Vials or septa: which causes more leaching?

First separate leaching, extraction and sample loss

Extractables are substances released from a material under a defined laboratory extraction. Leachables are substances that migrate under actual storage or use conditions. The distinction is established in packaging science; a deliberately aggressive extraction is not the same as your routine vial exposure. ^[1]

Adsorption is different: an analyte sticks to a surface and its measured amount may decrease without a new contaminant peak. Degradation changes the analyte chemically. A vial investigation should therefore consider both unexpected signals and missing recovery. ^[2]

What the septum contributes

The septum must seal, allow needle access and perform after the required punctures. In a PTFE/silicone laminate, the sample-facing PTFE layer provides a chemical barrier while the silicone supports resealing. Once the barrier is pierced, the exposed elastomer and puncture path also matter.

Pre-slit designs can reduce penetration resistance and help with pressure equalization during withdrawal, but the slit changes the barrier. Solid PTFE options generally do not reseal after puncture. Thickness, hardness, needle design and solvent compatibility must be considered together. ^[3]

Waters documents a specific LC-MS case where the first injection was clean but later injections showed siloxane-related contamination: solvent on the needle interacted with a pre-slit septum. That is evidence of one failure mechanism, not a claim that every silicone septum contaminates every method. ^[4]

What the glass contributes

Glass can release inorganic species or alter the sample environment. Waters describes sodium release into aqueous solutions and consequent pH shifts that can affect analyte stability. A signal caused by glass interaction may therefore look different from an organic septum-derived peak. ^[5]

Which effect dominates depends on what the detector measures and what the sample tolerates. A low-background organic analysis and a trace-element analysis may reveal different problems in the same container. Do not use a universal percentage or “septa always leach more” rule without a comparison under matched conditions.

A practical comparison for troubleshooting

The following is a proposed investigation framework, not a validated method. Define suitable replicates, acceptance limits and conditions with your laboratory’s method owner.

1. Check the baseline. Establish solvent, instrument and carryover backgrounds using the approved procedure.
2. Compare complete systems. Evaluate two candidate vials and two compatible closures as a small two-by-two comparison. Keep diluent, fill volume, temperature and hold time the same.
3. Separate contact time from puncturing. Use independent replicates for fresh and aged samples; compare the first injection with the routine repeat-puncture sequence.
4. Measure recovery as well as blanks. Include a low-level standard or representative sample. Check pH where it is relevant and the measurement is suitable.
5. Change one component at a time. A signal that follows the closure is a lead for further investigation; it is not automatic chemical identification.

Include enough samples to distinguish variability from a consistent effect. An open vial is usually a poor universal control because evaporation and airborne contamination create different conditions.

Choose the seal for the measurement

Ask for the septum formulation, facing layers, slit status, thickness, tested temperature range and the scope of any low-bleed claim. Evaluate the number of punctures and sample hold time you actually use. Consider evaporation alongside cleanliness; changing to a non-resealing closure may solve one interference while creating another. ^[4]

For specialized methods, follow their material restrictions instead of defaulting to PTFE/silicone. Keep vial and closure lot numbers together, and retain a reference combination that has already performed acceptably in the method.