In gas chromatography (GC) or liquid chromatography (HPLC) analysis, one of the most troublesome issues is the appearance of "ghost peaks"—unexpected chromatographic peaks in blank samples or solvents. These peaks not only interfere with the qualification and quantification of target analytes but can also mask low-concentration components, rendering data invalid.

Tracing the source of "ghost peaks" is a detective's work, and sample vials and septa are often the prime suspects. This article will delve into how they can become contributors to "ghost peaks" from the perspective of contamination source analysis.

I. Septa: A Major Contributor to "Ghost Peaks"

Chromatography septa can release or generate detectable compounds under high temperatures or solvent exposure.

1. GC Septum Bleed

   • Cause: The high temperature of the GC inlet (often >250°C) causes thermal degradation or direct volatilization of low-molecular-weight silicones, plasticizers, or antioxidants in the septum.

   • Characteristics of "Ghost Peaks":

     • Location: Typically appear early in the chromatogram as a series of continuous, regularly spaced "comb-like peaks" (silicone series).

     • Time Dependency: Particularly noticeable after installing a new septum or following multiple needle penetrations.

   • Solutions:

     • Use High-Temperature, Low-Bleed Septa: Choose silicone/PTFE composite septa processed with special techniques.

     • Optimize Inlet Temperature: Use the lowest permissible temperature while ensuring complete sample vaporization.

     • Replace Septa Regularly: Follow the instrument's recommended schedule and document baseline changes after replacement.

2. HPLC Septum Leachables and Adsorption

   • Cause:

     • Leachables: Organic solvents in the mobile phase (e.g., acetonitrile, methanol) may dissolve low-molecular-weight polymers or plasticizers from the septum.

     • Adsorption-Desorption: The septum may have adsorbed strongly retained compounds from a previous analysis and slowly release them in subsequent runs.

   • Characteristics of "Ghost Peaks": Broader peak shape, variable retention times, potentially co-eluting with target peaks.

   • Solutions:

     • Choose Highly Chemically Inert Septa: e.g., PTFE-faced/silicone-core septa with excellent resistance to organic solvents.

     • Perform Adequate Blank Rinses: Run multiple blank solvent injections before and after the sample sequence to observe if "ghost peaks" are eluted.

II. Sample Vials: Hidden Sources of Contamination

The vials themselves and cleaning residues are another common source of contamination.

1. Vial Body Contaminants

   • Residual Detergents: Incompletely rinsed detergents (e.g., SDS) are a common source of "ghost peaks" in LC-MS, appearing as a complex series of background peaks.

   • Leachables from Vial Walls: Low-quality plastic vials (e.g., PP, PE) may leach antioxidants, plasticizers (e.g., phthalates).

   • Metal Ions from Glass: Sodium, potassium, boron, silicon ions from glass can cause interference with specific detectors (e.g., ICP-MS, certain HPLC conditions).

2. Vial Caps and Sealing Components

   • Inner Coating of Aluminum Caps: Polymer coatings on the inner surface of some crimp caps may decompose at high temperatures.

   • Gasket Materials: Besides the main septum, elastomer rings used for auxiliary sealing inside the cap can also release impurities.

Conclusion: