Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry—III. The lanthanum modifier and the determination of phosphorus
Morphological studies on graphite surfaces by scanning electron microscopy are presented for platforms made from pyrolytic graphite, and for polycrystalline electrographite tubes with pyrolytic graphite coating in which phosphorus was determined without and with the addition of higher concentrations of lanthanum as the modifier. Lanthanum causes severe pitting and corrosion of the graphite surface already after relatively few determinations, and definite indication was found for the formation of intercalation compounds between lanthanum and graphite. No sign was found, however, for the formation of a dense coating of lanthanum carbide as proposed by several authors. The mechanism for the increase of phosphorus sensitivity is most probably the formation of a thermally stable compound involving lanthanum and phosphorus which leads to vaporization of phosphorus at high enough temperatures to obtain sufficient atomization and useful analytical signals. This is supported by the morphological changes of the graphite surface observed after application of higher lanthanum concentrations, and the resulting increased number of active carbon sites. Phosphorus alone also causes substantial corrosion of graphite, but with a completely different pattern. A very pronounced secondary coating of tube and platform wall is observed in the absence of lanthanum which is most probably supported by the formation and decomposition of compounds between phosphorus and graphite.