The Waterloo Membrane Sampler^™ (WMS^™), a passive sampler for monitoring volatile organic compound (VOC) vapor concentrations, was developed at the University of Waterloo by Drs. Tadeusz Górecki and Suresh Seethapathy.  The researchers had identified a need for a passive sampler with resistance to water vapor, which can saturate the sorbent present in passive samplers, for sampling in environmental vapor matrices where water vapor concentrations can be high, such as in soil gas, and they believed that the unique design of the WMS^™ gave it an exclusive position in the marketplace for passive samplers.

The WMS^™ design incorporates a polydimethylsiloxane (PDMS) membrane across the face of a vial filled with a sorbent medium. Volatile organic compound (VOC) vapors partition into and permeate through the membrane. The sorbent then traps the vapors, and the mass of each compound is determined by gas chromatography–mass spectrometry (GC-MS). The uptake rate has been experimentally measured for many common VOCs and can easily be calculated for other compounds because it is directly proportional to the gas chromatographic retention index, a property that is readily available in the scientific literature. Thus, the WMS^™ sampler can be used to measure time-weighted average concentrations for virtually any VOC.

• Lower cost
• Simpler sampling protocols
• Lower reporting limits without a premium price
• Longer time-integrated samples
• Very small size (discrete to deploy and easy to ship)

• Predictable uptake rates for less common compounds
• Ability to measure Total Petroleum Hydrocarbon/Gasoline Range Organic compounds
• Minimal effect of moisture (very advantageous for subsurface monitoring)
• Insensitive to wind velocity (very advantageous for outdoor and vent-pipe monitoring)
• Ability to modify configuration to avoid the “starvation effect” when collecting subsurface samples
• Small diameter (easy to put in vent pipes or sub-slab probes)
• Competitive pricing

The Waterloo Membrane Sampler^™ (WMS^™) contains a sorbent in an inert container with an opening of known dimensions covered with a membrane. VOC vapors pass through the membrane at a constant rate, called the uptake rate, over time.

Passive samplers can be classified into two general types based on the how the VOC uptake is controlled: (1) those that rely on diffusion through a stagnant air region (passive diffusion samplers) and; (2) those that rely on permeation through a nonporous membrane (passive permeation samplers). In the latter, VOCs permeate through the uptake-rate limiting membrane before they are collected by the sorbent.

The Waterloo Membrane Sampler^™ is a permeation-type passive sampler. When it is exposed to air (e.g., indoor or outdoor air, soil gas, vent pipes), VOCs in the air permeate through the membrane covering the top of the sampler vial, driven by a concentration gradient. The sorbent inside the sampler then traps the vapors, and the mass of each compound on the sorbent is determined in an analytical laboratory by GC-MS.

Learn more in the technical memorandum our staff produced for the United States Navy »

CALCULATION OF CONCENTRATION
Concentrations in the sampled air are calculated according to Equation 1, where:

C = concentration in sampled air (μg/m3)
M = mass on sampler (picograms)
t = sampling time (min)
UR = known analyte-specific uptake rate (mL/min)

EQUATION 1

REPORTING LIMITS AND SAMPLING TIME
The sampling time required to meet a desired reporting limit can be calculated by rearranging Equation 1, and using the minimum mass on the sampler that can be detected by the analytical method (MM) to replace M, and the reporting limit required (RL) to replace C. This results in Equation 2, where:

t = sampling time required to achieve the reporting limit (min)
MM = minimum mass on sampler that analytical method can measure (picograms)
RL = reporting limit required (μg/m3)
UR = known analyte-specific uptake rate (mL/min)

EQUATION 2