Fiber Optic Oxygen Sensors — How Do They Work?
- Tuesday, 01 May 2012
Fiber optic oxygen sensors use the fluorescence of a chemical complex in a sol-gel to measure the partial pressure of oxygen. The pulsed blue LED sends light, at ~475 nm, to an optical fiber. The optical fiber carries the light to the probe. The distal end of the probe tip consists of a thin layer of a hydrophobic sol-gel material.
A sensor formulation is trapped in the sol-gel matrix, effectively immobilized and protected from water. The light from the LED excites the formulation complex at the probe tip. The excited complex fluoresces, emitting energy at ~600 nm. If the excited complex encounters an oxygen molecule, the excess energy is transferred to the oxygen molecule in a non-radiative transfer, decreasing or quenching the fluorescence signal. The degree of quenching correlates to the level of oxygen concentration or to oxygen partial pressure in the film, which is in dynamic equilibrium with oxygen in the sample. The energy is collected by the probe and carried through the optical fiber to the spectrometer. This data can then be displayed using software such as OOISensors Software.
Oxygen, as a triplet molecule, is able to quench efficiently the fluorescence and phosphorescence of certain luminophores. This effect (first described by Kautsky in 1939) is called "dynamic fluorescence quenching." Collision of an oxygen molecule with a fluorophore in its excited state leads to a non-radiative transfer of energy. The degree of fluorescence quenching relates to the frequency of collisions, and therefore to the concentration, pressure and temperature of the oxygen-containing media.
In order to make accurate oxygen measurements of your sample, you must first perform a calibration procedure with your oxygen sensor system. Two major factors affect the calibration procedure of your system.
1. First, decide if you are going to
compensate for changes in temperature
in your sample. If you are working
with a sample where there are no
fluctuations in temperature, you do
not need to compensate for temperature.
Temperature affects the fluorescence
decay time, fluorescence intensity,
collisional frequency of the oxygen
molecules with the fluorophore,
and the diffusion coefficient of oxygen.
The sample should be maintained
at a constant temperature (± 3
°C) for best results.
2. Next, choose the algorithm you wish to use for your calibration procedure. The Linear (Stern-Volmer) algorithm requires at least two standards of known oxygen concentration while the second order polynomial algorithm requires at least three standards of known oxygen concentration.
Calibration curves are generated from your standards and the algorithms to calculate concentration values for unknown samples. The second order polynomial algorithm provides a better curve fit and, therefore, more accurate data during oxygen measurements, especially when working in a broad oxygen concentration range.
Linear (Stern-Volmer) Algorithm
The output (voltage or fluorescent intensity) of our fiber optic oxygen sensors can be expressed in terms of the Stern-Volmer algorithm. The Stern- Volmer algorithm requires at least two standards of known oxygen concentration. The first standard must have 0% oxygen concentration and the last standard must have a concentration in the high end of the concentration range in which you will be working. The fluorescence intensity can be expressed in terms of the Stern-Volmer equation where the fluorescence is related quantitatively to the partial pressure of oxygen:
I0 is the intensity of fluorescence at
zero pressure of oxygen,
I is the intensity of fluorescence at a pressure p of oxygen,
k is the Stern-Volmer constant