Traditional, ex situ nutrient monitoring efforts require the use of instruments such as autoanalyzers to perform reagent-based analytical techniques on sea water samples to determine nutrient concentrations. The time and cost of sample retrieval and preparation often leads to poor spatial and temporal resolution of nutrient measurements in remote environments. Microfluidic technologies enable in situ sensors to perform the same reagent-based analysis in these environments, with a comparable limit of detection (LOD) and repeatability to the bench top instruments [1].

Here, we present a fully automated in situ phosphate analyzer based on an inlaid microfluidic absorbance cell [2]. The cell is made from opaque polymethyl(methacrylate) (PMMA) inlaid into clear PMMA to attenuate any non-directional or scattered light. It uses embedded microprisms to direct light from an 880 nm LED through the interrogation channel, and then up into a photodiode to measure the resultant intensity. The sensor uses three independently controlled stepper motors to actuate four syringes: two for reagents, one for a standard and one for the sample. There are 10 solenoid valves in a separate oil filled pressure tolerant compartment to ensure reliable and repeatable fluid control. Power is supplied either by the optional separate battery back, or through an external source. The fluids and waste are all housed in a separate perforated container and connected by Teflon tubing.

Using colorimetric absorbance spectrophotometry to determine phosphate concentrations in sea water, the sensor achieves a practical LOD of less than 100 nM. Furthermore, an in situ verification with an on-board standard showed that the measurements were repeatable, with a relative standard deviation of less than 1.5 %. We determined the temperature dependence of the colorimetric reaction by carrying out four sequential calibrations, at intervals between 5 °C and 20 °C, with phosphate standards ranging from 0.2 to 10 µM. Next, we fixed the sensor to a jetty at a depth of 1.5 m to capture the phosphate fluxes during the diurnal tidal cycle in the Bedford Basin. To verify and bench-mark the sensor measurements, we used a Niskin bottle to retrieve samples from alongside the instrument, and then prepared them for analysis on an autoanalyzer. Finally, we mounted the phosphate analyzer to the Stella Maris multi-sensor platform 100 m off shore at a depth of 10 m for a multi-month deployment, in the Bedford Basin in Nova Scotia, Canada.