Characterization of proton-bound acetate dimers in ion mobility spectrometry

Abstract

Ionized acetates were used as model compounds to describe gas-phase behavior of oxygen containing compounds with respect to their formation of dimers in ion mobility spectrometry (IMS). The ions were created using corona discharge at atmospheric pressure and separated in a drift tube before analysis of the ions by mass spectrometry. At the ambient operational temperature and pressure used in our instrument, all acetates studied formed dimers. Using a homolog series of n-alkyl-acetates, we found that the collision cross section of a dimer was smaller than that of a monomer with the same reduced mass. Our experiments also showed that the reduced mobility of acetate dimers with different functional groups increased in the order n-alkyl :=:; branched chain alkyl zs cyclo alkyl < aromat. For mixed n-alkyl dimers we found that the reduced mobility of acetate dimers having the same number of carbons, for example a dimer of acetyl acetate and hexyl acetate has the same reduced mobility as a dimer composed of two butyl acetates. The fundamental behavior of acetate monomers and dimers described in this paper will assist in a better understanding of the influence of dimer formation in ion mobility spectrometry. on mobility spectrometry (IMS) is a versatile technique that has found widespread application in many areas, such as military and civil security applications, where it has been used to detect chemical warfare agents, explosives, and drugs [1-4]. The major advantages of IMS instruments are their small size, low power consumption, and capability to operate at atmospheric pressure, which makes them highly suited for real time analysis in the field [I, 5]. However, IMS spectra are sensitive toward changes in analytical conditions such as humidity, pressure, temperature, and matrix [6]. This feature makes it difficult to use the instrument for analysis of more than a limited number of prespecified compounds at a time, and reagent gases are often used to simplify the spectra either by suppressing certain ions [7,8] or by selective dimer formation [9].The creation of an IMS spectrum involves gas-phase ionization followed by a separation of the ions based on their mobility in an electrical field at pressures between -10 2 and 10 5 Pa. As a result of the ionization at atmospheric or near atmospheric pressure the ionized sample molecules may undergo several different transAddress reprint requests to Mr. C. S. Pedersen, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark. E-mail: csp@kemLku.dk formations such as fragmentation, rearrangement, and cluster formation. In particular molecules containing functional oxygen or nitrogen, atoms tend to make clusters with water molecules or other molecules with similar hydrogen bonding capabilities. Since the separation of ions in an IMS experiment depends upon both charge and size, cluster formation has a large influence upon the observed spectra and mobilities. The formation of clusters can be advantageous. For example, a problem with i...

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