Oxygen Sensing

oxygen_full Oxygen is one of the most important molecules on our earth. It is integral in the cycles of life and decay: photosynthesis in plants, respiration in animals, and most breakdown processes. Without oxygen, there would be no fire, no fermentation or biodegradation, and no oxidation and corrosion to complete the circle and enable new life.

Accurate, non-invasive measurement of oxygen is important not only for monitoring and studying these processes, but also for safety. Management of oxygen levels in fuel tanks can prevent explosions, while headspace measurements in food packaging can guarantee adequate shelf life. Monitoring oxygen can assure patient safety in point of care analysis and respiratory settings, or indicate a sterile seal on surgical instruments and drug packaging.

Optically-based oxygen sensors offer many advantages over traditional electrode oxygen sensors, including faster response and remote measurement. They do not consume any oxygen during measurement, and are chemically inert. Furthermore, advances in detection methods, miniaturization, and reduced calibration requirements make optical oxygen sensors easier than ever to use in the lab and the field.

Are you curious yet? Take a deep breath and dive into our world of oxygen sensors to see just how much we can do.

Visit Oxygen Sensors Product Hub

Advantages

Versatile: Able to detect molecular oxygen in gas and liquid phases across wide ranges of temperature and concentration.
Multiple form factors: Sensors are available as traditional probes as small as 300μm, non-invasive patches, and standard cuvette sampling cells.
Non-consuming: Oxygen is not consumed during measurement, giving true unaltered readings of your system.
Rapid response time: Less than 1 second response time with averaging minimized.
Low-level detection: Sensors can resolve to 0.004% oxygen at 1atm at the lowest oxygen concentrations.

Application & Technical Notes

Applications

Other Common Applications

Biological environments: biofermentation processes, cell culture and growth media monitoring, sterile environments
Biomedical & life sciences: blood oximetry, cellular analysis, tissue analysis, serum, respiration, medical devices
Food process & storage monitoring: beverage packaging, vacuum-packaged foods, vegetable oils, wine fermentation
Vacuum & semiconductor processes: controlled-environment glove boxes, ion deposition processes
Aerospace: cell growth in space, O₂ monitoring in confined spaces
Process monitoring: harsh chemical environments (hexane, toluene, and mineral oil), hydrocarbons (fuels, alcohols, hydraulic fluids), packaging and headspace, pharmaceutical fermentation
Environmental & ecological: aquaculture applications, marine organisms, natural waters (marine, surface), soils and sediments, wastewater treatment, cooling water
Aviation: fuel storage monitoring, fuel transportation

Application Notes:

Application Blog Post: Monitoring Oxygen and pH during E. coli Fermentation

Technical Note

How Can Oxygen Levels Be Measured?

A traditional electrode sensor measures oxygen in a liquid electrochemically. Oxygen diffuses into the sensor through a permeable membrane and encounters an anode and cathode. The current created when oxygen is reduced at the cathode is proportional to the oxygen levels in the liquid. The drawbacks of this method are that it consumes oxygen in the process, and it depends on the rate of oxygen diffusion into the sensor, necessitating regular stirring.

An optically based sensor (optrode), in contrast, works by coming into equilibrium with its environment. Oxygen diffuses into the sensor coating, where it alters the nature of the fluorescence of an indicator material. By looking at how the fluorescence intensity, lifetime, and phase are altered in the presence of oxygen, the oxygen level can be quickly and accurately determined without consuming any oxygen.

Optrodes have many other advantages: they have faster response, are chemically inert, offer long-term stability without the need for recalibration, and couple easily to optical fibers for remote measurements. The sensor material on which they are based can be applied to the tip of a fiber, a flat substrate, adhesive membrane, or other surface, allowing these sensors to be customized to the specific application. Optrodes can take a minimally intrusive format like an ultra-narrow probe, or can be placed directly within packaging or a cuvette for non-destructive, non-contact measurement. They can even be overcoated for deployment in harsh environments.

Our oxygen sensors measure the partial pressure of dissolved or gaseous oxygen. Because they consume no oxygen, they can be used in continuous contact with viscous samples; unlike with electrode oxygen sensors, continuous stirring is not required.

Technical Note

How Does an Ocean Optics Oxygen Sensor Work?

Our oxygen sensors are based on fluorescence of an indicator material that has been integrated into a matrix. The sensor matrix is then coated on a probe tip, a slide, the wall of a cuvette, or formed into an adhesive patch. We use a matrix with good thermal and mechanical stability, superior chemical compatibility, ease of production and rapid response.

We use two types of indicator materials that fluoresce – ruthenium and Pt-porphyrin complexes. Each works well for a specific range of applications like general laboratory use, high-sensitivity applications or hydrocarbon-rich sample environments. What they have in common is that both indicator materials are highly sensitive to partial pressure of oxygen.

In the presence of molecular oxygen, the fluorescence properties exhibited by these materials are altered. The most obvious change is a quenching of the fluorescence, or decrease in fluorescence intensity, as oxygen levels increase. The quenching happens because an excited indicator molecule has come into contact with an oxygen molecule, transferring its excess energy to the oxygen molecule in a non-radiative transfer (a gentle handshake of sorts). The degree of fluorescence quenching depends on the frequency of collisions, and therefore on the oxygen concentration of the sample, as well as its pressure and temperature.

Our particular oxygen sensors use a blue LED for excitation at 450 nm, transmitted to the sensor material using a fiber optic probe. Fluorescence at visible wavelengths is collected back through the same probe and routed to an avalanche photodiode for detection.

A more subtle effect of fluorescence quenching is that the average fluorescence lifetime of the indicator material decreases as oxygen concentration increases. By pulsing the excitation LED and looking at when maximum fluorescence is emitted relative to those pulses (the phase shift), the average fluorescent lifetime can be determined. This method is called phase-sensitive detection. This is the method used by all of our NeoFox systems.

Phase sensitive detection is a more accurate method than intensity-based detection, as it is insensitive to electronic and light source drift, scattering within the sample, and intensity variations due to fiber bending and ambient light. It is also unaffected by chemical degradation of the indicator material, and by refractive index changes caused by calibrating in gas prior to measurements in a liquid. Photobleaching is vastly reduced in platinum-based chemistries as compared to ruthenium-based.

Each probe has a slightly different response to oxygen concentration, so prior to use it must be calibrated using known levels of oxygen. Once the phase shift is measured, it can be related to the oxygen concentration or partial pressure using the Stern-Volmer equation. If pressure and temperature cannot be controlled and kept constant from calibration to measurement, they must also be factored into the calculations, and a multi-point calibration must be used.

It is important to note that this type of sensor requires oxygen to diffuse into the sensor material, so the sensor tip or patch must be kept in direct contact with the sample. However, since the indicator material is trapped in a matrix, it is immobilized and protected from exposure to the sample, making these sensors unusually chemically inert and robust when used with an overcoat. Notably, they have minimal response to environmental changes in pH, salinity and ionic strength.

Application Notes:

PDF: App Note – Smart Oxygen Cuvette for Optical Monitoring of Dissolved Oxygen in Biological Blood Samples

PDF: App Note – Noninvasive, Real-Time Monitoring of Oxygen and pH during E.coli Fermentation Using Optical Sensors

Blog Post: Bringing New Life to Old Arteries with Oxygen

Example Setups

Featured Products for Oxygen Sensing:

NEOFOX-GT	A compact optical instrument capable of measuring fluorescence lifetime, phase, and intensity for oxygen sensing.
RE-BIFBORO-2	Bifurcated optical fiber with borosilicate fiber bundle has one fiber leg to transmit excitation to the patch and one fiber leg to collect the response.
RedEye oxygen patches	Select from among patch diameters (4 mm and 8 mm) and sensor formulations (general purpose coating, high-sensitivity coating or highly robust coating). Single patches and packs of 5 patches are available.
NeoFox Viewer	Windows-based software allows users to collect, manage and analyze data with NeoFox systems.
Calibration Service	Recommended for applications where the sample cannot be maintained at a constant temperature (±1 °C). Calibration service covers environments from 0 – 80°C and is optimized for the sensor coating formulation being used and the temperature and oxygen concentration ranges of the sample environment.

Making Measurements

What system should I use for oxygen sensing?

The NeoFox® family of phase measurement systems are based on frequency domain electronics, using a blue LED for excitation and an avalanche photodiode for detection. The two ports connect to a bifurcated fiber which can be mated to a wide variety of sensing probes via a splice bushing. (Integrated probes are also available for some applications.) The sensor material may be resident on the tip of the probe, coated on a slide or cuvette wall, or in the form of a RedEye® adhesive patch.

These systems have been designed to reduce cost, improve stability and make calibration easier for a wide variety of oxygen sensing configurations. They report fluorescence lifetime, phase, and intensity for measurement of gaseous and dissolved oxygen in a variety of environments. The unique self-calibration feature allows the calibration of the probe and system to be fine-tuned at point of measurement for maximum accuracy and improved electronic stability.

Benchtop: NeoFox® Phase Measurement System

NeoFox® is a benchtop device which measures fluorescence phase shift and intensity. An onboard pressure transducer measures pressure of the environment, while temperature can be monitored through the system via optional external thermistors. It interfaces with the user’s computer via our NeoFox Viewer software. It is particularly well suited to applications where sensitivity to drift and system stability are critical.

What is the best sampling optic for my O₂ measurement?

The versatility of our sol-gel matrix allows oxygen measurements to be made using probes, patches, cuvettes, and even customized sampling optics. Probes can be as narrow as 200 μm, or as large as ¼” diameter. Probes are recommended for applications requiring the highest resolution and lowest detection limits. Our self-adhesive patches are available in 4 and 8 mm diameter, and offer excellent performance in a more flexible format. Probes and patches can also be customized to your specific needs as required.

Discrete sensor probes

The majority of our standard oxygen probes are designed to be mated to the phase measurement system using a bifurcated fiber and splice bushing. It is important to match the probe diameter to the recommended size of bifurcated fiber to maximize coupling efficiency.

In choosing a probe for a given application, consider the diameter of the fiber core (spatial resolution), as well as outer diameter and length (durability and fixturing). The choice of ferrule or jacket material may also be important for harsh or sensitive environments. All can be reconditioned if the sensor material is fouled or damaged.

Our –R probes are general purpose probes made using 1000 µm core fiber in a 1/16″ OD stainless steel ferrule that is 6″ (15.24 cm) long. The narrower AL-300 probe is ideal when fine spatial resolution is required. It is also good for tissue monitoring, as its light weight and narrow outer diameter minimize damage when inserted into tissue. It uses a 18 cm long, 300 µm core fiber.

For direct replacement of electrode probes, consider our 1/8″ diameter OR125 probes. They use a 1000 µm core fiber, are 2.5″ (6.35 cm) long, and come in stainless steel. The OR125-G also features an O-ring groove near the probe tip for sealed applications.

A polyimide probe like the PI600 may be a better choice for environments that are hostile to metallic probes, or when resistance to harsh chemicals is needed. It is available as 600 µm core fiber with silicone jacketing, cut to 18 cm in length.

RedEye™ oxygen sensor patch

The RedEye® oxygen patch is designed specifically to measure oxygen in packaging or vessels. It can be integrated into the inside of the package during the manufacturing process, or into the vessel prior to sealing. When used with a NeoFox® system and bifurcated fiber external to the package or vessel, it allows rapid, non-invasive quantitative oxygen measurements.

These self-adhesive patches are offered in standard 4 and 8 mm diameters, but can be custom-made to your specifications in sizes from a few millimeters to several centimeters. They can also be applied to the inside of cuvettes. They are ideal for applications like point of care analysis, blood bag testing, food and beverage packaging, bioprocess control, and cell culture monitoring.

Adhesion of these patches is excellent, and has been tested for mechanical hold and optical clarity during extended exposure to extreme pH and salinity levels, including pH 1-11, seawater, and deionized water.

RedEye® Oxygen Sensor Patch to Measure Oxygen Non-invasively in Food and Beverage Packaging

Which oxygen sensor formulation should I use?

We offer three different oxygen sensor formulations, each targeted to a specific range of applications. All provide rapid response time with 5% or better accuracy in gas or liquid when a multi-point calibration is used. Specifications for the sensors are given in terms of partial pressure of oxygen at 1 atmosphere (1 atm). If you are working at a different operating pressure, you must consider that in determining the percentage of oxygen at that pressure.

All three sensor formulations can be used for probes and patches, or applied to custom substrates like slides or cuvettes. Specifications for the sensor materials as applied to probes are discussed below. Performance in patch form is very similar, but is slightly more relaxed as regards resolution, detectable limit, and sometimes temperature range. Response time and accuracy are comparable. Please consult our specification tables for details.

FOSPOR oxygen sensor

The FOSPOR coating uses a very sensitive platinum-porphyrin complex embedded in polymer as its indicator to achieve excellent sensitivity and accuracy at low oxygen levels. Similar to ruthenium, fluorescence quenching of the platinum complex at ~650 nm can be directly related to the partial pressure of dissolved or gaseous oxygen. This sensor formulation is a popular choice for through-packaging measurements in the food and pharmaceutical industries where low oxygen concentration is essential to assuring shelf life. It offers exceptional long-term stability.

FOSPOR sensors are suitable for measuring oxygen in gases up to 100% concentration, with a lower limit of detection of 0.001 – 0.01%. In liquid, it can measure from 0 – 40 ppm with 0.4 ppb resolution at room temperature. Operating temperature for this formulation is somewhat narrower (0 °C to +60 °C), but it is still highly suitable for many packaging applications and benign environments. Response time in either case is 2 – 5 seconds (within 20 s with overcoat).

HIOXY oxygen sensor

Our proprietary HIOXY sensor material was developed specifically for compatibility with non-aqueous vapors and solutions. This ruthenium-based sensor formulation is ideal for use with oils, alcohols and hydrocarbon-based vapors and liquids. It has been tested successfully in commercial and military aviation fuels, gasoline, diesel, some alcohols, wine, vegetable oils, glycol, and hydraulic fluids.

HIOXY sensors are suitable for measuring oxygen in gases up to 20.9% concentration, with a lower limit of detection of 0.004%. In liquid, it can measure from 0 – 8 ppm with 4 ppb resolution at room temperature. Operating temperature for this formulation is very wide (-50 °C to +100 °C), making it suitable for many process, transportation, and industrial environments. Response time in either case is less than 1 second.

FOXY oxygen sensor

The standard FOXY sensor material is based on a fluorescent ruthenium indicator in a hydrophobic sol-gel matrix. When excited by blue light, the ruthenium complex fluoresces at ~600 nm. This general-purpose oxygen sensor works very well for benign environments over a wide temperature range (-50 °C to +80 °C for probes).

FOXY sensors are suitable for measuring oxygen in gases up to 100% concentration, with a lower limit of detection of 0.003%. In liquid, it can measure from 0 – 40 ppm with 4 ppb resolution at room temperature. Response time in either case is 1 – 2 seconds (within 20 seconds with overcoat).

Do I need an overcoat on my oxygen probe?

An optional black silicone overcoat is available for our oxygen sensor probes, designated MGB (Medical Grade Black). This silicone overcoat is applied to the probe to exclude ambient light, improve chemical resistance and eliminate refractive index effects. It does, however, increase the response time of the probe, as it takes longer for oxygen to diffuse through the overcoat and establish equilibrium between the environment and the sol-gel matrix.

As part of continuous improvement processes, this overcoat is now more uniform and reproducible than ever, with a guaranteed response time of less than 20 seconds for oxygen patches, and 30 – 60 seconds for probes. Once applied, the overcoat cannot be removed without damaging the sensor, but the probe can be reconditioned (recoated) if the response time is deemed to be too long.

Do I need temperature compensation for my oxygen probe?

If your sample is not held at constant temperature (within ±3-5 °C), then yes, temperature compensation is required to achieve accurate measurements. Temperature affects the excited state lifetime of the indicator complex, decreasing its fluorescence energy quantum efficiency as temperature increases. This affects the slope of the calibration curve, reducing the accuracy of the partial pressure reading.

Temperature measurement is possible using the rugged NEOFOX-TP thermistor, which connects to the NeoFox® system via cable.

To perform temperature compensation, the probe itself must be calibrated for temperature. This can be performed in the NeoFox Viewer software using multiple oxygen concentrations and temperatures, or the probe can be purchased pre-calibrated. Standard calibration covers 0 – 60 °C, but can span a wider range upon request.

What software do I use with my oxygen sensor?

Our NeoFox® measurement systems are controlled by the NeoFox Viewer, a stand-alone software program designed exclusively for oxygen measurement. This software controls APD voltage (gain), reports oxygen values, and allows data logging for single or multiple-channel systems. It provides two calibration algorithm options: a two point (linear) fit and a multipoint (2nd order polynomial) fit with temperature compensation.

Values reported by the software include percent oxygen in gas as a volume or mole percentage, and dissolved oxygen partial pressure in ppm, in mg/L, or as a percentage of 1 ATM. It also reports pressure measured onboard the phase fluorometer.

How do I calibrate my oxygen sensor?

Calibration of the probe or patch material is required in order to make accurate oxygen measurements with your oxygen sensor system. Two major factors affect the calibration procedure of your system: algorithm type and temperature range.

The Linear (Stern-Volmer) calibration algorithm requires two standards of known oxygen concentration. The most common calibration points are 0% and 20.9%, as linearity is a reasonable approximation in this range. For a wider oxygen concentration range, greater accuracy, or temperature compensation, a second order polynomial algorithm must be used. It requires at least three standard of known oxygen concentration.

Temperature is also an important factor in calibration. Temperature affects the fluorescence decay time, fluorescence intensity, collisional frequency of the oxygen molecules with the fluorophore, and the diffusion coefficient of oxygen. If working in a sample or environment where there are no fluctuations in temperature, temperature compensation is not needed. Keep in mind, however, that a sample should be maintained at a constant temperature (± 1 °C) for best results when temperature compensation is not being used.

O2-CAL-STANDARD / O2-CAL-CUSTOM

On-site calibration at Ocean Optics can be provided for sensor systems configured with temperature-compensation accessories. The O2-CAL-STANDARD service calibrates the sensor over the most typical ambient oxygen and temperature ranges, 0 – 25% oxygen at 1atm and 15 – 35°C. The O2-CAL-CUSTOM option allows you to specify exactly what range of conditions your sensor will be exposed to, and even lets you group points in specific regions for increased accuracy. Contact us for more information.

Advanced Measurements & Samples

What is the linear (Stern-Volmer) algorithm for intensity calibration?

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

For a given media, and at a constant total pressure and temperature, the partial pressure of oxygen is proportional to oxygen mole fraction.

The Stern-Volmer constant (k) is primarily dependent on the chemical composition of the sensor formulation. Our probes have shown excellent stability over time, and this value should be largely independent of the other parts of the measurement system. However, the Stern-Volmer constant (k) does vary among probes, and it is temperature dependent. All measurements should be made at the same temperature as the calibration experiments or temperature monitoring devices should be used.

If you decide to compensate for temperature, the relationship between the Stern-Volmer values and temperature is defined as:

The intensity of fluorescence at zero pressure of oxygen (I₀) depends on details of the optical setup: the power of the LED, the optical fibers, loss of light at the probe due to fiber coupling, and backscattering from the sample. It is important to measure the intensity of fluorescence at zero pressure of oxygen (I0) for each experimental setup.

It is evident from the equation that the sensor will be most sensitive to low levels of oxygen. The photometric signal-to-noise ratio is roughly proportional to the square root of the signal intensity. The rate of change of signal intensity with oxygen concentration is greatest at low levels. Deviations from the Stern-Volmer relationship occur primarily at higher oxygen concentration levels. Using the Second Order Polynomial algorithm when calibrating corrects these deviations.

Backscattering in the media can increase the collection efficiency of the probe, increasing the observed fluorescence. It is important to perform calibration procedures in the media of interest for highly scattering substances. For optically clear fluids and gases, this is unnecessary.

What is the second order polynomial algorithm for intensity calibration?

The Second Order Polynomial algorithm requires at least three 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 Second Order Polynomial algorithm is considered to provide more accurate data because it requires at least three known concentration standards while the Linear (Stern-Volmer) algorithm requires a minimum of two known concentration standards. The Second Order Polynomial algorithm is defined as:

I₀ is the fluorescence intensity at zero concentration

I is the intensity of fluorescence at a pressure p of oxygen,

K₁ is the first coefficient

K₂ is the second coefficient

If you decide to compensate for temperature, the relationship between the Second Order Polynomial algorithm and temperature are defined as:

How can I use Henry’s Law to convert between partial pressure and concentration?

It is possible to calibrate the system in gas and then use the probe in liquid or vice versa. In theory, the sensor probe detects the partial pressure of oxygen at 1 ATM. Henry’s Law can then be used to convert partial pressure to concentration. When temperature is constant, the weight of a gas that dissolves in a liquid is proportional to the pressure exerted by the gas on the liquid. Therefore, the pressure of the gas above a solution is proportional to the concentration of the gas in the solution. The concentration (mole %) can be calculated if the absolute pressure is known:

Oxygen mole fraction = oxygen partial pressure / absolute pressure

Since the sensor detects partial pressure of oxygen, the response in a gas environment is similar to a liquid environment in equilibrium with gas. Therefore, it is possible to calibrate the sensor in gas and then use the system with liquid samples using Henry’s Law.

X = mole fraction of oxygen

T* = T/100 in Kelvin

a = -66.7354

b = 87.4755

c = 24.4526

It is important to note, however, that Henry’s Law does not apply to gases that are extremely soluble in water. The table below shows how the solubility of oxygen in water varies with temperature.

T (ºC)	*T (T/100K)**	Mole Fraction of oxygen in water at 1 atmosphere p0₂	Weight Fraction (ppm) at 1 atmosphere p0₂ (pure 0₂)	Weight Fraction (ppm) at 0.209476 atmospheres p0₂ (air)
5	2.7815	3.46024 E^-05	61.4620	12.8748
10	2.8315	3.06991 E^-05	54.5289	11.4224
15	2.8815	2.7555 E^-05	48.9446	10.2527
20	2.9315	2.5004 E^-05	44.4146	9.30380
25	2.9815	2.2924 E^-05	40.7193	8.52972
30	3.0315	2.1220 E^-05	37.6926	7.89570
35	3.0815	1.9821 E^-05	35.2081	7.37526
40	3.1315	1.8673 E^-05	33.1686	6.94802

What is the maximum temperature my oxygen probe can withstand?

That depends on the sensor formulation. All of our sensor materials can be used safely up to 60°C in either probe or patch format. HIOXY-coated probes can be used up to 130 °C.

What is the maximum pressure my oxygen probe can withstand?

Most of our probes are rated to 300 psi.

How can I safely clean and sterilize my oxygen probe?

A variety of cleaning and sterilization methods are suitable for use with our oxygen sensor formulations. Be aware that a single-point recalibration is required after cleaning or sterilization, and that the procedure may reduce the lifespan of the probe or patch.

Cleaning

All of our oxygen probe formulations are robust enough to be cleaned with a variety of standard solvents, including isopropanol (IPA), and ethanol (EtOH).

Sterilization of probes

All sensor formulations may be safely sterilized using ethylene oxide (EtO). Only HIOXY probes may be safely autoclaved. Only FOXY probes may be sterilized using gamma radiation.

Sterilization of patches

FOXY and HIOXY patches may be safely sterilized using ethylene oxide (EtO).

Probe reconditioning

Typical probe life is one year, but it may be shortened due to harsh environments, biofouling, physical abrasion, and photobleaching. Rather than replace the entire probe, most probes can be reconditioned. In this process we strip and clean the probe, recoat it with new sensor material, and recalibrate.

Can I measure oxygen in solids?

No, but you can measure oxygen in soft materials, provided that direct diffusion contact can be maintained between the probe tip and the soft material. A good example is soft tissue in the body (such as brain tissue), which is surrounded by a liquid matrix of interstitial fluid that is in gaseous equilibrium with the tissue itself.

What chemicals are compatible with FOSPOR, HIOXY and FOXY sensors?

Do you wonder if your environment is compatible with our oxygen sensors? View our chemistry compatibility table. If you work in an environment that’s not listed, please contact us.

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