GB2311856A - Air sampling for analysis - Google Patents

Abstract

A sampler of environmental air capable of recovering particles less than 0.1~m to more than 20~m mean diameter comprises a chamber (10a), a bead bed (11) supported across the chamber, a liquid feed (14) adapted to deposit liquid droplets on the beads, a sample air inlet (13) ahead of the bead bed and air outlet (16) downstream thereof, and a liquid collection outlet (15). In passage through the sampler air yields its particles to the liquid on the beads and the liquid is then collected for analysis.

Description

ATMOSPHERE SAMPLING The present invention relates to atmospheric aerosol sampling, and is particularly concerned with apparatus for sampling environmental air for microbes.

A popular apparatus for sampling air for microbial particles is a cyclone device. This basically comprises a hollow cone adapted to be held with the apex lowermost and communicating with a collection chamber, and the base uppermost and associated with a tangential air inlet, there being an axially disposed outlet approximate the base, arranged for connection with a pump for drawing sample air through the cyclone. A liquid pool is arranged to entrap microbes. The apparatus is adapted to permit microbe burdened liquid to be removed from the collection chamber for analysis.

The cyclone sampling device relies upon centrifugal (inertial and impaction) forces to separate microbes from the air being sampled, and accordingly its efficiency is reduced the smaller and lighter are the particles being searched for. Also cyclone devices are quite expensive of manufacture, particularly when they incorporate means such as vanes in an endeavour to overcome the problem just mentioned.

Smaller, submicron particles can be collected in a different apparatus employing diffusion forces and vapour phase rehydration.

It can also be important to avoid subjecting the air being sampled to high shear forces, and can be quite difficult to arrange that the air inlet to a sampling apparatus draws in a representative sample whereby a reliable measurement can be obtained of the concentration of any particular microbe in the air. This can imply a need for low flow resistance. It is also valuable if the apparatus can be adapted for continuous operation.

The present invention provides an apparatus for sampling environmental air which is capable of recovering particles in the size range 0.5 to 20Fm diameter and also those less than 0.ism diameter with equal alacrity. No other single sampler in existence can do this; this sampler collects particles other single samplers cannot. Also it is readily adapted for continuous operation and imposes low sampling shear stresses. This latter facility is important when one wishes to avoid traumatising the sampled microbes and thus causing loss of biological activity.

According to one aspect of the present invention an air sampling apparatus comprises a chamber, support means supporting a bead bed comprising several layers of beads across the chamber, liquid supply means for so depositing fine liquid droplets onto the beads as to wet the surface of each one, a sample air inlet arranged for delivering sample air to an upper surface of the beads, liquid collection means for delivering liquid from the beads, and a sample air exhaust.

The beads may be spherical and preferably have a shiny surface. Suitably they may be of glass or plastics material, possibly hollow, and be 3 to 5 mm in diameter, preferably all the same.

The chamber may be arranged to have a plenum region between the air and liquid inlets and the beads. This has the advantage of allowing gentle vapour-phase rehydration of microbes before they land on the wet surface of the bead bed or are drawn through it.

Conveniently the beads are supported on a perforated base, such as a gauze, extending across the chamber. If there is a collector plenum region below the support means, then this readily permits ensuring that exhausting sample air does not entrain particle bearing liquid.

The liquid supply means is preferably arranged to deliver liquid in heavy mist form. The liquid may be water. The water is preferably distilled water but may contain additives, for example suitable detergents, proteins, or sugars to assist in the maintenance of the apparatus and the particles.

According to a second aspect of the invention a process for sampling air comprises wetting with a flow of liquid a bead bed in apparatus according to the first aspect of the invention, drawing a flow of sample air through the apparatus whereby it passes through the bed of wetted beads, and collecting the liquid flowing from the bead bed for delivery to analysis apparatus.

Taking as a basis a cylindrical chamber with the bead bed deployed across the diameter of the cylinder, a typical liquid flow rate is of the order of 1 to 3 ml per minute per 50 cm2 cylinder cross section area, with a typical sample air flow rate of 5 to 15 litres per minute per 50 cm2 cylinder cross section area. The flow rates are chosen in relation to bead bed characteristics such that the beads do not dehydrate in operation and sample air flow reentrainment of liquid leaving the bead bed is minimised. This may imply a varying flow rate to cater for temperature and/or pressure changes and ensuring that the bead bed does not dry out in use. The bead bed is preferably then deep enough for the air sample to spend at least 3 seconds in the bed.

It may be necessary to limit sampling time to avoid microbial growth in the apparatus affecting the sampling results. Similarly, process and apparatus may be adapted to permit occasional washing and sterilization, particularly of the bead bed. It is a particular advantage of the invention that the bead bed can be thoroughly and adequately washed and sterilized in situ.

Whilst a continuous gas and liquid flow is usually envisaged, where it is desired the more closely to mimic the operation of a human or animal lung, apparatus and process may be arranged and adapted for the provision of a fluctuating liquid flow. With animal and human lung operation the nose and major bronchi are adapted for filtering out the larger particles, assisted by the air fluctuation. Apparatus and process according to the present invention may additionally be arranged for the prefiltration or presampling of the larger particles.

Air sampling apparatus and process in accordance with the invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic cross section of a sampler, and Figure 2 is a schematic flow diagram of a process employing the sampler.

The air sampler shewn in figure 1 comprises a cylinder 10 across which is supported a bead bed 11 by a perforated support plate 12. The cylinder has an air inlet 13 above the bead bed 11, there being a plenum region 10a between the inlet and the beads. There is also a liquid injector 14 above the bead bed 11 arranged for delivering a heavy mist of liquid into the plenum region above the beads, whereby the surface of each bead can be wetted with the liquid.

Below the plate 12 is an egress plenum region 10b arranged for the collection of liquid flowing from the bead bed 11 and leading from which is a liquid outlet 15. There is also a sampler air outlet 16 to the plenum region 10b, arranged, for example by cowl 17, to minimise the reentrainment by the sampling air of liquid flowing from the bead bed 11.

Referring now to Figure 2, for use the sampler air inlet 13 is associated, as closely as possible and without sharp bends or restrictions, with an air sampling probe 20. The air outlet 16 is connected via an air pump 21, to a sampled air dump, eg the environment. The liquid injector 14 is connected to the outlet of a pump 22 arranged to draw the liquid, via a filter 23, from a liquid reservoir 24. The liquid outlet 15 communicates with a separator 25 where particles in the liquid can be removed and conveyed to an analyser 26, whilst the liquid is returned to the reservoir.

In use, air from the probe 20 is drawn into the cylinderl0 by the pump 21 where liquid, injected at 14, wets the surface of the beads in the bed 11. Passing through the bead bed 11 the air yields up its store of particles to the liquid. The liquid leaves the cylinder 10 at the outlet 15 and goes to the separator 25.

In a typical sampler according to the invention the cylinder is 8 cm diameter and 20 cm long and the beads are glass spheres 3 mm in diameter. The air flow rate is arranged to be 10 litres per minute and the liquid flow 2ml per minute.

In alternative apparatus employing samplers in accordance with the invention, the separator 25, sampler 26, and filter 23 are omitted, and the particle bearing liquid is arranged to be reinjected at injector 14. In this way the particle concentration can be increased. After a suitable period of operation the particle laden liquid is drawn off from the reservoir for analysis. In another version the liquid drawn off at 15 is introduced directly to a fraction collector wherein, for example, a turntable or belt holding a number of test-tubes advances by 1 tube after a certain volume has been collected or after a given time interval.

In an experiment using bioaerosols of Bacillus subtilis var. niger spores together with those of Klebsiella pneumoniae, this sampler gave approximately ten-fold higher viable recoveries than did the AGI 30 sampler, albeit with quite vigorous washing of the bead bed. This impinger type sampler is described in J Aerosol Science 1992 Vol 23 Supp 1 S675-S678.

It comprises an air inlet culminating in a capillary orifice held in a chamber 30 mm above the base of the chamber which contains a layer of collecting fluid. The capillary acts as a critical orifice.

Samplers in accordance with the invention are particularly valuable in mimicking the alveoli of the lung. A sampler which mimics the operation of the lung in collecting microbes is then able to detect which airborne microrganisms are able to infect via the lung. However, lung simulation represents only one of the possible areas of use of samplers in accordance with the invention. It may after all be important to be able to detect all microorganisms in a particular atmosphere.

Advantages that samplers in accordance with the invention have over prior art samplers include providing vapour phase rehydration, imposing low sampling shear stresses, capability of collecting particles of submicron size (less than 0.1pm diameter) and upwards, including 0.5 to 20pm diameter, being easy and relatively cheap to construct, and providing a low resistance to flow. They can be suitable for continuous sampling over long periods both indoors and out, indeed over a wide range of sampling conditions. They are suitable for simultaneous collection of mixed aerosols ranging from infectious particles such as virus to pollens and other allergens as well as inert dusts. The apparatus can readily be made as rubust as may be required.

Claims (15)

Claims

1. Air sampling apparatus comprising a chamber, support means supporting a bead bed with several layers of beads across the chamber, liquid supply means for so sprinkling liquid onto the beads as to wet the surface of each one, a sample air inlet arranged for delivering sample air to an upper surface of the beads, liquid collection means for delivering liquid from the beads, and a sample air exhaust.

2. Apparatus as claimed in claim 1 and wherein the beads are spherical.

3. Apparatus as claimed in claim 1 or claim 2 and wherein the beads have a shiny surface.

4. Apparatus as claimed in any ony of claims 1, 2 or 3 and wherein the beads are of glass.

5. Apparatus as claimed in any one of claims 1 to 4 and wherein the beads are of plastics material.

6. Apparatus as claimed in any one of claims 1 to 5 and wherein the beads are hollow.

7. Apparatus as claimed in any one of claims 1 to 6 and wherein the beads are similar spheres 3 to 5 mm in diameter.

8. Apparatus as claimed in any one of the preceding claims and having a plenum region between the air and liquid inlets and the bead bed.

9. Apparatus as claimed in any one of the preceding claims and wherein the beads are supported on a perforated base extending across the chamber.

10. Apparatus as claimed in any one of the preceding claims and having a collector plenum region below the support means.

11. Apparatus as aclaimed in any one of the preceding claims and wherein the liquid supply means is arranged to deliver liquid in heavy mist form.

12. Apparatus as claimed in any one of the preceding claims and wherein the liquid is water.

13. A process for sampling air comprising wetting with a flow of liquid a bead bed in apparatus as claimed in any one of the preceding claims, drawing a flow of sample air through the apparatus whereby it passes through the bed of wetted beads, and collecting the liquid flowing from the bead bed for delivery to analysis apparatus.

14. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

15. A process substantially as hereinbefore described with reference to the accompanying drawings.