Technical Abstract: Quantification of both digested and slurry samples were studied using ultrasonic slurry electrothermal vaporization inductively coupled plasma mass spectrometry (USS-ETV-ICP-MS). The results of external calibration using aqueous standards, method of additions, and In as an internal standard were compared. The elements studied include: Mn, Ni, and Cu. Pd was used as a physical carrier and oxygen ashing was used. Different degrees of matrix suppression effects were observed due to changes in para- meters such as skimmer-sampler separation, skimmer/sampler orifice align- ment, and separation between the skimmer sampling orifice and the first ion lens. Aging of the skimmer cone and loss of its circular symmetry results in differences in sampling of the ion beam and the degree of matrix effects observed. The presence of matrix suppression effects is evidenced by strong suppressions in the Ar2, C, and analyte signals. When matrix suppression effects were present external calibration provided low recoveries, there- fore it was necessary to use the method of additions to compensate provid- ing an average accuracy of 100+/-13%. When matrix effects were absent external calibration provided an average accuracy of 101+/-16%.