Molded GC Septa Proof

Low Bleed Profile

A molded Thermogreen LB-2 GC septum and popular molded septa from two competitors were solvent extracted using a proprietary in-house procedure. Dodecane was used as an internal standard to eliminate differences in detector response between analyses. This procedure was chosen because it gives a profile of the amount of silicone oil present on each septum, as an indication of bleed level. All three molded septa tested were a standard design without injection hole. The resulting chromatograms are shown in Figure 1. As seen, the bleed from the molded Thermogreen LB-2 septum is on par with a popular “red” septum. A competitor’s “green” septum performed very poorly when subjected to this bleed test. The unlabelled peaks represent silicone oils that were extracted from the septum. When placed on an injection port and heated, these silicone oils would inevitably bleed off, collect on the head of the column, and appear as contamination peaks in chromatograms.

Figure 1. Bleed Profiles of Several ‘High Performance’ Septa

Molded Thermogreen LB-2 GC Septum


Competitor's "Red" Septum


Competitor's "Green" Septum

Peak IDs

  1. Methylene chloride (solvent)
  2. Dodecane (I.S.)
Column Conditions
column: SLB-5ms, 30 m x 0.25 mm I.D., 0.25 µm (28471-U)
oven: 40 °C (3 min.), 15 °C/min. to 325 °C (15 min.)
inj.: 250 °C
det.: FID, 325 °C
carrier gas: helium, 25 cm/sec constant
injection: 1 µL, splitless (1 min.)
liner: 4 mm I.D., single taper, unpacked
sample: solvent extracts of septa, dodecane (as an internal standard) added to 100 µg/mL


High Puncturability

As a measure of durability, a molded Thermogreen LB-2 septa with injection hole was subjected to repeated injections. Column head pressure and carrier gas linear velocity/flow rate were monitored throughout the test to determine if the septum developed a leak. An autosampler was used for the test, ensuring that the syringe needle (tapered 23s-26s gauge) punctured the same location repeatedly. Figure 2 shows chromatograms from the initial run and after 2100 injections. As demonstrated, no artifact peaks were present in the final run to indicate coring of the septum and subsequent bleed from particles in the inlet. Additionally, retention, head pressure, and column flow remained constant, indicating that a leak free seal was maintained.

Figure 2. Molded Thermogreen LB-2 GC Septum Durability Test


Peak IDs

  1. Methylene chloride (solvent)
  2. 2-Octanone
  3. n-Decane
  4. 1-Octanol
  5. n-Undecane
  6. 2,6-Dimethylphenol
  7. 2,6-Dimethylaniline
  8. n-Dodecane
  9. n-Tridecane
Column Conditions
column: SLB-5ms, 30 m x 0.25 mm I.D., 0.25 µm (28471-U)
oven: 115 °C (15 min.)
inj.: 250 °C
det.: FID, 360 °C
carrier gas: helium, 30 cm/sec. @ 115 °C
injection: 1 µL, 100:1 split
liner: 4 mm I.D., split, cup design, unpacked
sample: Nonpolar Column Test Mix (47300-U), each analyte at 500 µg/mL in methylene chloride


Additional Resources

Title Publication ID.
Molded Thermogreen™ LB-2 Septa Product Information Sheet T407082 (353 Kb PDF)


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