Learning Materials·October 15, 2024
This EANET training presentation (Session 2: Methodology) by Yujiro Ichikawa covers the foundations of collecting VOCs with sorbent tubes and the linked thermal desorption–GC/MS workflow. It’s a practical, step-by-step primer intended for technicians and air-quality practitioners who need to set up reliable ambient VOC sampling programs.
Adsorption vs. absorption—why sorbent tubes work.
The deck starts with the distinction between absorption (bulk dissolution into another phase) and adsorption (retention on a surface). Sorbent tubes capture VOCs by adsorption, concentrating trace gases on an engineered packing for later thermal desorption.
Hardware choices: tube body and sorbent media.
Tube shells should be inert to minimize wall losses; the slides note inert stainless as easy to handle but ~1.5× costlier. Inside, the sorbent bed is selected to match target compound volatility:
Porous polymers for roughly C5–C30 hydrocarbons;
Graphitized carbon blacks for C3–C20;
Carbonized molecular sieves for C2–C8;
Zeolite molecular sieves for C2–C4.
Vendors can produce custom multi-bed tubes, and the slides emphasize that multi-bed tubes are directional—the arrowed end must face the sample flow.
Active sampling fundamentals.
This method actively pulls air through the sorbent using a pump. The deck recommends a general flow ≤250 mL/min and defines sampled volume as Flow × Time. Example hardware shown: a GL Science SP209-100Dual pump paired with a Camsco Air Toxics tube.
Breakthrough and how to avoid it.
Breakthrough—when analytes push through the sorbent without being retained—can occur at too-high flow or excessive sample volumes. The deck flags this explicitly and includes a separate “how to check breakthrough” step, underscoring the need for method validation in the field.
Humidity: the ever-present troublemaker.
Water vapor can mask active sites or displace compounds during sampling; during analysis it can distort chromatography, impair MS vacuum, and reduce MS sensitivity. Mitigations include choosing hydrophobic sorbents and installing a dehumidifying tube upstream under humid conditions.
Dry purge—useful but don’t overdo it.
A dry purge with inert gas helps remove adsorbed water, but excess purging can push VOCs off the sorbent. As rules of thumb, the deck gives approximate dry-purge volumes of ~0.25 L for porous polymer/graphitized carbon black tubes, and 0.5–3 L for carbonized molecular sieve tubes, scaled to humidity level.
Field setup and pre-sampling preparation.
A concise checklist covers:
Sorbent tube selection fit to application.
Sampling pump capable of 4–250 mL/min.
Optional dehumidifying tube (glass, ~15 g Mg(ClO₄)₂ with quartz wool), for very humid sites.
Optional ozone scrubber (MgO₂ or KI) to protect ozone-reactive VOCs.
Connection tubing: inert stainless steel is robust but unwieldy; pre-treated PTFE is acceptable after blank checks.
Tube conditioning before use: elevated temperature + ultrapure He or N₂ for several hours (per manufacturer).
At site: clean gloves, inlet shelter against weather, field blanks (~1/10), set flow & time, and complete a Field Data Sheet.
Storage after sampling: ≤4 °C or other cool, clean environment; store with activated charcoal.
Calibration and spiking standards.
For calibration curves, standard gas can be spiked by gas-tight syringe (or standard solutions by microsyringe). After spiking, pass inert gas for several minutes to promote adsorption—this also helps evaporate solvent when solution standards are used. Build calibrations across multiple masses/concentrations.
From tube to chromatogram: Thermal Desorption–GC/MS.
Sorbent-tube samples are typically analyzed by TD–GC/MS (or GC/FID). The deck lays out a five-step TD sequence anchored in US EPA good practice: (a) tube leak check, (b) flow-path leak check, (c) dry purge (tube & path), (d) primary desorption, (e) secondary desorption into the cryotrap/column. Trap types vary and can be customized by manufacturers.
Why these choices matter (and how to trade off).
Compound range vs. artifact risk: colder, stronger traps and aggressive multi-bed tubes extend the volatility window but raise water and breakthrough management challenges. The deck’s humidity section and dry-purge guidance give practical limits.
Sampling duration vs. flow: longer time at lower flow reduces breakthrough and water pickup for many targets; confirm with breakthrough tests and field blanks.
Protection for reactive VOCs: where ozone is high, add KI/MgO₂ scrubbers ahead of the tube (but validate no analyte loss).
Clean handling & storage: the checklists for gloves, shelters, cool storage, and activated-charcoal caddies are about contamination control and stability.
How this method fits the bigger VOCs toolbox.
While this deck focuses on sorbent tubes, it complements the programmatic overview covered elsewhere in EANET training—where agencies select among active (sorbent/canister), passive, and real-time options based on objectives and budget. Sorbent-tube active sampling remains a workhorse for compound-resolved ambient programs, especially when paired with TD–GC/MS and robust QA/QC.
Bottom line.
This is a hands-on “how-to”: choose the right multi-bed sorbent and inert tubing, set flow ≤250 mL/min, manage breakthrough and humidity, condition and handle tubes cleanly, verify with blanks and leak checks, and execute the TD–GC/MS sequence meticulously. Follow these basics, and you’ll generate defensible VOC data that feed exposure assessments, ozone/PM₂.₅ precursor control, and regulatory decision-making.
Keywords
Sorbent tube; multi-bed tube (directional); porous polymer (C5–C30); graphitized carbon black (C3–C20); carbonized molecular sieve (C2–C8); zeolite molecular sieve (C2–C4); active sampling (≤250 mL/min); breakthrough; humidity interference; hydrophobic sorbents; dehumidifying tube (Mg(ClO₄)₂); ozone scrubber (KI/MgO₂); tube conditioning (He/N₂); field blanks; cold storage (≤4 °C) with activated charcoal; calibration by syringe spiking; thermal desorption; leak checks; primary/secondary desorption; TD–GC/MS; GC/FID.