Description
In the spring of 2002, a new XRF instrument was installed, and a new sample preparation technique was established in the Geochemical Laboratory at San Diego State University (SDSU). Precisions and accuracy of data obtained from the new instrument and sample preparation technique are herein documented. Five replicates of a powder derived from one of sample collected from the C horizon of a weathering profile developed on the Cretaceous La Posta pluton were analyzed. The results of these replicates indicate that precisions of the sample preparation procedure, expressed as the 95% confidence interval about the mean, for each major element is as follows: SiO2 = ± 0.21, AhO3 = ± 0.05, TiO2 = ± 0.003, Fe2O3 = ± 0.01, CaO = ± 0.01, NaO = ± 0.01, K2O = ± 0.005, MgO = ± 0.009, MnO = ± 0.0004, P2O5 = ± 0.0007. The precision of the new instrument was determined by how well values were reproduced during a 10 day period. These results are as follows: SiO2 = ± 0.06, AhO3 = ± 0.02, TiO2 = ± 0.001, Fe2O3 = ± 0.004, CaO = ± 0.005, NaO = ± 0.005, K2O = ± 0.002, MgO = ± 0.002, MnO = ± 0.0002, P2O5 = ± 0.0003. Using these estimates for precision, analysis of GSP-1, a commonly used standard for granodiorite, resulted in the the following estimate of its composition: SiO2 = 67.53 ± 0.06, AhO3 = 15.1 ± 0.02, TiO2 = 0.66 ± 0.001, Fe2O3 = 4.28 ± 0.004, CaO = 2.01 ± 0.005, NaO = 2.74 ± 0.005, K2O = 5.52 ± 0.002, MgO = 0.97 ± 0.002, MnO = 0.04 ± 0.0001, P2O5 = 0.28 ± 0.0003. These values are close or overlap with the expected values of SiO2 = 67.22 ± 0.04, AhO3 = 15.1 ± 0.05, TiO2 = 0.65 ± 0.005, Fe2O3 = 4.29 ± 0.02, Cao = 2.07 ± 0.007, NaO = 2.8 ± 0.02, K2O = 5.51 ± 0.01, MgO = 0.96 ± 0.01, MnO = 0.04 ± 0.001, P2O5 = 0.28 ± 0.004. The results ofmy study therefore suggest that data derived from the laboratory at SDSU are precise and relatively accurate.
