Thermal aspects of laser desorption mass spectrometry

Two-laser infrared and ultraviolet matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization (MALDI) was performed using two pulsed lasers with wavelengths in the IR and UV regions. A 10.6 micro m pulsed CO(2) laser was used to irradiate a MALDI target, followed after an adjustable delay by a 337 nm pulsed nitrogen laser. The sample consisted of a 2,5-dihydroxybenzoic acid matrix and bovine insulin guest molecule. The pulse energy for both of the lasers was adjusted so that the ion of interest, either the matrix or guest ion, was not produced by either of the lasers alone. The delay time for maximum ion yield occurs at 1 micro s for matrix and guest ions and the signal decayed to zero in approximately 400 micro s. A mechanism is presented for enhanced UV MALDI ion yield following the IR laser pulse based on transient heating.

Design and calibration of an electrostatic energy analyzer-time-of-flight mass spectrometer for measurement of laser-desorbed ion kinetic energies

The design of a hybrid electrostatic energy analyzer-time-of-flight mass spectrometer for measurement of ion kinetic energies produced by laser desorption ionization is presented. The need for experimental evaluation of the calibration and performance of the instrument is discussed and a novel laser multiphoton ionization technique, which allows experimental calibration of the energy bandpass of the electrostatic energy analyzer, is described. Laser multiphoton ionization at varying electric field strengths also allows the effects of electric field distortions on energy resolution of the instrument to be probed. Measurement of the translational energies of ions produced by 266-nm laser desorption ionization at 48 mJ/cm(2) of material adsorbed to a stainless steel probe by using this instrument also is presented. Ion translational energies of +19±5, +10±5, and +10±5 eV are found for adsorbed Na(+), K(+), and m-xylene M(+), respectively.

Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP](+) reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP](+) formed by charge exchange with M(+), which is also present in the spectrum. Negative ion spectra are dominated by [TPP](-); which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP](+) with M and by a second pathway under plasma ignition conditions through reaction of TPP with M(+). In negative ion mode, metallation is only observed under plasma ignition conditions through reaction of [TPP](-) with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.

Potassium halide adducts as reagent ions in infrared laser desorption/ionization fourier transform ion cyclotron resonance mass spectrometry

Potassium halide adducts of the form K2X(+) (X = F, CI, Br, and I) desorbed from neutral salts by high power, pulsed, infrared laser radiation are detected in abundance by Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. FT-ICR detection of the K2X(+) adduct is favored at increased laser power densities (> 10(8) W/cm(2)) and at trapping potentials below 3 V, independent of X. In contrast, detection of K(+) is promoted at laser power densities below 10(8) W/cm(2) or at higher trapping potentials, with a threshold for trapping that is strongly dependent on X. When laser desorption/ionization (LDI)/FT-ICR is performed on 1:1 mixtures of KX and organic molecules, ejection pulses applied continuously at the cyclotron resonance frequency of K2X(+) inhibit formation of the cation-attached product, [M + K](+). Conversely, resonance ejection of K(+) enhances [M + K](+), apparently by reducing the matrix ion population trapped in the cell. In evaluating higher molecular weight adducts, only K3F 2 (+) formed in abundance by laser desorption of KF is found through double resonance experiments to contribute significantly to formation of [M + K](+). Finally, among the potassium halides, KI generates the highest ratio of detected K2X(+) to K(+) at low trapping potentials and is therefore best suited for cation-transfer reactions in infrared LDI/FT-ICR experiments performed at power densities in the 10(8) W/cm(2) range.

Time-of-flight mass spectrometry: an increasing role in the life sciences

Although available commercially since the mid-1950s, there has been a renewed and increasing interest in time-of-flight mass spectrometers during the last decade or more. Improvements have been made in mass resolution; and high-speed data acquisition systems have been developed which enable the recording of all ions in each time-of-flight cycle. Most importantly, these instruments have been coupled with several new ionization techniques, which are capable of desorbing the relatively large and intractable biopolymers whose structures are of interest to molecular biologists, biochemists and biophysicists. Primarily these are techniques which employ pulsed lasers, fission fragments and pulsed ion beams, for which a 'non-scanning' and/or high-transmission analyzer provides considerable analytical advantage. In this report we review some basic principles of the time-of-flight mass analyzer, highlighting efforts to improve dynamic focusing for instruments forming ions in the gas phase and static focusing for desorption instruments, and the progression from time-slice to time-array detection. We also review some of the accomplishments of instruments employing the time-of-flight analyzer, including: molecular weight determinations for peptides and small proteins; the analysis of tryptic digests, crude extracts and whole cells; the structural analysis of glycolipids, phospholipids and lipopolysaccharides; and the determination of covalent and metal-linked peptide dimers. We conclude with some recent developments in combining the time-of-flight analyzer with liquid chromatography using the continuous flow probe technique.

Mass Spectrometry of Large, Fragile, and Involatile Molecules

Desorption ionization makes it possible to obtain mass spectra of molecules whose vaporization by heating may lead to thermal degradation. Several methods are in use, but in general desorption is achieved by particle or photon bombardment of the sample and the mass spectra obtained by different methods are fundamentally similar. Desorption ionization techniques have been used to obtain mass spectra of biomolecules, including peptides, antibiotics, and oligosaccharides, for which normal mass spectral methods have been of limited power. Several examples are given of recent applications of these new techniques, and prospects for their further evolution are discussed.