Which VOC Sampling Technique is Right for You?
Thermal desorption tubes provide a complementary option to canisters for sampling VOCs. Both techniques have advantages and disadvantages and their features must be evaluated for suitability relative to the sampling environment and analytical capabilities. Enumerated below are the similarities and differences between these techniques; use this handy comparison to determine which equipment is best for you.
Similarities Between Thermal Desorption Tubes and Canisters
- Reusable sampling device.
- Long product lifetime.
- Long-term sample stability.
- Blank certification required prior to sampling.
- Sample concentration required before GC/MS analysis.
- Dry purge helpful to remove moisture before GC injection.
- Ppt sensitivity.
- Method acceptance.
- Collection of wide range of VOCs with single device.
- Useful for screening of unknowns.
- Leak tightness critical to maintaining sample integrity and preventing contamination of a clean device.
Differences Between Thermal Desorption Tubes and Canisters
| Thermal Desorption Tubes | Canisters | |
|---|---|---|
| Methods | US EPA TO-17 ASTM D6196 ISO 16017, ISO 16000-6 NIOSH 2549 | US EPA TO-14, TO-15 ASTM D5466 OSHA PV2120 NIOSH Protocol Draft |
| World-wide acceptance | Gold standard for US ambient air market | |
| Applications | Ambient air, indoor air, industrial hygiene Material emissions Food & flavor Chemical weapons | Ambient air, indoor air, vapor intrusion, emergency response |
| C3 to C30 | <C3 to ~C10 | |
| Handling | Light weight for personal monitoring and general ease of use | Larger and heavier; more costly to ship |
| Sampling | Active sampling with sampling pump or diffusive sampling without pump is possible with determined diffusion coefficients for each compound. | Passive sampling, no sampling pump required. Long-term sampling possible without battery to recharge. |
| Integrated sampling only | Grab & integrated sampling | |
| Concentrated sample | Whole air | |
| Proper sorbent selection recommended in methodology. | N/A | |
| Must sample below sorbent breakthrough volumes to avoid sample loss and irreversible adsorption on sorbent | N/A | |
| Large sample volumes >100L | Sample volume is function of canister size, 15L max | |
| Analysis | Tube dimensions are instrument specific | Compatible with all manufacturer sample concentrators |
| 1 injection, more injections possible for some instrumentation | Multiple sample injections | |
| Concentration range ppt to ppm | ppt to ppm | |
| Some sorbents prone to artifact formation. | Low blanks when properly cleaned. | |
| Storage | Sample storage at 4°C recommended for multi-bed tubes to prevent potential migration of compounds to more retentive sorbent which maybe difficult to recover. | Room temperature |
| Cleaning | Analytical process automatically cleans tube for reuse. Cleans as it analyzes. Conditioning/cleaning and analysis incorporated in one thermal desorption unit. | Canister cleaning requires separate equipment as additional step prior to background certification and sampling. |
| Cost | $50-130 each | $200-700 each |
