Comparing Chromophoric Dissolved Organic Matter Methodologies

Abstract

Chromophoric dissolved organic matter (CDOM) is a combination of plant and animal decomposition byproducts and the optically active component of dissolved organic matter (DOM) in water. CDOM measurements have been a focus in the literature related to aqueous environments since the 1980s. Current CDOM analysis is conducted using spectrophotometers that are large, bulky, and expensive (most upwards of $50,000 USD). In this study, the accuracy of a more compact, less expensive (~ $5,500 USD) field spectrometer (StellarNet) was tested for accuracy against a traditional spectrophotometer (Photon Technologies International (PTI)). Thirty-six samples were collected from the Neponset River Salt Marsh in Boston, Massachusetts and analyzed on both instruments with the same set of standards for comparison. The correlation between measurements taken by the two instruments was strongly linear (R² = 0.9278) and the two sets of data (StellarNet and PTI) were not statistically different (p-value > 0.05), indicating that the less expensive, smaller StellarNet spectrometer is reliable in addition to field appropriate. The StellarNet spectrometer requires additional analysis (compared to the PTI) to convert the output of the instrument (photons) to a concentration (QSU). Highly concentrated sample concentrations (3-fold dilutions required) were not as well-correlated between instruments (R² = 0.5027). However, this dilution error can be attributed to the length of time (1 year) between sample analysis (i.e., freezing/thawing effects) and/or sampling errors between analysts on the different instruments.