A new photometer system is described that allows very small traces of an element, such as cesium, to be determined quantitatively with greater precision than previously possible with flame-emission spectra. For example, with the new system it is possible to replicate measurements of the cesium in quantities that are normally present in one liter of sea water (about 0.3 μg) with standard error less than two percent. Dilute cesium standard solutions can be compared significantly within a few parts per billion. This, apparently, has not been reported for any previous flame photometer. The system consists of two photomultiplier tubes observing a flame simultaneously, from the same aspect, but each having a different optical filter interposed. Use is made of a beam splitter and two interference filters, one filter passing only a narrow band centered at the 8521.1-Å line of cesium, the other filter passing a nearby band not containing much of this emission but rather passing a sample of the background of the flame that contributes much of the total statistical fluctuation. Signals from these two phototubes are compared by suitable differential circuits, and are continuously recorded. A specialized automatic sample-changing system was developed to provide for precise timing of the burning of the sample and the standard solutions and to carry out aspirator washings immediately following the burning of each sample. Finally, means are provided for continuously monitoring the aspiration rate by recording the brightness of the background (off-peak) signal independently.
T. R. Folsom, C. Sreekumaran, W. E. Weitz, and D. A. Tennant, "Flame Photometer Specialized for Cesium," Appl. Spectrosc. 22, 109-114 (1968)