Septa Lot Control Process
Gas chromatography has relied on silicone rubber septa products for many years. Silicone rubber became the raw material of choice because of its ease of production, general cleanliness and affordability. The Environmental/Analytical industry incorporated the 22mm Teflon® bonded to Silicone septa on the standard EPA vial since inception and has specified this type of septa in standard methods.
General water testing applications have found the septa (open-top) vials to be a reliable component for providing a clean, inert and leak-proof seal on water sampling vials (40mL EPA vial). More recent methods for soil sampling and analysis (Method 5035, for example), and sample purging in fractional volumes with headspace, have increased the need for clean “low bleed” septa. However, syringe injection, sample heating and introduction of solvents has resulted in detection of artifacts in the silicone. Method refinements and more sensitive instrumentation have also contributed to the possibility of septa interference. Providing consistently clean septa in an ongoing industry issue.
Septa manufactures buy gum or base silicone rubber from one of several prominent sources, including Dow Corning (USA), Wacker (Germany) and Momentive (formally GE Silicone Products,USA).
The gum silicone rubber is supplied to septa manufactures in either large blocks or fiber drums. The septa manufacturers then process the raw silicone rubber through various steps of blending, calendaring and curing. For the standard EPA application septa, the silicone rubber is then rolled into sheets and bonded with a TFE Teflon® film.
The gum or base silicone rubber is made in large factories under industrial conditions. One of the properties that make silicone rubber attractive in the environmental analytical field is its ability to absorb compounds in instrumentation. Solvents, such as toluene and acetone, that are used in the manufacturing process to clean tanks, kettles and mixers can be easily absorbed into the silicone rubber raw material. The post conditioning processes that are performed by septa manufacturers are done to bake out volatile compounds such as these and siloxanes, which are inherent in silicone rubber. Curing is done to polymerize the rubber to ensure a bond between the Teflon® and the silicone.
The post conditioning process is done at high temperatures before and after the Teflon® has been bonded to the silicone. However, if trace levels of VOC’s have not been entirely baked out they can become encapsulated into the Teflon®/Silicone septa. Each step of the curing and baking process is in place to drive off unwanted VOC’s however, there is a potential for remaining traces of VOC’s to be encapsulated against the Teflon® after the bonding process. Syringe injection into a vial cores through the entire septum, including the portion of Silicone that is in contact with the Teflon®, which could expose samples to these trapped trace VOC’s. For this reason, ESS records the lot number(s) of every septa shipment it receives and submits every septum to a final baking process at 100o C for a 24 hour period.
Locating and sourcing the cleanest gum or base silicone rubber is a significant step to ensure that the septa vials will meet and exceed the requirements of the Environmental/Analytical industry. To this end, platinum-based silicones are beginning to replace peroxide-based silicones.
Cleaning, Testing and Tracking
Septa received by ESS are washed and baked to USEPA procedures specified in the Office of Solid Waste and Emergency Response (OSWER) Directive Number 9240.0-05A titled “Specifications and Guidance for Contaminant-Free Sample Containers”. Included in this directive are VOC testing requirements for the assembled 40mL EPA vials using Solid Waste Method 8260. However, compound detection levels have changed (lowered) since the OSWER directive was published in 1992. ESS analyzes lot representative vials using Drinking Water Method 524.2 plus fuel oxygenates. But while this water analysis meets all regulatory requirements it does not tell the whole story. The Teflon®/Silicone septa of QC blanks that are analyzed as water samples are not exposed to the conditions of other common vial applications such as fractional water purges and soil analysis. For this reason, ESS analyzes each Teflon®/Silicone septa lot it receives via GC/MS Heated Headspace Method TO-15. With this method, most VOC’s are reported to 10ppbv (which closely correlates to ppt levels) This is a significant improvement on the 524.2 analyses which only reports most analytes down to 1ppb. Method TO-15 provides ESS with the lowest Teflon®/Silicone septa QC levels offered in our industry for EPA vials.
ESS tracks every case of vial product it sells. The standard tracking number on a carton of our most popular EPA vial, the 0040-0300-QC, includes the following information:
QC Lot Number
This 8-character identification (seven digits followed by one alpha character) can be found on the Product Description Label located on the top flap of the carton and represents the production lot number that was assigned to a vial lot processing and the lot-related analytical testing. The QC Lot Number (and ESS Product Number) can also be found on our QC Class Certificate of Compliance that confirms the Independent Analysis using Method 524.2 plus fuel oxygenates.
This 12-digit number and bar code is located on the lower left hand corner of the top flap of our vial box. The first 6 digits of the Component Stamp reference the septa manufacturer’s lot number. This lot number is traceable to the GC/MS Heated Headspace Method TO-15, which is available upon request. The remaining 6 numbers of the Component Stamp reference the vial manufacturer and ESS’ production information.
This 6-digit number is located on the top flap of our vial box and represents the date that the product leaves our facility. The Date Stamp number assists laboratories in monitoring the products shelf life.
There are many presumptions as to the source(s) of contamination in septa vials including raw materials, septa exposure and storage (laboratory) environment. One common presumption is that contamination is a result of the Teflon®/Silicone septa adsorbing VOC’s during shipping from the supplier’s facility to the end user. While certainly possible, transportation is rarely been determined as the source. Some sample container suppliers, believing transportation to be the source of contamination, have gone to the extreme of packaging their septa vial products in foil bags. These suppliers still experience product contamination, further supporting the small role, if any, that transportation plays in product cleanliness.
During the 2007 National Environmental Monitoring Conference, Mark Gilbert of Columbia Analytical Instruments presented a paper titled “Mission Critical VOA Vials: Evaluation of and Protection from Compound Intrusion/Evacuation”. In his paper, Mr. Gilbert discussed how septa with a top vapor film reduced the potential for contamination when exposed to solvents. His data was valid but only addressed contamination through exposure. Mr. Gilbert’s paper did not take into consideration that contaminates are inherent in the raw gum or base silicone rubber.
ESS has analyzed multiple lots of gum or base silicone from different raw material suppliers and septa manufacturers and, at times, has detected as much as 250ppm to 500ppm of toluene. Raw material products specifications (<25ppb) and curing by septa manufacturers can get toluene levels down to 1ppb to 10ppb. Final septa washing and baking procedures can bring the toluene levels to below laboratory reporting limits of 1ppb to 0.2ppb. The effectiveness of each curing and processing step is dependant on a gum or base silicone rubber raw material that is low in VOC’s. Raw material specifications and prequalification is imperative to providing contaminate free septa vials.
For this reason, ESS has developed a Validation and Testing of Septa document that must be followed and supported by each of our septa manufacturers. For further information on ESS’ efforts and programs to provide the Environmental/Analytical industry with the septa vial products available on the market today, please contact Matthew Macy by telephone at 1-800-233-8425 ext. 120, by facsimile at 1-800-625-6095 or by e-mail at firstname.lastname@example.org.