In positive-ion mode, helium DART[1] generally produces protonated molecules [M + H]+ for compounds with a basic site, a heteroatom, or a site of unsaturation. In negative-ion mode, helium DART produces deprotonated molecules [M – H]- for compounds such as carboxylic acids or phenols that have a labile proton. Samples are introduced directly into the DART gas stream. By heating the helium gas, DART can detect volatile and semi-volatile compounds. Compounds with a molecular weight below 1000 can generally be detected by helium DART. Some compounds with molecular weights in excess of 2000 can be detected if they are sufficiently volatile.
Positive-ion helium DART mass spectrum showing additives in a synthetic motor oil.