Ocean Optics LPC-500CM - Manual (Page 10)

1: Introduction
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LWCC Properties
Similar to optical fibers, light is confined within the (liquid) core of an LWCC by total internal reflection
at the core/wall interface. Optical fibers are then used to transport light to and from the sensor cell.
Designed for use with fiber optics, both LWCCs require only small sample volumes and have a high
optical throughput.
The LWCC is made from fused silica tubing with an outer coating of a low refractive index polymer. The
core liquid is contained by the synthetic silica tube coated with the low refractive index cladding material.
Placing the refractive surface outside the silica protects it from undesirable effects of the liquid. In
addition, the fused silica wall is impermeable to gases.
Lateral Section of LWCC
Effective Pathlength and Linearity
Effective pathlength and linearity have been extensively studied with the LWCC. Effective pathlength is
defined as the equivalent pathlength of the cell, assuming the LWCC strictly follows Beer’s law.
Although there have been several reports in the literature in which calculation of effective pathlength has
been performed, the theoretical basis by which to calculate the effective pathlength of the LWCC has not
yet been established. It is, therefore, currently determined experimentally. The effective optical pathlength
was determined to be slightly shorter than the physical pathlength (0.94± 0.01 times of its physical
pathlength), dependent on the LWCC’s inner diameter and wall thickness. This is caused by the fact that
light is partially traveling in the fused silica wall of the LWCC. By Beer’s Law, the absorption of a liquid
sample in the LWCC bears a linear relationship to the concentration of an analyte. The LWCCs were
extensively tested and proved to be linear over a range of 0.01 to 2.0 AU (limited only by noise and stray
light from the measuring spectrophotometer). A detailed analysis of the effective pathlength and linearity
of the LWCCs has been published (Belz et al., 1999).
*
* Mathias Belz, Peter Dress, Aleksandr Sukhitskiy and Suyi Liu, “Linearity and effective optical pathlength of
liquid waveguide capillary cells,” Part of the SPIE Conference on Internal Standardization and Calibration
Architectures for Chemical Sensors, Boston, Massachusetts, September 1999, SPIE Vol. 3856, 271–281.