Anal. Chem. 2001, 73, 4983-4987
Generating Multiply Charged Protein Ions by Ultrasonic Nebulization/Multiple Channel-Electrospray Ionization Mass Spectrometry Jentaie Shiea,* Der-Yeou Chang, Chia-Hsin Lin, and Shiuh-Jen Jiang
Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424 Taiwan
An ultrasonic nebulization/multiple channel electrospray ionization (USN/MC-ES) source, which generates multiply charged peptides and proteins ions, was developed. The source is an ultrasonic nebulizer that is connected to a multiple channel electrospray ionization source. Aerosols were formed by ultrasonically nebulizing the sample solution. The aerosols were then purged into the central channel of a seven-channel ES source via nitrogen gas. A methanol solution that contained 1% trifluroacetic acid was electrosprayed through the outlying six electrosprayers. Detection of multiply charged peptide and protein ions indicated that electrospray was generated from the charged droplet containing analyte. The sample aerosol appeared to fuse with the charged methanol droplet in the air. Then electrospray ionization of the analyte occurred from the newly formed droplet. The peptide and protein prepared in deionized water were detected by this USN/ MC-ES-MS. By varying the electrospray solvents, the signals of certain components in the mixture were selectively suppressed. Since 1988, an electrospray ionization source with more than one electrosprayer has been employed in several chemical and biological applications.1-14 Among these applications, volatile (1) Ogorzalek Loo, R. R.; Udseth, H. R.; Smith, R. D. J. Phys. Chem. 1991, 95, 6412. (2) Ogorzalek Loo, R. R.; Loo, J. A.; Udseth, H. R.; Fulton, J. L.; Smith, R. D. Rapid Commun. Mass Spectrom. 1992, 6, 159. (3) Ogorzalek Loo, R. R.; Udseth, H. R.; Smith, R. D. J. Am. Soc. Mass Spectrom. 1992, 3, 695. (4) Ogorzalek Loo, R. R.; Smith, R. D. J. Am. Soc. Mass Spectrom. 1994, 5, 207. (5) Ogorzalek Loo, R. R.; Winger, B. E.; Smith, R. D. J. Am. Soc. Mass Spectrom. 1994, 5, 1064. (6) Cheng, X.; Gale, D. C.; Udseth, H. R.; Smith, R. D. Anal. Chem. 1995, 67, 586. (7) Rulison, A. J.; Flagan, R. C. Rev. Sci. Instrum. 1993, 64, 683. (8) Kostiainen, R.; Bruins, A. P. Rapid Commun. Mass Spectrom. 1994, 8, 549. (9) Takahashi, Y.; Fujimaki, S.; Kobayashi, T.; Morita, T.; Higuchi, T. Rapid Commun. Mass Spectrom. 2000, 14, 947. (10) Andrien, B. A., Jr.; Whitehouse, C.; Sansone, M. A. Proceedings of the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, 1998; p 889. (11) Jiang, L.; Moini, M. Anal. Chem. 2000, 72, 20. (12) Wang, C. H.; Shiea, J. J. Mass Spectrom. 1997, 32, 247. (13) Lee, C. Y.; Shiea, J. Anal. Chem. 1998, 70, 2757. (14) Hong, C. M.; Tsai, F. C.; Shiea, J. Anal. Chem. 2000, 72, 1175. 10.1021/ac0101609 CCC: $20.00 Published on Web 09/12/2001
© 2001 American Chemical Society
organic and biological compounds were detected via a multiple channel electrospray ionization source (MC-ES) connected to a gas chromatograph (GC) or a flow pyrolyzer (FP).13,14 In MC-ES, nitrogen was employed to conduct the gaseous analyte eluted from GC or FP into the central channel of a seven-channel electrospray ionization source. Concurrently, the surrounding six channels were electrosprayed with a methanol solution that contained 1% trifluoroacetic acid. Experimental results indicated that the protonated molecular ions (MH+) were on-line detected for (1) a series of methylated fatty acids that GC separated, (2) volatile dehydrogenation products from the slurry that contained dimethylhydrazine as the reactant and mercury oxide particles as the catalyst, and (3) reactive ketene monomers that were synthesized in the flow pyrolyzer.13,14 Applying MC-ES for chemical analysis has the following merits: (1) GC/MS and LC/MS can be performed in a single portion of the mass spectrometer; (2) headspace analysis can be performed for volatile organic compounds, which are presented in an impure sample; and (3) extremely reactive or unstable volatile organic compounds can be analyzed immediately after they are synthesized. The formation of MH+ in MC-ES is due to an ion-molecular reaction (IMR) that occurs between the analyte (M) and proton (H+) or methanol ions ((CH3OH)H+).13-16 Ion-molecule reactions can be conducted by directly reacting gaseous M with H+ or protonated methanol ions in the air. It is also suggested that the gaseous M, which was eluted from the central channel of a MCES source is absorbed into the fine droplets that the six outlying electrosprayers generate.13,14 The IMR responsible for forming MH+ then occurs in the new droplet.13,14 Our previous studies detected only singly charged ions in MCES-MS.13,14 This is because all of the examined analytes were small (molecular weight
