davey and the crew of SV Pelican 1 during the Two Bays cruise is gratefully acknowledged.

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