A piezoelectric pipet is used to dispense arrays of low-nanoliter aliquots of matrix and DNA into individual etched wells on <1 in. silicon chips prior to their semiautomated analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Spectrum acquisition is expedited relative to conventional “large-spot” MALDI since the resulting miniaturized samples are approximately the same size as the irradiation area of the ionization/desorption laser; thus, searching for crystal regions from which intense analyte signals may be obtained is not necessary. Using a linear TOF instrument designed for scanning high-density arrays of samples, mass spectra from as little as 0.2 fmol (45 nM) of a 36-mer DNA have been acquired from single miniature elements. Spot-to-spot reproducibility from microdispensed samples is superior to that using conventional pipets; in less than 6 min, spectra with high signal-to-noise ratios were acquired from 100 elements containing 8 fmol of a 25-mer. Low-nanoliter quantities of DNA diagnostic products generated in primer oligo base extension reactions from PCR templates were transferred to chips and analyzed by MALDI-TOF MS, giving accurate genotyping results for single base mutations and short tandem repeat polymorphisms (microsatellites). These procedures provide enabling capabilities for extremely accurate high-throughput DNA diagnostics and sequencing based on mass spectrometry.

The ability to simultaneously focus a wide mass range of metastable fragment ions formed after the initial ionization event in a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer has been made possible by the development of a new type of ion reflector. This coaxially designed time-of-flight instrument employs a modified single stage reflector whose axial voltage gradient rises differentially in order to produce an alignment of energy focal points for product ions. In contrast, product-ion focusing in conventional constant field reflectors occurs over a broad range of distances from the reflectron exit. Approximately 90% of the product-ion mass spectrum can be collected without adjustment, thereby eliminating the need to scan the reflector voltage. Design considerations of the curved field reflectron, its calibration properties and representative metastable spectra of several peptides are discussed.

A new type of curved-field reflectron has been developed for the energy focusing of ions formed after initial acceleration in time-of-flight (TOF) mass spectrometers. These include ions generated in the dissociation region of a tandem TOF, as well as metastable decay products formed in the field-free drift region prior to their reflection. Unlike conventional linear-field reflectrons, which focus product ions to different focal lengths that are proportional to the mass (energy) of the fragment, the new curved-field reflectron energy focuses all ions to within a small region near the exit of the reflectron. Thus, the collection of resolved fragment peaks is possible without scanning or stepping the potential gradient of the reflectron, enabling faster acquisition of the production spectrum and, more importantly, true multiplex recording of the spectra from each ionization event. Theoretical considerations of the design are discussed along with an evaluation of collision-induced dissociation data, demonstrating the performance of the new reflectron in a tandem TOF instrument.