CHEMICAL DERIVATIZATION FOR ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY .2. AROMATIC AND HIGHLY CONJUGATED MOLECULES

Analytes are typically detectable by electrospray ionization mass spectrometry (ES-MS) only if they are ionic in solution. Neutral, nonpolar analytes are not generally amenable to the technique. In this paper, neutral polycyclic aromatic hydrocarbons (PAHs), a heteroaromatic, a substituted aromatic, and the highly conjugated molecule buckminsterfullerene (C-60) are ionized (i.e., derivatized) in solution via reaction with the chemical electron-transfer reagents trifluoroacetic acid (TFA), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), or antimony pentafluoride and then detected in the gas phase as their respective radical cations by ES-MS. The nature of these electron-transfer reactions dictates selectivity for analytes of this type, i.e., analytes that are easy to oxidize. The oxidizing strength of the chemical electron-transfer reagents determines the degree of ionization/detection selectivity. Weak oxidants provide selectivity for the easiest to oxidize compounds while stronger oxidants provide for greater detectability (i.e., greater ionization efficiency) and more universal detection among analytes that undergo these reactions. For the ionization of a suite of PAHs, the relative oxidizing strength of the solvent/ oxidant systems investigated was methylene chloride/0.1% TFA (v/v) < methylene chloride/0.1% TFA/DDQ (v/v/60 mu M) < methylene chloride/0.1% TFA/0.5% antimony pentafluoride (v/v/v). The potential of this derivatization approach to be used on-line following a separation method for selective analyte ionization/detection in ES-MS is demonstrated using now injection experiments.