and thereby produce a nonlinear interference. Also, because iodide is a much more effective quencher than other ions, the interference effect might be negligible a t high Iconcentrations, especially if interferent concentrations are below the 1.0 M level used in this study.Fluorophore molecules immobilized on the surface of Teflon tape produced better figures of merit than immobilization on resin even though the resin-bound rhodamine 6G has a fluorescence lifetime which is twice as long. Thus, accessibility has proven to be a more important factor in optrode performance than the fluorescence lifetime of an immobilized fluorophore. Resin immobilization does provide an effective solid support, but for optimal optrode performance, surface adsorption or chemical bonding directly to the fiber's surface should be the method of choice.Vacuum ultraviolet (vacuum UV) radiatlon can be readily observed through a sampling orlflce Inserted dlrectly Into an Ar lnducttvely coupled plasma. Limits of detectlon are 50 ng mL-' for the resonance ilnes of Br and Ci during InJectlon Into the plasma of aerosols of aqueous solutions generated by pneumatlc nebulkatlon. With ultrasonic nebulization, the detection limits Improve to 8 ng mL-' for Br and 15 ng mL-' for CI. Analytlcal callbratlon curves are llnear over a range of 3.5-4 orders of magnltude In concentratlon. Each of the group I I I B (group 13 in 1985 notation) elements (AI, Ga, and In) possesses at least one intense line in the vacuum UV region that yields better lknlts of detectlon than the conventional ultraviolet llnes typically observed. Emission signals from Br, CI, and AI are not strongly affected by high concentratlons of Na. The Interference induced by Ca on AI emission (and vice versa) In the vacuum UV region Is sknllar under this mode of observation to that descrlbed In the ilterature for observation of the normal UV llnes elther axially or conventionally (Le., from the slde). l Present address: Dow Chemical Co., Midland, MI 48640.In a recent communication we described the direct optical viewing of an argon inductively coupled plasma (ICP) through a sampling orifice inserted directly into the plasma and the utility of this approach for plasma diagnostic studies (1). Because this optical viewing system does not use any lenses, mirrors, or windows, vacuum ultraviolet (vacuum UV) radiation can be detected down to 78.7 nm, the wavelength corresponding to the ionization energy of Ar. In the present paper, we evaluate the analytical figures of merit of the direct optical viewing approach.

Optical spectra are observed through a sampling orifice inserted directly into an inductively coupled plasma. The optical system does not use lenses, mirrors, or windows so that vacuum ultraviolet wavelengths as low as the ionization energy of Ar (78.7 nm) can be detected. These lines are readily observed with lateral spatial resolution. The intensity of Ar I 106.67 nm is studied for various plasma operating parameters and observation positions. The number density for the upper state for this line (3P1) is estimated to be of the order of 1011 cm−3 in the axial channel, which is similar in magnitude to the populations of the nearby metastable Ar levels measured previously. The Ar I line at 104.82 nm is self-reversed, indicating that Ar resonant radiation can definitely be trapped in the inductively coupled plasma. The results support the radiation trapping hypothesis but do not quantitatively determine the magnitude of energy transfer from the induction region to the axial channel by this mechanism.

A supplementary electrical discharge is generated by extraction of the axial channel of an Ar inductively coupled plasma (ICP) into a small vacuum chamber. The spectral background levels and background noise emitted by this discharge are similar to those from the ICP alone. The discharge enhances the intensities of ion lines by factors of up to 13 relative to intensities observed from the ICP alone. Neutral atom lines from elements with high ionization energies (≤9 eV) are also enhanced but by less than ion lines; neutral atom lines from easily ionized elements are suppressed by the discharge. Metal oxides in the ICP can be seen to dissociate into atomic species as they enter the discharge. These effects are attributed to more efficient atomization, excitation, and ionization in the discharge and to the tendency of analyte species to be constricted or concentrated closer to the central axis of the ICP as they flow into the discharge. Under the same operating conditions a 40-MHz plasma generates a more intense discharge and yields higher intensity enhancements than does a 27-MHz plasma.