Direct Determination of Manganese in Gasoline by Atomic Absorption Spectrometry in the Nitrous Oxide-Hydrogen Flame

The nitrous oxide-hydrogen flame provides efficient combustion of aspirated gasoline without the formation of luminescent unburned carbon particles. Direct aspiration of gasoline into this flame is practical even at very high nebulizer uptake rates and is thus the basis for rapid direct determination of manganese by atomic absorption spectrometry. The atomic absorption spectrophotometer, however, must be calibrated by using standards prepared from the same organometallic manganese compound present in the samples. Atomic absorption response for manganese naphthenate, manganese octoate, and methylcyclopentadienylmanganese tricarbonyl (MMT) can differ by as much as 45% relative depending on the nebulizer uptake rate. At the optimum nebulizer uptake rate used, 6 ml/min, the difference between absorption response of the most sensitive compound, manganese naphthenate, and the least sensitive, MMT, is 15%. Very high nebulizer efficiencies and relatively high uptake rates result in high sample transport to the flame. Absorption at a given concentration for all compounds increases in a linear fashion with the amount of sample transported to the flame up to about 3.0 ml/min. At 4.0 ml/min equivalent liquid phase volume delivered to the flame maximum absorbance occurs; absorbance diminishes rapidly thereafter. Plots of absorption vs equivalent liquid phase volume delivered to the flame passed through the origin for manganese naphthenate and manganese octoate. The same plot for MMT shows a positive intercept on the volume delivered axis, which indicates a consistent loss of absorption for MMT relative to the naphthenate and the octoate for all rates of sample delivery to the flame.