Importance of impaction velocity for sampling viable organisms

We are often asked ‘how can MicroBio samplers be that good at sampling compared to others costing more than twice as much?’

The simple answer: the impaction velocity of organisms is kept low enough, between 9.8 and 11 metres per second for MicroBio sampling heads, so their viability is not compromised when impacting upon sample plates, but not too slow as to flow through the air stream and miss the plate altogether!

Our sampler design concentrates on viable bioaerosol collection rather than the aesthetics of the product.  What’s important is capturing a viable sample, rather than just looking the part.

Research has shown (Stewart et al, 1995 Apr;61(4):1232-9) that particles with too high an impaction velocity have their viability affected.  Imagine running into a wall. At a slow jog it wouldn’t hurt too much, but running at a sprint then serious injury will occur.  It’s no different when an organism impact upon the agar  surface of a contact plate.

Often forgotten in the real world of sampling are the multitude of environmental effects.  Organisms need a source of nutrients, something air normally lacks.  Depending on humidity, temperature, existing bioaerosol and particle loading, the bioaerosol’s viability is not easy to predict (Walter et al, 1990 Nov;56(11):3468-72). Even when sampling in the same location with apparently the same conditions, results can vary widely.

By designing an air sampler with emphasis on enabling high viability collection, results can be more meaningful.

By this simple common sense realisation we keep our particle velocity in a range that most organisms will survive impaction and be viable for culturing and counting.  This need is also emphasised in ISO-14698 that mentions impaction velocity should be less than 20 metres per second. MicroBio MB2 is designed to meet the sampling requirements of ISO-14698 standard.

Some manufacturers concentrate on particle collection efficiency and by pulling air through at higher velocity will collect more particles, but organism survival is then compromised, hence viable collection efficiency is reduced.  With bioaerosol sampling, the important measurement is viable organism count.

How do we achieve the ideal particle velocity?  

By understanding the mechanics of bioaerosol sampling.

Having developed mathematical models over the years we are able to design sampling heads and predict the particle d50 cut-off points and impaction velocity for any combination of sampling head hole count, hole size and volumetric flow.

In reality, no sampling device is perfect for all particles.  Different organisms may be more robust than others and sizes vary widely so any design of sampling head can be no more than the best possible compromise. By realising this compromise we have developed two basic sampling heads that cover most customers needs (d50 = 1.35um and d50 = 1.7um) that offer high viable particle efficiency.

Another aspect our sampling heads is the way the holes are produced. There should be no sharp edges on the inlet of the holes.  Turbulence is created around these points and they can physically damage organisms.  We have tried various manufacturing techniques over the years and have found punching the holes produces a smooth form in the surface of the metal for air to flow, while keeping them affordable.  Simply drilling a hole creates a sharp entry point to the hole, unless expensive additional machining of each hole is undertaken.  We’ve also experimented with etching techniques and laser cutting, but under the microscope the edge finish of such holes is rough and sharp.

What to look for in a sampler

When considering which bioaerosol sampler to purchase remember to consider the organism viability first rather than list of sampler features that may never be used and how good it looks.  The science is more important than the style of the instrument.

  • Some key features to investigate are:
  • d50 cut-off – this determines how well the sampler will collect certain sizes of organism.
  • Particle velocity, keep the organisms safe! ISO-14698 required <20m/s.
  • Does not use proprietary media – can it use standard contact plates and petri dishes?
  • Can sample at 100 litres per minute and up to at least 1000 litres per sample.
  • Removable sampling head that can be autoclaved.
  • Battery powered, highly portable and simple to use.
  • Low total cost of ownership (spares, calibration, accessories)

Special sampling needs

Over the years customers have made special requests so they can sample particular size organisms.  This we are happy and able to do due to our ability to model various combinations to achieve specific needs.

For a fee, we can design sampling heads for specific customer needs and small quantities fabricated within 8 to 12 weeks.  There are practical limits with the sampler but for a free no obligation assessment of what’s possible please contact us with your specific needs.