Short Path™ Thermal Desorption TD-5 (Catalog C3)

C3

Short Path™ Thermal Desorption TD-5

FAX: 908-806-6631

www.sisweb.com

Theory of Operation

T

he Thermal Desorption Unit sits directly on top of the

injection port of most GC's, where it provides direct des-

orption of both volatile and semi-volatile samples into the

GC injection port and column. The system delivers samples into the

GC along an optimal “short path,” thereby eliminating

transfer lines

and cross-sample contamination issues found in previous systems.

Samples to be analyzed are collected into stainless steel

desorption tubes (or silco-treated)(Fig.1).

A solid sample of inter-

est may be packed directly into the tube and subjected to direct ther-

mal desorption.

Alternately, the tube may be packed with adsorbent

resin such as Tenax™ TA or activated carbon for indirectly trapping

analytes from liquid or air samples.

To analyze a prepared desorption tube (Fig.2), a syringe nee-

dle is first attached to the desorption tube, which is then attached to

the connector tube on the Desorption Unit (#1). The desorption

blocks heat to initial temperature (#2). Carrier gas is sent through the

desorption tube for an initial purge time (#3). The system then injects

the desorption tube into the GC injection port (#4) where flows are

readjusted as required by the method of analysis, i.e. split/splitless,

etc. The hinged heating blocks close around the desorption tube (#5)

to ballistically heat the sample tube, optionally with a temperature

ramp. The combination of the heat applied and the carrier gas flow

through the desorption tube will purge the desired components into

the GC injection port and onto the front of the GC column.

To obtain

sharp chromatographic peaks when desorption typically lasts 5 to 15

minutes, it can be desirable to focus desorbed components at the

head of the GC column. This focusing can be improved by installing

a Cryo-Trap, which cryogenically cools the head of the GC column

(with liquid CO2 or LN2) during desorption to trap desorbed analytes.

After desorption, the Cryo-Trap rapidly heats to volatilize the trapped

analytes and quickly release them through the GC column for sepa-

ration (#6).

Purge

Gas Off

Desorb

Blocks Off

Cryo Off

Purge

Gas Off

Desorb Blocks

Heating

Desorb Blocks

Heating

Cryo Cooling

Cryo Cooling

Cryo Cooling

Purge

Gas On

Purge

Gas On

Desorb Blocks

Heating

Cryo Cooling

Purge

Gas On

Desorb Blocks

Heating

Cryo Heating

Purge

Gas Off

Desorb

Blocks Off

1

2

Detachable

Needle

Quartz Wool

Quartz Wool

Screw Threads

Solid Sample or

Adsorbent Resin

Figure 1 - Desorption (sample) tube

Desorption Unit mounted on an Agilent 6890 GC

Sample Loaded

Temperatures ON

Purge Gas ON

Injecting

Desorbing Sample

GC Analysis

Figure 2 - TD-5 Run Cycle

X

X

3

4

5

6

X

Cross section of thermal desorption tube

Loading of Desorption Tubes for sample analysis

Silco Coated

Stainless Steel

Outer Shell

Silco Coated SS Desorption Tube