Fluorescent Dissolved Organic Matter (FDOM)

Fluorescent Dissolved Organic Matter (FDOM) is a primarily anthropogenic source of carbon (ie. pesticides, herbicides, and fungicides). The larger group carbon falls under is Total Organic Carbon (TOC) which also includes naturally occuring fulvic and humic acids derived from decaying plant debris. As a chemical class, most forms of organic carbon readily absorb ultraviolet light and their presence in water can be inferred by conventional UVA/UVT measurement (absorbance at 254nM).

Some of the compounds that contribute to the TOC pool also fluoresce. In science literature, the fluorescent part of the TOC load is called Fluorescent Dissolved Organic Matter. FDOM is composed of both natural and man-made substances, but the complex cyclic molecules contained in fungicides, pesticides, and herbicides tend to fluoresce far more brightly than the simpler, straight-chain natural substances found in plant debris.

Such exotic cyclic chemicals normally exist in much lower concentrations (on the order of micrograms per liter) than TOC (mg/L) and therefore have a minimal effect on UV  absorbance. Fluorescence measurements like FDOM are more sensitive than absorption measurements of TOC. The combination of these two factors means that while the LiquID Station is extremely sensitive to changes in TOC (±10 µg/L), this level of detectability cannot compete with fluorescent compounds that can be detected at the sub-parts-per-billion level (<1 µg/L).

Moreover, since the LiquID Station measures FDOM and TOC, it can also produce continuous records of another useful parameter called FTOC which is the ratio of FDOM to TOC. By measuring FTOC, the LiquID Station is able to present a view of how the percentage of man-made chemicals in the TOC mix varies with time. The TOC/FDOM/FTOC combination provides plant operators with a powerful event detection tool.

  • Parameter Detection Range/Sensitivity Accuracy
    Fluorescent Dissolved Organic Matter (FDOM) 0.5 to >100 DFU ±5%

  • To learn more about the optical approach used by the LiquID to measure FDOM please read the How It Works – FDOM located in the Library and Resources section of our website.
  • Please Contact Us if you have additional questions about measuring FDOM or general inquires into the LiquID Station.