Principles For Successful TD Operation

Figure 1 - Single stage thermal desorption 

During the thermal desorption process, heat and a flow of inert gas are used to extract volatile and semi-volatile organics retained in a sample matrix or on a sorbent bed. The analytes desorb into the gas stream and are ultimately transferred to the analyser. Although compounds can be transferred directly from the original sample to the analyser in one thermal desorption step, this simple, single-stage approach has limited practical application. The elution volume required for complete extraction of typical 100 mg – 1 g samples is too large giving poor analytical resolution and relatively low sensitivity.

For this reason most commercial thermal desorbers are two-stage – i.e. they contain a focusing mechanism for concentrating analytes desorbed from the sample tube before releasing them into the analytical system in as small a volume of vapour as possible. Two basic types of refocusing mechanism are used:

• Capillary cryofocusing
• Cold trapping

Figure 2: Two stage desorption with capillary cryofocusing


Capillary cryofocusing does produce excellent, capillary-compatible chromatography, but it can be extremely costly in terms of liquid cryogen consumption. More importantly, such systems are prone to blocking with ice during the desorption of humid samples. Analytically, this spells disaster -----thermal desorption is a dynamic process. Any blockage or restriction of the desorption gas flow has a significant impact on the efficiency of the process.

Figure 3: Two stage desorption using an electrically-cooled sorbent trap

Refocusing on a small electrically-cooled sorbent trap, which is then heated rapidly - in a reverse flow of carrier gas (backflush) - to desorb 99% of analytes in a few seconds, is invariably the technique of choice for thermal desorption. Such systems have been shown to quantitatively retain analytes as volatile as C2 hydrocarbons while at the same time being able to desorb fast enough to produce uncompromised, high resolution capillary chromatography with low, or even zero, split ratio. There is the obvious benefit of eliminating costly liquid cryogen and little risk of blocking a typical 2 mm internal diameter secondary cold trap with ice.  Backflush desorption of the focusing trap also facilitates simultaneous analysis of very volatile and semi-volatile compounds.

Concentration Enhancement

TD has significant concentration enhancement potential. When used in conjunction with sorbent tubes and 2-stage desorption for monitoring trace level organic vapours in air or gas; concentration factors can reach as high as 10⁶.

If sample has been collected using a primary sorbent tube, target compounds are typically extracted/desorbed from it in ~100-200 ml of inert gas. Extracted compounds are then refocused on the cold trap. Once primary desorption is complete, the cold trap is itself desorbed with complete extraction typically achieved in as little as 100-200 µl of vapour. It is this tiny plug of vapour that is introduced to the analyser with minimal band spreading thus optimising detector response.

If the original sample collected was 100-200 L of air or gas the concentration factor achieved is 10⁶. If a more typical 10-20 L sample was collected the concentration effect is still high – 10⁵.

If whole air or gas samples are introduced directly to the focusing trap from canisters, bags or on-line air/gas streams, the concentrating power of the primary desorption stage is lost. Maximum concentration factors are typically in the order of 10³ – 10⁴ in this case. 

Applications

Thermal desorption offers advantages to almost every application involving the measurement of trace level VOCs (volatile organic chemicals) and can also be used for some semivolatile determinations. It offers great versatility with regard to analyte concentration (ppt to %) and can save many hours manual sample preparation. To understand the potential of thermal desorption it helps to know the limitations of the technique: -

Thermal desorption cannot usually be used for…

• Inorganic gases, except N₂O and SF₆
• Compounds which are too unstable for conventional GC analysis.
• Compounds less volatile than n-C₄₀, or 6-ring polyaromatic hydrocarbons (PAHs).
• Organics in a sample matrix where the temperatures required for quantitative thermal desorption would cause severe degradation of the matrix itself.

Apart from this limited number of exceptions, thermal desorption has wide application. Key examples include:

• Industrial hygiene –  monitoring the personal exposure of workers to toxic chemicals in the workplace
• Environmental monitoring –
  • Ambient urban air
  • Indoor air
  • Emissions from building materials and related consumer products
  • Tracer gas tests of building ventilation
• Chemical warfare agents - monitoring CW agents such as nerve gases (VX, Sarin) and blister agents (mustard)
• Materials QA/QC – residual solvents, residual monomer, taint, etc..
• Flavour and fragrance – odour profiling, shelf life, etc..
• Organic composition – water-based paints, syrups, medicinal ointments, adhesives, etc..
• Accelerants in fire debris
• Biogenic emissions – plant volatiles, insect pheremones, body odour, VOC profiles from moulds/fungi/bacteria, etc.
• Volatiles in soil and water
• Organic artifacts on silicon wafers and related electronic components
  

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