Spectrochimica Acta Part B 58 (2003) 1647–1662 0584-8547/03/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/S0584-8547Ž03.00140-X Spatially resolved ground-state number densities of calcium and strontium ion in an inductively coupled plasma in contact with an inductively coupled plasma mass spectrometry sampling interface Scott A. Lehn , Mao Huang , Kelly A. Warner , Gerardo Gamez, Gary M. Hieftje* 1 2 3 Department of Chemistry, Indiana University, Bloomington, IN 47405, USA Received 14 October 2001; received in revised form 16 June 2003; accepted 27 June 2003 Abstract Radial profiles of Ca and Sr ion number densities in an ICP at 6, 7 and 8 mm above the load coil (ALC) and at 1.25 kW of input rf power were measured by saturated fluorescence induced by an Nd:YAG laser-pumped dye laser at 396.85 nm and 421.55 nm, respectively. The measurements were performed in the presence and in the absence of an ICP-MS sampling interface. When in place, the orifice of the sampling cone was positioned 13 mm ALC on the axis of the plasma torch. The results show that the interface can either raise or lower the ion number densities, depending on the central-gas-flow rate, and can cause a vertical shift of their entire radial profiles with respect to the ICP axis. The introduction of Li, Cu and Zn as matrix elements reduced the ion number densities of the analytes, both in the presence and in the absence of the interface. This effect became more significant at higher central-gas- flow rates. In addition, the peak value of the radial ion number density was found to depend strongly on the central- gas-flow rate maximum occurred at 1.1 lymin for both Ca ion and Sr ion under the ICP operating conditions used in this study. This behavior is very similar to the mass spectrometric signals previously observed downstream and reported in the literature.  2003 Elsevier B.V. All rights reserved. Keywords: Laser-induced fluorescence; ICP-MS sampling interface; Matrix effects 1. Introduction The inductively coupled plasma (ICP) has become a popular source for both atomic emission and atomic mass spectrometry (MS). Like many *Corresponding author. Tel.: q1-812-855-2189; fax: q1- 812-855-0958. E-mail address: Hieftje@indiana.edu (G.M. Hieftje). Currently, at Pacific Northwest National Laboratory. 1 On leave from Chinese Academy of Sciences. 2 Currently, at Harvard University, Department of Education. 3 techniques, both kinds of ICP spectrometry suffer from matrix interferences. Luckily, there are ways to overcome such interferences. One is to use a matrix-matched standard. Yet, since it is not always possible to know everything that is present in a sample, this procedure is not often viable. Another approach to overcome matrix interferences is by the use of standard additions. However, standard additions are time-consuming and often costly. A better way is rationally to change operating con-