Studies on Structure and Physical and Chemical Properties of Organic Compounds by Liquid Ionization Mass Spectrometry

Abstract

Liquid ionization mass spectrometry developed for ionizing a liquid sample at atmospheric pressure is described as a chemical laboratory. One of advantages of the method is easy sample handling. In general, an intense MH⁺ peak of an organic compound is observed and makes it easy to determine molecular weight of the compound. A CAD (collisionally activated dissociation) spectrum measured at an optimum temperature for obtaining MH+ as the base peak shows fragment ions due to the loss of a neutral molecule from the MH+ and the simple cleavages of the MH+ ions, which are characteristic of the structure of the compound (Figs.2∼7). Information on dissociation energies of various bonds in a molecule may be obtained from CAD spectra measured at several different pinhole voltages (Vp) which are related to the ion energies (Fig.3).
Mass spectra obtained at several dif ferent temperatures of a sample give information about the thermal properties, such as stability, melting point and decomposition scheme, and the structure of the compound (Fig.4). The use of additives (matrix, solvent and reagent) is very important to obtain a reproducible mass spectrum, to understand interaction between molecules, and to measure proton affinities of compounds involved. When a mass spectrum is measured, a reagent (R) like hydrazine is introduced into the ion source and produces the adduct ions (MH⁺R) which provide information about positions of functional groups, that is useful to distinguish geometrical isomers (Figs.6 and 7). Use of a labelled compound like D₂O gives information about H-D exchange reactions and the number of active hydrogens (Fig.8).
We can obtain several different mass spectra for one compound in a short period of time by using several reagents. Systematic studies on properties of compounds by the method should be done not only for analytical purpose but also for other fields of chemistry.