How It Works
The Spot Sampler™ particle collectors use well-known laminar-flow water condensation technology and gentle impaction for wet or dry collection. The inherently pre-concentrated particle samples are ready for laboratory chemical or biological analysis.
Schematic of the growth tube using a three-stage water condensational growth approach.
The figure above depicts dry particle collection in a sample well.
Temperature, saturation ratio, and droplet size under standard operating conditions (Hering et al.2014).
Aerosol particles enter the collector where they are exposed to a supersaturated water vapor environment. A moderated 3-stage growth tube consists of three temperature regions to control the vapor profile.
The initial cold “conditioner” establishes a controlled vapor saturated aerosol stream largely independent of the incoming sample flow conditions.
Supersaturation occurs in the second region as a result of the difference in the diffusive rates of water vapor and heat transport. The warm walls of the “initiator” provide a region high of partial pressure of water vapor. Because the mass diffusivity of water vapor is larger than the thermal diffusivity of air, the water vapor diffuses into the flow faster than it warms. To reach a desired state of equilibrium, the supersaturated air condenses water vapor on the tiny “seed” particles. The relative humidity in the initiator reaches values of 130-140% which is enough to overcome the surface energy associated with the curvature of the particle surface, and activate condensational growth of particles as small as 5-10nm diameter. The particles are enlarged into water droplets ~ 3µm diameter effectively forming a fog.
The final cool “moderator” region allows continued droplet growth while reducing the flow temperature and water vapor content. This lowers the vapor content below saturation levels at ambient conditions and avoids unwanted condensation downstream of the collection region. Droplet growth occurs at temperatures close to ambient (25-30ºC in the flow stream) providing robust collection for volatile constituents, reducing changes in the particle chemical composition, and maintaining microorganism viability.
The water-encapsulated particles gently impact onto the warmed surface of a 75µL collection well, whereby the water evaporates and leaves a dry, 1-mm diameter “spot” of particles. Heat applied to the collection well is user adjustable to accommodate the collection objectives and heat transfer rates of the various sample plate materials. A single wetted-wick is easily removed and replaced from the top of the growth tube.
A stepper motor at the base of the growth tube rotates a multi-well plate for time-resolved sample collection. Users can program the instrument to advance to a new collection well at defined intervals providing a set of 30 uninterrupted, time-resolved samples, plus blanks and a set-up well. The collection plate is easily transported back to the laboratory for analysis while the instrument remains in the field ready to collect new samples.
For wet chemical analysis, the multi-well sample plate is placed directly into an autosampler, without requiring user extraction or preparation. The autosampler adds extraction solvent and handles mixing, extraction, and sample injection into the analytical system without requiring any user manipulation. A single analytical system (such as an ion chromatograph, IC, or high-performance liquid chromatograph, HPLC) can be used to analyze samples from multiple particle collectors. Sample plates can be cleaned and reused.
Collection into Liquids
For collection into liquids, the water-encapsulated particles exiting the growth tube are directed through three nozzles into a small vial containing 0.5ml of liquid. Droplets gently splash impact with the liquid surface. The particles are efficiently captured into the liquid suspension without break up, liquid spray, or re-aerosolization. The mild turbulence created in the liquid mixes the particles throughout the suspension. Inlet and exit ports on the vial allow users to configure the sampling for semi-continuous chemical analysis.
Several sequential Spot Sampler prototypes were tested in the field and compared to standard filter collection methods and near-real time instruments. Hourly ambient PM2.5 samples were collected for inorganic anion measurement and the results compared well with those obtained using a Particle-into-Liquid System (PILS) and URG-filters. Particle-bound polyaromatic hydrocarbons (PAHs) were also measured with 12-hr time resolution for an uninterrupted period of 4 months. Ambient particle concentrations measured with prototype samplers compared well with those obtained using standard Teflon® filters. See the Reference section for publications and presentations that document the collector development and field test results.
Multi-well sample plates with dry 1-mm deposits forming a particle “spot” in each well. Shown here is a prototype sample plate made of PEEK® (left) and aluminum (right) containing ambient PM2.5 samples collected over 12 hours and 6 hours respectively.
A multi-well collection plate placed in a prototype autosampler system for PAH analysis.