Ref. No. [UMCES]CBL 2013-018
ACT VS12-04
Vertical profiling results for all six casts are presented in figures 11-13. Each panel
displays the Hach FP 360 sc response in ppb Oil along with a continuous trace of temperature
and salinity for that cast. In addition, total hydrocarbon, CDOM, Turbidity, and Chlorophyll
concentrations from the discrete reference samples are plotted on the same graph. It should be
noted that despite the presence of a visible sheen of oil on the surface of the water above the
leaking barge, all total hydrocarbon concentrations were reported as below detection for all
reference samples collected (stated method of detection limit ≤ 25 ppb). The instrument
response was essentially at background reading either 0 or 36 ppb Oil, with most of the non-zero
response relating to when the CTD was paused at depth to collect a discrete water sample. These
readings are below the instruments likely detection limit of approximately 30 ppb Oil (based on
average proportionality to a specified detection limit of 1.2 ppb PAH), and consistent with the
analytical non-detects for the reference samples. The FP 360 sc was unresponsive to differences
in CDOM, Turbidity, and Chlorophyll concentrations with depth, where differences were
minimal, or to differences between sites where they were quite large. While a few reference
sample EEM characterizations revealed some possible evidence of hydrocarbons (e.g. the surface
sample of site 2, cast 5 and mid-depth sample for site 2, cast 3 (Fig. 14), even those peaks fell
mostly outside the optical window of the FP 360 sc. Only minimum levels of fluorescent signal
were observed throughout the EEM maps of the other samples. As noted earlier all TPH
concentrations were below detection and therefore it was not possible to evaluate any
relationship between instrument response and hydrocarbon concentrations (Fig. 15, panel A). In
addition, there was no consistent relationship between instrument response and the
corresponding EEM
QSE
intensities from the same reference samples (Fig. 15, panel B).
SUMMARY of INSTRUMENT RESPONSE ACROSS ALL TEST APPLICATIONS
Overall, the Hach FP 360 sc UV fluorometer exhibited a linear response to challenge
analyte concentrations for the variety of compounds tested, however, clear differences in the
measurement response occur based on having fluorescence properties best match the optical
configuration of the instrument package (Fig. 16). “Packaging” of oil droplets also appeared to
have a significant impact on response level as seen by differences in the dispersed versus non-
dispersed trials at BIO-COOGER and as seen within the spread of the BIO-COOGER data in
figure 16.
It is unknown whether having the instrument logged through a second party data
logger impacted the response of the instrument in the Gulf of Mexico.
Care should be taken in
specific interpretation of environmental fluorescence signals in absence of analytical
reference samples.
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