A rapid birefringence method for measuring suspended CaCO3 concentrations in water


Guay. C.K. and J.K.B. Bishop (2002)

Deep-Sea Research I, 49, 197-210 [LBNL #46895]

Abstract. The extreme birefringence of calcium carbonate (CaCO3) relative to other major components of marine particulate matter provides a basis for making optical in situ measurements of particulate inorganic carbon (PIC) in seawater. This concept was tested with a benchtop spectrophotometer equipped with a 1- and 10-cm path length sample cell and modified with linear polarizers to measure the birefringence of suspended particles. Sample suspensions containing 3-100% CaCO3 (by weight) were prepared from calcareous marine sediment material and varying amounts of non-birefringent diatomaceous earth. The samples ranged in total suspended material from 0.003 to 249 mg l(-l) and PIC from 0.03 to 1820 mu mol CaCO3 l(-1). A positive relationship was observed between birefringence and PIC, with response falling off as the total particle concentration and the relative abundance of non-CaCO3 particles in the sample increased. Sensitivity increased linearly with optical path length, and absolute detection limits of 0.2-0.4 and 0.04-0.08 mu mol CaCO3 l(-1), respectively, were determined for path lengths of 1- and 10-cm based on the intrinsic signal noise of the modified spectrophotometer. Conventional (i.e., non-polarized) transmittance measurements were used to correct the birefringence signal for the sensitivity loss due to interference from scattering and absorption. Without further modification, this spectrophotometer-based method can be used (with a 10-cm cell) to quantify PIC in most surface ocean waters-including those influenced by coccolithophore blooms. The spectrophotometer results define performance requirements and design parameters for an in situ instrument capable of operating over the oceanic range of PIC.

Figure: Crossed Linear Polarizers and a Spectrophotometer are the main elements of a simple method for sensing particulate calcium carbonate particles in seawater

PIC sensor concept

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