Quantitative analysis versus semiquantitative optical emission spectrographic analysis of chromium, nickel, zinc, vanadium, and molybdenum in organic-rich Paleozoic marine strata, Western United States

Comparison of analytical results between the quantitative method and the semiquantitative six-step optical-emission spectrographic method commonly shows significantly lower values for the semiquantitative method. The lower values are most obvious for splits from the same samples of fine-grained organic-rich rocks where the concentrations of the metals chromium, nickel, zinc, vanadium, and molybdenum are greater than about 200 ppm (parts per million). Samples compared are Paleozoic marine rocks from the Western United States; they include the Permian Phosphoria Formation (Idaho, Montana, Utah, Wyoming), Mississippian Chainman Shale (Nevada and Utah), Mississippian Heath Formation (Montana), Mississippian Doughnut Formation (Utah), Devonian Woodruff Formation (Nevada), and Ordovician Vinini Formation (Nevada).

The semiquantitative method consistently underestimates the concentration of the five metals studied by a factor of 1.7 to 2.7 and, thus, cannot be considered reliable for determining the concentration of metals in these rocks if the reported semiquantitative values exceed about 100 ppm. It is noteworthy that the most comprehensive analytical and statistical studies of metal concentrations in black shales of the United States have used the semiquantitative six-step optical-emission method of analysis.