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Tubes & Traps
The design of the sorbent tubes used for sample collection and thermal desorption in CDS / Dynatherm thermal desorbers is a major factor contributing to the overall performance of the equipment.
Glass versus Stainless
It takes 70 seconds for a glass tube to reach 300°C, compared to 110 seconds for stainless steel. One can inspect the condition of the adsorbent through glass, and see ...
- color changes, indicators of polymer decomposition
- moisture condensate,
- dirt and particulates,
- loose adsorbent bed
... all problems that affect sampling and recovery.
Chemical silanization deactivates the tube surface and frit, offering significant benefits when performing trace analysis of chemical agents.
Dynatherm sampling tubes are precisely engineered to snugly fit into Vespel® Graphite ferrules, offering a balance between elasticity (seal in a pressurized environment) and durability (resist deforming under pressure). The 1 mm tube wall thickness is rugged and robust.
Unground Pyrex® tubing has an elliptical surface that can deform the ferrule, causing a leak around subsequent tubes inserted in the desorption chamber.
The initial cost of a ground surface on heavy walled tubing is offset by eliminating analytical errors due to leaks.
CDS tubing is ground to an outer diameter of 0.236" +.000"-.001", matching a similar tight tolerance on the internal bore of 6 mm Vespel® Graphite ferrules.
Porous Glass Frit
The frit at the sampling end of the tube:
- Keeps adsorbent from shifting during high volume vacuum sampling
- Protects expensive internal valving from glass wool particles and stray adsorbent fragments
- Provides a precisely positioned inert surface directly in the heated zone ensuring that any analyte captured there is transferred to the GC for analysis.
Packing and Conditioning
The glass frit and spring clip at opposite ends of the packing eliminate shifting adsorbent during sampling operations.
Channeling caused by adsorbents that shift freely, ebbing and flowing at the bottom of the tube with voids above, contributes to analyte breakthrough, skewing retention and recovery data.
CDS conditioners independently set carrier gas flow rate and temperature for every tube, ensuring that contaminants released during heating are removed.
Large conditioning ovens with carrier gas manifolds suffer from low flow rates in random tubes and temperature gradients that compromise the conditioning process. Random conditioning failures are difficult to pick up on batch tests, allowing inadequately conditioned tubes to enter the sampling queue or necessitating 100% cleanliness testing, an expensive alternative.
CDS tubes are sealed after conditioning in a N2 atmosphere and remain usable for months.
Unsealed tubes shipped in plastic bags become contaminated through diffusion, causing a significant artifact level even if the tubes were properly conditioned before shipment.
Pressure Drop Across Tubes
Flow restrictions and maximum vacuum sampling flow rates are a function of several factors:
- Shape and mesh size of the adsorbent. 20:35 mesh Tenax-TA is less restrictive than 60:80 mesh
Chromosorb 106 or HayeSepD.
- The internal diameter of the tube, coarseness of the glass frit, and the mechanics of fusing it
to the tube influence the maximum flow rate that can be obtained during sampling
- Vacuum pump specifications and method of sampling rate flow control ultimately determine
the maximum flow rate.
Low cost battery powered industrial hygiene sampling pumps typically fail to maintain adequate flow volume over extended sampling periods.