INFLUENCE OF GRAPHITE SUBSTRATE ON ANALYTICAL SIGNALS IN ELECTOTHERMAL VAPORIZATION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

This paper describes a series of electrothermal vaporization inductive ly coupled plasma mass spectrometry (ETV-ICP-MS) experiments for vapor ization 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 p yrocoated and oxygenated pyrocoated graphite substrates. During these experiments several species originating from the graphite surface (C-1 3(+), CO+ and C-2(+)) were monitored. When samples were vaporized from a pyrocoated graphite surface, precisions of about 20% for Ar-2(+), 2 00% for C-13(+) and 60% for CO+ were observed in their integrated ion intensities. For vaporization from an oxygenated pyrocoated graphite s urface, the variation in integrated ion intensities was significantly reduced; less than 5% for Ar-2(+), less than 20% for C-13(+) and 40% f or CO+. The results obtained from these experiments indicate that ETV- ICP-MS can be used effectively for trace and ultratrace quantitative a nalysis. For best analytical results with ETV-ICP-MS, it is prudent th at a new oxygenated graphite surface be used along with matrix modifie rs and the method additions technique. Surface origin of the C-2(+) io n was also investigated. The results indicate that the surface species responsible for the C-2(+) ion are not the major analyte carrier spec ies from the ETV to the ICP-MS. This was substantiated by the: observa tion that the C-2(+) signal appears as a well behaved peak, regardless of the surface condition or the analyte used. There is evidence to su ggest that the C-2(+) comes from an aldehyde-type molecular surface sp ecies. The C-2(+) signal is only observed when metals were present in acidic sample solutions or when the graphite surface was oxygen treate d. Metal catalyzed surface oxidation is one possible explanation for t he 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 b y Elsevier Science B.V. All rights reserved.