Espectroscopia UV-Vis

In addition to the above study on the mass throughput of the studied THES disperser, fluorescent nanoparticles were used to examine the uniformity of the nano aerosol

concentrations at all the nose ports of the animal cages in the TSE inhalation exposure

chamber. The experimental procedure for the fluorescence analysis was given in Section 2.2.

Figure 4.8 shows the percentage of concentration deviating from the mean mass

concentration among all the evaluated nose ports. It is observed that deviations of aerosol

concentration at the majority of the nose ports were within ±13% of the mean value. Only

one nose port located nearby the side exit port of the exposure chamber showed a slightly

high value of deviation (i.e., ~27%). It is possibly because of the local variation of flow field near the side exit port. In reviews of aerosol exposure chambers (O’Shaughnessy et al., 2003;

Dabish et al., 2010), the coefficient of variability (CV) of nose/head-only chamber (defined

as the ratio of the standard deviation to the mean value) usually ranges from 4.8% to 16.3

%. Based on the above CV definition, the overall coefficient of variability for the studied

exposure chamber was calculated as 11.4%, indicating a good spatial uniformity of the studied TSE inhalation exposure chamber for nanoparticles. The possible reason for slightly

higher CV value in this study could be due to the fact that the studied flow rate of 10.0 lpm

is lower than the optimal flow rate recommended for the 20-port nose-only animal exposure

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Figure 4.8: Deviation of the particle mass concentrations measured at 20 nose ports of inhalation exposure chamber, referenced to the mean of all measured mass concentrations.

4.4 Summary

As the recent development of nanotechnology, nanoparticles of various sizes and

compositions have been synthesized and proposed for industrial applications. At the same

time, the concern of adverse health effects due to the exposure of nanoparticles has been

increasing. Tools are in demand to perform the toxicity studies of nanoparticles, particularly in its individual form (not in the agglomerate form). Unfortunately, such a tool for studying

the toxicity of nanoparticles in their individual form has not been commercially available

until now. In this study, the performance of a twin-head electrospray nanoparticle disperser

developed by TSE Systems Inc. was evaluated. In addition, the spatial uniformity of nano

-30% -20% -10% 0% 10% 20% 30% 40% 50% 1 -1 ₁-2 -3_{1 1}-4 -5_{1 1}-6 ₁-7 ₁-8 ₁-9 1 -1 0 2 -1 ₂-2 ₂-3 ₂-4 -5_{2 2}-6 ₂-7 ₂-8 ₂-9 2 -1 0 D e v ia ti o n % Sampling Port #

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aerosol concentration in the 20-port nose-head-only animal exposure chamber was further

evaluated when a nanoparticle stream was introduced in.

Electrospray technique has been recently proposed and applied in many applications

involving particles, because of its capability of producing un-agglomerated droplets/particles

with sizes ranging from nanometers to super-micrometers. With the presence of a DC

electrical field for its operation, particles produced by electrospray are highly charged in the

same polarity. Such charged particles are in general difficult to be kept airborne because of

the electrostatic effects. The charge reduction is thus necessary for electrosprayed particles

in order to minimize the loss during the particle transport. Without using radioactive

materials or corona discharge as the bi-polar ion sources for reducing charges on

electrosprayed particles, the twin-head electrospray technique was implemented in the

studied disperser to achieve the same task. In the disperser, positive high voltage was applied

to one spray head (i.e., capillary), producing positively charged particles, and negative high

voltage applied to the other, generating negatively charged ones. The mixing and collision

of particles in both polarities in general reduced the charge level on particles while increasing

the mass concentration of particles exiting the studied disperser.

A systematic study has been performed on the studied electrospray disperser regarding to its mass throughput and quality of particle size distribution. The performance

of the disperser was studied by varying the spray capillary tip distance, carrier-to-sheath flow

rate ratio, total gas flow rate, liquid feeding rate, nanoparticle suspension concentration and

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uniformity of particles in the inhalation exposure chamber was also investigated. It was

found that the maximal mass concentration of nanoparticles exiting from the studied disperser was achieved by having a capillary tip distance of 3.0 cm, and operating it at the

total flow rate of 10.0 lpm with the carrier-to-sheath flow rate ratio of 4:3 and the suspension

feeding flow rate of 20 µl/min. The applied voltages on both spray capillaries ranged from

7.5 to 8 kV. Under this setting, nanoparticles in high mass concentrations (i.e., > 10 mg/m3₎

could be produced when spraying a suspension of nanomaterial concentrations higher than

8.0 g/L. The linear relationship between the mass throughput and the mass concentration of

sprayed nanoparticle suspensions was also observed in this study. Further, the mass

throughput of the studied THES disperser depended on the nanoparticle composition. As a

result, a mass concentration monitor is thus recommended to be included in the system for

measuring the actual mass concentration of particles exiting from the disperser. Finally, the

fluorescence study on the spatial uniformity gave a CV value within the range of uniformity

susgested by previous research groups, indicating the acceptable particle uniformity in the

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