Spectrochimica Acta Part B 58 (2003) 1453–1471 0584-8547/03/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/S0584-8547Ž03.00137-X Simulation of droplet heating and desolvation in inductively coupled plasma—part II: coalescence in the plasma Craig M. Benson , Jiaqiang Zhong , Sergey F. Gimelshein , Deborah A. Levin , a,1 b b b Akbar Montaser * a, Department of Chemistry, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA a Department of Aerospace Engineering, Pennsylvania State University, 233 Hammond Building, University Park, PA 16802, USA b Received 22 January 2002; accepted 19 June 2003 Abstract A numerical model is developed to consider for the first time droplet coalescence along with transport, heating and desolvation in an argon inductively coupled plasma (Ar ICP). The direct simulation Monte Carlo (DSMC) method and the Ashgriz–Poo model are used, respectively, to compute droplet–droplet interactions and to determine the outcome of droplet collisions. Molecular dynamics (MD) simulations support the use of the Ashgriz–Poo coalescence model for small droplet coalescence. Simulations predict spatial maps of droplet number and mass densities within an Ar ICP for a conventional nebulizer-spray chamber arrangement, a direct injection high efficiency nebulizer (DIHEN), and a large bore DIHEN (LB-DIHEN). The primary findings are: (1) even at 1500 W, the collisions of the droplets in the plasma lead primarily to coalescence, particularly for direct aerosol injection; (2) the importance of coalescence in a spray simulation exhibits a complex relationship with the gas temperature and droplet size; (3) DIHEN droplets penetrate further into the Ar ICP when coalescence is considered; and (4) droplets from a spray chamber or the LB-DIHEN coalesce less frequently than those from a DIHEN. The implications of these predictions in spectrochemical analysis in ICP spectrometry are discussed.  2003 Elsevier B.V. All rights reserved. Keywords: Droplet coalescence; Droplet collisions; Numerical model; Direct simulation Monte Carlo simulations; DSMC; Molecular dynamics simulations; Inductively coupled plasma; ICP; Direction injection high efficiency nebulizer; DIHEN; Spray chamber; Droplet transport, heating and desolvation 1. Introduction Inductively coupled plasma (ICP) spectrometry is a powerful technique for elemental and isotopic *Corresponding author. Tel.: q1-202-994-6480; fax: q1- 202-994-2298. E-mail address: montaser@gwu.edu (A. Montaser). Present address: Naval Research Laboratory, Remote Sens- 1 ing Division, Washington, DC 20052, USA. analysis w1,2x. In this technique, liquid droplets are commonly introduced into the ICP by using a conventional nebulizer-spray chamber arrangement w3,4x. Direct introduction of sample by nebuliza- tion devices such as the direct injection high efficiency nebulizer (DIHEN) is gaining interest, especially when the sample is limited, expensive, or hazardous w5–23x. Direct injection of sample into the plasma is currently more challenging than