Observation of Molecular Ions and Analysis of Nonpolar Compounds with the Direct Analysis in Real Time Ion Source

Abstract

Positive ions in the direct analysis in real time (DART) ion source are commonly formed by proton transfer. However, the DART source is similar to atmospheric pressure photoionization (APPI) in that it can produce molecular ions as well as protonated molecules, although the two sources differ in the initial ion formation process. This report discusses some of the factors that influence molecular ion formation in DART and shows how the DART source can be used to analyze “difficult” or nonpolar compounds such as alkanes and cholesterol. Trace reagent ions including NO⁺ and O₂⁺· formed from atmospheric gases are shown to play important roles in DART ionization. The use of the DART source as a gas chromatography/mass spectrometry (GC/MS) interface is demonstrated to show the difference between mass spectra obtained using conditions that favor proton transfer and those that favor molecular ion formation.