Ambient aerosol particles affect both the earth's climate and human health. Both effects depend on the chemical composition of the particles including the binding state of specific elements. Modern on-line aerosol mass spectrometry is capable of measuring aerosol composition with high temporal resolution, avoiding artifacts often introduced by classical off-line methods. The two most common types of on-line aerosol mass spectrometers, laser desorption/ionization mass spectrometers (LDI-MS) and thermal desorption electron impact ionization mass spectrometers (TD-EI-MS) provide reliable information on the most common ambient inorganic and organic aerosol species with high temporal resolution. However, for less common aerosol species identification with both types of instrument is frequently associated with large uncertainties. Here, we provide an overview of the element speciation capabilities of current on-line aerosol mass spectrometry for both carbonaceous and non-carbon-containing aerosol species. We describe limitations and other issues for this type of on-line aerosol analysis.