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SASS® 2300 Air Sampler

INTRODUCTION

The SASS® 2300 is our latest and most exciting portable aerosol particle collection system. This advanced product incorporates all features of the successful SASS 2000 and SASS 2100 air sampler lines, plus much more. The SASS 2300 continues to employ the highly successful wetted wall aerosol collection method that has received U.S. Department of Homeland Security Certification under the U.S. Safety Act of 2002. Extremely long collection periods continue to be a product highlight — sample fluid is maintained at a constant amount independent of collection time, ambient air temperature, or relative humidity, providing unsurpassed monitoring capabilities in environments ranging from farmyards to hospitals to battlefields.

This is the only wet-type air sampler technology that has been shown able, in concert with real-time PCR, to efficiently collect and identify airborne viruses. It has, for example, been successfully used to detect the airborne viral pathogens that cause exotic Newcastle disease and hoof-and-mouth disease, as well as strains of avian flu virus (1).

Notable features of the SASS 2300 include:

• An onboard sample vial filling station;
• A water reservoir sufficient for 24 hours of operation;
• Impact- and water-resistant case;
• Peristaltic pump-based fluidics;
• Firmware upgradeable/customizable via the RS-232 port
• Convenient threaded inlet and outlet ports
• 30,000 hour blower
  

OVERVIEW

The SASS 2300 collects both particulates and water-soluble chemical vapors from the air. These materials are extracted from sampled air and trapped in a small volume of liquid that can be removed, in part or in total, at any time for analysis. Distilled water is typically the sample liquid of choice—no additives or surfactants are required for high efficiency. Trace aerosol concentrations can be amplified by extending the sampling time from minutes to hours or more.

The SASS 2300 draws air through a convenient threaded adapter on the unit’s exterior. This adapter allows the mounting of useful accessories such as flexible intake tubes, filters and nozzles. A built-in peristaltic pump can be used to transfer liquid samples to an external analyzer for immediate analysis, or to a sample vial filling station integral to the unit. The filling station is convenient for dispensing all or a part of the liquid sample into a dropper bottle of the type used with lateral flow bioassay tickets.

The air sampler is microcontroller-based and can function as a stand-alone unit or connected to other sampling, detection or communication systems via RS-232 or wireless link. Purpose-designed software allows for streamlined integration with Research International’s RAPTOR and BioHawk biodetectors. The extensive use of microcontroller-based circuits allows overall system operating characteristics to be easily tailored to customer requirements. Reprogramming of sampler operation may be performed at any time over the RS-232 link without having to disassemble the unit.

Electric power consumption is minimized by operating the unit’s blower at peak electric-to-pneumatic efficiency conditions and by using natural airflow through the cyclone structure to drive water re-circulation. Weight and power consumption figures are far below other aerosol collection systems of comparable sophistication and collection efficiency, and may be further reduced for specialized applications such as Unmanned Air Vehicle (UAV) applications.

OPERATING PRINCIPLES

A schematic representation of the SASS 2300 is shown in Figure 1. The cyclone has four main sections: a cyclonic cup, stripping column, cistern, and water feedback loop. A high-efficiency, brushless centrifugal blower at the cyclone exit pulls air into the unit. When the SASS is turned on, the blower is activated and a water charge injected into the cyclonic cup from an on-board fresh water reservoir. Incoming air enters at the cup perimeter, creating strong vortex action and a rapidly swirling film of water on exposed surfaces. The water film passes across the air inlet region, forming a water curtain through which air must pass.

Figure 1: SASS 2300 fluid management system.

Concurrently, a centrally located nozzle projecting from the cup base injects additional water. This location is subject to high air shear and fluid discharged from the nozzle into the cup will be in the form of a fine spray. The cup plays a major role in collection of sub-micron particles and molecular species due to the intimate two-phase contact provided, whereas larger respirable particles are captured in both the cup and a stripping column to be described.

The air stream then flows from the cup into a stripping column connected to the cup’s upper surface. As air enters the smaller diameter stripping column it increases in rotational velocity, enhancing particulate collection through centrifugal action. The inner surface of the stripping column is also wet by fluid outflow from the cup. The airflow rate and column inside diameter have been designed so that adequate shear force is produced to create a cocurrent upward flow of water on the column wall. The stripping column is operated beyond the so-called ‘flooding limit,’ meaning that liquid introduced at the base of the column cannot flow opposite to the upwelling air, and, in fact, must flow up the stripping column.

Water flows from the stripping column into a larger diameter cistern section located above the tube. Due to parasitic shear forces created by the rotating airflow, water transitioning from the tube to the cistern is flung outward to a water trap zone where a rapidly rotating water ring is formed. Water in the ring flows back to the cyclonic cup by way of a liquid feedback tube. This water is re-injected into the cup via the spray nozzle, where it is once again available to collect additional particulates. Fluid re-circulation rates have been measured to be in the range of 30 to 100 cc/minute. This means a typical 5 cc water inventory is re-circulated through the unit from 6 to 20 times per minute.

Aerosol collection characteristics are similar to those of the SASS 2000, but with significant improvements in particle retention over long collection periods. Figure 2 shows the effect of water inventory on the concentration and total number of 1 micron particles in the water phase. From this Figure it can be seen that about 5 cc of water provides optimum performance. The effect of fluid charge is modest as long as very lean, low water charges are avoided.

Figure 2: The effect of water inventory on the total number of particles collected and particle concentration in the water sample. Polystyrene microbeads, 1 micron diameter.

The similar SASS 2000 system has been tested at a number of facilities including Dugway Proving Grounds, Aberdeen Proving Grounds, Lawrence Livermore National Laboratory, Battelle Columbus, and the U.S. Naval Research Laboratory (NRL). A test of nine 'personal' sampler designs was conducted at Dugway Proving grounds in April 1997 against airborne Bacillus globigii in a controlled atmosphere chamber. In these tests, the number median aerodynamic spore diameter was 0.9 microns while the mass median diameter was 2.9 microns. At that time, Research International's prototype system tied with two cyclone-based devices for highest concentration factor versus time.

A second set of tests was run at BIO911 in December 1997 in which 11 different samplers were examined. In the period between the two test series, Research International made several improvements to the design that reduced water inventory and made the interior less likely to trap particles. In these tests, the SASS 2000 came in second, performing slightly lower in overall performance than the front-runner, but both it and the first-place system were substantially better than other systems tested. Its favorable performance is particularly notable since the number one system weighed four times as much and consumed 42 times more electric power.

Detailed examinations of SASS2000 capture efficiency have been performed by Lawrence Livermore Laboratory and Battelle Columbus over the past five years. A compilation of these test results, plotted as capture efficiency versus particle size, is provided in Figure 3. Finally, in portable air sampler tests performed by Battelle Columbus in 2004, no other portable unit was found to be more efficient that the SASS2000 series.

Figure 3: Collection efficiency for various particle types at low aerosol concentration.

Fluid Control Subsystem

The particle extraction process involves intimate mixing of incoming air with re-circulating sample water. This water would be lost in a short time if no attempt were made to compensate for evaporation. To prevent dry out, liquid volume is monitored with a proprietary sensor attached to the water feedback tube. When the sample water inventory falls below a preset level, a microprocessor-controlled peristaltic pump meters a small amount of clean water into the re-circulation loop from an onboard water reservoir to bring the level back to set point. Water inventories can be maintained within a range of about 4 cc to 5 cc with an accuracy of a few tenths of a cc for periods of hours to days (U.S. Patent 6,532,835). Since the re-circulation loop does not use a mechanical pump, power consumption is minimized, delicate organisms are not damaged, and cleaning is simplified.

Samples may be removed at any time using an onboard peristaltic pump. When the unit is operated manually, air flow is stopped during the sample transfer process to allow fluid films to collect as a pool in the bottom of the unit. If the unit is being operated remotely via the serial digital link, air sampling may continue during the sample transfer process. The user can in either case elect to discharge the sample to an onboard sample vial or to a discharge spigot at the rear of the instrument.

Electronics

The microcontroller-based SASS 2300 has been designed to communicate with and be controlled by various triggers, bioanalyzers and data collection networks via its RS-232 serial data link. System firmware is located in flash memory and can be modified, customized and upgraded electronically at any time. There is no need to disassemble the unit. A program that operates under Windows® is supplied with the instrument that allows the SASS 2300 to be operated from a PC. This program also allows users to change a large number of default settings resident in flash memory, such as fan speed and liquid inventory- essentially customizing operation to their purposes.

Accessories

Air inlet and outlet ports feature an industry-standard screw thread that allows many different types of input and output hoses, nozzles and filters to be mounted to the unit. System electronics and firmware are designed for ease of customization. Please contact us if you are interested in GPS, weather, long-range RF, visual, thermal imaging, or any other specific add-on feature.

Research International reserves the right to change specifications without prior notice.

(1) Sharon K. Hietala, et al., J. Vet. Diagn. Invest., 17, 198 (2005).

For more information, see:

• SASS 2300 specifications (PDF, 543 KB)

• SASS Series Collection Efficiency chart (PDF, 31 KB)

• Product bibliography

• A new addition to our product family is the Biolink™ RF serial data link. Biolink™ provides plug-and-play wireless communication between Research International's air samplers/detectors and a PC or laptop. View and print data sheet (PDF, 253 KB).

Adobe® Acrobat® Reader is necessary for viewing and printing .pdf files and is available to download at no charge from Adobe Systems.