Analytical intercomparison between type I and type II long‐pathlength liquid core waveguides for the measurement of chromophoric dissolved organic matter

Abstract

In the past decade, technological advances in optical sensors have facilitated an increased understanding of the relationship between optical characteristics and biogeochemistry of our oceans. In particular, long‐pathlength liquid core waveguide cells (LCWs) are being used to “map” chromophoric dissolved organic matter (CDOM), as a biogeochemical tracer, in various coastal and open ocean regions. At present, two LCW cell types are used in the study of marine CDOM, and concerns about discrepancies in data collected with the different cell types and problems with baseline offsets have arisen. We conducted a direct comparison of absorption coefficient spectra of a dissolved spectrophotometric standard, molecular weight standard (MWS), and dilution series of natural seawater obtained in type I and type II LCWs to assess data agreement and potential colloid‐mediated biases. Although no statistical difference was observed for the dissolved standard, we found the type I to have a slight bias toward higher absorption coefficient values for MWS 14–150 kDa (within 95% confidence interval) and natural seawater. Seawater CDOM spectral slopes differed significantly between type I and type II LCWs, with a maximum difference in slope of 0.0006 nm−1. Fastidious elimination of microbubbles from the capillary cells greatly reduced baseline offsets and markedly improved CDOM spectral slope precision.