An article is sulfonated by forming a sulfonating gas composition containing a gaseous sulfur-containing compound provided from a gas source. The gas source may utilize a chemical feed stock that includes a precursor sulfur-containing compound. The article is then contacted with the sulfur-containing gas until a surface treatment is effected. Next, the article is then optionally contacted with a gaseous neutralization composition, and then a liquid neutralization composition. In another sulfonation method, a first test sample is sulfonated and then analyzed by inductively coupled plasma analysis and X-ray fluorescence spectroscopy. A calibration relationship between the two spectroscopic techniques is determined with the inductively coupled plasma atomic spectroscopic value taken as the accurate value. In subsequent sulfonation runs, test samples are included and monitored by X-ray fluorescence.