Details are given for simple modifications of a laboratory monochromator so that selected internal standard line pairs from flame spectra may be isolated and measured simultaneoualy with a multiplier phototube differential photometer. The basic design is applicable to most spectrometers or spectrographs. An atomizing system with provisions for sample recycling and recovery is also described. This equipment has been applied to the determination of calcium in the rare earths in the concentration range of 0.025 to 2.5%. Average deviation for a series of repeated determinations at various concentrations is about plus or minus 1.5%
