Atomic Nitrogen Spectra in the Argon Inductively Coupled Plasma (ICP)

Nonresonant, low-energy (visible-near infrared) atomic nitrogen transitions from high-energy (11 to 14.4 eV) doublet and quartet excited N(I) states are observed in the 1.5 to 2.2 kW argon inductively coupled plasma (ICP) when gaseous compounds containing this element (N) are introduced as samples. A Table of ICP excited nitrogen (I) lines and relative emission intensities from 2225 to 10 950 Å is presented. A number of new nitrogen lines are observed for which transitions have not been assigned. The ICP nonresonance emission spectrum of atomic nitrogen [N(I)] is found to be especially good in the near infrared region. The use of near infrared nonresonant N(I) lines for the analytical detection of nitrogen is reported. The relative intensity of N(I) emission (derived from N₂ samples) is at a maximum between the turns of the rf load coil. An “equal intensity” contour “map” of N(I) emission in the ICP is presented using N₂ as the sample. The present limit of nitrogen detection in argon for nonoptimized conditions is 1.0 ppm (v/v) using continuous sample introduction (limited by photomultiplier noise). For gas sampling loop injections, the present detection limit is 0.3 μg (limited by atmospheric leakage or permeation contamination into the sampling loop system). Considerable improvement is expected in the future. The relative contributions to the baseline signal of: (1) atmospheric nitrogen entrainment in the argon plasma, (2) atmospheric permeation into the present Teflon gas sampling loop system, and (3) argon tank contaminants are evaluated. Suggestions for further improvement are given. The response is linear, and the short-term precision for repetitive introduction of samples containing 20 μg of N₂ is 0.4% RSD using a 157 μl Teflon gas sampling loop.