Seals for Desorption Tubes

Seals are utilized in a variety of locations in the Short Path Thermal Desorption Unit. These seals are used (1) in the Desorption Tube Stainless Steel Caps, (2) in the Desorption Tube Syringe Needle, (3) at the connection of the Desorption Tube to the connector tube in the Desorption Unit, (4) to form seals in the Desorption Tube Conditioning Oven, (5) to form seals in the Desorption Tube Sampling Oven, (6) to form seals in pumping systems for sampling systems utilizing the Desorption Tubes, and (7) in other accessories to which the Desorption Tube is to be connected and a leak tight connection is required.

Seals are used on the inside of the Desorption Tube Stainless Steel Caps to provide a contaminate free surface and to form an air tight seal between the Desorption Tube and the cap. These seals are important to maintain the integrity and prevent the contamination of conditioned Desorption Tubes before being subjected to sample collection as well as after sample collection. The preferred seals for this application are normally PTFE washers (S.I.S. part # 781004).

Seals are used at the top end of the Desorption Tube against the connecting Tube (Top Sealing Washer) and at the bottom end of the Desorption Tube next to the needle cap (Needle Sealing Washer), when the Desorption Tube is installed in the Desorption Unit. It is imperative that these seals are leak tight even at pressures up to 30 P.S.I. for the proper operation of the Desorption Unit and to provide for the complete delivery of desorbed sample into the GC injection port. The seals used to seal the needle cap assembly to the Desorption Tube (Needle Sealing Washer) normally have a small I.D. to seal snugly around the needle shaft and minimize metal surface exposure to the samples being analyzed. The Top Sealing Washers have a larger I.D. to prohibit seal collapse and hindrance of the gas flow through the Desorption Tube. When soft materials such as graphite are used for the top seals, a metal tube is inserted inside the center hole in the washer to prevent the graphite from closing and restricting gas flow (part # 781015). This same seal is also utilized in Desorption Tube conditioning handles and Desorption Tube adaptors to provide good seals particularly at elevated temperatures.

Graphite seals are physically soft but have excellent sealing properties and temperature limits and are recommended for most applications with the Desorption System. The graphite seals have good sealing properties when tightened finger tight, have a fairly good life, and provide for the highest temperature without contamination of the system by foreign matter.

Graphitized Vespel seals contain 40% graphite and physically are harder than graphite seals but with lower temperature limits. They require slightly more force to form a good seal in the Desorption Unit. Normally about 1/16 turn past finger tight will form a good seal. Since these seals are harder than graphite, they hold their shape better, have a longer life, and are easily removable.

Viton seals are easy to use and remove. However, their main limitation is the bleed that may be seen at higher temperatures, much like the bleed seen from GC septa. In addition, the constant turning of the Desorption Tube when screwed into the fittings tends to abrade and wear these seals, causing leaks after 10 or 12 sample runs.

PTFE seals have temperature limitations like the Viton seals, but are good general seals for lower temperature applications. They have good sealing characteristics, good abrasion resistance, and are usable to about 230 degrees C. At temperatures above this they tend to flow and then are quite difficult to remove from the fittings.

The double seals on both ends of the desorption tube as shown above are recommended. The Graphitized -Vespel seal should always be against the desorption tube due to its good abrasive and wear characteristics. The graphite seal should be against the needle and connecting tube due to graphites excellent flowing and sealing properties. This combination will provide optimum performance, life and leak free seals. Tighten all tubes as needles, finger tight then 1/8 turn past finger tight with the pliers. This will maximize seal life and provide for consistent leak free seals.

Material          Max. Temp.      Sealing Properties   Abrasion Properties
Viton            250 degrees C         Good                 fair
PTFE             230 degrees C         Good                 good
Graphite         450 degrees C         Excellent            poor
Graph Vespel     350 degrees C         Good                 good

Indicate # of units. Viton and PTFE have 10 washers per unit, the rest have 1 washer per unit.

Thickness (in)=0.06
Part No.	OD (in)	ID (in)	Cap Seal	Top Seal	Needle Seal	Acc Seals	pkg of	UOM	est wt.	Quantity In Stock	Price / Pkg.	Quantity (pkg) to order
781003	Viton Washers
	0.21	0.08	Yes	Yes	Yes	Yes	10	PK	1	0(#)	$22.05	1
781004	PTFE Washers
	0.21	0.12	Yes	Yes	Yes	Yes	10	PK	0.25	82	$9.24	1
781015	Graphite Top Sealing Washer
	0.21	0.06	No	Yes	No	Yes	---	EA	0.25	344	$13.50	1
781016	Graphite Needle Washer
	0.21	0.04	Yes	No	Yes	No	---	EA	0.25	PART NUMBER OUTDATED. New number is 781018(#)	---	---
781017	Grap Vesp Top Sealing Washer
	0.22	0.06	No	Yes	No	Yes	---	EA	0.25	7814	$10.50	1
781018	Grap Vesp Needle Seal Washer
	0.22	0.04	No	No	Yes	Yes	---	EA	0.25	4991	$10.50	1
(#) Standard lead-time for out-of-stock items is approximately two weeks.
Ordering information. International (non-US) prices available through your local dealer.

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