Electrothermal Vaporization Atomic Spectrometry/Mass Spectrometry For Trace Analysis

Inductively coupled plasma atomic emission spectrometry / mass spectrometry (ICP-AES/MS) has several advantages for trace element determination: low detection limits, wide linear dynamic range, multi-element detection capability, and high sample throughput. Therefore, it has a widespread application in an extensive field such as life science, food science, clinical chemistry, material technology, environmental science, geological chemistry, metallurgic chemistry, and nuclear technology. And it is worthy of note that the appearance of ICP-MS is a significant breakthrough in atomic spectrometry.Electrothermal vaporization (ETV), which converts both the solution and the solid samples to the vapor state, is considered to be a useful technique of sample introduction for ICP-AES / MS over conventional pneumatic nebulization. The advantages of ETV can be described as follows: (1) high efficiency of sample introduction ( 80%), which results in low detection limit; (2) separation processes of vaporization and excitation / ionization of analyte, which is beneficial to the optimization of experimental parameters and alleviation of interferences caused by matrix; (3) small size of sample requirement, which could be applied to the analysis of samples at micro-level; (4) capability of direct solid analysis, which reduces sample pretreatment and gives low risk of contamination and analyte loss; (5) and the sensitivity and selectivity of the method can be promoted further with the usage of chemical modifier.The objective of this dissertation is to study the vaporization behaviors of refractory / carbide forming elements in a graphite furnace with / without the use of polytetrafluoroethylene (PTFE) emulsion as chemical modifier; to explore the possibility of in-situ elimination of spectral interferences and matrix effect in the graphite furnace; and then to develop a new method of fluorination assisted electrothermal vaporization (FETV)-ICP-MS for the direct analysis of micro-amount of refractory solid powder and biological samples. For the analysis of ultra-trace elements and elemental species, some new separation / preconcentration techniques was developed and combined with ETV-ICP-MS. The main interests of this dissertation are summarized as follows:(1) A method of slurry sampling FETV-ICP-MS was developed to directly determine the trace impurities in niobium pentaoxide solid powder in which a PTFE emulsion was used as fluorinating modifier to promote the vaporization of refractory elements from a graphite furnace and to avoid the formation of thermal stable carbides. For one analysis run, only 0.002 mg of Nb₂O₅ sample was required. The electrothermal vaporizer device was connected to the ICP system by a laboratory-built interface, and the flow rates of sample carrier gas and auxiliary carrier gas were optimized experimentally. Thepotentially polyatomic interferences resulting from the pyrolysates of PTFE were evaluated. A new method by stepwise diluting the sample matrix is proposed for investigating the matrix effect and the results showed that no obvious matrix effect was observed with the sample matrix (Nb2O5) concentration of less than 200 mg I/1. The proposed method has been applied to determine the trace impurities in M^Os powder, and both the results obtained from the external calibration curves and the standard addition curves were in a good agreement with those obtained by conventional nebulization ICP-MS.(2) A method of in situ removal of matrix was proposed for direct determination of the trace refractory elements in human serum by ETV-ICP-MS with the use of PTFE as fluorinating reagent. Attention has been paid to investigating the vaporization behaviors of both refractory elements of interest and matrix elements (Na, K, Ca, Mg, Cl, S, P) in a graphite furnace with the PTFE modifier presence or not. It was shown that the potential interferences of the organic and inorganic matrices in serum sample could be eliminated or reduced to a neglectable level with properly diluting the human serum and deliberately optimizing the ETV temperature program. The proposed method has been applied to the direct simultaneous determination of V, Cr, Mo, Ba, La, Ce and W in human serum.(3) Anew method for the determination of La3+ and La complexes in solution using a nanometer-sized titanium dioxide as solid-phase extractant and fiuorination assisted electrothermal vaporization (FETV)-ICP-AES as sensitive detector has been developed. The effect of pH on the adsorption characteristics of nanometer TiC>2 for La3+ and La-complexes of citric acid, 2-hydroxyisobutyric acid, and humic acid has been investigated and optimized. Based on the difference in volatility between fluorides of analyte (lanthanum) and matrix (titanium), an in-situ removal of the adsorbent matrix (TiCb) from a graphite furnace was realized. Therefore, the La3+ in solution and La-complexes absorbed on nanometer-sized titanium dioxide could be determined respectively by FETV-ICP-AES without any chemical pretreatment. The proposed method has been applied for the determination of free La3+ and La-complexes in synthetic solutions and soil extracts with satisfactory results.(4) Anew method is described for single-drop microextraction of trace rare earth elements La, Y and Yb with l-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) / benzene solution as organic phase. ETV-ICP-MS was used to determine the analyte in post-extraction organic drop. Several parameters, which influence the microextraction behavior of analyte, such as sample flow rate, microdroplet volume, solvent and extraction time, were investigated and the optimized experimental conditions were established, hi order to explore the process of microextraction, the conventional pneumatic nebulization ICP-MS was used for on-line monitoring the analyte concentration change in the aqueous phase. Based on the obtained experimental results, a dynamic model for continuous-flowmicroextraction of inorganic elements has been supposed. Under the selected experimental conditions, the preconcentration factors with a 25 min cycle-flow microextraction of a 1 mL sample were 233, 242 and 244 for La, Y and Yb, respectively. The proposed method was applied to the analysis of biological materials (GBW07605 Tea leaves and GBW07601 Human hair), and the results obtained were in good agreement with the certified values.(5) A procedure for copper, zinc and manganese determination in food samples by slurry sampling graphite furnace atomic absorption spectrometry (GFAAS) is proposed. The analysis results of food reference materials (NIST SRM1567, 1568) agreed with the reference value and the spiked recoveries of real samples were in the range of 95% -106%.(6) The mechanism of vaporization / atomization of Cr, Ni, Zr, Nb and Yb has been studied using ETV-ICP-MS and GFAAS. The behavior of vaporization / atomization of Cr, Ni, Zr, Nb and Yb in a graphite furnace with and without chemical modifiers (Pd, Mg, PTFE) were compared. The effect of some operation parameters such as atomization temperature and ICP power on the signal intensities and the signal profiles of Cr, Ni, Zr, Nb and Yb in ETV-ICP-MS were investigated. It was found that: (1) Pd and Mg have no obvious modification on the behavior of vaporization and atomization of Cr, Ni, Zr, Nb and Yb in the graphite furnace. (2) In the presence of PTFE, the behavior of vaporization of Cr, Ni, Zr, Nb and Yb were remarkably changed, the formation of refractory carbides were successfully circumvented, and the vaporization temperatures were decreased significantly; the signal intensities of Cr and Ni in GFAAS were enhanced in some extent, however, those of Yb, Nb and Zr were restrained greatly. It was attributed to the high bond dissociation energies of the fluorides of Yb, Nb and Zr, which caused ineffective dissociation / atomization of analytes in the graphite furnace.