A cathode layer dc arc method of determining microgram amounts of calcium in latex of <i>Hevea brasiliensis</i> is described. Using lithium carbonate and lanthanum sulphate as buffers, a working curve covering 0.032% to 10.0% Ca in ash of latex can be constructed with palladium as internal standard. Phosphate and silica suppress Ca emission but silica has a considerably greater effect. Of the four matrices examined, the composition with 44.7% K<sub>2</sub>O, 22.3% P<sub>2</sub>O<sub>5</sub>, and 13.4% MgO, which is most representative of the composition of latex ash, gives reproducible Ca values and these are in closest agreement with those obtained by atomic absorption. The spectrographic technique may be extended to simultaneously determine trace amounts of Mn, Fe, Cu, and Si.
S. K. Ng, R. C. H. Hsia, and P. T. Lai, "Spectrographic Determination of Calcium in Latex of Natural Rubber (Hevea brasiliensis)," Appl. Spectrosc. 24, 583-587 (1970)