Oceanography | Vol. 38, No. 2
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is used for solutions of PC (Jaeschke et al., 2021) and PE (Ardiles
et al., 2020), and deionized water is used for RWT. The PIXIE is
compatible with the above solvents but cannot tolerate the sul
furic acid that dissolves quinine sulfate, the stand-in for crude
oil calibration. Sulfuric acid is used almost exclusively in lit
erature to prepare fluorescence standards of quinine sulfate
(Kristoffersen et al., 2018).
Sensor Design
Design notes for the PIXIE, available in the PIXIE Complete
User Guide document provided at the GitHub link, are described
briefly here for context. The PIXIE was designed with a focus
on user configurability, and customization if desired. Users can
simply acquire the hardware and assemble the default configura
tion or use the PIXIE’s documentation if more detailed custom
ization is needed.
The PIXIE performs fluorometry using standard optics
through an O-ring-sealed glass window. The user configures the
targeted fluorophore for each of the four channels by selecting
the appropriate optical filters and excitation LEDs. The PIXIE
described in this work was equipped with hardware to target
PC, RWT, Chl-a, and crude oil, though only the RWT and Chl-a
channels were used. The PIXIE cannot measure using more than
one channel at a time, but it can cycle between channels fast
enough to achieve quasi-simultaneous measurements.
To extract only the fluorescence excited by the device itself,
the PIXIE modulates the brightness of its excitation LEDs sinu
soidally. While the change in brightness is imperceptible to the
eye, the resulting fluorescence will have a synchronous bright
ness that can be distinguished from other sources of light. This
allows PIXIE measurements even in bright laboratory or sun
lit outdoor conditions. The process of measuring the sinusoidal
fluorescence and converting it to a measure of fluorophore con
centration, or lock-in amplification, is implemented digitally and
can therefore be adjusted by more technically inclined users. The
PIXIE detects the sinusoidal fluorescence using an AC-coupled
transimpedance amplifier with a software-configurable gain of
400 MΩ, within a sample volume of 0.1 mL. More technical
details can be found on our GitHub page.
Calibration
The PIXIE’s RWT and Chl-a channels were calibrated in the
laboratory using a set of temperature-controlled standards.
Fluorescence is known to depend strongly on temperature
(Smart and Laidlaw, 1977), so the PIXIE was calibrated across
a range of temperatures and concentrations. The set of tempera
tures and RWT concentrations were chosen to parallel previ
ous work in this area (Park et al., 2023) for comparison’s sake.
The protocols used were adapted from a US Environmental
Protection Agency Method 445 on Chl-a fluorometer calibra
tion (Arar and Collins, 1997). An effort was made to adapt the
protocol to keep the number of expensive lab instruments and
equipment to a minimum, though the protocol applied to Chl-a
required a fume hood. The PIXIE was suspended above a beaker
such that its sensing end was submerged without overflowing
or trapping air bubbles (see Figure 1, bottom inset). A complete
description of the calibration protocols is available in the PIXIE
Complete User Guide document on GitHub.
Field Deployment
A 10 L Niskin bottle (General Oceanics, Miami, FL, USA) was
prepared for use during an RWT tracer release experiment in
Halifax Harbor, as depicted in Figure 1. The bottle was equipped
with a proprietary datalogger that powered an array of exter
nal sensors, including for temperature and depth. The PIXIE
was also mounted to the Niskin bottle, pointed downward, and
constantly streamed its fluorometry data to the logger via the
RS-232 protocol at a frequency of 16 Hz.
On August 10, 2023, a pre-set amount of RWT was released
from the Tufts Cove Power Generation Station to study the
dye plume. Multiple sensors and techniques were used, includ
ing the PIXIE-integrated Niskin bottle and an ecoCTD (Dever
FIGURE 2. The PIXIE, in assem
bled view (left) and in labeled
exploded view (right). For clar
ity, only one channel is popu
lated with optics. Cable, exci
tation LEDs, and some O-rings
are omitted.