Products shown here are concerned with sample extraction from the vapour phase.
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.
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
- 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.
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.
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.