Jon W. Carnahan

Acousto-optic tunable filters: fundamentals and applications as applied to chemical analysis techniques

Abstract Acousto-optic tunable filters (AOTFs) are solid state wavelength tunable optical filters. Modern AOTFs are constructed by attaching piezoelectric transducers to an appropriate crystalline material. By driving the transducers at the appropriate frequency, a series of perturbations traverse the material. Interactions of photons with these perturbations allow the AOTF to selectively diffract a single narrow-bandpass wavelength. The wavelength may be varied by changing the applied frequency. The focus of this review is to detail chemical analysis applications of AOTFs. A brief historical background of AOTF development is presented. Mathematical and physical descriptions of AOTF behavior are summarized. Currently used materials and characteristics of these materials are described. AOTFs are compared with other wavelength selection devices including: monochromators, absorption filters, transmission filters and liquid crystal tunable filters. Atomic, molecular, fluorescence and Raman spectrometry applications, as well as the use of AOTFs, for remote and proximal imaging applications are highlighted. Lastly, the combination of AOTF and fiber optic technology to make a new device known as an integrated AOTF is considered.

A microwave induced plasma system for the maintenance of moderate power plasmas of helium, argon, nitrogen and air

Abstract A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM 010 cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diffuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath of coolant air. Details of the design and preliminary characterization of each plasma system is presented.

Trace Determination of Cd, Cu, Br, and Cl with Electrothermal Vaporization into a Helium Microwave-Induced Plasma

An electrothermal vaporization (ETV) system was used to introduce aqueous samples into a helium microwave-induced plasma (He-MIP). The plasma was operated at 2.45 GHz and 500 W. The sample introduction system consisted of a carbon-cup-type electrothermal vaporizer in a glass chamber. A vapor restriction device was used to enhance the efficiency of sample transport to the plasma. For obtaining the best detection limits, matrix modifiers were used for the nonmetals. Lead nitrate as a matrix modifier was found to be particularly efficacious for the determination of bromide and chloride. Detection limits with the ETV-He-MIP with 15-ML samples were: Cd, 10 pg (0.6 ppb) at 228.8 nm; Cu, 30 pg (2 ppb) at 327.4 nm; Br, 300 pg (20 ppb) at 470.5 nm; and Cl, 120 pg (8 ppb) at 479.5 nm. Although the detection limits for the two metals studied are greater than those that have been obtained by ETV-ICP-AES, the detection limits for Br and Cl are the best that have been obtained by an ETV-AES technique to date in the UV-VIS spectral region. Linear responses of analytes were obtained for each element. Matrix effects of selected metals and nonmetals on chloride were investigated and found to be insignificant.

Design Considerations and Preliminary Characterizations of a Kilowatt-Plus Microwave-Induced Plasma

A high-powered helium microwave-induced plasma was developed, and preliminary characterizations are presented. This kilowatt-plus microwave-induced plasma (KiP-MIP) was contained in a helical flow quartz torch. Power was transferred via waveguides from a 3.0-kW, 2.45-GHz, 120-pulses-per-second microwave generator to a waveguide-to-coaxial converter. A modified Beenakker TM010 resonator cavity was used to focus the energy. Effects of the magnitude of applied power on helium line emission intensities were studied, as well as comparisons of the temporal output of generator output and helium line emission. Introduction of gaseous halogens (Cl2, CBr4) produced intense ion emission which follows the intensity ratios commonly observed in He-MIPs.

Characterization of an Acousto-Optic Tunable Filter and Use in Visible Spectrophotometry

The use of a paratellurite acousto-optic tunable filter (AOTF) is demonstrated as a monochromator for spectrophotometry. This device operates in the range of 400 to 700 nm with a resolution from 2.5 nm (at 633 nm) to 0.5 nm (at 400 nm). Results of the initial characterizations indicate that behavior follows the theoretical model in terms of frequency/wavelength relationships, dependence of the output wavelength upon the incident radiation angle, and bandpass as a function of wavelength. Initial results of spectroscopic characterizations of permanganate, cobalt(II), holmium(III), praseodymium(III), and neodymium(III) solutions are presented. Placing a polarizer in the light path reduced the amount of stray light reaching the detector. Two types of photomultiplier tubes were used as detectors. The sensitivity of the PMTs varied at different wavelengths, which affected spectral responses and the effects of scattered light.

Characteristics of a Kilowatt-Plus Helium Microwave-Induced Plasma Utilizing 2- and 3-cm-Depth TM010 Resonator Cavities

A kilowatt-plus microwave-induced plasma (KiP-MIP) system was modified for better analytical performance. Modifications included the use of a redesigned plasma torch and a new plasma resonator cavity. The use of the redesigned plasma torch improved the detection of aqueous chloride by 2 orders of magnitude. With the redesigned plasma torch, the performance of the KiP-MIP utilizing the original 3-cm-depth and 2-cm-depth resonator cavities were studied. Results indicated that the KiP-MIP with the 2-cm-depth resonator cavity exhibited better characteristics in terms of plasma energy coupling, excitation characteristics, and determination of aqueous chloride.

Analytical Figures of Merit and Interelement Effects with Air and Nitrogen Microwave-Induced Plasmas

A microwave (2450 MHz)-induced plasma (MIP) with nitrogen nebulization and plasma gases was developed and compared with an air-MIP similar to that previously reported in our laboratories. These plasmas were maintained at 300 to 500 W of forward power and were stable with pneumatic nebulized aqueous solution introduction. In general, metals with high metal-oxygen bond strengths, high excitation energies, and low ionization potentials exhibited superior detection limits in the N2-MIP. Several elements with emission lines in N2 and N2+ spectral regions yielded better detection limits in the air plasma. The effects of high concentrations (2%) of calcium, potassium, sodium, and phosphate ions upon the analytical signal intensities were examined. During direct solution nebulization, the analytical signals of both plasmas exhibited significant but predictable interferences from metals of low ionization potential and plasma loading. Matrix effects were more significant in the N2-MIP.

Simulated supercritical fluid chromatography mobile phase introduction to a helium microwave-induced plasma : effects on nonmetal emission and spectral behavior

The effects of the introduction of capillary supercritical fluid chromatography mobile phases, CO2 and N2O, into a helium microwave-induced plasma (He-MIP) are investigated. The plasma and mobile-phase flow rates are maintained at conditions comparable to those typically used for these systems. Spectral characteristics are monitored in both the ultraviolet-visible and near-infrared regions. Molecular bands from these “foreign” gases are assigned and their relative intensities are noted as a function of the doping gas flow rate. Possible molecular band interferences are noted for nonmetal atomic emission determinations with He-MIPs. Nonmetal atomic emission reduction by these gases is studied with the use of chlorine atom and ion lines as test signals. Decreases in chlorine atom and ion emission are seen as CO2 and N2O flow rates are increased. It is found that ion line emission intensity is reduced more significantly than atom line emission. Reduction of both atom and ion line emission is more pronounced with CO2 than with N2O. The electron density of the plasma does not change as the CO2 flow rate is increased from 0 to 0.25 mL (STP)/min.

Adirectly Coupled Microwave-Induced Plasma Atomic Emission Liquid Chromatograpy Detector for Nonmetals: Preliminary Characterization with Halides and Oxohalogen Salts

Abstract A liquid chromatography system is directly coupled to a moderate power helium microwave induced plasma for the selective determination of no metals in aqueous solutions. The detector is a large volume helium microwave-induced plasma operated at 500 watts with a helium support gas flow of 21L/min. A sample set of halides and oxohalgen salts are separated by ion exchange chromatography and introduced into the plasma as mist generated with an ultrasonic nebulizer. Detection limits range from 1.5 to 6 μg. Calibration plots are presented. Selectivity is observed when monitoring the elemental signals of co-eluting peaks.

Helium-chlorine charge transfer: an explanation for chlorine ion emission behavior in helium discharges

Abstract Emission from chlorine ions in helium discharges is intense and cannot be explained on the basis of Boltzmann and Sana considerations alone. Presented is a study which reconciles observed ion emission based upon charge-transfer ionization-excitation of ground state chlorine atoms by helium ions. The 3 s 3 p 5 3 P 0 chlorine ion state populated by this reaction serves as a reservoir to populate nearby electronic states which are involved in the observed emission. This model explains why intense chlorine ion emission is observed in helium discharges, but not in those of argon.