Effect of torch size on a 148-MHz inductively coupled plasma

Physical parameters and analytical performance are presented for three analytical ICPs operated at 148 MHz. The torch size has been varied in these three systems in order to investigate more closely the influence of the ratio of plasma radius (r) to skin depth (s) on the plasma characteristics. The electron number density appears to be directly related to the r/s ratio, while the excitation temperatures and ion to atom intensity ratios only follow a general trend. A10 mm i.d. torch at 148 MHz provides conditions most similar to a standard 18 mm i.d. torch at 27 MHz. An intermediate-size torch of 13 mm i.d. provides a good balance between the increased ease of sample handling of a large r/s ratio torch and the improved sensitivity of a small r/s ratio system. The r/s ratio is shown to be a convenient means of understanding the effects of changes in the plasma operating frequency and in the torch size.