LIQUID CHROMATOGRAPHY-ELECTRON IONIZATION MASS SPECTROMETRY: FIELDS OF APPLICATION AND EVALUATION OF THE PERFORMANCE OF A DIRECT-EI INTERFACE Achille Cappiello,* Giorgio Famiglini, Pierangela Palma, and Antonella Siviero Istituto di Scienze Chimiche ‘‘F. Bruner’’ Universita` degli Studi di Urbino‘‘Carlo Bo’’ Piazza Rinascimento, 6-61029 Urbino, Italy Received 20 January 2005; received (revised) 28 February 2005; accepted 1 March 2005 Published online 17 May 2005 in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/mas.20054
A comprehensive evaluation and a thorough discussion of the fields of possible applications of the Direct-EI interface are described in this review. Direct-EI allows the direct introduction of the effluent from a capillary HPLC column into the electron ionization (EI) ion source of a mass spectrometer. Thanks to the reduced liquid intake and an in-source nebulizer, the interfacing process occurs smoothly and entirely into the ion source. No intermediate interfacing mechanisms of any sort are interposed between the column and the mass spectrometer, thus circumventing any undesired sample loss and minimizing the effort for instrument modification. Theoretically, any GC-MS system can be converted into an LC-MS for EI amenable compounds. Several parameters, crucial for a successful integration of liquid chromatography and mass spectrometry, have been considered in the evaluation of the functioning of such an interface: limit of detection, linearity of response, reproducibility, and chromatographic compatibility. Different mobile phases, also containing non-volatile buffers, were taken into account, demonstrating an outstanding separation flexibility. The entire set of experiments was carried out at different flow rates and temperatures of the ion source. The interface behavior was also tested in real world applications, with mixtures of pesticides, hormones, nitro-PAH, and endocrine-disrupting compounds, allowing picogram level detection and the possibility to record library-matchable, readily interpretable electron ionization mass spectra, for prompt compound characterization and confirmation. # 2005 Wiley Periodicals, Inc., Mass Spec Rev 24:978–989, 2005 Keywords: nano-column liquid chromatography; electron ionization mass spectrometry; Direct-EI; trace analysis
I. INTRODUCTION High performance liquid chromatography (HPLC) and electron ionization mass spectrometry (EIMS) are two analytical techniques that, in principle, seem to be incompatible. However, because these two approaches share a great deal of applications in the analysis of suitable molecules, typically less than 1000
———— *Correspondence to: Achille Cappiello, Istituto di Scienze Chimiche ‘‘F. Bruner’’ Universita` degli Studi di Urbino‘‘Carlo Bo’’ Piazza Rinascimento, 6-61029 Urbino, Italy. E-mail:
[email protected]
Mass Spectrometry Reviews, 2005, 24, 978– 989 # 2005 by Wiley Periodicals, Inc.
Daltons, a large effort has been devoted by the scientific community to attempt to develop a reliable, easy-to-use, and flawless interface. The first successful and commercially available device to combine EI and HPLC was developed by Willoughby and Browner (1984). It was based on the conversion of the liquid effluent into a gas phase and transformed into a beam of solute particles, after the evaporation of the spray droplets and the elimination of the solvent vapors through a multi-stage momentum separator. The initial success of EI in coupling liquid chromatography and mass spectrometry, offered by the particle beam interface, was not followed by any new, advanced, commercially available designs and solutions, based on the assumption that electron ionization is better suited for gas chromatography-amenable compounds. The development of new soft ionization techniques for all of the HPLC-possible applications generated a family of Atmospheric Pressure Ionization-based interfaces (API), although a considerable number of small-medium molecular weight analytes can provide very good EI spectra. The simple operating principle of EI can induce the ionization of any molecule present in the gas phase, regardless of its chemical background, as long as it can withstand the typical EI source conditions (high vacuum and temperature). This attribute represents a point of strength that brings to an end all of those weaknesses that are typical of API-based interfaces such as signal suppression, influence of the mobile phase composition, need for desalting, or post-column solvent modifications. In fact, in EI the mobile phase does not, in principle, influence the ionization process, and no molecule-ion interactions are normally observed. Moreover, the typical EI spectrum is highly informative, and its high reproducibility allows an easy comparison with thousands of spectra from commercially available sources (such as NIST or Wiley). EI detection can benefit from recently developed sophisticated algorithms such as, for instance, the one developed by the National Institute of Standards and Technology (NIST) and called AMDIS (Automated MS Deconvolution and Identification System), which extracts the analyte’s mass spectra in complex chromatographic mixtures with several overlapping peaks (Ausloos et al., 1999). The fact that EI- and HPLC-operating conditions remain distant cannot, however, be ignored, and it might explain some of the frustration observed in the everyday use of a typical LC-EIMS interface of the past: poor sensitivity, signal instability, reduced linearity, and disappointing optimization. The growth of new
LC-EIMS WITH A DIRECT-EI INTERFACE
instruments that rely on electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) solved most of those drawbacks and widened extraordinarily the field of possible applications; however, on the other hand, they also introduced new analytical nightmares such as the signal suppression or adduct-ion formation that are often found in certain applications. Although thousands of studies have been published on new and improved API interfaces, only a few authors have contributed to the attempt of efficiently generating EI spectra from a liquid effluent: Kientz et al. (1996) were able to generate EI spectra based on an eluent-jet formation by means of inductive heating of the micro-LC effluent, and momentum separation in a jet separator; Dijkstra et al. (1998) improved that technique with a particular emphasis on its use under chemical ionization (CI)-MS conditions; and Amirav and Granot (2000) developed a new apparatus to obtain high-quality, library-searchable EI spectra based on supersonic molecular beam mass spectrometry (SMBMS) approach. That valuable attempt allows conventional mobile phase flow rates and enhances molecular ion response for an improved molecular weight determination. Our research group has devoted a considerable effort to update and improve the design and the performance of an EIbased LC-MS system (Cappiello & Bruner, 1993). Significantly improved sensitivity for any mobile-phase composition was achieved by drastically reducing the mobile-phase intake and adapting the design of the interface accordingly. The microparticle beam (PB) interface and its evolution, called capillary EI (CapEI, commercialized by Waters, Milford, MA), use microscale flow rates (1–5 mL/min), and were successfully employed in the analysis of several classes of compounds and with different modifiers added to the mobile phase (Cappiello, Famiglini & Bruner, 1994; Cappiello & Famiglini, 1995a, 1998; Cappiello et al., 1995b,d, 1996, 1999a, 2000; Cappiello, Famiglini, & Tirillini, 1995c; Cappiello, 1996). It is noteworthy to point out that this approach showed, for the first time, a significantly improved tolerance for non-volatile buffers, as demonstrated in two applications (Cappiello et al., 1997, 1999b). Because the major obstacle in coupling HPLC and an EI source is represented by the mobile phase vapors, we designed a new and alternative interface, in which the eluate is directly introduced into the EI ion source (Cappiello et al., 2001a,b, 2002; Cappiello, Famiglini, & Palma, 2003a) at a nanoscale flow rate (
