Experimental
2.1 instrumentation
all the measurements were performed with a perkin-elmer(norwalk,ct,usa)model optima 4300 DV inductively coupled plasma optical emission spectrometer. a scott type double-pass spray chamber and a cross-flow nebulozer were used throughout. the determination of element concentrations was optimized using different default parameters of the instrument (nebulizer flow 0.5-11 L,auxiliary gas flow 0.2L,plasma gas flow 15L and plasma power of 1300 or 1400 W). Two wavelengths for each of the elements investigated were tested by both axially and radially viewed plasmas. The appropriate wavelengths with the correct plasma viewing used in the final determination are shown in table 1.