A Novel Lab-on-Chip Spectrophotometric pH Sensor for Autonomous In Situ Seawater Measurements to 6000 m Depth on Stationary and Moving Observing Platforms

Abstract

We report a new, autonomous Lab-on-Chip (LOC) microfluidic pH sensor with a 6000 m depth capability, ten times the depth capability of the state of the art autonomous spectrophotometric sensor. The pH is determined spectrophotometrically using purified meta-Cresol Purple indicator dye offering high precision (<0.001 pH unit measurement reproducibility), high frequency (every 8 min) measurements on the total proton scale from the surface to the deep ocean (to 600 bar). The sensor requires low power (3 W during continuous operation or ∼1300 J per measurement) and low reagent volume (∼3 μL per measurement) and generates small waste volume (∼2 mL per measurement) which can be retained during deployments. The performance of the LOC pH sensor was demonstrated on fixed and moving platforms over varying environmental salinity, temperature, and pressure conditions. Measurement accuracy was +0.003 ± 0.022 pH units (n = 47) by comparison with validation seawater sample measurements in coastal waters. The combined standard uncertainty of the sensor in situ pH T measurements was estimated to be ≤0.009 pH units at pH 8.5, ≤ 0.010 pH units at pH 8.0, and ≤0.014 pH units at pH 7.5. Integrated on autonomous platforms, this novel sensor opens new frontiers for pH observations, especially within the largest and most understudied ecosystem on the planet, the deep ocean.

Field Application of Automated Spectrophotometric Analyzer for High-Resolution In Situ Monitoring of pH in Dynamic Estuarine and Coastal Waters

“…The automated real-time observation of carbonate chemistry dynamics with sensors like LOC offer a potentially substantial improvement in that regard. While the performances of the LOC sensor for high-resolution spectrophotometric pH measurements has been demonstrated in surface waters of shelf seas with salinities above 24 (Rérolle et al, 2018;Yin et al, 2021), this is the first report on deployments of the sensor in dynamic estuarine waters (S<20).…”

“…Under oceanic conditions, spectrophotometric pH measurement systems have demonstrated excellent performance (Clayton and Byrne, 1993;Bellerby et al, 2002;Aßmann et al, 2011;Rérolle et al, 2016). Commercially available, submersible spectrophotometric sensors (see Table 1, Lab-on-Chip (LOC) and SAMI-pH) have been successfully tested for fine-scale, autonomous and in situ monitoring of pH in a wide variety of environments, from freshwater (Martz et al, 2003) to seawater (Seidel et al, 2008;Rérolle et al, 2018;Yin et al, 2021). Indicator impurities and wavelength accuracy of spectrophotometers are some of the potential sources of uncertainty in spectrophotometric pH measurements, which DeGrandpre et al (2014) has reported in detail.…”

“…Recently, the capabilities of the LOC pH sensor have been successfully demonstrated on fixed and moving platforms under different environmental conditions, including deployments in surface waters of Southampton (S>24) for several months, and in deep ocean waters (~4820 m) (Yin et al, 2021). However, the performance of this LOC pH sensor in estuarine and coastal waters has not yet been thoroughly investigated.…”

“…The ratio of absorbances at the absorbance maxima of the acidic and basic forms of the indicator dye is then used to assess the pH value of a sample (Clayton and Byrne, 1993). In continuous operation mode, the sensor requires low power about 3 W or 1300 J per measurement and one measurement takes approximately 8 minutes (Yin et al, 2021). For each measurement, 3 µl of indicator dye is added to 700 µl of sample according to a pre-programmed measurement routine stored internally on the sensor unit.…”

“…The inlet of an acid cleaned (with 1 M HCl) 6.4 mm ID C-Flex tubing (Cole Palmer Masterflex) was attached to the frame close to the sample inlet of the LOC pH sensor. Discrete seawater samples for direct spectrophotometric laboratory analysis of pH T (Yin et al, 2021) were pumped into 250 mL borosilicate bottles and sealed with a ground stopper. Analyses of unpreserved samples were performed within few hours in the laboratory using a benchtop spectrophotometric CONTROS HydroFIA pH analyzer (4H-Jena engineering GmbH, Aßmann et al, 2011).…”