Design and construction of LC/MS interfaces utilizing fused-silica capillary tubes as vacuum nebulizers

Direct-EI in LC-MS: towards a universal detector for small-molecule applications

This review article will give an up-to-date and exhaustive overview on the efficient use of electron ionization (EI) to couple liquid chromatography and mass spectrometry (LC-MS) with an innovative interface called Direct-EI. EI is based on the gas-phase ionization of the analytes, and it is suitable for many applications in a wide range of LC-amenable compounds. In addition, thanks to its operating principles, it prevents unwelcome matrix effects (ME). In fact, although atmospheric pressure ionization (API) methodologies have boosted the use of LC-MS, the related analytical methods are sometime affected by inaccurate quantitative results, due to unavoidable and unpredictable ME. In addition, API's soft ionization spectra always demand for costly and complex tandem mass spectrometry (MS/MS) instruments, which are essential to acquire an "information-rich" spectrum and to obtain accurate quantitative information. In EI a one-stage analyzer is sufficient for a qualitative investigation and MS/MS detection is only used to improve sensitivity and to cut chemical noise. The technology illustrated here provides a robust and straightforward access to classical, well-characterized EI data for a variety of LC applications, and readily interpretable spectra for a wide range of areas of research. The Direct-EI interface can represent the basis for a forthcoming universal LC-MS detector for small molecules.

Trace level determination of organophosphorus pesticides in water with the new direct-electron ionization LC/MS interface

A new LC/MS method for the determination of organophosphorus pesticides in water, based on the use of direct-electron ionization (EI) interface, is presented. Direct-EI is a new device that, in a very simple fashion, couples a nano-HPLC system with a mass spectrometer equipped with electron ionization capability. The nanoscale liquid flow allows for a direct introduction of eluate into the ion source and, after nebulization, for its ionization under typical EI conditions. Library-matchable EI spectra are generated for a choice of full scan or SIM detection of the analytes. In our case, a selection of organophosphorus pesticides, commonly distributed in local sugar beet cultivation, were considered. The new interface permits a very sensitive detection of the analytes in a wide range of linear response (0.09-9 ng). When applied to a real sample, the method allowed detecting four different pesticides at a concentration level of approximately 3 ng x L(-1).

A simple approach for coupling liquid chromatography and electron ionization mass spectrometry

A miniaturized, aerosol based interface for directly coupling a liquid chromatograph with a mass spectrometer is presented. The interface is entirely within the electron ionization (EI) source of the mass spectrometer and no additional, external devices are needed. This simple and effective approach exploits micro-flow nebulization technology providing a new interface suitable for a variety of applications of environmental and biological interest. The new interface provides necessary linearity, ruggedness, sensitivity, and reproducibility of response for trace level analysis, and readily interpretable mass spectra for unambiguous identification of unknown compounds of small to medium molecular weight.

New trends in the application of electron ionization to liquid chromatography-mass spectrometry interfacing

I. Introduction 88 II. Cap-EI Interface 90 A. Interface Performance 92 III. Direct-EI Interface 93 A. Interface Performance 97 IV. Conclusions 103 Acknowledgments 103 References 103 --Two recent approaches for coupling capillary scale liquid chromatography and electron ionization mass spectrometry are reviewed and discussed. The first one, Cap-EI, is the latest evolution of the micro-scale particle beam interface, in which the nebulizer has been optimized to overcome the limitations of the former approach, in terms of sensitivity and linearity. It can be easily hosted in pre-existing instruments without major modifications and can use helium and the less-expensive nitrogen to generate library-matchable electron ionization spectra. The second one is a miniaturized interface for nano- and micro-HPLC, in which the interfacing process takes place into a suitably modified ion source. Because the eluate from the column is completely transferred into the ion source for ionization, superior sensitivity, linearity, and reproducibility are obtained. No signs of chemical ionization are observed at flow rates up to 1.5 microL/min. These two interfaces demonstrate that electron ionization can be successfully used for the analysis of small-medium molecules of various polarities, and also at the trace level. The possibility to record library-matchable electron ionization spectra offers the analyst a powerful tool that can be particularly useful in real-world applications.

Characterization of benzo(a)pyrene metabolites by high performance liquid chromatography-mass spectrometry with a direct liquid introduction interface and using negative chemical ionization

Polynuclear aromatic hydrocarbons are ubiquitous pollutants that may become highly carcinogenic during their metabolism by organisms. Benzo(a)pyrene is an example of such a compound. Benzo(a)pyrene metabolite standards, which are labile, polar compounds, have been used to evaluate high performance liquid chromatography-mass spectrometry as an analytical technique for polynuclear aromatic hydrocarbon metabolites. The chromatography used microbore (1 mm i.d.) C18 columns with acetonitrile/water as the elution solvent which became the reagent/moderator gas. A direct liquid introduction interface was used and characteristic spectra were obtained in the negative chemical ionization mode. Molecular or pseudomolecular ions were obtained for five phenols (m/z 268 and m/z 267) and three trans-dihydrodiols (m/z 286, m/z 285, m/z 284). Useful spectra were also obtained for a tetrahydrotriol, four tetrahydrotetrols and three conjugated metabolites. Fragmentation usually involved loss of water and anions at m/z 284 and m/z 268 were common to most of the spectra. Variations in the source/desolvation chamber temperature between 175 degrees C and 275 degrees C caused significant alterations in the relative abundance of molecular and fragment ions. Dissociative electron capture was the dominant ionization process.