Technical note: Measurements of fluorescent dissolved organic matter (FDOM) in seawater (Filter blanks, pore sizes, and storage)

Abstract. Fluorescent dissolved organic matter (FDOM) provides crucial information regarding the sources and characteristics of DOM in oceans. However, the FDOM measurements vary depending on filter blanks, pore sizes, and sample storage. To develop more reliable methods for FDOM measurements, in this study, we examined the uncertainties associated with different preparation methods for seawater samples. These samples identified three primary components using parallel factor analyses: terrestrial humic-like peak (C peak), marine humic-like peak (M peak), and protein-like peak (T peak). Relatively high procedural blank values were obtained from samples filtered through the pre-combusted glass fiber filter (0.7 μm pore size) and membrane filter (0.2 μm pore size) without pre-cleaning. However, the blank values became negligible when the filter was pre-washed with 5 mL of 0.1 M HCl or 20 mL of distilled water. The effects of different filter pore sizes were not observed for the C and M peak concentrations (FDOM_H), but relatively low T peak values were observed for filtered samples (0.7 or 0.2 μm) relative to unfiltered samples. For all samples, FDOM_H showed consistent results for 21 days (8 % ± 3 %) when stored in a refrigerator or a freezer. However, T peak concentration decreased rapidly in both filtered (15 %–50 %) and unfiltered samples (10 %–40 %) after five days, indicating considerable bacterial degradation of protein-like components within three days. Therefore, our results suggest that reliable FDOM_H values can be obtained either unfiltered and filtered samples stored in either a refrigerator or freezer for three weeks, but careful sample filtration, storage, blank controls are necessary for T peak measurements.