Written By Melissa Henry – Laboratory Analyst I
Helium leak testing is a highly sensitive, deterministic method for evaluating container closure integrity. In pharmaceutical packaging, it can detect leaks in a container closure system and, when appropriately developed and validated, provide quantitative information about package integrity.
ASTM F2391-22, Standard Test Method for Measuring Package and Seal Integrity Using Helium as the Tracer Gas, describes helium-based procedures for evaluating package and seal integrity using both Sniffer Mode and Vacuum Mode. The standard can be applied to flexible, semi-rigid, and rigid packages. However, selecting helium leak testing requires more than choosing the most sensitive instrument. Package design, fixture configuration, helium introduction, test parameters, acceptance criteria, and validation all need to be considered together.
What Is Helium Leak Testing?
Helium leak testing uses helium as a tracer gas to determine whether a package has a pathway through which gas can escape. A mass spectrometer leak detector measures helium passing through a potential weak point. Because helium is chemically inert and can be detected at very low concentrations, it is useful when high leak-detection sensitivity is needed. ASTM F2391 describes applications including package design, tooling qualification, process setup, process validation, quality assurance monitoring, and research and development.
How Helium Leak Testing Works
The basic principle is to place helium on one side of a potential leak pathway and detect any helium that passes through it. Depending on the package and test configuration, helium may either be introduced through an inlet point after sealing or incorporated into the package headspace before sealing. ASTM F2391 describes large, moderate, fine, and ultra-fine leak-rate ranges. These classifications describe relative leak-rate ranges and do not establish pass/fail criteria. The appropriate acceptance limit must be determined and scientifically justified for the specific application. An instrument's ability to detect a very small leak does not automatically make that leak rate the appropriate acceptance criterion for every container closure system.
Vacuum Mode vs. Sniffer Mode
ASTM F2391 describes two helium leak testing procedures.
Vacuum Mode places the helium-containing package inside a sealed fixture. A vacuum is drawn on the fixture, creating an enclosed space around the package where escaping helium can collect and be measured by the leak detector. This approach is useful for container closure integrity testing because the fixture can be designed around the package and specific area being evaluated, helping provide consistent test conditions.
Sniffer Mode detects helium escaping from the outside of a package by moving a probe along the package surface. It can be useful for locating the source of a leak or testing package configurations that are better suited to external scanning. However, results can be more dependent on operator technique. Scan speed, probe position, distance from the package, background helium, and package permeability should be considered and controlled during method development.
Sample Preparation
Sample preparation depends on the package design and how helium will be introduced. One approach is to introduce helium into a sealed package through an intentional inlet point and reseal the opening before testing. Because this modifies the package, this type of testing is generally considered destructive. Another approach is to introduce helium into the package headspace before sealing. When practical, this may allow the package to be tested without creating an inlet point after sealing. Regardless of the approach, samples should be prepared consistently. Differences in helium introduction, sealing, or sample configuration can contribute to variability in the measured response.
Typical Instrumentation
A typical helium leak testing system may include:
- Helium supply or controlled helium source
- Helium mass spectrometer leak detector
- Vacuum pumps for Vacuum Mode
- Package-specific test fixtures
- Reference or calibrated leak standards
Fixture design is an important part of the system. ASTM F2391 notes that fixture design is outside the scope of the standard because configurations vary between applications. The fixture should provide a reliable seal, isolate the area of interest, provide an appropriate enclosed volume for helium collection, and allow consistent sample positioning.
Advantages of Helium Leak Testing
Helium leak testing offers several potential advantages:
- High leak-detection sensitivity
- Quantitative leak-rate measurement when appropriately configured
- Applicability to flexible, semi-rigid, and rigid package configurations
- Potential for destructive or non-destructive testing, depending on the configuration
- Ability to evaluate package and seal integrity
- Use during package development and process validation
- Potential to locate leaks when an appropriate test configuration is used
These characteristics make helium testing useful when a highly sensitive deterministic CCIT method is needed.
Limitations of Helium Leak Testing
Helium leak testing is not appropriate for every package. ASTM F2391 states that the method is not applicable to breathable or porous packaging because helium permeability can interfere with the measurement, producing false positive responses in otherwise intact samples. Helium testing may also be unsuitable when helium cannot be reliably introduced or retained, the package cannot be adequately accommodated by a suitable fixture, or the proposed configuration cannot demonstrate the required leak limit.
In these situations, another validated CCIT method may be more appropriate for the package, product, or study objective. FDA guidance emphasizes that the selected CCIT method should be appropriate for the specific product and container closure system and supported by appropriate validation.
Common Mistakes in Helium Leak Testing
An unexpected helium leak result does not always indicate a package leak. Common sources of variation include:
- Excessive background helium
- Fixture or vacuum-system leaks
- Improper fixture sealing
- Inconsistent sample preparation
- Incorrect test parameters
- Insufficient baseline stabilization
- Inconsistent Sniffer Mode scanning technique
- Failure to verify test-system performance before testing
Checking these factors can help distinguish a true package failure from an issue with the instrument, fixture, sample, or test procedure.
Troubleshooting Helium Leak Testing
When unexpected helium leak results occur, start by checking the test system before changing method parameters or acceptance criteria. A calibrated leak or other appropriate reference standard can help verify that the leak detector is responding as expected. Next, check the fixture for leaks or poor seals and confirm that the vacuum system is reaching and maintaining the required conditions. Review the sample setup, helium introduction, package sealing, and background helium levels to identify potential sources of variability.
If the test system is performing properly, review the sample and test procedure. Positive and negative controls can help determine whether an unexpected result is associated with the package, fixture, instrument, or test procedure. This systematic approach can help distinguish a true package leak from an issue with the test setup or method.
Frequently Asked Questions
What kind of CCIT method is helium leak testing?
Helium leak testing is a deterministic container closure integrity testing method that detects and measures helium passing through a leak path in a package. It uses an instrument-based measurement rather than relying on a visual or subjective response. The method can be used to evaluate package and seal integrity when appropriately developed and validated for the specific container closure system.
What is ASTM F2391?
ASTM F2391-22 is the Standard Test Method for Measuring Package and Seal Integrity Using Helium as the Tracer Gas. It describes procedures for evaluating package and seal integrity using helium as a tracer gas, including both Sniffer Mode and Vacuum Mode.
What is the difference between Vacuum Mode and Sniffer Mode?
Vacuum Mode places a helium-containing package inside a sealed fixture. A vacuum is applied, allowing escaping helium to collect around the package and be detected by the instrument. Sniffer Mode scans the outside of a helium-filled package for escaping helium and can help identify the location of a leak.
Is helium leak testing destructive?
It can be. When helium is introduced into an already sealed package through an inlet point, the package must be modified and the opening resealed for testing. If helium is incorporated into the package headspace before sealing, non-destructive testing may be possible depending on the package and test design.
Is helium leak testing suitable for pharmaceutical packaging?
Yes, when the method is appropriate for the specific container closure system and has been properly developed and validated. FDA recognizes validated physical CCIT methods as potential alternatives to sterility testing for appropriate stability applications. The package, product, test method, and acceptance criteria should all be considered when determining suitability.
Does a more sensitive helium leak test mean a better CCIT method?
Not necessarily. The goal of CCIT is to demonstrate that the container closure system provides the required level of protection for the product. ASTM F2391's leak-rate classifications do not establish pass/fail criteria. The appropriate acceptance limit should be scientifically justified for the specific application.
Conclusion
Helium leak testing provides a highly sensitive approach to container closure integrity testing, with ASTM F2391 describing both Vacuum Mode and Sniffer Mode procedures. Reliable testing depends on more than instrument sensitivity. Package design, fixture configuration, helium introduction, test parameters, sample preparation, and test-system verification can all affect the result. A successful helium leak testing method should be appropriate for the specific container closure system, capable of detecting the relevant leak condition, and supported by appropriate method development and validation.