Samplers Collecting onto Solid or Semi-solid Surfaces

Casella Slit Sampler

This sampler collects air at either 30 or 750 L.m in'l, through a narrow slit onto a rotating agar plate, relying on the growth of the microorganisms for their detection. Samplers are supplied with either 1 or 4 slits arranged so the air impacts radially onto the agar surface. The distance between the slit and the agar is 2 mm. The agar plate is rotated by an electric motor which completes a single rotation in 30, 120, or 300 seconds, for a 4 slit sampler. No indication of the size distribution of the particles in the sample is given. The Casella slit sampler has not been fully characterised in terms of physical collection efficiencies. This sampler was used by Tinnes and Hoare (1992) to monitor release from a disc-stack centrifuge.

Anderson Microbial Sampler (AMS)

This is a cascade impactor, it collects airborne particles onto a series of nutrient agar plates, giving a size distribution. It has not been fully characterised. Detection is based on the cell's ability to grow following impaction onto the agar. The sampler consists of 6 sieve plates, each with 400 precision drilled holes, mounted

over a 96 mm diameter agar plate containing a defined depth of agar.

Air is drawn at 28.3 L.min"! through the sampler, multiple jets o f airborne particles are directed onto the collection plates, particles with high inertia cross the airstream lines and strike the agar surface. Smaller particles with less inertia remain in the airstream and pass onto the next stage. The smaller holes in the next stage cause the air velocity to increase, and therefore the inertia of the airborne particles. The largest of the remaining particles may have enough inertia to be collected here. Large particles are impacted onto the agar plates at the top of the sampler and the smaller particles at the bottom. The size o f the particles collected at each stage can be related to deposition in the human respiratory tract.

After incubation o f the agar plates, the number of colonies/plate and CFU.L"! of sampled air are calculated. Each colony corresponds to one hole in the sieve plate. A positive hole conversion (Anon, 1984) is used to calculate corrected CFU .L'l of sampled air. As the number of viable particles in the sampled air increases, so the

Chapter Three: Mass Balancing in Contained Environments

sieve plate decreases, and the probability of more than one particle impacting through the same hole and forming a single colony. Barrett et a l (1984) found this sampler to have a collection efficiency >90% for particles with an aerodynamic diameter <2.5 pm, although this efficiency decreased dramatically with increased air movement. Zimmerman et a l (1987) used the AMS when monitoring airborne

E. coli during simulated waste water spray irrigation dispersal studies. Kastelein and Logtenberg (TNO, 1989) used the AMS to monitor releases from a centrifuge as part of a risk assessment study.

Surface Air System Sampler (SAS)

This is a portable single stage sieve impactor which collects particles onto an agar filled ROD AC (Replicate Organism Direct Contact) plate (Lach, 1985). There are two models which differ in the number of holes in their sieve plates, and the size of the ROD AC plate. Air is sampled at a rate of 180 L.min"^, each sampling run is timed by the sampler itself. Lach (1985) compared the performance of the SAS with the Casella Slit Sampler to collect B. subtilis var. niger spores. For particles <4 pm, the effective sampling rate of the SAS decreased, with 50% efficiency for particles of 2 pm diameter. Therefore it is not advisable to use this sampler where single cell aerosols may be present. The plate can also become overloaded quickly. This sampler has not been characterised in a wind tunnel.

Biotest RCS Sampler

Originally used by Macher and First (1983), this is a portable hand-held sampler which collects particles onto an agar strip, relying on the growth of the microorganisms for detection. No indication of the size distribution o f the particles in the aerosol is given. The sampler consists of an open ended drum housing an impeller, an agar strip is inserted into the drum around the impeller blades. Air is drawn into the sampler at 40 L.min"!, the impeller rotates at 4096 rpm. Airborne particles are subjected to centrifugal acceleration and impact onto the agar strip at a high velocity.

The major advantage of this sampler is that it is small, portable, and easy to use. Stewart et a/. (1988) observed higher recoveries o f aerosolised B. subtilis var niger

spores when sampling with the RCS compared to the AMS. A disadvantage is that it does not give information on the size distribution o f the microorganisms. Clark

Chapter Three: Mass Balancing in Contained Environments et al. (1981) assessed its performance with environmental samples and aerosols of known size to look at the effective volumetric sampling rate and particle size range over which it is maintained. Air flow measurements show a higher air flow rate drawn into the sampler than that claimed by the manufacturers. The effective sampling rate for larger particles is greater than that claimed by the manufacturers, although for particles <0.5 pm it is lower. So although the RCS is convenient to use, any results obtained should be interpreted with caution (Decosemo, 1992).

HouAvink (1988) used the Biotest RCS for daily air monitoring in laboratories and biotech plants, finding it to be most practical. This sampler was updated in 1992 and underwent trials carried out by Bennett (1992). When the RCS plus was compared with the Casella slit sampler for aerobiological monitoring, the results showed that the two samplers have similar efficiencies particles over 4 pm. For particles less than 4 pm the sampling efficiency falls off gradually, the efficiency of sampling 1 pm particles being 50 %.

Settle Plates

The use of settle plates to sample airborne microorganisms has long been criticised as being coarse (Ambler and Vernon, 1951; du Buy, 1945). The ability of settle plates to collect airborne microorganisms is governed by the gravitational force on the particle, which decreases with velocity. Since particle velocity is mass- dependent, settle plates are biased towards collecting larger particles and are also sensitive to wind movement. The method also relies on the growth of the microorganisms for detection. Other disadvantages are that settle plates will become overcrowded very quickly, making them difficult to enumerate accurately. The agar surface o f the plates also dries out after a short time.

Marple Personal Cascade Impactor

This sampler (Sierra Anderson Div., Anderson Samplers Inc., Atlanta.) works on the same principles as the AMS, but it is designed to be worn on the body within the breathing zone. Each stage (4, 6, or 8 are available) has 6 nozzles arranged

radially, staggered from stage to stage. The area between the nozzles form the impaction surface for the preceding stage. Stainless steel and Mylar collection media are available. Air is drawn into the sampler at 2 L.min"!. Macher and First

Chapter Three: Mass Balancing in Contained Environments

(1984) used membrane and gelatin filters with this sampler to collect B. subtilis

and E. coli from aerosols.

May Ultimate Impactor

Designed by May (1975), this is a static sampler which separates airborne particles into 7 size stages and deposits them on microscope slides. It works using the same principle as cascade impactors, air is drawn through stages and collected in series. Air velocity and particle inertia increase at each stage through the jet. The largest particles impact on the first slide and the smallest on the last. The particles are separated into the following size bands: 32, 16, 8, 4 ,2 , 1, 0.5 pm. Lacey and Lacey

(1987) used this impactor to collect airborne microorganisms in cotton mills.

lOM Personal Inspirable Dust Spectrometer

Aerosols are separated into 8 size fractions (Gibson et al. 1987) onto aluminium

plates. Within each plate there are circular orifices which act as impactor jets. Each stage acts as a collection plate for the preceding stage. The application of a sticky surface minimises particle bounce on the stages. The sampler operates at 2 L.min"^ and is worn in the breathing zone. It has been designed to collect the inhalable fraction of aerosols consistent with ISO recommendations (ISO, 1983), and to provide a size distribution. The inlet is designed to collect the inspirable fraction and minimise wall losses between entry and the first stage of the impactor. It has been characterised using polystyrene latex beads (Vincent, 1989) and was used to show worker exposure to coal dust, but has not yet been used for airborne microorganisms.