Autonomous in situ calibration of ion-sensitive field effect transistor pH sensors

Ion-sensitive field effect transistor-based pH sensors have been shown to perform well in high frequency and long-term ocean sampling regimes. The Honeywell Durafet is widely used due to its stability, fast response, and characterization over a large range of oceanic conditions. However, potentiometric pH monitoring is inherently complicated by the fact that the sensors require careful calibration. Offsets in calibration coefficients have been observed when comparing laboratory to field-based calibrations and prior work has led to the recommendation that an in situ calibration be performed based on comparison to discrete samples. Here, we describe our work toward a self-calibration apparatus integrated into a SeapHOx pH, dissolved oxygen, and CTD sensor package. This Self-Calibrating SeapHOx is capable of autonomously recording calibration values from a high quality, traceable, primary reference standard: equimolar tris buffer. The Self-Calibrating SeapHOx's functionality was demonstrated in a 6-d test in a seawater tank at Scripps Institution of Oceanography (La Jolla, California, U.S.A.) and was successfully deployed for 2 weeks on a shallow, coral reef flat (Lizard Island, Australia). During the latter deployment, the tris-based self-calibration using 15 on-board samples exhibited superior reproducibility to the standard spectrophotometric pH-based calibration using > 100 discrete samples. Standard deviations of calibration pH using tris ranged from 0.002 to 0.005 whereas they ranged from 0.006 to 0.009 for the standard spectrophotometric pH-based method; the two independent calibration methods resulted in a mean pH difference of 0.008. We anticipate that the Self-Calibrating SeapHOx will be capable of autonomously providing climate quality pH data, directly linked to a primary seawater pH standard, and with improvements over standard calibration techniques.