Products shown here are concerned with sample extraction from the vapour phase.
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.
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.
- Ozone precursor monitoring
- Odours in urban air
- Fenceline monitoring of petrochemical plants and other industrial installations
- Odour from sewage treatment works
Passive sampling (diffusive sampling) onto sorbent media uses the principle of Fick’s first law of diffusion to target known sample environments. Using this approach, analytes migrate to the surface of a sorbent bed at a rate that depends on the conditions.
Find out more about Passive sampling in our new blog, Monitoring the air we breathe with passive sampling or contact our Sampling technologies team to discuss your applications.
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.
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.
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.