An evaluation of ISFET sensors for coastal pH monitoring applications

Abstract

The accuracy and precision of ion sensitive field effect transistor (ISFET) pH sensors have been well documented, but primarily by ocean chemistry specialists employing the technology at single locations. Here we examine their performance in a network context through comparison to discrete measurements of pH, using different configurations of the Honeywell DuraFET pH sensor deployed in six coastal settings by operators with a range of experience. Experience of the operator had the largest effect on performance. The average difference between discrete and ISFET pH was 0.005 pH units, but ranged from-0.030 to 0.083 among operators, with more experienced operators within ± 0.02 pH units of the discrete measurement. In addition, experienced operators achieved a narrower range of variance in difference between discrete bottle measurements and ISFET sensor readings compared to novice operators and novice operators had a higher proportion of data failing quality control screening. There were no statistically significant differences in data uncertainty associated with sensor manufacturer or deployment environment (pier-mounted, flowthrough system, and buoy-mounted). The variation we observed among operators highlights the necessity of best practices and training when instruments are to be used in a network where comparison across data streams is desired. However, while opportunities remain for improving the performance of the ISFET sensors when deployed by less experienced operators, the uncertainty associated with their deployment and validation was several-fold less than the observed natural temporal variability in pH, demonstrating the utility of these sensors in tracking local changes in acidification.

Characterization of the CO2 System in a Coral Reef, a Seagrass Meadow, and a Mangrove Forest in the Central Red Sea

“…The mean (± SD) difference between the spectrophotometric pH T in samples and the SeaFET measures are −0.001 ± 0.012, 0.003 ± 0.050 and 0.02 ± 0.20 in the coral reef, seagrass meadow and mangrove forest, respectively (Figure S1.). The precision and accuracy of the pH T measurement in the coral reef and seagrass meadow were comparable to other studies in coastal ecosystems (McLaughlin et al, ). The precision and accuracy were, however, 1 order of magnitude lower in the mangrove forest waters, probably due to the extreme deployment conditions (high temperature and salinity, low water level with periods of emersions, exposure to sediment) and the high potential for large differences of the CO 2 system within a few cm between sampling point and sensor.…”

Variability of Seawater Chemistry in a Kelp Forest Environment Is Linked to in situ Transgenerational Effects in the Purple Sea Urchin, Strongylocentrotus purpuratus

“…To compare benthic relationships between pH, oxygen, and in situ temperature inside and outside of a kelp forest, SeapHOx R sensors (equipped with a Honeywell DuraFET R III pH electrode, an Aanderaa R 3835 oxygen optode, and Seabird R SBE-37 microCAT CTD) (Martz et al, 2010;McLaughlin et al, 2017) were deployed 1 m off the bottom at both the inside and outside spar buoy locations for 4 weeks (March 10-April 08, 2015). Sensors were programmed to log every 20 min.…”

Perspectives on in situ Sensors for Ocean Acidification Research

“…Wanninkhof et al (2013) have provided further guidance on incorporating in situ sensors into the SOCAT QC framework, including details such as degree of in situ calibration necessary for desired confidence and levels of accuracy. A number of published papers address practical suggestions and demonstrations for Durafet R pH sensor QC (Bresnahan et al, 2014;Johnson et al, 2016;Rérolle et al, 2016;McLaughlin et al, 2017;Gonski et al, 2018;Miller et al, 2018). Similarly, there is considerable ongoing research into complications with in situ spectrophotometric pH analyzers, especially those resulting from indicator dye impurities and the moving parts within these systems (e.g., Liu et al, 2011;Lai et al, 2018).…”