AC Research
Anal. Chem. 1998, 70, 205-212
Accelerated Articles
Ionization within a Cylindrical Capacitor: Electrospray without an Externally Applied High Voltage Houle Wang and Murray Hackett*
Department of Medicinal Chemistry, University of Washington, Box 357610, Seattle, Washington 98195
An interface was developed for mass spectrometry based on dc induction. Ions were produced as liquid flowed through a concentric cylindrical capacitor at room temperature. No externally applied high voltages affecting the spray tip were used, thus eliminating corona discharge problems in negative ion mode. The voltages applied to the capacitor were lower than those used for electrospray on the same instrument. Stable signals were observed for positive ions infused at flow rates from 50 nL/min to 200 µL/min, anions in the range 50-500 nL/min, without addition of sheath liquid or gas. The minimum concentration required to generate an interpretable product ion spectrum from the [M - 2H]2- precursor, for infused Salmonella typhimurium diphosphorylated lipid A, was improved by a factor of ∼500. Positive ion capillary LC/MS results for a horse apomyoglobin tryptic digest were similar to those observed for an optimized electrospray interface. Factor XIII b subunit, an 83-kDa protein, yielded an interpretable positive ion multiple charge envelope, under buffer conditions that did not allow collection of useful data with our low-flow electrospray source. Mass spectra were also generated for a single-stranded DNA 15-mer and 2′,4′-dideuteriotrifluoropropranolol, a small molecule that had been difficult to ionize by either electrospray or APCI.
A common problem encountered with low flow LC/MS or infusion type API atmospheric pressure ionization (API) inlet designs1-4 is unstable operation with negative ions, especially in aqueous solution. The addition of [H2O]n peaks and other noncovalent adducts are symptomatic of corona discharge at * Corresponding author: e-mail,
[email protected]. (1) Wilm, M.; Mann, M. Anal. Chem. 1996, 68, 1-8. S0003-2700(97)01036-6 CCC: $15.00 Published on Web 01/15/1998
© 1998 American Chemical Society
nanoliter flow rates, where the more obvious indications seen at higher flows, such as excessively high electrospray current and disruption of the normal baseline, are often missing. In this paper, we describe a method for generating a charged solution for mass spectrometric applications that offers advantages for negative ion work, including adduct-free and stable ion currents with nanoliter flow rates. The method is also capable of efficiently generating positive ions from small molecules that, at least in our hands, have not been amenable to either electrospray or atmospheric pressure chemical ionization (APCI). Although we have not studied the issue rigorously, our experience with electrospray ionization (ESI) using small inner diameter stainless capillaries,5 which work extremely well for positive ions, suggests that stainless steel has problems with signal stability at low flows with negative ions, especially in aqueous solutions with less than 20% or so organic solvent content. The majority of our negative ion work has been with hydrophobic glycolipids (lipid A),6 dissolved in chloroform/methanol, where the adduction problems are less severe, due to the electron scavenging properties of chloroform and the lower electrospray voltage required, relative to water. However, detection limits were still poor compared to what is routinely achieved with many positive ion protein and peptide applications, and flow rates below 500 nL/min were problematic.7 Because of our interest in the (2) Jonscher, K. R.; Yates, J. R., III. Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland, OR, May, 12-16, 1996; p 1149. (3) Davis, M. T.; Stahl, D. C.; Hefta, S. A.; Lee, T. D. Anal. Chem. 1995, 67, 4549-4556. (4) Emmett, M. R.; Caprioli, R. M. J. Am. Soc. Mass Spectrom. 1994, 5, 605613. (5) Wang, H.; Lim, K. B.; Lawrence, R. F.; Howald, W. N.; Taylor, J. A.; Ericsson, L. H.; Walsh, K. A.; Hackett, M. Anal. Biochem. 1997, 250, 162-168. (6) Raetz, C. R. H. J. Bacteriol. 1993, 175, 5745-5733. (7) Lim, K.; et al. Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, June 1-5, 1997; p 1371.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998 205
analysis of trace quantities of bacterial glycolipids, our pursuit of a better way to form negative ions has focused on low flows that allow the highest possible S/N ratio for a given concentration. About 20 years ago, Irebarne and Thompson developed an API source that relied on an external “induction electrode” to facilitate ionization.8 Electrospray, as developed by Dole,9,10 and then by Fenn’s group,11-13 employed an external electric field for purposes of both creation of a spray of fine droplets and formation of gasphase ions. The electrospray ionization process has been described as essentially electrophoretic in nature, and its fundamental aspects, as they relate to mass spectrometry, have been reviewed.14-18 Various approaches to augmenting pure electrospray have been explored, including ultrasonic sprayers14,19 and pneumatically assisted electrospray.20 APCI relies on an external electrode to deliberately create a corona discharge, creating reactive charged species that in turn charge the analyte,21 and is used primarily for LC/MS analysis of small molecules (
