A portable fiber optic system for measuring particle absorption using the quantified filter technique (QFT)

Richard L. Miller, Christopher Buonassissi, Carlos E. Del Castillo, Mathias Belz

Limnol. Oceanogr. Methods 9:554-564 (2011) | DOI: 10.4319/lom.2011.9.554

ABSTRACT: The Quantitative Filter Technique (QFT) has been shown to be an effective method for determining absorption coefficients of aquatic particles retained by filters. Although many investigators use the QFT, its routine use has been limited due in-part to the required use of a high performance spectrophotometer and prescribed protocols for handling, storage, and transport of filters obtained in the field for later laboratory analysis. We describe a portable fiber optic system for conducting QFT measurements designed to overcome these limitations. The system (QFT2) consists of a high intensity light source, filter holder, and a photodiode array spectrometer configured as a single-beam optical system. A set of 52 samples consisting of natural samples and cultures were analyzed following standard QFT protocols on a conventional spectrophotometer and the QFT2 system. Optical Density (absorbance) rather than absorption spectra were compared to avoid errors introduced by determining the path-length amplification factor, b. Optical density (OD) spectra normalized at 400 nm measured using the fiber optic system were very similar to spectrophotometer spectra over a wide range of particle loadings. Strong linear relationships of normalized OD values at 443, 520, 550, 650, and 670 nm were observed for total particulate (r2 = 0.996; P < 0.001, n = 52), detrital particulate (r2 = 0.991, P < 0.001, n = 38), and pigmented particles or phytoplankton (r2 = 0.968, P < 0.001, n = 38). The greatest difference between the two systems was observed at ca. 683 nm where the QFT2 underestimates absorption due to pigment fluorescence.