Note 40: Comparison of Septa by Direct Thermal Extraction

INTRODUCTION

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 300 deg C. 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 of operation 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.

Instrumentation

A new technique called "Direct Thermal Extraction" which utilizes a thermal desorption apparatus attached to the injection port of a GC/MS system 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 sample is placed inside a preconditioned glass-lined stainless steel desorption tube between two glass wool plugs which simply hold the sample in place. The desorption tube containing the sample is attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube with sample is injected into the GC injection port, ballistically heated and together with the carrier gas flow through the sample the volatiles are outgassed into the injection port and onto the front of the GC column for subsequent analysis.

All experiments were conducted using a Scientific Instrument Services model TD3 Short Path Thermal Desorption System accessory connected to the injection port of an HP 5890 Series II GC interfaced to an HP 5971 Mass Selective Detector. The mass spectrometer was operated in the electron impact mode (EI) AT 70 Ev and scanned from 50 to 550 daltons during the GC run for the total ion chromatogram.

A short 0.5 meter by 0.53 mm diameter fused silica precolumn was attached to the injection port end of a 60 meter x 0.25 mm i.d. DB-5MS capillary column containing a 0.25 m film thickness. The GC injection port was set to 260 deg C and a direct splitless analysis was used. The head of the column was maintained at -100 deg C using a GC Cryotrap model 961 (Scientific Instrument Services, Ringoes, NJ) during the desorption and extraction process and then ballistically heated to 200 deg C after which the GC oven was temperature programmed from 35 deg C (hold for 5 minutes) to 80 deg C at 10 deg C/min, then to 200 deg C at 4 deg C/min and finally to 250 deg C at a rate of 10 deg C/min.

Experimental

Several brands of septa from different manufacturers were analyzed by "Direct Thermal Extraction" to determine the optimum type of septa commercially available. Samples of septa measuring 5-6 mg were placed into an inert thermal desorption tube on top of a glass wool plug. The desorption tube with sample was then attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube was injected into the GC injection port at desorption block temperatures of 100 deg C, 150 deg C, 200 deg C, 250 deg C and 300 deg C for 5 minutes each. The extracted organics are subsequently cryo-trapped at the front of the GC injection port using the GC Cryotrap at a temperature of -100 deg C. After the 5 minute desorption period, the Cryotrap was heated to 200C to elute the volatiles and begin the GC analysis and identification via the mass spectrometer.

Results and Discussion

Figure 1 - Supelco Thermogreen Septa Analysis At Various Temperatures

Septa from 5 different manufacturers (HP, SGE, J&W, Restek and Supelco) were analyzed by "Direct Thermal Extraction" to identify and compare the volatile organics present. A series of studies were conducted to examine which septa had the lowest amount of bleed and which were the most suitable type of septa commercially available. Samples from Supelco Thermogreen and HP gray septa were desorbed at temperatures of 100 deg C, 150 deg C, 200 deg C, 250 deg C and 300 deg C, in order to evaluate the effect of desorption temperatures on septum bleed (Figs. 1&2). Septa from the other manufacturers were desorbed at 250 deg C. Thermogreen septa from Sulpelco were found to contain low concentrations of the compounds pentamethyl-disiloxane and a propanoic acid derivative at a desorption temperature of 250 deg C (Fig. 1); whereas, higher concentrations of the compounds dimethyl and diethyl phthalate, several siloxane peaks and the propanoic acid derivative were detected in the HP gray septa at the same desorption temperature (Fig. 2). The phthalates and the propanoic acid compound were detected in the HP gray septa at temperatures as low as 150 deg C. At temperatures greater than 250 deg C, both the septa from Sulpelco and HP were found to contain numerous straight and branched chain hydrocarbons, additional siloxane peaks as well as higher concentrations of the compounds previously identified at the lower desorption temperatures. The relative intensities of these compounds increase as the temperature increases. This is due to the hot carrier gas passing through the desorption tube and flushing the underside of the GC septa. The higher the temperature, the more of these componds that will be purged off the underside of the septa.

Figure 2 - HP Grey Septa Analysis At Various Temperatures

As with Sulpelco Thermogreen septa, both Restek Thermolite (Fig. 3) and HP red septa (Fig. 4) contained pentamethyl-disiloxane at low concentrations when desorbed at 250 deg C; however, neither exhibited the propanoic acid derivative. Upon closer examination of Sulpelco Thermogreen septa, this propanoic acid derivative may be the result of a contaminated injection port and not derived from the septa. This was supported by another analysis at 250 deg C after the injection port was thoroughly baked out (Fig. 5). Numerous siloxane peaks were identified in both J&W High Temperature red (Fig. 6) and blue (Fig. 7) septa with increasing concentrations found in the blue septa. SGE 3-layered septa exhibited high GC background at a desorption temperature of 250 deg C (Fig. 8) which was very similar to that for HP gray septa.

Figure 3 - Restek Thermolite Septa

Figure 4 - HP Red Septa

While using the thermal desorption technique, these background compounds are trapped in the cryotrap section of the column during the desorption step and are subsequently chromatograped as distinct GC peaks. From previous studies, these background peaks have long been associated as resulting from the silicone used in the manufacture of GC septa. These peaks can be minimized by using high quality preconditioned GC septa. We have determined from this study that Sulpelco Thermogreen , Restek Thermolite and HP red septa to be the best septa available for thermal desorption applications due to their low bleed. All of the background peaks identified in these septa are quite weak in comparison to the normal sample sizes that are analyzed via the thermal desorption technique. These weak peaks are only of concern when very low levels of samples are being analyzed (< 1.0ng) or when these peaks co-elute with compounds of interest.

Figure 5 - Supelco Thermogreen Septa

Figure 6 - J&W High Temperature Red Septa

Figure 7 - J&W High Temp Blue Septa

Figure 8 - SGE Three Layered Septa

Conclusion

Background peaks detected in the thermal desorption techniques can be minimized by using high quality preconditioned GC septa such as Sulpelco, Thermogreen ,Restek Thermolite and HP red septa. The higher the desorption temperature and the injection port temperature, the higher that these peaks will appear in the total ion chromatogram. In addition, background peaks can be minimized by use of the septum purge, use of the split technique, and operating the desorption system and injection port at the lowest temperatures possible. High levels of these background peaks, as siloxanes, indicate that the septum needs to be changed. Septa should be changed periodically in order to ensure the cleanliness of the system and to assure that background peaks are not interfering with the analysis of the samples.