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.
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.
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.
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.
As no power source is required, passive samplers can be used for long-term, time-weighted-average monitoring to determine exposure levels in line with occupational health guidance. When used for environmental
air monitoring, their relatively low cost makes them suitable for large-scale deployment.
- Axial diffusive samplers are used for measuring volatile organic compounds (VOCs) in air over a wide concentration range. Depending on the concentration of the target analytes, axial samplers are exposed for periods between 8 hours and 4 weeks.
- Radial diffusive samplers are constructed so that the orientation of the diffusion path is parallel to the radius of the sorbent cartridge. They have a cylindrical diffusive surface area, and so have an effective sampling rate typically 100 times that of axial samplers. Because of this, radial samplers are often used for short-term deployment (4 hours to 1 week), and are not always suited to sampling high-concentration atmospheres.
- Speciality passive samplers are available for certain non-volatile organic compounds.
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.
Sorptive extraction is an equilibrium process involving interaction between a sorbent and a sample, either by immersion in the sample or through contact in the headspace above the sample. It is commonly used as an alternative to liquid–liquid extraction (LLE) or solid-phase extraction (SPE). Sorptive extraction is a convenient approach for complex samples and for monitoring persistent organic pollutants in foods, beverages and other products derived from natural sources.
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.
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.
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.
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.
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.