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 colloidmediated 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.

Interferences in the analysis of nanomolar concentrations of nitrate and phosphate in oceanic waters

“…Byrne and Kaltenbacher [28] postulated that type II waveguides will have greater wavelength dependency and shorter effective path-length than type I. This is refuted by both theoretical [29] and experimental studies [27,29,30] which showed neither shortened effective path-length nor wavelength dependency. Despite showing that there was no significant difference between the types when Table 1 Instrument performance characteristics.…”

“…Sampling frequency (Hz) Resolution (nM Hz −1/2 ) Detection limit (3 × ) (nM Hz −1/2 ) measuring standard solutions, Floge et al found that type I LWCCs measured higher (9-16%) absorption values than type II in natural and diluted seawater [30]. They postulated that this was due the interaction of the waveguide design and particles which caused scattering.…”

“…The relative merits of type I and type II LWCCs have been debated in the literature [27][28][29][30]. Both types of waveguide require careful cleaning, elimination of micro bubbles and matching of refractive index of standards, reagents and sample [11,27,[29][30][31]. Byrne and Kaltenbacher [28] postulated that type II waveguides will have greater wavelength dependency and shorter effective path-length than type I.…”

“…Type I LWCCs (not used in this study) consist of a Teflon AF tube directly in contact with the sample. The relative merits of type I and type II LWCCs have been debated in the literature [27][28][29][30]. Both types of waveguide require careful cleaning, elimination of micro bubbles and matching of refractive index of standards, reagents and sample [11,27,[29][30][31].…”

A calibration/validation protocol for long/multi‐pathlength capillary waveguide spectrometers

“…Two types of liquid core waveguide cells are commonly used, one with Teflon AF tubing (used in the UltraPath and focus of this work) and a second with fused silica capillary tubing and an outer coating of Teflon AF. Comparisons of these cell types have been conducted previously (Floge et al ), along with measurements of linearity, precision, accuracy, and effective pathlength (Belz et al ; Miller et al ; Belz et al ; Gimbert and Worsfold ; Lefering et al ). However, consistent results among instruments are still not always achieved (Novak ).…”