Lab-on-Chip Measurement of Nitrate and Nitrite for In Situ Analysis of Natural Waters

Abstract

Microfluidic technology permits the miniaturization of chemical analytical methods that are traditionally undertaken using benchtop equipment in the laboratory environment. When applied to environmental monitoring, these "lab-on-chip" systems could allow high-performance chemical analysis methods to be performed in situ over distributed sensor networks with large numbers of measurement nodes. Here we present the first of a new generation of microfluidic chemical analysis systems with sufficient analytical performance and robustness for deployment in natural waters. The system detects nitrate and nitrite (up to 350 μM, 21.7 mg/L as NO(3)(-)) with a limit of detection (LOD) of 0.025 μM for nitrate (0.0016 mg/L as NO(3)(-)) and 0.02 μM for nitrite (0.00092 mg/L as NO(2)(-)). This performance is suitable for almost all natural waters (apart from the oligotrophic open ocean), and the device was deployed in an estuarine environment (Southampton Water) to monitor nitrate+nitrite concentrations in waters of varying salinity. The system was able to track changes in the nitrate-salinity relationship of estuarine waters due to increased river flow after a period of high rainfall. Laboratory characterization and deployment data are presented, demonstrating the ability of the system to acquire data with high temporal resolution.

Trends in microfluidic systems for in situ chemical analysis of natural waters

“…[48][49][50] Of these, colorimetry -in which the sample is mixed with an analyte-specific reagent to produce a measurable colour -has proved to be by far the most popular method and has been used for in situ analysis of a range of chemical parameters including nitrate and nitrite, [21,22,24,29,31,34,35,38] phosphate, [27,38] iron, [23,26,28,36,39,[42][43][44] manganese, [25,26,28,37,40] sulfide [32,33,35,39,41] silicate [30,32,33,38] and pH. [51][52][53][54][55] Colorimetry lends itself well to microfluidic in situ analysers as it is chemically robust, offers excellent analytical performance (limits of detection typically in the order of 10 nM [21,25,36]) and requires relatively small, cheap and easily-sourced components.…”

“…[21,22] In contrast to solenoid pumps, syringe pumps can deliver pulse-free, stable low-volume flow (down to µL/min) by using a hightorque rotary stepper-motor which linearly drives syringes via a screwthread (see Fig 2d). The high torque motor means they can tolerate the elevated backpressures associated with small channels (allowing channel dimensions to be decreased to a couple of hundred µm [21,22,60]) whilst still offering a significant power saving compared to peristaltic pumping. For example the syringe pump used in the example discussed below typically uses 1.5 W during sensor deployment.…”

“…For example the syringe pump used in the example discussed below typically uses 1.5 W during sensor deployment. [22] Consequently they offer optimum flexibility, allowing long term deployments with excellent analytical performance.…”

“…An example of an in situ sensor that utilises syringe pumping is the colorimetric nitrate/nitrite sensor reported by Beaton et al in 2012, [22] shown in Fig 4a. In contrast to the systems described earlier which used off-the-shelf capillary tubing, microfluidic channels were milled into a plastic substrate and sealed [61] to form a compact monolithic microfluidic chip ("lab on a chip").The use of syringe pumping meant that small (150 x 300 µm) channels could be used to minimise fluid volume. The pump was directly mounted onto the chip (see Fig 4a) and, importantly, the chip also hosted all optical measurement components.…”

Development and Deployment of a Microfluidic Platform for Water Quality Monitoring

“…Mowlem and co-workers at the National Oceanography Centre, Southampton, UK, have developed in-situ sensors for a range of chemical parameters [23][24][25][26][27] including nitrite, nitrate, ammonia, phosphate and iron. The first of this series of microfluidic chemical analysis systems [23] was used to detect nitrate and nitrite with a limit of detection (LOD) of 0.025 µM for nitrate (0.0016 mg L -1 as NO 3 − ) and 0.02 µM for nitrite (0.00092 mg L -1 as NO 2 − ).…”