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Products shown here are related to the application of thermal desorption for defence and forensic. Or find out more about defence applications and forensic applications.

Active sampling

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.

Microchamber sampling

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

  • Screening of products prior to long-term certification tests e.g. consumer products
  • Checking raw materials.
  • Comparing products to those of competitors.
  • Monitoring odour and emission profiles e.g. car trim.
  • Kinetic studies such as shelf-life tests or monitoring aroma or fragrance profiles as they change over time for the food and consumer industries.

Online air or gas sampling

‘Whole air’ sampling refers to collection of samples in the field using canisters or bags, or their direct introduction to the focusing trap of the thermal desorption instrument without the use of sorbent tubes (online sampling). The compounds remain in the gas phase throughout.

Air sampling bags are commonly used to collect whole air samples for landfill gas and soil gas. A sample can be collected in the container using

either pumped or passive sampling, and subsequently sealed and transported to the laboratory for analysis.

Where the compounds of interest are too volatile to be retained by sorbent tubes at ambient temperature (such as hydrogen sulfide), either on-line or canister analysis is required (as opposed to tube sampling).

On-line air monitoring is the method of choice for real-time monitoring of changes in vapour concentrations and for continuous remote monitoring.

Applications include:

  • Ozone precursor monitoring
  • Odours in urban air
  • Fenceline monitoring of petrochemical plants and other industrial installations
  • Odour from sewage treatment works

Soil gas sampling

Quantitative purge-and-trap with GC(–MS) is a common approach for the analysis of VOCs and SVOCs in soil, but it involves labour-intensive sample preparation and pre-screening analysis. Soil probes are a complementary approach to purge-and-trap for analysis of VOCs in soil vapour that overcomes many of these issues. Their use supports compliance with ASTM D7758.

Stainless-steel assemblies are inserted into the ground, allowing soil gas to be sampled in-situ onto a

sorbent tube, either pumped or passively.

Established VOC air monitoring technology has also been applied to the monitoring of contaminated land, but the frequently-used approach of canister sampling excludes many heavier organic pollutants. Sampling onto sorbent tubes, followed by thermal desorption analysis, allows a much wider volatility range to be addressed. Soil probe technology therefore has the potential to significantly speed up and reduce the cost of surveys of contaminated land.

Sorptive extraction

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

    platform

SPME

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.

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.

  • Check-standards are sorbent tubes pre-loaded with a suite of analytes that can be used to check instrument performance during set-up or troubleshooting.
  • CRS tubes are loaded with a certified level of analytes and are designed to validate calibration for QA, as described in international standards. They are supplied with a shipping blank, and example chromatograms of the sample and shipping blank.
  • Tubes for liquid calibration contain a short bed of sorbent for analysts wishing to inject a liquid standard directly onto the sorbent tube, rather than loading the tube using a flow of gas (e.g. with the CSLR).

Headspace sampling

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.

Starter kits

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.

Whole air canister sampling

Canisters for air sampling (often referred to as SUMMA® canisters) have long been used to monitor volatile organic ‘air toxics’. Whole air canister sampling is a simple form of ‘grab’ sampling and is useful for sampling very volatile, non-polar compounds such as C2 to C12 hydrocarbons and the most volatile freons, which can be difficult to retain quantitatively on sorbent tubes at ambient temperature.

For many analysts, the most familiar

canister method is US EPA Method TO-15. This method involves sampling of ambient air using evacuated canisters followed by thermal desorption preconcentration, and GC–MS or GC–FID analysis. It is widely used by commercial laboratories for analysis of VOCs in urban and industrial settings, especially in the USA and Asia.

Preconcentration/trapping is still required before analysis to allow injection of the sample in a small volume of carrier gas, and to eliminate the bulk constituents of air (especially oxygen) and water, which would otherwise adversely affect the performance of the GC column and detector.