A new method for the direct flame calibration of nebulized additive concentrations

Abstract

A new technique is described to calibrate the flame delivery flux of an aerosol-producing nebulizer. The method is based on flame deposition. It centers on the nature of deposition onto a cooled surface immersed in the burned flame gases that contain an alkali salt (Na, K, Rb, or Cs) that has been nebulized into the flame. This deposition process has been shown to be controlled solely by the alkali concentration in the flame and for the same alkali flux the rates of deposition are invariant being independent of the nature of the alkali flame speciation, the fuel, equivalence ratio, flame or probe temperature (in the 350-800 K range for the latter), probe material, or the deposit’s molecular composition. In flames containing, for example, sodium and sulfur or chlorine, there is a pronounced preferential ranking of Na 2SO 4 > NaCl > Na 2CO 3 > NaOH for the resulting composition of the deposit. This dominance of sulfate formation is so pronounced that it produces a linear titration method for the alkali, and fractional addition of sulfur reflects into a corresponding fractional formation of Na 2SO 4. In flames containing solely sodium and sulfur additives, insufficient sulfur for a Na 2SO 4 formulation will result in a fractional mixture of the sulfate, with the remaining sodium forming carbonate. Collecting a deposit from a flame containing a nebulized alkali salt solution aerosol and a known trace addition of SO 2, and then analyzing the deposit using an inductively coupled plasma/atomic emission spectrometer (ICP/AES) for its alkali to S ratio, provides a measure of the extent of sulfate formation. This then translates into an absolute calibration of the nebulizer delivery flux of alkali in the burned gases. A variation of the method is to use instead a Raman scattering spectrometer to measure the ratio of sulfate to carbonate in a deposit, in this case for flames free of halogens. The method has the advantage that it can be used in any flame, turbulent or laminar, requires only a trace of deposit for ICP analysis, and is accurate. Moreover, the probe can intercept any partial part of the flame gases. A limited number of other techniques currently are available for nebulizer calibration. However, other than the curve of growth method, these have not been discussed in combustion texts. Their various limitations and accuracies are outlined together with that of the presently suggested method.