Anal. Chem. 1995, 67, 1950-1954
High-Resolution Matrix-Assisted Laser Desorption/ Ionization in a Linear Time=of-FlightMass Spectrometer Randy M. Whittal and Liang Li* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
A time-lagfocusing method is developed for the improvement of mass resolution in a linear time-of-flightmass spectrometer for matrix-assistedlaser desorptiodionization (MALDI). In this technique, the ions generated by the lMALDI process are extracted by a pulsed voltage. A short time delay (280 ns) is inserted in between the laser desorptiodionizationevent and the ion extraction. The region between the repeller and extraction grid is fieldfree during the delay. The time-lag extraction allows the ions generated in the region between the repeller and the extraction grid to separate according to their velocity (energy). Application, to the repeller, of the appropriate pulse voltage provides the energy correctionnecessary to simultaneously detect all ions of the same mass/charge regardless of their initial energy, resulting in improved mass resolution. It is demonstrated that mass resolution in the range of 3000-6000 fwhm can be obtained. With this mass resolution, isotopically resolved mass spectra are observed for peptides with masses up to 3000 Da. For proteins, such as bovine insulin, cytochrome c, and apomyoglobm, resolution in the range of 800-1000 fwhm is observed with a mass measurement accuracy better than 0.01%. Matrix-assisted laser desorption/ionization (MALDD has become an increasingly important technique for the detection of biopolymers.lS2 At present, the ions generated by the MALDI process are commonly analyzed with the use of a time-of-flight mass spectrometer (TOFMS). A TOFMS is inexpensive to construct and simple to operate. It has no theoretical mass limit. The sensitivity of TOFMS can be very high due to its efficient ion transmission and ion extraction duty cycle with a pulsed ionization source. A major drawback of TOFMS is the low mass resolving power for the detection of ions in MALDI. Low mass resolution can be attributed to several factors including the initial spatial, temporal, and kinetic energy distribution of the ions. Metastable decay in the flight tube may also play a significant role.3 Several approaches have been developed to reduce the contribution of one or more of these factors in order to enhance the mass resolution of MALDI TOFMS. For example, a TOFMS that is equipped with an ion reflector for compensation of the ~~~
(1) Chait, B. T.: Kent, S. B. H. Science 1992,257, 1885-1894.
(2) Hillenkamp, F.; Karas, M.; Beavis, R C.; Chait, B. T. Anal. Chem. 1991, 63, 1193-1203. (3) Cotter, R. J. In Time-ofFllight Mass Spectrometry; Cotter, R J., Ed.; ACS Symposium Series 549; American Chemical Society: Washington DC, 1994; pp 16-48.
1950 Analytical Chemistry, Vol. 67, No. 13, July 7, 1995
kinetic energy distribution provides much improved mass resolution? Resolution as high as 6OOO fwhm (defined as t/(2At),where t is the k h t time of the ion and At is the peak width at half height) has been reported for peptides with masses up to 3000 Da in a commercial ~ y s t e m In . ~ contrast, a simple linear TOFMS normally provides a mass resolution of no greater than 1000 fwhm. Reported here is the observation of mass resolution in the range of 3,000-6,000 fwhm in a simple linear TOFMS. With this high resolution, isotopically resolved mass spectra are observed for peptides with masses up to 3000 Da. To attain this resolution, a time-lag focusing (TLF) method is used for ion extraction. In conventional MALDI instruments, the ions generated by the laser beam near the surface of the sample probe are extracted by a dc potential. In TLF, a short time delay (
