In the world of oxygen permeation testing of packaging materials, not all instruments are created equal. This is why ASTM approved two distinct test methods for determining oxygen transmission rates of packaging materials: ASTM D3985 – 05 (Reapproved 2010) – “Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor”, and ASTM F2622 – 08 – Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using Various Sensors. This white paper explains the difference between these two methods and discusses when either method could be used, and when one method would be better than the other.
Oxygen adversely impacts the shelf life of many foods by causing oxidation, bacteria growth, and other changes in quality. To protect the product’s quality, oxygen barrier packaging materials are used. Consequently, accurate Oxygen Transmission Rate (OTR) measurement is important when assessing oxygen barrier properties during the selection of packaging materials.
Sensor technology is critical to obtaining accurate, repeatable and reliable OTR results from which important commercial and quality control decisions are made.
ASTM methods differ. Only MOCON oxygen permeation systems with the patented COULOX® coulometric sensor comply with ASTM D3985 as was reapproved by ASTM Committee F02 on Flexible Barrier Materials in 2010.
Theory of the Coulometric Sensor
The Coulometric or the Coulox® oxygen sensor is a fuel cell that performs in accordance with Faraday’s Law. When oxygen enters the coulometric sensor, the sensor reacts with the oxygen molecules to release four free electrons. This reaction is detected by the sensor as an electrical current, the magnitude of which is proportional to the amount of oxygen flowing into the sensor per unit of time.
Note: Each oxygen molecule entering the Coulox® results in four free electrons creating an electrical current. One mole of oxygen (22.4 liters at 0C and 760 mmHg) would produce four Faradays of current. Because one Faraday = 96,500 Ampere-seconds, each mole of oxygen will produce 4 x 96,500 = 3.86 x 105 Ampere-seconds. In more practical terms: One cc of oxygen in 24 hours = 0.000199 Amperes of current.
Each and every oxygen molecule that enters the sensor is analyzed, allowing a measurement of the oxygen transmission rate (OTR) that is 95-98% efficient; therefore, no calibration is required.
Figure 1. Coulometric Sensor
ASTM D3985 was devised for dry testing, but it can be modified for testing at different relative humidity conditions when combined with its sister method ASTM F1927-07 “Standard Test Method for Determination of Oxygen Gas Transmission Rate, Permeability and Permeance at Controlled Relative Humidity Through Barrier Materials Using a Coulometric Detector.”
For a sensor to be considered coulometric, 100% of the oxygen that passes through the test sample must be analyzed. If an instrument claims to have a coulometric sensor, it is imperative to confirm whether 100% of the oxygen does in fact pass through the sensor (Figure 1).
Because these test methods are considered “intrinsic” standards that do not require calibration, they can be used as reference methods to verify the accuracy of non-coulometric OTR testing.
About Non-coulometric Sensors
ASTM F2622 intentionally has been prepared to allow for the use of various sensors, devices, and procedures. The sensor could be any electrochemical or other type of detector.
To repeat, a coulometric oxygen sensor is one which is at least 95% efficient or one which literally sees and counts all the oxygen permeating through the test sample. Certain electrochemical sensors are configured with a membrane over the sensor to protect the sensor from seeing all the oxygen molecules (Figure 2). Only a fraction or portion of the stream is analyzed. Although, as the name implies, an electrochemical reaction takes place and the number of electrons are measured to relate the quantity of oxygen in the stream, those electrochemical sensors only measure the concentration of oxygen. They are not 100% efficient. Therefore, these instruments do not fit the definition of coulometric rather the method described in ASTM F2622. They could be called “non-coulometric” as they require calibration.
Figure 2. Non-Coulometric Sensor
To reach the sensor, the permeant must pass through a film that itself is a barrier. Consequently, these instruments have difficulty measuring the OTR of high oxygen barriers because the amount of oxygen received by the sensor is so small. Also, to ensure the total transmission rate is calculated accurately, the electrochemical sensor must make a comparison to a known amount of oxygen which makes the system calibration-dependent.
When calibrating, it is important to use a material that has a similar OTR to the test material to ensure the accuracy of the test results. However, for calibration-dependent systems, the nearest reliable calibration level is magnitudes higher than many barriers. Moreover, the calibration process could be operator-dependent. Thus, when measuring high oxygen barriers, large errors and poor reproducibility can result.
Comparison of other aspects for both methods
1. Sensor size
Coulometric sensors are much larger than non-coulometric ones because they are designed to handle a higher volume of test gas. As they are larger, they can contain more reagent and last much longer. While a non-coulometric sensor may only last a few months, depending on how often it is used, coulometric sensors can last for up to two years. This decreases the cost of ownership associated with permeation testing.
2. Influence by ambient conditions
True Coulometric sensors such as the COULOX® used in most MOCON oxygen permeation systems are not affected by flow, pressure or temperature changes. However, calibration-dependent Non-coulometric sensors are affected by these variables. As these variables are constantly in flux, a system that is affected by them will exhibit reduced repeatability. When a secondary film is used to protect the sensor, environmental conditions that would affect the barrier property of this protective film alter the number of oxygen molecules seen by the sensor. Consequently, the accuracy of measurements from the Non-coulometric sensor is reduced.
Which method is better for your application?
Coulometric and Non-coulometric methods have different applications. For example, if the application is produce/fruit packaging where high levels of oxygen are needed for products’ respiration, or other products that are not oxygen sensitive, then the packaging material used is usually polyolefin or other low oxygen barrier materials. In this case, instruments with non-coulometric sensors suit the purpose. An example is the recent developed MOCON OX-TRAN® 2/12.
When working with high barrier materials to package foods or other products that are easily oxidized, the low OTR level packaging material requires a more accurate sensor. Best practice is to select an instrument conforming to the Coulometric method.
Refinements regarding the handling of temperature, relative humidity, system leaks and other parameters means the difference between a right answer and a wrong one. When business decisions are made based on results generated from a permeation measuring device, accuracy, repeatability and reliability of the results must be dependable.
Appendix: Summary table of method comparison