Standard Practice for Selection of Sorbents, Sampling, and Thermal Desorption Analysis Procedures for Volatile Organic Compounds in Air
|Publication Date:||10 May 2003|
|ICS Code (Ambient atmospheres):||13.040.20|
This practice is intended to assist in the selection of sorbents and procedures for the sampling and analysis of ambient (1), indoor (2) and workplace (3, 4) atmospheres for a variety of common volatile organic compounds (VOCs). It may also be used for measuring emissions from materials in small or full scale environmental chambers or for human exposure assessment.
A complete listing of VOCs for which this practice has been tested, at least over part of the measurement range (1.6), is shown in Tables 1-9. For other compounds this practice shall be tested according to EN 1076 (pumped); Practice D 6246, ISO 16107, ANSI/ISEA 104, EN 838 or EN 13528-1/EN 13528-2 (diffusive); or other appropriate validation protocols (Sections 13 and 14). (5,1)
This practice is based on the sorption of VOCs from air onto selected sorbents or combinations of sorbents. Sampled air is either drawn through a tube containing one or a series of sorbents (pumped sampling) or allowed to diffuse, under controlled conditions, onto the sorbent tube or tubes (diffusive or passive sampling). The sorbed VOCs are subsequently recovered by thermal desorption and analyzed by capillary gas chromatography.
This practice applies to three basic types of samplers that are compatible with thermal desorption: (1) pumped sorbent tubes containing one or more sorbents; (2) axial diffusive samplers (typically of the same physical dimensions as standard pumped sorbent tubes and containing only one sorbent); and (3) radial diffusive samplers.
This practice recommends a number of sorbents that can be packed in sorbent tubes, for use in the sampling of a wide range of different volatile organic compounds boiling in the range 0 to 400°C (v.p. 15 to 0.01 kPa at 25°C).
For pumped sampling, sorbent selection is based on breakthrough capacity. Single-bed tubes containing for example sorbent Type A are appropriate for normal alkanes from n-C6(hexane) to n-C10(decane) and substances with similar volatility (v.p. 15 to 0.3 kPa at 25°C). More volatile materials should be sampled on stronger sorbents, such as sorbent Type B. Other sorbent types than those specified may be used, if their breakthrough capacities are adequate and their thermal desorption blanks are sufficiently small. Examples are given in Appendix X2. A broader range of VOCs may be sampled using multi-bed tubes.
Guidance given for the selection of sorbents for pumped monitoring tubes can be applied equally well to axial diffusive sampling tubes. The restriction to a single sampling surface (hence single sorbent), limits the target analyte range that can be monitored by a single tube. However, the unobtrusive nature and low cost of diffusive samplers usually means that two or more samplers containing different sorbents can be used in parallel without impacting study objectives.
The high sampling rate and associated risk of back diffusion associated with radial diffusive samplers typically restricts the use of these samplers to compounds of equal or lower volatility than benzene. It also means that stronger sorbents are generally required for these samplers when compared with either axial diffusive or pumped sorbent tubes.
This practice can be used for the measurement of airborne vapors of these volatile organic compounds over a wide concentration range.
With pumped sampling, this practice can be used for the measurement of airborne vapors of VOCs in a concentration range of approximately 0.1 µg/m³ to 1 g/m³, for individual organic compounds in 1-10 L air samples. The method is also suitable for the measurement of the airborne concentrations of individual components of volatile organic mixtures, provided that the total loading of the mixture does not exceed the capacity of the tube. Quantitative measurements are possible when using validated procedures with appropriate quality assurance measures.
With axial diffusive sampling, this practice is valid for the measurement of airborne vapors of volatile organic compounds in a concentration range of approximately 2 mg/m³ to 10 mg/m³ for individual organic compounds for an exposure time of 8 h or 0.3 mg/m³ to 300 mg/m³ for individual organic compounds for an exposure time of four weeks. The method is also suitable for the measurement of the airborne concentrations of individual components of volatile organic mixtures provided that the total loading of the mixture does not exceed the capacity of the tube.
With radial diffusive sampling, this practice is valid for the measurement of airborne vapors of volatile organic compounds in a concentration range of approximately 0.3 mg/m³ to 300 mg/m³ for individual organic compounds for exposure times of one to six hours. The method is also suitable for the measurement of the airborne concentrations of individual components of volatile organic mixtures provided that the total loading of the mixture does not exceed the capacity of the tube.
The upper limit of the useful range is set by the sorptive capacity of the sorbent used, and by the linear dynamic range of the gas chromatograph, column and detector, or by the sample splitting capability of the analytical instrumentation used. The sorptive capacity is measured as a breakthrough volume of air, which determines the maximum air volume that must not be exceeded when sampling with a pump.
The lower limit of the useful range depends on the noise level of the detector and on blank levels of analyte or interfering artifacts, or both, on the sorbent tubes.
Artifacts are typically <1ng for typical sampling tubes (7.2) containing well-conditioned sorbent Type C and carbonaceous sorbents such as graphitized carbon, carbon molecular sieves and pure charcoals; at 1 to 5 ng levels for sorbent Type D and at 5 to 50 ng levels for other porous polymers such as sorbent Type A and sorbent Type E. Method sensitivity is typically limited to 0.5 µg/m³ for 10 L air samples with this latter group of sorbent types because of their inherent high background.
This procedure can be used for personal and fixed location sampling. It cannot be used to measure instantaneous or short-term fluctuations in concentration. Alternatives for on-site measurement include, but are not limited to gas chromatography and infrared spectrometry.
The sampling method gives a time-weighted average result.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.