Thesis (PhD)--University of South Australia, 1996

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

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An atmospheric pressure radio frequency capacitively coupled plasma (CCP) has been developed and characterized for applications in atomic emission spectrometry (AES), atomic absorption spectrometry (AAS) and gas chromatography (GC). The CCP torch was initially designed as an atom reservoir for carrying out elemental analysis using atomic absorption. Functionally, the device consists of two parts, the CCP discharge tube and the tantalum strip electrothermal vaporization sample introduction system. The torch design provides for very effective energy transfer from the power supply to the plasma by capacitive coupling. Therefore, the plasma can be generated at atmospheric pressure with a flexible geometry. The plasma can be operated at very low rf input powers (30-600 W) enabling optimal conditions for atom resonance line absorption measurements. Absorption by the analyte takes place within the plasma discharge which is characterized by a long path length (20 cm) and low support gas flow rate (0.2 L/Min). Both of these characteristics ensure a relatively long residence time. The device exhibits linear calibration plots and provides sensitivities in the range of 3.5-40 pg. A preliminary measurement gave a Fe I excitation temperature of approximately 4000 K for the discharge. At this temperature, potential chemical interferences are likely to be minimal. Chemical interferences for Fe, Al, As, Ca, Co, Cd, Li, Mo and Sr were negligible in the determination of silver. Chloride interference, which is prevalent in GF-AAS, was not found. The amount of Ag found in a SMR#1643b (NIST) water sample was 9.5 ± 0.5 ng/g which fell in the certified range of 9.8 ± 0.8 ng/g. Spikes of 30 ng/g and 60 ng/g of silver were added to the SRM and recoveries were found to be in a range from 105 % to 96.2 %. The RSD obtained for 7 replicates of 270 pg silver was 4.6 %. The results for the CCP AES are even more promising. The interferences of thirteen elements are negligible in the determination of silver. The chloride interference was not found. The detection limits for Ag, Cd, Li, Sb and B are 0.7, 0.7, 2, 80 and 400 pg respectively. The amount of silver found in a SRM#1643b (NIST) water sample was 9.3 ± 0.5 ng/g which also fell in the certified range of 9.8 ±0.8 ng/g. Spikes of 30 ng/g and 60 ng/g of silver were added into the SRM#1643b (NIST) samples; the recoveries were found to range from 97 % to 104 %. The RSD obtained for 7 analyses of 270 pg silver were 1.5 % for CCP-AES. It was also found that the signal to noise ratios (S/N) are higher in the AES mode than those in the AAS mode in the same CCP atomizer. In order to exploit advantages inherent in both GF-AAS and I CP-AES, an atmospheric pressure capacitively coupled plasma sustained inside a graphite furnace was developed. This source combines the high efficiency of atomization in furnaces and the high efficiency of the excitation in atmospheric pressure plasmas. In general, plasma sources are able to effectively excite high-lying excited states for most metals and non-metals and can also ionize vaporized elements. Therefore the possibility exists of using non-resonance lines to avoid the effects of self-absorption at high analyte concentrations. Ion lines may also be used in cases where they provide better sensitivity or freedom from spectral interferences. This source also offers the ability to independently optimize vaporization and excitation. However, the most important aspect of this new source is that it can be used for simultaneous, multielement determinations of small sized samples in a graphite furnace atomizer, a design which has been proven to be effective over many years of use. Preliminary quantitative characteristics of this new atmospheric pressure plasma emission source have been studied. The detection limit for Ag of 0.3 pg is lower than the value of 0.4 pg reported for GF-AAS. Variants of the CCP, including a gas chromatography (GC) detector, combinations of laser ablation - CCP, rf sputtering - CCP direct solid analysis, and its application as an intense spectral lamp have been developed and are reported in this dissertation. / Science, Faculty of / Chemistry, Department of / Graduate

It has been said that inductively coupled plasma (ICP) is the panacea for the determination of metals in environmental samples. The ease of sample introduction for liquids coupled with excellent limits of detection of this spectrometric method provide the analyst with the ability to perform rapid, multi-element determination of most elements in the periodic chart. However, when samples have to be introduced in solid form such as suspensions of finely powdered material (slurries), in order to avoid lengthy extraction procedures and the use of strong acids, the method must be modified so that it can handle solid, often refractory material. Due to large size of the particles (≥ 10 μm), two problems are encountered: poor sample introduction efficiency in the conventional, concentric nebulizer; and poor vaporization efficiency at the argon plasma. The nebulizer tends to clog and a large fraction of particles is lost in the spray chamber due to their weight. The conventional argon plasma is not energetic enough to vaporize the analyte. In this project, a clog free Babington nebulizer was used. A surfactant/thickening agent, polyethylene oxide (PEO), was added to alter such physical properties of the slurry as surface tension, viscosity, and aerosol droplet size. Mixed gas plasma containing small amounts of nitrogen were used. Results showed that, by adding about 5 ppm of PEO, the emission intensity of an analyte increased significantly. Further experiments demonstrated that the signal enhancement resulted from an increase in the nebulizer efficiency brought about by a slight increase in viscosity of the slurry. The use of mixed gas plasma (Ar + 4% N₂) further improved the emission intensity. Temperature diagnostic measurements of such plasmas indicated that rotational and excitation temperatures are higher than those in a pure argon plasma. The improved temperature is believed to result from the higher thermal conductivity of molecular gases. Nitrogen added to the cooling gas works better than when added to the injector gas. Hydrogen does not seem to work as well as nitrogen, probably because its thermal conductivity is 14 times less than nitrogen. Further studies of the excitation temperature using Fe as the thermometric species, have been helpful in elucidating the mechanism of slurry vaporization in the plasma. There is evidence in this study that mass-transfer rather than heat-transfer is the limiting factor. With these improvements in the sample introduction and atomization cell, slurries having particle diameters up to 7 μm have been successfully analyzed. This value is 3 times larger than particles injected into pure Ar-plasma without a surfactant. The percent recovery of Ca, Fe, Mg and Pb, are comparable to that obtained from the same samples analyzed as solutions following acid digestion. / Ph. D.

A unique spectrometer system, the Multi-Mode Spectrometer (MMS), has been developed. The MMS integrates a scanning Michelson interferometer, a flat-field grating, and a linear photodiode array detector into a single spectrometer system. With these components, the MMS is capable of applying dispersive, interferometric, or combined dispersive/interferometric techniques for enhanced spectrometric flexibility. The effects of source fluctuation and redistributed photon noise can be reduced. In addition, the MMS has unique capabilities in data compression, application of internal standards, and noise spectrum analysis. / Ph. D.

The direct current plasma is an excitation cell that should be well suited to rapid analysis of coal slurries by virtue of its tolerance for various sample matrices. Problems which are encountered in coal analysis by emission spectrometry include incomplete atomization of analyte by formation of metal oxides, lack of adequate methodology for sulfur analysis, and ineffective sample transport for coarse coal slurries. Atomization of metal oxides can be improved by addition of small amounts of propane (Ca 45 mL/min) to the nebulizing argon of the direct current plasma. However, the improved atomization is manifested above the normal viewing zone, and the enhancement effect of propane on analytical signals is offset by severe depression of emission signals caused by temperature reduction in the lower regions of the plasma. Sulfur in coal can be determined by direct current plasma emission spectrometry. Emission lines accessible to the echelle grating of the DCP are not suitable for such analysis, so that the deep-UV lines from 180-183 nm must be utilized for such work. A relatively simple purge system with low argon consumption (5 L/min) is adequate for sulfur analysis, and the beforementioned analytical lines provide detection limits that are adequate for sulfur determination in 1% slurries of coal. However, transport of the coal sample to the plasma is incomplete when compared to that of aqueous solutions, precluding the use of such solutions as calibration standards. Transport of the coal can be improved by increasing the viscosity of sample and standards, which increases the droplet size from the nebulizer and hence the particle size transportable• The increased droplet size causes a decrease in sensitivity due to reduced desolvation/vaporization, but does permit the use of aqueous solutions and as calibration standards for determination of sulfur, iron, aluminum, silicon in coal. / Ph. D.

Very early in the history of atomic emission spectroscopy (AES) it was understood to be a powerful analytical tool. Until the 1930's the usefulness of atomic spectroscopy was not utilized very extensively even though its fundamental power was understood. The breakthrough that placed it in the standard chemistry laboratory was the discovery and implementation of the photoelectric effect. Since this discovery there has been a revolution in atomic spectroscopy which has brought it from the role of a humble servant used for primary elemental screening to an outstanding leader in applications of elemental analysis. Atomic emission spectroscopy of complex samples has long suffered from matrix effects which result in overlapping of spectral lines, fluctuating backgrounds and changing conditions in the source. Investigations employing an echelle polychromator with a two dimensional solid state array detector show great promise in minimizing the effects of these interferences on multielement analyses of complex samples. The Charge Injection Device (CID) detector used exhibits many characteristics which make it uniquely qualified for simultaneous, multielement detection in AES. With only slight modifications to the optics of a commercial spectrometer and the employment of a CID detector, detection limits for a number of elements are quite favorable. Dynamic ranges of over seven orders of magnitude are obtainable with this experimental system. The reduction of matrix effects by utilizing the huge wealth of information available from over 60,000 individual detector elements are demonstrated through results from several complex matrix standards. This CID-polychromator system was also employed for the element selective detection of gas chromatographic (GC) effluents. A microwave-induced plasma (MIP) based on the Surfatron design was built. A helium plasma from this device has shown to have resilience to organic samples and give good emission response to non-metallic atoms. A number of studies with this GC-AES-polychromator system are presented. This system is capable of monitoring atomic emissions from C, H, F, Cl, Br, I, O, N and S all simultaneously, and the selectivity of this system is unsurpassed. Elemental ratios for separated compounds are also presented as a precursor to empirical formula prediction.