Direct Thermal Desorption (DTD)

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Contributors: Catherine Stephens, Michael Samide, Eric Monroe, Greg D. Smith

Overview

Technique: DTD
Formal name: Direct Thermal Desorption
Sample image of the data

What this techniques measures: Direct Thermal Desorption measures volatile organic compounds released from a material at a defined temperature over a defined collection time. The released compounds are typically separated via gas chromatography and detected with a mass spectrometer. Several different, but related, methodologies and equipment are capable of performing this technique and have some variance in sample processing/handling and capabilities which result in variations in time & temperature of desorption as well as method of collecting of the desorbed volatile compounds. The temperature of the thermal desorption process is selected to evolve volatile compounds but not degrade the sample material, unlike pyrolysis. Both dedicated thermal desorption and pyrolysis equipment (operated at lower temperatures) have been used for DTD. Identification of the emitted volatile compounds may lead to a better understanding of pollutants that may be released into the museum environment which may impact artwork. Some chemical intuition is required to interpret the chromatogram and link chemical identity with pollutant potential.

Limitations of this technique: Non-volatile compounds will not be detected. Very volatile compounds may not be detected unless cryo-focusing is employed. Even then, the detection of compounds such as formaldehyde, carbonyl sulfide, and acetic acid may be hampered by an inability to separate these compounds using the gas chromatograph. Often, the use of a selected-ion chromatogram will allow one to detect the presence of the very volatile compounds.

Can/how can this technique be made quantitative?: Semi-quantitative analysis is possible if a material is prepared and thermally desorbed and measured in a manner to ensure that all volatile compounds are collected from the sample material. The technique can be a broadly qualitative methodology to examine what volatile compounds would be released from a material over time. Quantitation can be performed such that a mass of pollutant per mass of sample per time (ng/mg-sec) can be determined. Using this information, one can compare the amount of pollutant emitted under specific conditions and compare that to other materials studied under identical conditions. In this way, one can perform comparative analyses to help identify a best product for use.

Samples

Phases it can be used to examine: Samples are present in the solid phase during the direct thermal desorption step. Compounds that are detected are those that were desorbed as gases during sample processing.
Is this technique non-destructive?: No, physical samples are needed and are heated to release volatile compounds. Typically only a few milligrams of material is necessary for study. For sample that are not homogeneous in nature (e.g., a multi-layer carpet sample) may need cryo milling in order to obtain a representative sample
How invasive is this technique? Physical samples are needed, although these can be relatively small.
Minimum size of sample necessary to use this technique? Typically, 1 to 20 mg of material is needed in most applications. More material can be useful when a sample is complex or higher sensitivity is needed. The amount of material is often a function of the equipment used to perform the analysis.
Sample preparation methods: Often a small sample can be cut from a larger piece of test material. The small sample of a material is placed into a sampling apparatus (metal cup, glass tube, etc.) and heated in an oven at a defined temperature for a defined period of time. Desorbed compounds are introduced to the inlet of a GCMS for separation and detection. This inlet may have cryogenic focusing capabilities that can trap and concentrate desorbed compounds, allowing for a longer, potentially more complete collection of volatile compounds as well as better separation and identification of compounds.

Applications

Examples of how this technique is used in the field? Quality assurance measurements of construction and encasement materials, analysis of degrading materials from collections.
Risks associated with using this technique? Physical samples are required for analysis. Thermal exposure can damage the samples. While the technique can identify what compounds are emitted from a sample, some chemical intuition is required to understand if that compound and the amount of that compound make it a potential pollutant.

The AIC Materials Working Group has a written protocol for sampling VOCs using DTD. This technique requires the use of a gas chromatograph-mass spectrometer to analyze the data.

Budgetary Considerations

Approximate cost to purchase this instrument? Thermal desorption equipment can range from $ 35K to ~100k depending on the complexity and level of automation needed for the equipment. This is in addition to a GCMS for the analysis of the desorbed compounds ($60-150k additional.)
Annual cost to maintain this instrument? Service support costs could be expected to be ~10% of the equipment cost annually and consumables in the $1k range. Costs would be higher if cryotrapping is utilized as liquid nitrogen is needed.
Sample analysis costs? Semi-quantitative DTD of materials following standard techniques VDA278/GMW15634 are ~$700 a sample.

Case Studies

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Additional Information

Complementary Techniques [describe and link] Variations of this technique [describe and link]

References

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