Ignacio A. Zuleta

Desorption Electrospray Ionization: Achieving Rapid Sampling Rates

The sampling rate and imaging capabilities of desorption electrospray ionization (DESI) are examined using a rotating sample platform combined with Hadamard transform time-of-flight mass spectrometry (HTTOFMS), a multiplexed time-of-flight technique that allows for millisecond acquisition of full mass-to-charge ratio scans. DESI-compatible dyes are used to produce spatially defined sample patterns on poly(methyl methacrylate) discs. Control of disk rotation rate sets the residence time of the sample spots in the DESI plume, and thus the sampling rate. Surface patterns of alternating analytes are spectrally resolved up to 80 samples/s and single-analyte spots up to 50 samples/s. The rapid movement of the surface under the DESI plume allows for high DESI solution flow rates without blurring the chemical information on the surface. Data from multiple rotations can be additively combined, generating a chemical image of the surface with improved signal-to-noise characteristics. This multipass data enables analysis of the rising and falling edges of the analyte signal, placing a lower limit on both the temporal resolution of DESI and the maximum achievable sampling rate. Multipass analysis is proposed as a method for DESI surface imaging.

Continuous Time-of-Flight Ion Imaging: Application to Fragmentation

We have designed and constructed a continuous imaging reflectron time-of-flight mass spectrometer (TOFMS) that provides a mass spectrum at every pixel of a two-dimensional image with a 100% duty cycle. The technique is based on pseudorandom ion beam modulation and three-dimensional ( x, y, t) ion imaging. We use a multichannel plate detector with a delay-line anode that provides x, y positions and flight times t of every ion arrival event. The precision of the peak heights in the 100% duty cycle mass spectra is shown to be enhanced even at short (10 ms) acquisition times, which should prove useful for the study of solution kinetics or fast chromatographic separations. As a demonstration of the systems capability, we have imaged the fragmented ions that underwent surface-induced dissociation inside the reflectron and the ions that fragmented spontaneously through postsource decay.

Computer-controlled, variable-frequency power supply for driving multipole ion guides.

A high voltage, variable-frequency driver circuit for powering resonant multipole ion guides is presented. Two key features of this design are (1) the use of integrated circuits in the driver stage and (2) the use a stepper motor for tuning a large variable capacitor in the resonant stage. In the present configuration the available frequency range spans a factor of 2. The actual values of the minimum and maximum frequencies depend on the chosen inductor and the capacitance of the ion guide. Feedback allows for stabilized, computer-adjustable rf amplitudes over the range of 5-500 V. The rf power supply was characterized over the range of 350-750 kHz and evaluated by driving a quadrupole ion guide in an electrospray time-of-flight mass spectrometer.