Until recently, it has not been possible to measure O(2) concentrations in oxygen minimum zones (OMZs) with sufficient detection limits and accuracy to determine whether OMZs are anoxic or contain 1-2 μM O(2). With the introduction of the STOX (switchable trace oxygen) sensor, the level for accurate quantification has been lowered by a factor of 1000. By analysis with STOX sensors, O(2) can be prevented from reaching the sensing cathode by another cathode (front guard cathode), and it is the amplitude in signal by polarization/depolarization of this front guard that is used as a measure of the O(2) concentration. The STOX sensors can be used in situ, most conveniently connected to a conventional CTD (conductivity, temperature, and depth analyzer) along with a conventional oxygen sensor, and they can be used for monitoring O(2) dynamics during laboratory incubations of low-O(2) media such as OMZ water. The limiting factors for use of the STOX sensors are a relatively slow response, with measuring cycle of at least 30 s with the current design, and fragility. With improved procedures for construction, the time for a complete measuring cycle is expected to come down to about 10 s.
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