Full Paper Formation and fundamental characteristics of novel free-running helium inductively coupled plasmas Lisa A. Iacone, Wellington R. L. Masamba,{ Sang-Ho Nam,{ Hao Zhang,§ Michael G. Minnich, Akitoshi Okino and Akbar Montaser* Department of Chemistry, The George Washington University, Washington, DC 20052, USA. E-mail: montaser@gwu.edu Received 16th November 1999, Accepted 28th February 2000 Published on the Web 27th April 2000 This report is the ®rst study on the formation, stabilization and fundamental investigation of free-running helium inductively coupled plasmas (He ICPs). The plasma was operated at 600±900 W with a total helium gas ¯ow of less than 10 L min 21 . The shape and the physical appearance of free-running helium plasmas differ markedly from the He ICP sources reported previously using a crystal-controlled generator. Although the free- running helium plasma appears annular, high-speed video studies reveal that the plasma rotates at a frequency of 75±275 Hz. Lateral pro®les of emission spectra of the free-running discharge viewed end-on provide rotational temperature (T rot ), excitation temperature (T exc ), and electron number density (n e ) of 1600±1800 K, 3800±4300 K, and 3.0 to 12610 13 cm 23 , respectively. These fundamental characteristics are compared to those reported previously for crystal-controlled He ICPs and Ar ICPs. 1 Introduction This study focuses on the ®rst application of a free-running generator to form new types of helium inductively coupled plasmas (He ICPs) and on the investigation of fundamental and physical characteristics of these discharges. From a fundamental standpoint, temperature 1±15 and electron number density 16±24 constitute two of the most important properties of ICP discharges. These properties are commonly used to compare analytical performance indices of plasma- based techniques and to understand vaporization, atomization, excitation, and ionization processes in various discharges. The distribution of energy among the various energy states of the atoms and molecules de®nes the temperatures of the dis- charge. 1,4 Electrons play a prominent role in sustaining and in transporting energy throughout the discharge. Their density gives an indication of the degree of ionization in the plasma and is often related to the ionizing power of the plasma. Additionally, the appearance, physical characteristics and stability of the plasma can affect analytical performance indices of the instrument. Plasma ¯uctuations may be identi®ed in numerous ways, from measurement of noise power spectra to high-speed photography. 25±33 In our previous reports, 5,6,10,13,16,17,23,25,34±44 we examined analytical and fundamental characteristics of atmospheric- pressure helium ICPs formed with crystal-controlled genera- tors. Helium is attractive as a plasma gas because it possesses a higher thermal conductivity compared to argon, thus improv- ing desolvation, vaporization, and atomization ef®ciencies. Helium plasmas also offer higher ionization ef®ciencies for hard-to-ionize elements than argon discharges. 34±47 Addition- ally, helium is monoisotopic and occurs at a low mass, thus reducing mass spectral interferences that currently prevent the use of argon plasmas in certain applications. 34±44,48±51 The limitations of helium as a plasma gas include dif®culty in plasma formation and containment, relatively low gas temperature and electron number density, 6,16,17,23,34,35,37,42,43 and interface-linked discharges in mass spectrometry (MS). 34±36,39,41±43,51 The occurrence of the secondary discharge in ICP-MS depends on the type of rf generator used and on the way of powering the coil. 52 Considering that all prior studies on the He ICP have been conducted with crystal-controlled generators, used in conjunction with an impedance matching network, it is prudent to investigate whether some of these drawbacks can be eliminated when a free-running He ICP is used. The free-running generator has been useful, along with an electronically-balanced load coil, in reducing/eliminating the secondary discharge in argon ICP-MS, 52,53 but its utility in He ICP-MS is untested. The purpose of this research is thus to form the free-running helium ICP and to examine its fundamental characteristics in terms of plasma appearance, rotational temperature (T rot ), excitation temperature (T exc ), and electron number density (n e ). 2 Experimental 2.1 Helium ICP torch The He ICP system, imaging optics, spectrometer, and detection systems used in this study are listed and described in Table 1. For all studies, a 14.5-turn load coil is used to produce the He ICP. The diagram of the demountable, low- gas-¯ow torch used to generate the He ICP is presented elsewhere. 34,44 This tangential-¯ow torch consists of three components: a 13 mm id plasma con®nement tube, a Macor insert, and a Macor base. The plasma is formed within the con®nement tube of the He ICP torch. The outer surface of the insert contains four threads and provides a channel to direct the plasma gas to the torch. 34,44 The insert, centered within the Macor base, also contains a central passageway, which directs sample aerosol into the plasma through a 0.5 mm ori®ce. {Present address: Chancellor College, Chemistry Department, P.O. Box 280, Zomba, Malawi. {Present address: Department of Chemistry, College of Natural Sciences, Mokpo National University, 61, Dolim-Ri, Chungkye- myon, Muan-gun, Chonnam, 534-729, South Korea. §Present address: Millipore Co., 80 Ashby Road, Bedford, MA 01730, USA. DOI: 10.1039/a909063k J. Anal. At. Spectrom., 2000, 15, 491±498 491 This journal is # The Royal Society of Chemistry 2000