What Are Extractables and Leachables in Chromatography Septa?

When selecting chromatography consumables, laboratories often focus on vial size, cap design, septum material, or compatibility with a particular instrument. But one factor that can have a significant impact on analytical results is often overlooked: extractables and leachables.

A septum may represent only a small component of a chromatography consumables workflow, but it can be part of the sample environment and, depending on the vial and closure configuration, may be exposed to the sample or its headspace. Under certain conditions, compounds from the septum can migrate into the sample and potentially interfere with analysis.

Understanding extractables and leachables is therefore an important part of selecting the right septum, particularly for trace-level analysis, GC-MS, LC-MS, pharmaceutical applications, environmental testing, and other analytical methods where contamination can compromise results.

What Are Extractables?

Extractables are chemical compounds that can be removed from a material when it is exposed to specific solvents, temperatures, or other controlled laboratory conditions. For chromatography septa, extractables can originate from several sources, including:

• Residual compounds from the manufacturing process
• Processing aids
• Additives or stabilizers
• Unreacted components
• Low-molecular-weight silicone species
• Residual curing-related compounds
• Other substances associated with the formulation or processing of the elastomer

Extractables testing is generally performed under controlled and often deliberately aggressive conditions to determine what substances have the potential to migrate out of a material. The presence of an extractable does not automatically mean that it will contaminate a sample during normal use. Instead, extractables testing helps characterize what substances a material could release under defined conditions.

What Are Leachables?

Leachables are compounds that actually migrate from a material into a sample or surrounding environment during normal or reasonably foreseeable use. This distinction is important:

• Extractables = what can potentially come out of the material under defined test conditions.
• Leachables = what actually migrates into the sample during use.

For example, a silicone septum may contain compounds that can be extracted under a laboratory test using an aggressive solvent. Whether those compounds will migrate into a particular sample during an actual chromatographic application depends on factors such as solvent composition, temperature, contact time, and the septum formulation.

Why Do Extractables and Leachables Matter in Chromatography?

Chromatographic analysis is often performed at very low detection levels. When an analytical method is designed to detect compounds at trace or ultra-trace concentrations, contamination from laboratory consumables can become significant. A compound introduced by a septum may appear as an unexpected peak, elevated background, or additional signal. This can lead to:

• Unexpected chromatographic peaks
• Elevated background
• Increased noise
• False positives
• Reduced sensitivity
• Poor reproducibility
• Difficulty identifying unknown compounds
• Potential interference with quantitative analysis

The concern applies to both GC-MS and LC-MS, although the mechanism can be different. In GC-MS, volatile and semi-volatile compounds released from a septum during high-temperature analysis can contribute to chromatographic background. This is commonly referred to as septum bleed.

In LC-MS, the concern is generally less about thermal septum bleed and more about extractables and leachables that can migrate into the sample or solvent. Because LC-MS can detect compounds at extremely low concentrations, even small amounts of contamination from a vial closure can potentially produce unexpected signals. Depending on the compound and analytical method, these signals can contribute to background, interfere with analyte detection, or complicate identification and quantification.

GC-MS vs. LC-MS: Why the Septum Matters Differently

The analytical environment of GC-MS and LC-MS is different, so the potential impact of a septum is different as well.

GC-MS: Think About Bleed

During GC analysis, the septum is exposed to elevated inlet temperatures. Volatile and semi-volatile compounds can be released from the septum and enter the carrier-gas stream. These compounds can travel through the column and reach the detector or mass spectrometer, potentially creating unwanted peaks or increasing background. For sensitive GC-MS applications, Ultra Low Bleed silicone septa can help reduce the potential contribution of the septum to chromatographic background.

LC-MS: Think About Extractables and Leachables

LC-MS does not subject the vial septum to the same high-temperature GC inlet environment. Instead, the concern is often the potential migration of compounds from the septum into the sample or solvent. This becomes particularly important when:

• Samples remain in vials for extended periods
• Organic solvents are used
• Samples are stored before analysis
• Trace-level compounds are being measured
• Highly sensitive mass spectrometric detection is used
• The sample matrix is particularly sensitive to contamination

In LC-MS, unwanted compounds originating from a septum may appear as additional mass spectral signals or chromatographic peaks. Depending on their chemical characteristics and concentration, they may also complicate peak identification, contribute to background, or potentially interfere with sensitive quantitative methods. Therefore, LC-MS septum selection should consider extractables, leachables, chemical compatibility, and material purity, not simply whether the septum provides a good physical seal.

What Is Septum Bleed?

Septum bleed refers to volatile or semi-volatile compounds released from a septum, particularly when exposed to the elevated temperatures associated with gas chromatography. These compounds can enter the carrier-gas stream and reach the GC column and detector. In a mass spectrometer, they can contribute to background signals or produce peaks that are unrelated to the sample. The result can be an analytical “fingerprint” that has nothing to do with the compounds the laboratory is trying to measure.

The risk becomes more significant when:

• GC inlet temperatures are high

• Long temperature programs are used
• Trace-level compounds are being analyzed
• The method relies on mass spectrometric detection
• Low background is critical
• The septum is subjected to repeated injections

Why Ultra Low Bleed Septa Matter

For sensitive GC and GC-MS applications, choosing a standard silicone septum may not provide the level of cleanliness required by the analytical method. Ultra Low Bleed (ULB) silicone septa are specifically designed to minimize the release of volatile and semi-volatile compounds during high-temperature GC applications. This can help reduce the contribution of the septum to chromatographic background and minimize the potential for unwanted peaks.

SSP’s Ultra Low Bleed silicone septa are designed for applications where background contamination and septum bleed are critical considerations. By controlling the silicone formulation and manufacturing process, ULB septa provide laboratories with a cleaner septum option for demanding GC and GC-MS applications.

It is important to recognize, however, that Ultra Low Bleed and low extractables are related but not identical properties. ULB technology primarily addresses the release of volatile and semi-volatile compounds under GC conditions. For LC-MS and other applications where the septum is exposed to solvents for extended periods, laboratories should also consider extractables, leachables, solvent compatibility, and overall material purity.

Why Does This Matter?

Consider a laboratory analyzing a compound at a very low concentration. If the sample produces a small chromatographic peak, the laboratory needs confidence that the peak is actually coming from the sample.

In GC-MS, excessive septum bleed can contribute background that makes this determination more difficult. In LC-MS, compounds that migrate from the septum into the sample can potentially produce additional signals that may be mistaken for sample components.

Reducing these potential sources of contamination helps improve the analytical environment and can increase confidence in the results. It does not eliminate every possible source of contamination, but it can reduce the risk that laboratory consumables contribute unwanted compounds to the analysis.

Standard Silicone vs. Ultra Low Bleed Silicone

Not every application requires an Ultra Low Bleed septum. For routine applications where sensitivity is less demanding, a standard silicone septum may provide excellent performance. However, as analytical sensitivity increases, the requirements placed on the septum also increase.

The important point is that septum selection should be based on the analytical method, not simply on whether the septum physically fits the vial.

Where Do Septum Extractables Come From?

The chemistry of the septum plays an important role. Silicone elastomers are widely used in chromatography because they offer a valuable combination of flexibility, temperature resistance, chemical compatibility, and resealing performance. However, silicone is a complex polymeric material, and its formulation and manufacturing process can influence its extractable and bleed profile.

Factors that can affect extractables and bleed include:

• Silicone formulation: Different silicone formulations can contain different raw materials, additives, and processing components. Two septa that appear identical may therefore have different extractable and bleed characteristics.
• Curing process: The curing chemistry and conditions used to convert silicone from a raw material into a finished elastomer can influence residual compounds in the final product.
• Post-curing: Post-curing can be used to reduce certain volatile or low-molecular-weight species remaining after the primary curing process.
• Manufacturing conditions: Processing, handling, cleaning, and packaging can all contribute to the overall cleanliness of a finished septum.
• Storage and handling: Even a high-purity septum can be exposed to contaminants during storage or handling. Packaging and environmental controls therefore matter as well.

Does Silicone Always Have High Extractables or Bleed?

The answer is No. It is important not to assume that all silicone septa have the same level or type of extractables or bleed. Silicone is a material family, not a single standardized formulation. The performance and cleanliness of a septum depend on the specific formulation, manufacturing process, curing conditions, post-treatment, and quality controls used by the manufacturer. This is one reason laboratories should evaluate the specific septum product rather than making a material selection based solely on the word “silicone.”

Factors That Influence Leachables

Whether a compound leaches into a sample depends on the interaction between the septum and the analytical method.

Important factors include:

• Solvent compatibility: Different solvents interact differently with elastomeric materials. Organic solvents can behave very differently from aqueous solutions.

• Temperature: Higher temperatures can increase molecular mobility and potentially accelerate migration of compounds from a material.
• Contact time: The longer a sample remains in contact with a septum, the greater the opportunity for migration.
• Surface area: The amount of septum material exposed to the sample environment can influence potential contamination.
• Material formulation: The specific silicone formulation and manufacturing process influence the compounds that may be present.
• Sample composition: A complex sample matrix can interact with the septum differently than a simple solvent.

How Can Laboratories Reduce the Risk?

Selecting an appropriate septum is an important first step.

1. Match the septum to the analytical method: A septum intended for routine GC analysis may not be the best choice for highly sensitive GC-MS, pharmaceutical, or environmental applications.
2. Consider Ultra Low Bleed silicone for GC-MS: For GC-MS applications, particularly those involving trace-level analysis, an Ultra Low Bleed silicone septum can help minimize background contributed by the septum. This is especially important when the analytical method is sensitive enough to detect compounds released from laboratory consumables.
3. Evaluate available analytical data: When contamination is a concern, ask the manufacturer about available extractables, bleed, and background testing.
4. Use appropriate blanks: Running solvent blanks and system blanks can help laboratories identify whether unexpected signals may be coming from consumables rather than the sample.
5. Use high-quality solvents: If the solvent itself contains trace contaminants, it becomes difficult to determine whether other components of the workflow contributed any contaminants

What Should You Ask Your Septum Supplier?

If extractables, leachables, or septum bleed are important to your application, consider asking your supplier:

• What silicone formulation is used?
• Is the material high purity?
• Is it designed for low-bleed or Ultra Low Bleed applications?
• What curing process is used?
• Is the material post-cured?
• Has the septum been tested for extractables?
• Is GC-MS background data available?
• What temperatures were used during testing?
• Are application-specific data available?
• Are there differences between standard and Ultra Low Bleed versions of the product?

A knowledgeable manufacturer should be able to explain the material and manufacturing process and help determine whether a particular septum is appropriate for your application.

The Bottom Line

Extractables, leachables, and septum bleed are important considerations when selecting chromatography septa, particularly when analytical methods involve trace-level detection or highly sensitive instrumentation. The specific concern depends on the analytical technique.

• For GC-MS, septum bleed from volatile and semi-volatile compounds at elevated temperatures can contribute to chromatographic background. Selecting an Ultra Low Bleed silicone septum can help reduce this potential source of interference.
• For LC-MS, the focus shifts toward extractables, leachables, solvent compatibility, and material purity. Compounds migrating from the vial closure into the sample can potentially introduce unexpected signals into an otherwise sensitive analysis.

In both cases, the septum is only one part of a larger analytical system. High-purity solvents, appropriate vial and closure materials, compatible septum formulations, proper blanks, and suitable analytical controls all contribute to cleaner and more reliable data.

SSP’s Ultra Low Bleed silicone septa are designed for demanding GC and GC-MS applications where minimizing septum bleed is critical. For laboratories working with sensitive LC-MS applications, selecting appropriate high-purity septum materials and considering extractables, leachables, and solvent compatibility can help reduce potential sources of contamination. Ultimately, your analytical consumables should help you see what’s in your sample, not introduce another peak for you to investigate.

Your septum should protect your sample, not become part of the chromatogram.