Zero Drift and HCR Optical O2 sensors
Our optical Oxygen sensors measure pO2 in gases, liquids and mixed systems such as slurries, biological samples and process streams. The fundamental mechanism is the collisional quenching of a fluorescent molecule by molecular O2.
In water and gases in general, the quenching is identical. However certain materials can cause problems by affecting the quenching efficiency of the O2. This is seen as a reversible change in calibration. In these cases the probes can still be used but must be calibrated in the particular solvent, or vapor.
Our AP chemistry behaves remarkably well in a variety of hydrocarbons. For more aggressive hydrocarbons like toluene and acetone we use a special HCR chemistry as the sensor material.
Here are some results with testing of a few different materials
AP chemistry in Ethanol
Ethanol is a common solvent used for cleaning and disinfection and of course is present in fermentation tanks and brewing of alcoholic beverages. We tested our AP sensor in air saturated with ethanol vapor. A typical test involves measuring the probe signal in air (20.9% O2) and N2 (0% O2), exposing the sensor to the material being tested for a period of time, and then re-testing. The results below show no affect on the signal.
As a consequence the SEOX AP sensors can be calibrated in gases (N2 and O2) and used in ethanol.
The signal values in the graph are Tau (useconds) – the fluorescence decay period. Tau is highest in the absence of O2 and decreases from quenching as O2 increases.
Zero Drift AP sensor in ethanol – no ill effects observed
AP Chemistry in propane
The Zero Drift AP chemistry also works well in low molecular weight alkanes, propane and methane. There is no affect on the signal. The tau value in N2 was the same as in propane (both being 0% O2). This means the probes can be easily calibrated and used in gas environments with natural gas, propane etc.
HCR chemistry in Toluene
The AP sensor cannot be used in toluene, in fact it will dissolve in this solvent. Our Hydro Carbon Resistant or HCR sensor was designed for use in aggressive solvents like toluene, jet fuels, crude and refined petroleum products.
Using HCR sensors is a little more complex as toluene affects the quenching efficiency of the O2.. The figure below illustrates what happens.
The high Tau values (to the left of numbers 3, 6 10, 13) are in N2 gas with and without toluene.
The low Tau values to the left of the other numbers are with air with and without toluene.
The N2 values show a slight decrease in the presence of toluene vapor. This may not be due directly to quenching by toluene, but may be the influence of residual O2 in the experimental setup.
In contrast, toluene has a substantial effect on Tau in air. To the left of #4 the probe is in pure air. The right of 4 its put into a tube of toluene being bubbled by air. The Tau value rises slowly to a new level (at #5). The slow response is associated with the sensor going from a “dry” state to a “wet” one where it has been exposed to the solvent. Once a sensor has been acclimated it can be used in the solvent and it will respond quickly to changes in pO2. If it is removed from the presence of the solvent for a few hours, it will “dry out” and return to its original condition.
To use the sensor in toluene, then, it is simply calibrated in toluene or toluene vapor. That can be done at the factory or by the user. Its recommended to soak the probe in toluene a few hours before using it.
AP Chemistry in Olive Oil
The response of an AP sensor in olive oil shows a different pattern of changed calibration compared to toluene. In this case there was not a long wetting process, the change in calibration was very quick.
The figure below is showing pO2 on the vertical axis (not Tau) so highs and lows are opposite the previous figures.
The reading in N2 gas (left of #3) has a higher Tau (lower O2) than the reading in N2 bubbled olive oil (left of #6).
The reading in air (left of #2) has a higher Tau (lower O2) than the reading in air bubbled olive oil (left of #8)
Readings are quickly restored when the probe is moved back to gases only.
These data may indicate that there is not a direct influence of the oil on quenching, but rather some optical effect, perhaps of index matching between the sensor material and oil leading to loss of signal. In any event the calibration in oil is reproducible and the probe is well suited for looking at inerting gases used in preparation and modified air packaging of these oils.
AP chemistry influenced by riboflavin – black overcoat solves the problem
Many biological samples have natural or experimentally controlled levels of riboflavin. The presence of riboflavin was observed to cause a decrease in tau (falsely elevating reported O2 values).
The mechanism is not exactly understood, however riboflavin is itself a fluorescent compound whose fluorescence can be quenched by O2.
In experiments we were unable to detect any fluorescence decay signal from riboflavin with our electronics, but there was an effect on our AP sensors. The problem was solved by adding a black overcoat of O2 permeable material. The black pigment blocks any extraneous fluorescence signal from the sample and also acts as a barrier to prevent riboflavin from interacting with the AP sensor or matrix.
Optical oxygen sensors have a wide range of applications. Using them successfully requires initial experiments to determine compatibility and potential changes in signals that will require in-situ calibration.
If you have unusual materials you want to measure, its best to check to see if we have already worked in that media. Otherwise we can do testing to determine feasibility.