With new nanomaterials being introduced every day, there is a growing need to monitor the airborne release of nano particles. Particle Measuring Systems has a device capable of size-selective sampling.
Health risks associated with the inhalation of airborne particles are known to be influenced by particle size. Studies have shown that certain nanoparticles – those with diameters less than 100nm – have increased toxicity relative to larger particle sizes of the same substance. Therefore, a reliable size-resolving sampler able to collect a wide range of particle sizes, including particles with sizes in the nanometer range, would be beneficial in investigating health risks associated with the inhalation of airborne particles.
Size-resolved sampling of airborne particles has previously required various pieces of equipment and techniques to be employed to cover both the broad nanometer (nm) and micrometer (µm) particle regime at any one time. A significant benefit would arise from a single sampler that could reliably collect size-resolved samples of the entire particle size range considered relevant to health effects.
Cascade impactors have been employed for more than half a century to fractionate aerosol particles according to their aerodynamic diameter.1 Usually, a cascade impactor enables aerosol particles to be collected onto 5 to 10 successive impactor stages with decreasing cut-off diameters. For instance, in a May cascade impactor2,3 at 20 litres/min flow rate, particles are separated into seven fractions (defined as diameters at which the collection efficiency is 50%) with aerodynamic diameters ranging from about 20 to 0.25µm. The majority of other cascade impactors collect similar size ranges.
However, because collecting particles smaller than ~0.25µm is not possible using standard impactors, low pressure cascade impactors have been developed that enable particles considerably smaller than 0.25µm to be fractionated. Although low pressure impactors can extend the range of size selective sampling to particles with sizes of 20–30nm, they cannot be accepted as ideal instruments for sampling particles in the nano-size range for the following reasons:
• The high pressure drop in low-pressure cascade impactors corrupts size distributions and causes condensation of water as well as other atmospheric constituents on substrates.
• Low pressure may cause volatilisation of some constituents and change the chemical composition of the collected substances – especially with organic compounds.
• High flow velocity in a low-pressure impactor causes particle bouncing. Rotating substrates is one technique that is used to try to increase the uniformity and to reduce bouncing; however, this also increases the cost and weight of impactors.
• Impactors fractionate particles according to aerodynamic equivalent diameter. In the nanometer-size ranges, deposition of particles in the respiratory tract depends primarily on diffusion. For this reason, mobility equivalent diameter is a better measure of nanoparticle sizes when evaluating health effects.
• Low pressure cascade impactors are based upon inertial deposition and therefore provide data on aerodynamic equivalent diameters as noted above. This creates uncertainties with interpretation of data in the nano range.
Thus, there is a need for a wide-range aerosol sampling instrument that enables more reliable data to be gathered on the size-resolved chemical composition of aerosols. If a sampler based on diffusion could be combined with a cascade impactor many of the disadvantages of the low pressure impactor could be overcome, providing a means for fractionating aerosol particles over a wide range of sizes.
It is clear that human health risk is influenced by deposition of airborne particles at specific sites in the respiratory tract. The efficiency of deposition in the respiratory system has been the subject of numerous experimental research and modelling tests.4,5,6,7 It is well-established that the deposition efficiency in the respiratory tract is influenced by the size of particles and forms a “V-curve” comprising two branches caused by two main mechanisms of particle deposition: inertial deposition for large particles with diameters greater than 200nm, and diffusion for smaller particles (dia. <150nm).8,9,10,11 Using both diffusion and impaction sampling methods correctly accounts for both processes, without implication as to where in the respiratory tract deposition occurs.
Combination device
However, combining a diffusion sampler with an impactor requires a number of issues to be addressed. In existing diffusion samplers particles are collected onto nets. These diffusion samplers have a V-shaped dependence between collection efficiency and the size of particles12 (see Fig 2), which creates a number of important implications:
• The V-shape means that there is not a one-to-one relationship between size and deposition efficiency. For example, the same collection efficiency corresponds to two different sizes as seen in Figure 2. Mass size distributions cannot, therefore, be easily retrieved from the mass deposited onto the substrates. (Note: This is not a problem when obtaining number concentration size distributions because fine and ultra-fine particles are usually present in much greater numbers than coarse particles and the number of coarse particles can be neglected; it is not possible to neglect the mass).
• In addition to the above, many existing diffusion samplers utilise a large number of screens (50+) to capture particles for mass analysis. For mass measurements, the mass of particles collected on a section needs to be sufficient to be detected by an appropriate analytical technique (Atomic Absorption Spectroscopy (AAS), Gas Chromatography-Mass Spectrometry (GC-MS) or Inductively Coupled Plasma Mass Spectro-metry (ICP-MS)). The diffusion sampler should therefore be designed to maximise the mass deposited on screens by employing the smallest possible number of screens.
• Diffusion samplers, such as those described in literature or those commercially available as stand-alone products, operate at flow rates of ~5 litres/min. However, cascade, impactors are often operated at flow rates of about 20 litres/min to minimise measurement errors, particularly if mass measurements are being made.
• Even if a diffusion sampler has been developed with sufficient resolution, the question remains: is the mass in the nano-size region sufficient to be detected using widely available techniques?
The mass to number ratio rapidly decreases when reducing particle sizes by a factor of 103 per decade. Consequently, between 1 and 100nm the aerosol mass concentration is too small to be readily detected using most common techniques. Thus, it is necessary to demonstrate that enough mass is collected, within a reasonable time period, to allow mass determination.
Based on the information detailed above, the ideal product that would sort and capture a wide range of aerosol particles would combine a cascade impactor and diffusion sampler capable of operating efficiently at high flow rates (20 litres/min) while also minimising the number of nets used within the diffusion sampler.
Verified theories of diffusion deposition onto cylindrical fibres were used to determine the optimal geometry and regimes of the diffusion particle sampler. A cascade impactor combined with a diffusion sampler was designed and assembled to maximise simplicity and ease of use. The integrated instrument – the Nano-ID – was calibrated using a number of standard techniques. Finally, laboratory and field tests were carried out to test the Nano-ID and to demonstrate situations where sufficient aerosol particle mass is present in the nano-size region, to be further characterised using conventional analytical techniques.
Using nets allows the size selective deposition only for aerosols of nano-particles, when particles greater than 0.25µm are not present. However, there is one important feature of the curve in Figure 2: the minimum is located at ~0.25µm, which is the size that is close to the lowest size stage of an ordinary cascade impactor. This gives the opportunity for combining a cascade impactor and a diffusion sampler. The size-selective deposition of particles smaller than 0.25µm is possible if larger particles are removed from the flow by a cascade impactor upstream of the diffusion sampler.
After removing the larger particles from the flow the subsequent collection of the rest of the particles (smaller than 0.25µm) is governed by a monotonous function of collection efficiency versus size, which enables those smaller aerosol particles to be collected selectively by nets.
An example of the Nano-ID, with a cascade impactor and a diffusion sampler is shown in Figure 1. First, aerosol particles have to be drawn into the cascade impactor where particles greater than 0.25µm in diameter are collected onto seven stages of glass substrates (other substrate media can be used as well). Collection efficiency of particles within an inertial cascade impactor increases with particle size.
Therefore, the largest particles are removed from the flow within the first stage and the smaller particles are deposited onto the following stages. The airflow emerging out of the cascade impactor contains particles smaller than 0.25µm.
These particles smaller than 0.25µm can then be size-selected and captured by a series of different nets. The collection efficiency of particles in a diffusion sampling unit decreases with increased size. Therefore, the smallest particles are collected at the first net and the larger particles are deposited onto the following nets. A diffusion sampler has to be designed to collect sequentially particles in the size range from 1nm to 0.25µm.
In practice, capturing particles from 1nm to 0.25µm is achieved by selecting nets with different fibre diameters and fibre densities, by varying the number of nets per stage, and by choosing the appropriate flow velocity. The Nano-ID contains five diffusion sampling stages, and each stage contains either one or several Nylon nets.
In a conventional diffusion sampler, the same type of screen/net (in terms of mesh size, threads per mm etc.) is usually used for different stages. In the Nano-ID different types of nets are required to cover the whole size range with cut-off diameters distributed equally across the range. In practice, there are a limited number of nets available. Nylon net filters with various mesh openings (from 11 to 180µm) were used in the diffusion size-selective particle sampler because of their relative low cost and their compatibility with AAS and other analytical techniques. These nets have been chosen following numerical modelling and confirmatory calibration, and are standard materials used for particle retention and analysis.
One of the main benefits of using different types of net filters includes reducing the number of nets needed to capture nanoparticles effectively. It was found that using the same type of nets for all stages meant the number of nets would be very large, making their use very expensive and impractical (assembly and analysis). For instance, using a net type with 180µm mesh opening requires almost 100 net filters to catch aerosol particles of 0.25µm diameter. Obviously this is impractical for a cost-effective and user-friendly particle sampler.
Integrated unit
In the Nano-ID, a wide range and size-selective diffusion sampler and cascade impactor were designed to perform seamlessly as a single integrated unit. This is achieved through designing the particle cut- off sizes of both parts to correspond to a single set of size sections without large gaps or overlaps in the deposition range of sections.
The correct interface of both parts, which is based upon different mechanisms of deposition, is important because the size-selective diffusion sampler collects smaller particles first but the cascade impactor, on the contrary, collects larger particles first. The cascade impactor therefore has been designed such that the lowest stage cut-off size is equal to the highest section diffusion sampler cut-off size.
Figure 3 shows how this interface between the two methods of collection (cascade impactor and diffusion sampling) is achieved within the Nano-ID. This version of the Nano-ID instrument has 11 stages including five stages for the diffusion aerosol sampler. The cut-off diameters for the diffusion sampler were calculated for a 20 litre/min flowrate and ds = 4.3x10-2m. Note that the cut-off diameter of the 5th diffusion sampler stage (the dashed line in figure 3) of 0.24µm is very close to the cut-off diameter of the fifth stage of the cascade impactor at 0.25µm. The cut-off sizes for stages 5 and 5* are very close, but the shapes of the curves are slightly different. It is almost impossible to achieve the ideal fit of these curves because of differences in the mechanism of the deposition.
In summary, the Nano-ID can be employed to sample size-selectively aerosol particles in the entire aerosol size range down to nanometer-size particles without using a high-pressure drop. Therefore, sampling artifacts caused by evaporation/ condensation of volatile and semi-volatile compounds are minimal for the instrument. In addition, the diffusion sampling unit was calibrated in the same way as for a traditional diffusion sampler by measuring aerosol particle number concentrations with a condensation particle counter (Figure 4).
The Nano-ID operates at 20 litres/min and does not require low pressure to collect nanoparticles. Data calculated using diffusion deposition theory were found to be in a good agreement with experimental data and other techniques, and useful results have been identified in field measurements.
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Footnote This is a condensed version of the paper by B. Gorbunov, N. D. Priest, R.B. Muir, P.R. Jackso*, H. Gnewuch, published in the Annals of Occupational Hygiene 2009. 53(3), 225-237