Wednesday, May 24, 2004 -- Poster 118

Practical Considerations Related to Increasing MALDI-TOFMS Throughput
Noah P. Christian; Kirk S. Boraas; James P. Reilly; Inproteo, Bloomington, IN; Indiana University, Bloomington, IN;

Introduction
Commercial MALDI-TOF mass spectrometers currently operate at laser repetition rates on the order of 50-200 Hz and this represents a limitation on the overall throughput of a mass spectrometer. To date, limited success has been demonstrated with higher repetition-rate lasers on linear TOF instruments. This has been attributed to the inability to get useful signal without obliterating a sample. By designing the instrument to be insensitive to laser position changes and by rapidly steering a laser over the sample spot, we have been able to demonstrate useful single-shot spectral acquisition at continuous rates in excess of 1 kHz.

Methods
A linear TOF system was constructed with ion optics specifically designed to remove any bias of signal intensity versus laser position. Following this change, a computer-controlled laser positioning system was implemented in the laser optics path, directing light from a diode-pumped Nd:YLF laser into the sample chamber in a rapidly addressable format. In order to maximize the on-time of the laser, and therefore the overall throughput of the instrument, a continuously fed substrate system was also developed. This allows for new samples to be positioned in the source without waiting for sample plates to be unloaded and loaded into the instrument.

Preliminary Results
In contrast with systems such as atmospheric pressure MALDI that utilize ion trapping or collection prior to TOF analysis, linear high-vacuum MALDI-TOF apparati reveal individual shot-to-shot differences. These variations can be due to the inhomogeneity of the sample, variations in laser power, and in the worst case, from complete sample ablation from previous laser pulses. In the newly presented system, a complete acquisition of 1000 shots may be accomplished in one second where no two laser pulses hit the same location of the sample. We have been able to obtain consistent MALDI mass spectra of single shots over an entire sample using this laser steering system. A signal intensity profile as a function of irradiated position is readily obtained. Comparison of high and low repetition rates offers insight into the potential for high-throughput laser steered MALDI sources. We will also discuss practical considerations associated with a linear TOFMS system at these higher repetition rates. Due to the velocity distribution of desorbedions, delayed extraction performs best with higher voltage pulses in the source region compared to orthogonal TOF systems. Results of simulations and calculations will be shown demonstrating this, coupled with practical limitations due to capacitances of the pulsing systems. A high throughput pulser which reduces capacitance by an order of magnitude over commercial pulsers has been constructed, allowing lower current, higher stability power supplies to be used in the system. Other efforts at increasing power supply stability, which relates directly to mass calibration stability, will be discussed.

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