Active sampling (pumped sampling) onto sorbent tubes is one of the most versatile TD sampling methods, used to target both known and unknown sample environments. This approach involves use of a pump to actively draw an air sample through the sorbent tube to trap the target analytes.
Active sampling is key to US EPA Method TO‑17, which involves pumping the sample atmosphere onto the sorbent tubes, which are then capped
and transported to the laboratory, for analysis by thermal desorption. It is also the recommended sampling approach in other standard methods, including ISO 16017-1, ASTM 6196, Chinese EPA HJ 644 & HJ 734, EN 14662-1, CEN/TS 13649, NIOSH 2549, UK Environment Agency LFTGN 04, and UK HSL MDHS 72.
Breath sampling is a valuable approach for obtaining information used for disease diagnosis, monitoring of metabolic stress and personal exposure to chemicals. Breath samples can be collected using a number of techniques, which can then be used with thermal desorption to extract and concentrate the VOCs, maximising sensitivity for analysis of trace-level target compounds while minimising interferences.
With the growing interest in breath monitoring, alternatives have emerged to the long-established
use of Tedlar® bags for sampling breath.
Markes’ BioVOC-2™ is an easy-to-use device for sampling of VOCs in breath that transfers the volatiles directly onto sorbent tubes for analysis by thermal desorption.
Direct desorption/thermal extraction
Direct desorption is a variation on the principle of dynamic headspace that can be used to rapidly screen small quantities of solid or semi-solid materials. Small samples are placed directly into empty TD tubes or via tube liners. These are then heated in a stream of inert gas to sweep volatiles directly from the sample matrix onto the focusing trap of the thermal desorption instrument, for subsequent analysis by GC.
Direct desorption is typically applied to homogeneous waxes, powders or pastes. Analytes monitored are generally in the range C3 to C30. Applications of direct desorption include the odour profiling and quality control of a range of materials:
- Residual volatiles in ointments, packaging films, pharmaceuticals/drug powders and polymer beads.
- Emissions from consumer goods, such as paints, car trim components, moulded PVC and adhesives.
- Characteristic vapour profiles from foodstuffs and fragrant products, including soap powder and shampoo.
Microchambers are compact, stand-alone test chamber units that allow rapid sampling of vapour-phase organic compounds from a product or material, complementing small-scale chamber testing by third-party laboratories. Microchamber sampling is a type of dynamic headspace sampling and a powerful tool for increasing laboratory productivity. The technique is usually used in combination with sorbent tubes and off-line thermal desorption–GC–MS.
Applications for microchamber sampling include:
- Quality-control of chemical emissions from products
and materials, such as construction materials
Sample extraction supplies
A range of supplies are required for the busy lab technician needing to perform sample extraction, whether automated or manual. A reliable supply of quality assured items such as vials, caps, liners, syringes, fibers, and probes supports throughput.
The commonly used sorbent tube sampling approach can be complemented by other sampling methods such as headspace, solid-phase microextraction (SPME) and HiSorb™ high-capacity sorptive extraction.
These sampling techniques all benefit from being
simple, easy to automate, and compatible with most commercial GC platforms.
Featured product: HiSorb Starter kit
- A flexible alternative to methods such as stir-bar sorptive extraction (SBSE)
- Detection limits are lower than for SPME
- Easier and quicker to use than solvent extraction
- Re-usable probes and tubes minimise cost per sample
- Robust and easy to use with multiple phases available
- Exclusive to Markes International
- Automated by the Centri sample extraction and enrichment
Solid-phase microextraction (SPME) uses a small fiber coated with absorptive phase(s), to extract volatiles in either headspace or immersive modes. SPME extraction is an equilibrium process dependent on the partitioning of analytes between the sample matrix (either liquid, solid or headspace) and the absorptive phase. Analytes that become absorbed into the SPME phase are subsequently desorbed using a heated injector, before transfer to the GC column.
The simple, solvent-free nature
of SPME has led to its increased popularity across a range of application areas, particularly in food and beverage analysis, clinical studies and environmental analysis. Compared with static headspace, SPME can offer improved selectivity and concentration capability, resulting from the sampling capacity of the sorptive phase. A variety of sampling phases are available to extend the applicability of SPME over a range of target analytes.
Supplies for standard methods
Markes’ application specialists are active on committees working on standard methods, and also in the development of products to support method compliance. A variety of standard methods are available for monitoring specific VOCs often in prescribed sample types. For example, an automobile industry standard method might focus on materials such as plastic, leather or foam.
Organisations developing standard methods can be grouped into government agencies such as US EPA and
UK HSE, international standards agencies such as ISO and CEN, and consensus-based standards organisations such as ASTM. When selecting an appropriate method to adopt, the following issues should be considered:
- Any specific regulatory requirements/guidance.
- Method scope e.g. ambient, indoor, workplace, material emissions test chambers.
- Sampling and analytical requirements.
- General guidance (e.g. choice of sorbents, selection of GC column).
- Validation protocol.
- Method limits – list of target analytes (if applicable), concentration range, detection limits, analyte volatility range, etc.
See our Standard Methods page listing all relevant methods.
Standards & calibration
Calibration of the complete analytical process is of paramount importance when conducting quantitative studies. For TD analysis, additional considerations should be made regarding the loading of standards to be representative of samples, with a variety of tools available to support this.
In addition to gas standards for both canister and tube-based methods, a number of products have been developed to support laboratories with their calibration and validation:
- The Calibration
Solution Loading Rig (CSLR) was developed to optimise the introduction of liquid standards onto sorbent tubes, by transferring the standard in a flow of gas.
Headspace (HS) has been widely used for quantitative evaluation of VOCs in solid and liquid samples for many years. Samples are placed into vials, sealed and incubated. Once equilibrium is reached VOCs partition into the headspace at levels that are proportional to their concentration in the original liquid or solid sample. A precise aliquot of headspace vapour is then transferred/injected directly into the GC (traditional methods) or via an electrically-cooled
focusing trap for enhanced performance.
Headspace sampling can be enhanced in combination with a preconcentration focusing trap step. Headspace–trap results in enhanced sensitivity by allowing larger injection volumes to be used, as well as multiple extractions from a single sample to be ‘concentrated’ in one GC–MS run. It also offers improved peak shape, optional sample splitting, and re-collection for repeat analysis without the need to repeat extraction steps.
Common application areas of headspace sampling include environmental monitoring, and analysis of foods, beverages and fragranced products.
Markes’ starter kits are designed to provide the essentials needed to get a thermal desorption system up and running quickly, by providing a single package with items such as tubes, traps, tools and other accessories.
Markes offers a selection of starter kits for a range of standard methods such as US EPA Method TO-17 and US EPA 325, as well as application-specific starter kits such as the Material emissions starter
kit and the Direct desorption starter kit.
Laboratory managers and technicians using the UNITY–ULTRA-xr or TD100-xr systems may also find the popular essential automated TD starter kit and the automated TD booster pack useful.
Other starter kits have been designed to help customers get up and running with tube tracking technology (TubeTAG) or adopting a new approach to sampling, such as HiSorb sorptive extraction.