Diamine reagents in thermospray mass spectrometry

Abstract

Determination of the molecular weight for an unknown by thermospray liquid chromatographic/mass spectrometric analysis using ammonium acetate mobile phase can be both confusing and misleading. This is due to the uncertainty of whether an ion is either a protonated species or an ammonium adduct or both. Deuterium oxide and ammonium acetate buffers have been used to elucidate molecular species of unknown compounds with some success. We have investigated an alternative method which utilizes diamine reagents. This method simplifies the identification of unknown molecular species in an inexpensive manner. Diamines form adduct ions readily with many substances. A comparison of spectra obtained for an unknown using ammonium acetate or diaminoethane in separate analyses is useful for identifying molecular species. Using ammonium acetate as a mobile phase, a molecule is often detected as a protonated molecular species. However, using diaminoethane mobile phase, this same molecule is most often detected as a diaminoethane adduct. Comparison of each spectrum reveals a mass difference of 60 for a molecule detected as a protonated species and a diaminoethane adduct. In addition, a mass difference of 43 will be obtained in a similar spectral comparison if a molecule forms ammonium adducts using ammonium acetate mobile phase and diaminoethane adducts using diaminoethane. An analysis of an unknown substance with several molecular species is presented as an example of the utility of diamine reagents in thermospray mass spectrometry. By observing mass spectral differences, molecular species can be identified as either a protonated ion or an adduct ion.