Elsevier

Journal of Aerosol Science

Volume 31, Issue 12, December 2000, Pages 1389-1395
Journal of Aerosol Science

A differential mobility analyzer and a Faraday cup electrometer for operation at 200–930 Pa pressure

https://doi.org/10.1016/S0021-8502(00)00037-9Get rights and content

Abstract

We have developed a differential mobility analyzer (DMA) based on the DMA devised by Seto et al. (1997) and a Faraday cup electrometer for measurement of nanometer-sized particles at a few hundred Pa and examined the operating characteristics of the DMA using the tandem DMA technique. The tandem DMA calibration establishes that the DMA successfully classifies particles in the 200–930 Pa pressure range. It was also found that the transfer function of the DMA follows the triangular transfer function and the resolution of the DMA is close to that given for an ideal case. As a standard of a minimum pressure that may be probed with the present DMA system, 400 Pa is estimated when the DMA operates with a 3nlmin−1 sheath flow and a 1nlmin−1 aerosol flow rate.

Section snippets

INTRODUCTION

A differential mobility analyzer (DMA) has been used as a size spectrometer for an aerosol in the atmosphere. Recently, Seto et al. (1997) succeeded in reducing the DMA operating pressure to several kPa. This study suggests the potential of a DMA for a particle monitor in the new field of the semiconductor fabrication industry where nanometer-sized particles formed at low pressures are a serious problem (Ziemann et al., 1995). In this paper, we report the successful development of a DMA and a

EXPERIMENT

Figure 1a and b show the schematics of the DMA (hereafter referred to as a very low-pressure DMA, VLPDMA) and the total system, respectively. The VLPDMA is a modified version of the low-pressure DMA reported by Seto et al. (1997). The column length of the DMA is 18 mm, and the radii of its external and internal electrodes are 33 and 25 mm, respectively. To reduce the pressure drop between the DMA and vacuum pumps, which is crucial for low-pressure operation, small radii and excessive lengths of

RESULTS

Figure 3a–d show the normalized mobility distributions n2(Z2)/N0 as a function of Z2/Z1 obtained for various particle sizes by changing the voltage applied to DMA 2 operating at 200, 400, 665, and 930 Pa, respectively. As both the electrical mobility Z related to the particle diameter in the free molecular regime and the centroid mobility Z classified by the DMA are inversely proportional to the pressure P, i.e., Z=ZatmPatm/P and Z=ZatmPatm/P where the subscript atm indicates the value at

SUMMARY

We have succeeded in lowering the operating pressure of a DMA to a few hundred Pa by devising a system that features a low-pressure drop and a large evacuation capacity. It was demonstrated that the performance of the present DMA might be approximately described by a triangular transfer function. The system maintains sufficient resolution at a few hundred Pa for accurate mobility classification in this pressure regime. To our knowledge, the present DMA system presents the lowest operating

References (8)

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