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]
A rapid birefringence method for measuring suspended CaCO3 concentrations in waterGuay. C.K. and J.K.B. Bishop (2002)Deep-Sea Research I, 49, 197-210 [LBNL #46895]
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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.
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Figure: Crossed Linear Polarizers and a Spectrophotometer are the main elements of a simple method for sensing particulate calcium carbonate particles in seawater
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