Influence of graphite substrate on analytical signals in electothermal vaporization inductively coupled plasma mass spectrometry

This paper describes a series of electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) experiments for vaporization of Na, Mg, P, K, Ca, V, Mn, Ni, Cu, Ga, Mo, Pd, Ag, Cd, In, W, Au and Pb in single element and mixed element solutions; using both pyrocoated and oxygenated pyrocoated graphite substrates. During these experiments several species originating from the graphite surface (C-13(+), CO+ and C-2(+)) were monitored. When samples were vaporized from a pyrocoated graphite surface, precisions of about 20% for Ar-2(+), 200% for C-13(+) and 60% for CO+ were observed in their integrated ion intensities. For vaporization from an oxygenated pyrocoated graphite surface, the variation in integrated ion intensities was significantly reduced; less than 5% for Ar-2(+), less than 20% for C-13(+) and 40% for CO+. The results obtained from these experiments indicate that ETV-ICP-MS can be used effectively for trace and ultratrace quantitative analysis. For best analytical results with ETV-ICP-MS, it is prudent that a new oxygenated graphite surface be used along with matrix modifiers and the method additions technique. Surface origin of the C-2(+) ion was also investigated. The results indicate that the surface species responsible for the C-2(+) ion are not the major analyte carrier species from the ETV to the ICP-MS. This was substantiated by the: observation that the C-2(+) signal appears as a well behaved peak, regardless of the surface condition or the analyte used. There is evidence to suggest that the C-2(+) comes from an aldehyde-type molecular surface species. The C-2(+) signal is only observed when metals were present in acidic sample solutions or when the graphite surface was oxygen treated. Metal catalyzed surface oxidation is one possible explanation for the production mechanism of the species leading to the C-2(+) ion. The origin of C-13(+) and CO+ ions is also explained. (C) 1998 Published by Elsevier Science B.V. All rights reserved.