No abstract

No abstract

No abstract

No abstract

Early in the development of electrohydrodynamic mass spectrometry (EH-MS), it was realized that ion source performance improved when the sample liquid was supplied discontinuously to the capillary emitter. Based on this realization, an ion source was designed that operates without an external sample supply system. Its essential part is a metal capillary that serves both as the liquid reservoir and the field anode and is mounted exchangeably on a pushrod. As the liquid flow is field induced, the flow rates are extremely low, in the range of 0.2 nl min-'. Thus, in spite of high analyte concentrations needed, sample consumption is only a few picomoles per mass scan. Moreover, the emission of larger droplets is avoided, which contributes to the stability of the ion signal. As the capillary emitter is mounted on a standard emitter carrier known from field desorption (FD) MS, the new ion source is compatible with conventional F D ion sources. EH mass spectra of analytes could be obtained from substances whose solubility in glycerol was >0.5 mol I-'. The substances investigated with the new ion source include sugars, amino acids, peptides and an antibiotic. The general characteristics of the mass spectra are essentially the same as those known from other EH ion sources. Besides abundant solvent cluster ions, only cationated or protonated molecules of the analytes could be detected, frequently with one or more solvent molecules attached to the ions. Fragment ions were not observed. INTRODUCTIONIn recent decades, a variety of so-called 'soft' ionization methods have been developed for mass spectrometry (MS) with the aim of making non-volatile or thermally labile organic compounds amenable to mass spectrometric analysis. A common feature of these methods is that mass spectrometric ions are released directly from a condensed phase (liquid or solid) into the gas phase. By a group of methods, field desorption mass spectrometry (FD-MS),' electrospray (ES) MSZ and electrohydrodynamic (EH) MS,3 this is accomplished by applying high electric fields to the sample material. In EH-MS, an electrolyte-containing sample liquid is passed through a metal capillary biased at high potential relative to a counter electrode. The high electric field at the tip of the capillary emitter causes electrohydrodynamic disintegration of the liquid, which results in the release of mass spectrometric ions and small charged droplets directly into vacuum. The ions released by this process consist of protonated or cationated analyte and solvent molecules and are assumed to reflect solution equilibria: Mass analysis is usually performed with a double-focusing mass spectrometer. The observed mass spectra are typically free of fragment ions, thus showing that the energy transferred by the ionization process is low.EH-MS was introduced in 1972 by Evans and cow o r k e r~,~.~ originally as a method for producing ions from liquid metals [a so-called liquid-metal ion source (LMIS)], and was used for the analysis of organic sub-* Paper prepared for Publication in...