Measurement of Dissolved Organic Matter Fluorescence in Aquatic Environments: An Interlaboratory Comparison

Abstract

The fluorescent properties of dissolved organic matter (DOM) are often studied in order to infer DOM characteristics in aquatic environments, including source, quantity, composition, and behavior. While a potentially powerful technique, a single widely implemented standard method for correcting and presenting fluorescence measurements is lacking, leading to difficulties when comparing data collected by different research groups. This paper reports on a large-scale interlaboratory comparison in which natural samples and well-characterized fluorophores were analyzed in 20 laboratories in the U.S., Europe, and Australia. Shortcomings were evident in several areas, including data quality-assurance, the accuracy of spectral correction factors used to correct EEMs, and the treatment of optically dense samples. Data corrected by participants according to individual laboratory procedures were more variable than when corrected under a standard protocol. Wavelength dependency in measurement precision and accuracy were observed within and between instruments, even in corrected data. In an effort to reduce future occurrences of similar problems, algorithms for correcting and calibrating EEMs are described in detail, and MATLAB scripts for implementing the study's protocol are provided. Combined with the recent expansion of spectral fluorescence standards, this approach will serve to increase the intercomparability of DOM fluorescence studies.

Moving towards a technical specification for fluorescence excitation-emission mapping and absorbance analysis of colored dissolved organic matter

“…Three recent publications from NIST researchers have particular significance for EEM correction, including a reference to the recently released ASTM standard guide (E2719) for fluorescence calibration and correction [9], a reference to CDOM EEMs and all aspects of instrument and IFE correction, [2] and a reference to the validation of the Fluorolog ® as the true accurate fluorometer for its use in generating and validating a series of Standard Reference Materials [7]. Furthermore, a recent paper has been accepted outlining the results of a major international interlaboratory comparison for CDOM IHSS standard samples; this study was headed by researchers and the United States Geological Survey (USGS), and was the focused outcome of a recent 2008 Chapman conference sponsored by the American Geophysical Union [8]. In terms of potential applications of CDOM analysis two reviews of the literature exist of note, including one paper explaining the potential of CDOM for monitoring all stages of water-recycling [14], and another focusing on analysis of natural and wastewater sources [1].…”

“…2A) for a CDOM sample involves scanning the excitation wavelength from 240-500 nm and acquiring the emission spectra from 250-600 nm. Spectral-bandpass and resolution conditions are now generally accepted to be 5 nm by the research community [8]. Because the excitation axis is scanned and the excitation source exhibits wavelength-dependent intensity features, the emission detector signal must be divided by the respective excitation-source intensity monitored using the reference (R) detector at each excitation coordinate.…”

“…Figures 4A and B show a comparison of EEMs collected using both a concentrated (Abs 254 nm ≈ 0.8) PLFA sample to one diluted with ultra-pure water (Abs 254 nm ≈ 0.2) in Figures 4C and D. The absorbance value at 254 nm is an industry standard for evaluating the total CDOM concentration [8]. The EEMs on the left show the uncorrected (F obs ) while the EEMs on the right show the IFE corrected (F ideal ) for the corresponding samples.…”

“…Obviously, both intra-and interlaboratory comparisons must rely on traceable and reproducible spectral-correction procedures for the identifications to be validated [8]. This is of great concern given the diversity of instruments and conditions under which EEMs are reported in the literature for CDOM.…”

Calibration, standardization, and quantitative analysis of multidimensional fluorescence (MDF) measurements on complex mixtures (IUPAC Technical Report)

“…0.2 μm) and stored refrigerated in dark coloured bottles. An inter-laboratory study [61] indicated that short term storage (<2 months) presented no significant storage issues, although this is dependent on the origin of the samples and its susceptibility to bacterial regrowth. 2.…”