Performance of a Fiber Optic Spectrofluorometer with Applications for In Situ Sensing and Eddy Correlatio

A novel optical fiber spectrofluorometer has been created capable of high-speed, high-resolution in situ measurements of naturally fluorescing compounds. This low-power (<5 W) spectrofluorometer utilizes an LED for excitation, with optional modulation to reject ambient light. A pair of 1000 um optical fibers is used to carry light to and from the sensing volume. Detection is accomplished by a tunable monochromator coupled to a photomultiplier tube and a custom photon-counting circuit board. The instrument is compact and battery-operated to allow deployment in the field, and is controlled by a wireless-enabled microcomputer.

The speed and sensitivity performance of the instrument has been characterized based on humic acid and fluorescence concentration measurements. For example, in a 3 ppm humic acid solution, 1,900 photons were measured in 50 ms (20 Hz), with a stdev/avg error of 2.9%. This compares favorably to the theoretical counting statistics sqrt(N)/N error of 2.3%. The calibration curve for humic acid is highly linear at low to moderate concentrations (e.g. at < 2 ppm total organic carbon (TOC), R² = 0.98), and TOC concentrations as low as 0.2 ppm were detected. The calibration curve for fluorescein is extremely linear (R²= 1) and concentrations as low as 1 ppb were detected.

The tunable monochromator allows scans of the entire emission spectrum from ~185 ~ 650 nm, with resolution of the order of 10 nm. Thus, the instrument can potentially be used to target a variety of compounds, as well as to characterize the fluorescent chemical composition of the environment. The use of optical fibers to interrogate a distal sensing volume allows great flexibility in physical setup, making the instrument suitable for a variety of marine applications. In particular, its adaptable geometry and fast response time render it particularly suitable for use in aquatic eddy correlation (EC) systems. EC is a relatively new technique for determining benthic fluxes that to date has mainly been used to determine dissolved oxygen fluxes. By coupling this spectrofluorometer to an Acoustic Doppler Velocimeter, it will be possible to determine benthic fluxes of naturally fluorescing compounds. Expanding the capabilities of EC in this way to a much wider range of biogeochemical fluxes is one exciting potential application of this new instrument.