Vacuum x-ray fluorescence spectrometers are used routinely in our laboratory to analyze automotive exhaust particulate emissions. However, a problem has developed in the analyses of samples collected from cars equipped with catalytic converters. The catalytic converters which were developed to decrease the carbon monoxide and unburned hydrocarbon emissions from auto exhausts, also produce sulfuric acid through the oxidation of part of the SO2 emissions. The sulfuric acid aerosol which is collected with the particulate emissions causes problems in the use of a vacuum spectrometer. If the samples are analyzed as , collected, the sulfuric acid would be pumped off the collection filters resulting in the loss of sulfur from the samples and contamination of the spectrometer. A technique was needed to fix the sulfuric acid so that it would remain on the filters. Exposure of the samples to ammonia gave the needed stability. Details of the technique in cluding standard preparation procedures are described. The results obtained with samples exposed to ammonia at various concentrations and periods of time are presented.