Learning Materials·October 15, 2024
This on-demand EANET training, delivered by Junya Hoshi (Tokyo Metropolitan Research Institute for Environmental Protection), teaches the diffusion (passive) sampler method for ambient VOCs—how it compares with active methods, how to compute concentrations, how to determine a sampling rate (SR), and how to handle field collection, extraction, and GC/MS analysis.
Active vs passive: when to use what.
Slides contrast active adsorption-tube sampling with passive diffusion-tube sampling. Active methods are less sensitive to temperature/humidity/wind, allow short sampling times and high preconcentration, but require pumps, power, skill, and higher cost. Passive methods are simple, no power, great for simultaneous multi-site mapping, but short sampling times are difficult in ambient air and you must know SR to convert adsorbed mass to concentration.
Core equation: turning adsorbed mass into concentration.
Ambient concentration is computed as
𝐶
=
𝑀
𝑑
−
𝑀
𝑏
𝑡
×
𝑆
𝑅
C=
t×SR
M
d
−M
b
where Md is the mass adsorbed on the exposed tube, Mb the mass in a blank (unused) tube, t the exposure time, and SR the sampling rate (m³/h). The training notes three ways to obtain SR: (1) parallel active–passive measurements in real air; (2) calculation (needs accurate diffusion length; difficult in practice); and (3) exposure tests (accurate but equipment-intensive).
Best-practice SR derivation: parallel canister × passive.
The deck emphasizes deriving SR by co-locating a passive sampler with an active canister method site and regressing
(
𝑀
𝑑
−
𝑀
𝑏
)
(M
d
−M
b
) against
(
𝐶
can
×
𝑡
)
(C
can
×t). The slope is SR (m³/h); high R² values indicate strong linearity. This approach explicitly captures site-specific meteorology and micro-scale effects, which can drive differences from vendor-stated SRs.
Case study: BTX in Tokyo (Kōtō-ku rooftop, 24-hour samples, 20–27 July 2022).
Co-measurement used canister + low-temperature preconcentration GC/MS and diffusion tubes + CS₂ extraction GC/MS. Regressions for benzene, toluene, ethylbenzene, m,p-xylene, o-xylene (two brands: Supelco and SIBATA) produced SRs with R² ≈ 0.87–0.999. For benzene and toluene, the Supelco SRs matched the manufacturer values; for ethylbenzene and xylenes, measured SRs differed from the catalog—hence the recommendation to determine SRs at each monitoring site. The study found no significant difference in accuracy for BTX between the two brands.
Field collection workflow (practical details).
Remove the passive sampler from its sealed bag (this starts sampling). Handle with gloves.
Mount in a holder and place in a shelter; secure strings to avoid wind-induced flipping.
After exposure, return the sampler to its original bag, label location/date/time, and refrigerate or freeze. Exposure time is bag-out → bag-in.
These simple steps let teams deploy many sites at low cost; the shelter standardizes exposure and protects from weather.
From sampler to vial: solvent extraction.
Analysis begins by opening the tube, removing PTFE plug/urethane foam, and transferring adsorbent to a vial. Add CS₂, stir to remove bubbles, stand ~2 hours, then transfer the supernatant (use an insert vial; manage static to prevent losses). An example method lists HP-5MS (30 m × 0.25 mm, 0.25 μm), oven 60 °C → 200 °C (8 °C/min) → 280 °C (15 °C/min, hold 5 min), He carrier (1.2 mL/min), 200 °C injector/ion source and 220 °C transfer line, with calibration ranges 20–1000 ng/mL for BTX (chlorobenzene-d5 as IS).
Chromatograms, SIM ions, and a benzene quirk.
Examples show SIM m/z 78 (benzene) and m/z 91 (toluene/ethylbenzene/xylenes), with clean peaks and linear calibration curves (R² ≈ 0.999). One caveat: benzene exhibits an intercept in CS₂-based calibration (a solvent blank effect), so analysts must subtract blanks and keep solvent handling consistent. Compounds whose retention overlaps CS₂ cannot be quantified in this configuration.
Takeaways & method positioning.
Why passive? It’s ideal for wide-area screening and multi-site mapping where power is scarce or budgets are tight.
What to watch? SR is the critical parameter—derive it locally with parallel active–passive tests when possible; meteorology and site setup matter.
Lab discipline: Manage solvent blanks, follow SIM strategies for sensitivity, and validate ranges with multi-level standards.
Brand choice: For BTX, Supelco and SIBATA performed similarly; still, verify SR per site and compound.
Limits: Passive methods handle long integrations well but struggle with short episodes; for temporal dynamics, pair with canisters or on-line instruments.
Bottom line.
Diffusion tubes offer a low-cost, no-power path to city-wide VOC mapping—provided you (i) use the correct concentration equation, (ii) determine SRs under local conditions (ideally via canister–passive parallels), and (iii) run a disciplined CS₂ extraction + GC/MS workflow with blank correction and appropriate SIM ions. Done right, the method delivers robust BTX data and a scalable backbone for health risk screening and ozone/PM₂.₅ precursor management.
Keywords
Passive samplers; diffusion tubes; sampling rate (SR); site-specific SR via parallel canister regressions; BTX (benzene, toluene, ethylbenzene, xylenes); Kōtō-ku rooftop (Tokyo) deployment; 24-h sampling; CS₂ extraction; HP-5MS column; SIM ions m/z 78 and 91; calibration 20–1000 ng/mL; benzene intercept (solvent blank); brand comparison (Supelco, SIBATA); wide-area mapping; no-power monitoring.