Chemical and ionization interferences in the atomic absorption spectrophotometric measurement of sodium,potassium, rubidium,and cesium

Although chemical and ionization interferences significantly affect the atomic absorption signal of the alkali metals, suitable corrective measures permit accurate analysis of these elements. The observed interferences are affccted in opposite ways by flame temperature, chemical depression of absorption produced by anions decreasing, and ionization enhancement produced by cations increasing with increasing flame temperature. Anionic depression is small in an acetylene-air flame and moderately large in a propane-air flame, increasing in the sequence sulfate < chloride < perchlorate < phosphate. Phosphate affects the cations in the order Na < K < Rb < Cs, with 20 mM phosphate depressing cesium absorbance approximately 40%. Conversely, ionization enhancement by cations is small in a propane-air flame and large in an acetyleneair flame, the effect on rubidium absorbance increasing in the sequence Mg < Ca < Li < Na < K < Cs, with 20 mM cesium producing a twoflod increase in absorbance. This is in the order of decreasing ionization potentials, indicating a direct relationship between ionization potential, degree of ionization, and enhancement produced. From consideration of the over-all effect of flame temperature on various interferences, we conclude that the propane-air flame is probably the most satisfactory for alkali metal analysis, especially for rubidium and cesium. Recovery studies on dialyzed and ashed rat liver microsomes and known controls demonstrate that addition of 15 mM lanthanum, to minimize anionic interferences, and addition of moderate concentrations of cesium, rubidium, or potassium, to minimize cationic enhancement, permit accurate and reliable measurement of the alkali metal cations in biological materials in the presence of potentially interfering cations and anions.