On the efficiency of impingers with fritted nozzle tip for collection of ultrafine particles

Abstract

The aim of this study was to determine the collection efficiency of ultrafine particles into an impinger fitted with a fritted nozzle tip as a means to increase contact surface area between the aerosol and the liquid. The influence of liquid sampling volume, frit porosity and the nature of the sampling liquid was explored and it was shown that all impact on the collection efficiency of particles smaller than 220 nm. Obtained values for overall collection efficiency were substantially higher (∼30–95%) than have been previously reported, mainly due to the high deposition of particles in the fritted nozzle tip, especially in case of finer porosity frits and smaller particles. Values for the capture efficiency of the solvent alone ranged from 20 to 45%, depending on the type and the volume of solvent. Additionally, our results show that airstream dispersion into bubbles improves particle trapping by the liquid and that there is a difference in collection efficiencies based on the nature and volume of the solvent used.

Introduction

Due to their small size, high number concentration and ability to penetrate deeply into the alveoli, ultrafine particles (D < 100 nm) are considered to be the most hazardous fraction of urban atmospheric particles (Oberdörster, 2000). To date, ultrafine particles have been collected for chemical and toxicological analysis mostly by filtration, for reasons of practicality and their excellent collection efficiency. Nevertheless, there are three major drawbacks of this approach – poor recovery of particles from the filter, evaporation of semi-volatile particulate-phase compounds during the sampling, and adsorption of gas-phase compounds onto the filter (Turpin et al., 2000). Particle collection methods, like Particle-Into-Liquid-Sampler (PILS) (Weber et al., 2001), based on growing of particles into droplets due to mixing with saturated water vapour and then collecting them by inertial techniques, have been developed, but due to their high cost are not so widely in use. Another method for the collection of particles is through liquid impingement. The collection device for this sampling approach is called an impinger or a bubbler. So far, impingers have been mainly used for collection of biological aerosols (D > 1 μm) (Grinshpun et al., 1997, Henningson et al., 1988, Lin et al., 1997), where the inertial impaction of particles into the liquid was considered to be the dominant removal mechanism. Apart from that, the diffusion mechanism also plays a role in removal of particles by bubbling through liquid and it becomes particularly important in the case of submicron and especially ultrafine particles. Liquid impingement is convenient when testing particle surface reactivity, preparing samples for toxicological studies, or when ageing of particles due to long-term sampling may alter their chemical properties.

In order to conduct quantitative chemical analysis on the particles collected by the impingers, it is important to know the portion of the particles being collected in the liquid (collection efficiency). Several studies have reported collection efficiencies of impingers for particles in the micrometer (D > 1 μm) and submicrometer (D < 1 μm) size range (Dart and Thornburg, 2008, Grinshpun et al., 1997, Henningson et al., 1988, Lin et al., 1997, Spanne et al., 1999). However, to date, there are only two studies reporting the efficiencies of impingers for the collection of ultrafine particles (Hogan et al., 2005, Spanne et al., 1999). Spanne et al. (1999) found that collection efficiency for particles between 20 and 700 nm in size in an all-glass midget impinger with a standard nozzle was less than 20%. Hogan et al. (2005) reported collection efficiencies less than 10% for an All Glass Impinger 30, SKC BioSampler and a frit bubbler sampling particles in the 30–100 nm size range.

Herein, we investigate liquid impingement as a method for collection of ultrafine and near-ultrafine particles (D < 220 nm) using impingers with a fritted nozzle tip. A fritted nozzle tip increases the contact surface between the aerosol and the liquid and should, therefore, increase the collection efficiency of an impinger. As mentioned previously, the diffusion mechanism for particle removal becomes significant for ultrafine particles, so parameters that may increase the possibility of particle removal were also explored.