Development of an in-situ total phosphorus analyzer

Abstract

A low cost autonomous, in-situ analyzer for orthophosphate and total phosphorus was developed and tested for near real time monitoring in aquatic and marine environments. The system applies the colorimetric stannous chloride-molybdenum blue method following oxidative digestion with acidic peroxodisulfate. A (fluorinated ethylene propylene) FEB tubing reactor coil is wrapped around a germicidal UV (254nm) light to accelerate the digestion reaction. A Nickel chromium wire (NiCr) wound between reactor coil wraps provides controlled heating for digestion. Reactor temperature is monitored continuously with a small temperature probe and controlled with a PID. A sequential injection analysis (SIA) method was chosen where the analyzer operates as a sequential batch reactor. Water sample and reagents are delivered through the microfluidics using syringe pump driven by linear actuators that are controlled with a Teensy microprocessor. To equalize pressures, reagent storage and waste are contained in collapsible reagent bags maintained at hydrostatic pressure outside the submersible housing. Following digestion and colorimetric reactions the sample is transferred to flow cell (5 cm path length) where a photodiode measures sample absorbance at 600 nm. For orthophosphate phosphorus measurements, the sample is not treated with digestion reagents, heat, or UV. Potassium dihydrogen phosphate and adenosine monophosphate (AMP) were used to assess analyzer performance for measuring orthophosphate and total phosphorus, respectively. The prototype analyzer produced a linear response during bench testing to total phosphorus with a precision of ±4.3% over a detection range concentration range of 30–200µg P/L. To evaluate the performance with natural waters, dilutions of algal culture with known phosphorus concentration were measured and found to be within 90% of nominal phosphorus concentrations. A preliminary cost analysis indicates that the operating components of the system (minus submersible housing) can be purchased for under $3000 compared to $30,000 for commercially available systems. At this low cost of construction, this sensor design may be a viable option for use in nutrient monitoring programs. Considering that system components have been designed to withstand internal pressures equivalent to a water depth of 100 m with a designed duty capacity of 2 weeks suggest the possibility of applications in coastal waters and relatively deep lakes.