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Evaluation of Septa Using a Direct Thermal Extraction Technique

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Santford V. Overton & John J. Manura. Scientific Instrument Services, Inc., 1027 Old York Road, Ringoes, NJ 08551

The most common source of contaminants in the GC injection port is the septum. Background or "memory" peaks can originate from both septa bleed as well as from previous samples trapped on the septa. The problem of septa bleed can be minimized by using low bleed high temperature septa which produce the lowest amount of bleed even when used at temperatures up to 300C. Septa bleed can also be minimized by using a septum purge at all times, even when using the splitless technique. In addition, sample splitting in the split mode will minimize the effect of septum bleed. The purpose of this study is to examine septa from several different manufacturers by a new technique entitled "Direct Thermal Extraction" and determine the optimum type of septa commercially available. This new technique utilizes a thermal desorption apparatus attached to the injection port of a GC/MS system and permits the direct thermal extraction of volatile and semi-volatile organics directly from small sample sizes (mg) without the need for solvent extraction or other sample preparation. The samples are ballistically heated and together with the carrier gas flow through the samples the volatiles are outgassed into the injection port and onto the front of the GC column for subsequent analysis via the GC and/or GC/MS. The volatile organics present in the septa were quantified using matrix spiked deuterated internal standards.