Sorbent-tube sampling is widely used to collect volatile and semi-volatile organic compounds from indoor air for laboratory analysis. Air passes through a tube containing a material that retains selected compounds, after which the trapped chemicals are released by heat and introduced into a gas chromatograph.
A sorbent tube contains one or more beds of material chosen to retain compounds from a moving air stream. During active sampling, a calibrated pump draws air through the tube. During diffusive sampling, compounds move into the tube because of the concentration gradient between ambient air and the sorbent surface. In both cases, the collected mass is related to the concentration in air and the effective volume sampled.
Retention is not uniform across all organic compounds. Stronger sorbents tend to retain more volatile compounds, while weaker sorbents may be better suited to compounds that would otherwise be difficult to release during thermal desorption. Water vapour can also affect performance, especially with sorbents that retain moisture. The tube therefore acts as a selective collection system rather than as a universal trap for every airborne chemical.
The sampling direction, tube orientation and sealing arrangement are method-specific. Tubes are normally capped promptly after sampling to limit contamination and loss. The laboratory needs enough information to connect the tube with its location, duration, flow or diffusive exposure period, and any unusual conditions that could have influenced collection.
Tenax is a commonly used single-bed sorbent for a broad range of indoor-air VOCs. It has useful thermal stability and relatively low water retention, which makes it suitable for many building investigations. Its limitations are equally important: very volatile compounds may not be retained adequately, while some heavier or reactive compounds may be recovered poorly or behave differently during analysis.
A multi-bed tube combines sorbents of different strengths. Air first encounters a weaker sorbent and then progressively stronger material, allowing a wider volatility range to be collected in one tube. During thermal desorption, the direction is generally reversed so that compounds move from the stronger beds towards the weaker bed and then into the analytical system. This arrangement reduces the chance that less volatile compounds become irreversibly trapped on the strongest sorbent.
Sorbent selection therefore defines the compound window of the method. A tube designed for very volatile organic compounds may not be optimal for heavier semi-volatile compounds, and a tube selected for a broad VOC suite may not include highly polar or reactive substances. Formaldehyde is a notable example because it is generally collected by derivatisation on a dedicated coated cartridge rather than treated as part of the ordinary thermal-desorption VOC suite.
Pumped sampling provides an actively controlled flow through the tube. The sampled volume is calculated from flow and time, with checks before and after sampling used to confirm that the flow remained acceptable. The selected flow must be compatible with the tube, pump and method. Excessive flow can increase pressure drop or disturb retention, while an unnecessarily low flow may fail to collect enough mass for reliable analysis.
Diffusive sampling does not use a pump. Instead, the concentration is calculated from the collected mass, exposure time and the validated uptake rate for the compound and sampler configuration. Uptake rates can vary by compound and may be influenced by air movement, temperature, exposure duration and the geometry of the diffusion path. A nominal rate should not be assumed to apply to every chemical in a mixed indoor-air sample.
Pumped and diffusive results can both be valid when the relevant method has been followed. Pumped sampling is often useful where an eight-hour result or a shorter event-specific sample is required. Diffusive sampling can reduce disturbance and simplify multi-location deployments, but longer exposure may be needed to collect sufficient mass, particularly where concentrations are low.
Breakthrough occurs when a compound is no longer retained effectively by the primary sorbent bed and begins to pass farther through the tube. It may result from excessive sampled volume, high concentration, unsuitable sorbent selection, moisture, temperature or competition between compounds. Once significant breakthrough occurs, the calculated concentration may underestimate the amount present in air.
A backup section provides evidence about whether material has moved beyond the front collection section. In a two-section tube, the front and backup portions may be analysed separately. Detectable mass on the backup section does not always mean that the sample is unusable, but the amount and method acceptance criteria must be considered. A substantial proportion on the backup section can indicate that the selected volume or sorbent was inappropriate.
Breakthrough risk is assessed before sampling by considering expected compounds, likely concentrations, humidity, temperature and total air volume. Field notes are valuable because a tube exposed beside a strong source, in a hot plant area or during an unexpected spill may have experienced conditions very different from those assumed in the original plan.
In thermal desorption, the tube is heated under a controlled flow of carrier gas so that retained compounds leave the sorbent. They are usually focused in a secondary trap before being released rapidly into the gas chromatograph. Focusing narrows the introduced band and improves chromatographic separation, especially where a relatively large volume of air has been sampled.
The gas chromatograph separates compounds according to their interaction with the analytical column and their volatility. Mass spectrometry then provides a spectrum that can support identification by comparison with reference spectra and retention behaviour. A mass-selective detector can distinguish many compounds in a complex indoor-air mixture, although co-elution, low abundance and similar spectra can complicate identification.
ISO 16000-6:2021 covers determination of organic compounds in indoor air and test-chamber air, including VVOC, VOC and SVOC, by active sampling on sorbent tubes, thermal desorption and gas chromatography using mass spectrometry or mass spectrometry with flame-ionisation detection. USEPA Compendium Method TO-17 also uses active sampling on sorbent tubes followed by thermal desorption and gas chromatographic analysis. TO-17 superseded TO-1 and TO-2.
Identification asks what compound produced a chromatographic peak. Quantification asks how much of that compound was collected and what air concentration that mass represents. Confident identification generally uses retention information, spectral quality and reference materials. A library match alone may suggest a candidate, but it does not always establish identity where peaks overlap or the spectrum is weak.
Quantification is normally based on calibration with external standards containing known amounts of target compounds. The laboratory establishes the detector response across a suitable range and applies the resulting calibration to sample peaks. Internal standards may be added to check analytical performance and compensate for variation in transfer or detection. Results below the reliable quantification capability should not be presented with the same certainty as fully quantified values.
A TVOC sum may be reported using a toluene-equivalent convention. Under this approach, compounds within a defined chromatographic range are quantified using the response factor for toluene unless individual calibration is available. The value is therefore a method-defined aggregate rather than the literal mass of every organic compound in the air. Differences in sorbent, integration range, excluded compounds and calibration convention can produce different TVOC results from the same environment.
Field blanks travel with the sampling tubes and are handled in the same way as samples without being exposed to sampled air. They help reveal contamination from transport, storage, handling or the tube itself. Laboratory blanks and instrument blanks serve related purposes at later stages. Blank findings should be reviewed compound by compound because a small background contribution may matter greatly where indoor concentrations are low.
Samples should be sealed with appropriate caps, protected from heat and contamination, and stored under conditions specified by the method or laboratory procedure. Strong-smelling products, fuel containers and solvent-based markers should be kept away from tubes during transport. Chain-of-custody information should record sample identity, times, flow data, location and any deviation from the plan.
Holding time is not a single universal number for all sorbent tubes and compounds. Stability depends on the analyte, sorbent, loading, storage temperature and sealing. The laboratory should therefore define an acceptable period supported by the applied method and validation data. Prompt dispatch and analysis reduce uncertainty associated with loss, reaction or contamination.
ISO 16000-6:2021 covers determination of organic compounds in indoor air and test-chamber air, including VVOC, VOC and SVOC, by active sampling on sorbent tubes, thermal desorption and gas chromatography using mass spectrometry or mass spectrometry with flame-ionisation detection. USEPA Compendium Method TO-17 also uses active sampling on sorbent tubes followed by thermal desorption and gas chromatographic analysis; it superseded TO-1 and TO-2. A toluene-equivalent TVOC value is method-defined, so the analytical convention should accompany the result.
ISO 16000-6:2021 and USEPA Compendium Method TO-17 — sorbent selection defines the compound window, and a toluene-equivalent TVOC value is method-defined
No. Sorbent strength, moisture behaviour and thermal-desorption performance define the compounds that can be collected and recovered. A wider target range may require a multi-bed tube or a separate sampling method.
It helps indicate whether a compound has passed through the primary collection section. Significant mass on the backup section can suggest breakthrough and possible underestimation.
Not inherently. A validated diffusive sampler can provide reliable results when the uptake rate, exposure duration and environmental conditions are suitable. The method must be appropriate for the target compounds and required concentration range.
They may use different sorbents, chromatographic ranges, integration rules, calibration compounds or treatment of unidentified peaks. A toluene-equivalent TVOC value is method-defined, so the analytical convention should accompany the result.