General Principle Of Operation - Hach ORBISPHERE G1100 User Manual

Specifications

2.2 General principle of operation

Optical sensing of oxygen originates from the work of Kautsky in 1939 where he demonstrated
that oxygen can dynamically quench the fluorescence of an indicator (decrease the quantum
yield). This principle has been reported in various fields of application such as monitoring
aquatic biology in waste water, tests for blood gas analysis and cell culture monitoring. The
method is now recognized by ASTM (American Society for Testing and Materials) for the
measurement of oxygen in water. Compared to classical oxygen detection using
electrochemical sensors, luminescent technology offers several advantages such as no oxygen
consumption, independence from sample flow velocity, no electrolyte and low maintenance.
Optical sensing of oxygen is based on the measurement of the red fluorescence of a
dye/indicator illuminated with a blue light as shown in Figure 1.
Figure 1 Principle of optical oxygen detection using fluorescent dye
The dye fluorescence is quenched by the presence of oxygen. The oxygen concentration can
be calculated by measuring the decay time of the fluorescence intensity as shown by Figure 2
left. The higher the oxygen concentration is, the shorter the decay time will be. By modulating
the excitation, the decay time is transformed into a phase-shift of the modulated fluorescence
signal, which is independent of fluorescent intensity and thus of potential aging (Figure 2 right).
Figure 2 Fluorescence decay time (left) and modulated signals (right)
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