New development in ICP spectrometry: low flow torch for axial viewing ICP-AES & studies in semiconductor analysis by ICP-MS

A low argon flow torch has been developed for axially viewed ICP-AES. Outer argon flow has been drastically reduced by cooling the torch wall with air passing through a cooling jacket added to the outside of a Fassel torch. The outer argon gas flow can be reduced to 7 L min"1 with no compromise in performance or torch lifetime. The plasma is viewed axially through a cooled cone interface centered on the axial channel. The low flow plasma does not reach out of the torch; but the cooled cone interface facilitates collection of photons from the axial plasma before exposure to air. For the particular spectrometer used, the plasma exhibits the same "robustness index" and interference effects from Na as the conventional, high-flow ICP supplied with the particular spectrometer used. Although detection limits (DL) for lines at ~ 200 nm are poorer by about a factor of two, those for lines at ~ 400 nm are actually much better than values typically seen for the same lines by axial viewing of a conventional, high-flow ICP. Extraordinary detection limits of the elements with high wavelength emission lines, e.g., Ca, Ba, and could be of great value in the semiconductor industry. Both quantitative and isotope ratio measurement of silicon in organic solvents are essential in semiconductor manufacturing process. Silicon measurement at m/z 28, 29 and 30 by ICP-MS is complicated by the spectral interference caused by organic molecular ions, nitrogen and nitrogen oxides. A method has been developed to measure silicon by reducing polyatomic interferences at m/z 28 to 30. Cryogenic desolvation of organic solvent from the wet droplets of a low flow nebulizer removes most of the solvent and makes silicon determination possible in the parts per million level by low resolution ICP-MS. Krypton and xenon have been added to the sample gas flow of plasma for further removal of polyatomic ions present in the plasma. A few ml/min of Kr or Xe greatly reduces polyatomic ion intensity in the m/z range of 28 to 30. Isotope ratios can be measured by medium resolution (R=4000) ICP-MS by direct nebulization. To avoid deposition on the interface cone, the flow of oxygen in the spray chamber is adjusted to titrate carbon in the plasma. Despite the sharply pointed peaks at R = 4000, ratio precision of ±0.1% RSD can be obtained using approximately 1 |ig of silicon. Detection limit of the analyte element by modern ICP-MS is generally limited by the purity of the blank. For detecting very trace contaminants in a high purity substance, like semiconductor materials, a clean blank is necessary. Electrochemically modulated liquid chromatography (EMLC) column has been adapted online with ICP-MS to clean up blanks to reduce background signals and thereby improve detection limits of analyte elements. Metal ions can be retained from the flow of blanks in the micrometer size glassy carbon particle packing with appropriate negative potential. A Ag/AgCl (satd. NaCl) reference electrode in dilute nitric acid supporting electrolyte was used for electrochemical cell combination. The supporting electrolyte is placed outside the porous stainless steel column with a Nafion ion exchange resin coating inside, so that supporting electrolyte does not contaminate the stream of blank flow through the column. Several metal ions have found to be retained with >99% extraction efficiency from the blank solution. Cleaning of the column can be done easily by stripping the deposited metal with a high positive potential.