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2007 vol. 2
Tech Tips
Preventing SeptumProblems
By Donna Lidgett, GC Accessories Product Marketing Manager and Scott Grossman, GC Accessories Chemist
• Avoid extraneous peaks with proper septum handling & maintenance.
• Handy size chart & septum choice guidelines.
• Optimize performance by choosing the right septum for the job.
All septa, regardless of their composition, puncturability, or resistance to
thermal degradation, will be a source of problems if they are mis-
handled or used inappropriately. Poor septum choice and
improper treatment can significantly compromise both quali-
tative and quantitative analytical results. Proper septum choice
and careful handling can minimize septum bleed and septum
coring, two of the most common septum problems that affect
chromatography.
Septum bleed occurs when volatiles from the septum (e.g., sili-
cone oils, phthalates) enter the column and then elute, creating
elevated baselines (for isothermal analyses), baseline disturbances, or
extraneous (but consistent) peaks in the chromatogram. Either baseline
rise or extraneous peaks can interfere with identification and quantification of target
analytes. This problem is prevalent in temperature-programmed analyses, because
the septum volatiles collect on the column during the oven cool-down and initial
hold periods.
To avoid septum bleed, either condition your septum prior to running your analysis,
or use a pre-conditioned septum that is ready for immediate use. All Restek septa are
preconditioned and ready to use. Allowing the septum to condition at operating tem-
peratures for a few hours is an excellent way to assure optimum performance. Also always use
clean forceps or wear clean powderless latex gloves, or cotton gloves when handling septa. Do not handle them with bare fingers or
with powdered latex gloves since contaminants such as finger oils, perfumes, make-up, fingernail polish, skin creams, hand soaps,
and talcum can be absorbed into the septum and bleed out during analysis.
Septum coring is another common problem that can diminish chromatographic performance. Coring occurs when the septum has
been punctured too many times, the needle is damaged, or the wrong needle tip type is used. In these cases, small particles may be
cored from the body of the septum and fall into the inlet liner. Once in the liner, they are subjected to higher temperatures, causing
the release of septum volatiles which are swept into the column and can appear on the chromatogram (see “How Hot is Your
Septum?” on page 22).
To prevent septum coring, always follow the septum and instrument manufacturers’ installation recommendations and take
care not to over-tighten the septum nut. Over-tightening the septum nut invariably reduces septum lifetime by increasing
coring and splitting. Routinely replacing your septum and inspecting your syringe needle (manual or autosampler) for
tip damage also help prevent septum damage.
Softer septa, such as Ice-Blue™ septa, are less likely to core than firmer septa. However, softer septa usually have a lower
maximum operating temperature than firmer septa, so consider your method requirements carefully before
deciding to switch. Changing syringe needle styles also can help reduce coring. For example, a point-style
#2 needle (beveled point) is much more likely to cause coring (especially when the tip has become bent or
dull) than a point-style #5 needle (conical needle with side-port).
A septum that can be penetrated cleanly and easily by the needle is less prone to coring and has a longer
life. Moreover, consistent injections made through such a septum help ensure accurate results. The soft silicone
rubber from which all Restek septa are manufactured is specially formulated for chromatographic performance,
which ensures our septa are easy to puncture. However, in cases in which a small degree of pliability is sacrificed
for high-temperature optimization, the CenterGuide™ dimple will help guide the syringe, for clean, consistent injections,
minimizing septum coring.
Careful consideration of instrument and method requirements should dictate your septum choice, but proper handling
and maintenance are the keys to minimizing septum damage and maximizing the accuracy of your analyses. Restek offers
septa for all major brands of gas chromatographs and injectors. Use our handy septum size chart to determine the septum
diameter for your instrument or contact us at
1-800-356-1688 (ext. 4)
to discuss your application.