Characterization of a novel autonomous analyzer for seawater total alkalinity: Results from laboratory and field tests

Katharina Seelmann,Steffen Aßmann,Arne Körtzinger

doi:10.1002/lom3.10329

Abstract

AbstractHigh‐quality seawater total alkalinity (AT) measurements are essential for reliable ocean carbon and acidification observations. Well‐established manual multipoint potentiometric titration methods already fulfill these requirements. The next step in the improvement of these observations is the increase of the spatial and temporal measuring resolution with minimal personnel and instrumental effort. For this, a rapid, automated underway analyzer meeting the same high requirements as the traditional method is necessary. In this study, we carried out a comprehensive characterization of the flow‐through analyzer CONTROS HydroFIA® TA (Kongsberg Maritime Contros GmbH, Kiel, Germany) for automated seawater AT measurements in the laboratory and in field with overall more than 5000 measurements. Under laboratory conditions, the analyzer featured a precision of ± 1.5 μmol kg−1 and an accuracy of ± 1.0 μmol kg−1, combined in an uncertainty of 1.6 – 2.0 μmol kg−1. High precision (± 1.1 μmol kg−1) and accuracy (−0.3 ± 2.8 μmol kg−1), and low uncertainty (2.0 – 2.5 μmol kg−1) were also achieved during field trials of 4 and 6 weeks duration. Although a linear drift appears to be the typical behavior of the system, this can be corrected for by regular reference measurements giving consistent measurement results. Another advantage of regular reference measurements is the early detection of any kind of malfunction due to its direct impact on the measurement performance. Based on the present study, recommendations for automated long‐term deployments are provided in order to gain optimal performance characteristics, aiming at the requirements for AT measurements.

Highlights

The root mean square error (RMSE) was determined with Æ 5.5 μmol kg−1; afterward, it is improved to Æ 1.0 μmol kg−1
For an autonomous analyzer with spectrophotometric pH determination, such a level of precision is still in favorable comparison to the standard AT titration methods. Uncertainty approximations both in the laboratory and in field are in full agreement with the “weather” goal requirements by Newton et al (2015) for ocean acidification observations
The very high requirement of the “climate” goal is achieved in the laboratory and almost be achieved in the field by being only 0.5 μmol kg−1 higher than the target of 2 μmol kg−1

Summary

Introduction

Other disadvantages of the traditional method are the relatively long-measurement time per sample (approximately 10–20 min), the need of well-trained technicians in an air-conditioned laboratory, and the fact that the Testing an autonomous total alkalinity analyzer measured seawater must be provided as a bottled and typically poisoned discrete sample. (2) The “Global Ocean Acidification Observing Network: Requirements and Governance Plan” by Newton et al (2015) provides uncertainty targets for AT measurements in order to identify relative spatial patterns and short-term variations (“weather” goal), and to assess long-term trends with a defined level of confidence (“climate” goal), respectively.

Objectives

Methods

Results

Conclusion