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Positive-mode atmospheric pressure chemical ionization tandem mass spectrometry (APCI-MS(n)) was tested for the differentiation of regioisomeric aromatic ketocarboxylic acids. Each analyte forms exclusively an abundant protonated molecule upon ionization via positive-mode APCI in a commercial linear quadrupole ion trap (LQIT) mass spectrometer. Energy-resolved collision-activated dissociation (CAD) experiments carried out on the protonated analytes revealed fragmentation patterns that varied based on the location of the functional groups. Unambiguous differentiation between the regioisomers was achieved in each case by observing different fragmentation patterns, different relative abundances of ion-molecule reaction products, or different relative abundances of fragment ions formed at different collision energies. The mechanisms of some of the reactions were examined by H/D exchange reactions and molecular orbital calculations., (© American Society for Mass Spectrometry, 2011)

We report here the construction and characterization of a high-power laser-induced acoustic desorption (LIAD) probe designed for Fourier transform ion cyclotron resonance mass spectrometers to facilitate analysis of nonvolatile, thermally labile compounds. This "next generation" LIAD probe offers significant improvements in sensitivity and desorption efficiency for analytes with larger molecular weights via the use of higher laser irradiances. Unlike the previous probes which utilized a power-limiting optical fiber to transmit the laser pulses through the probe, this probe employs a set of mirrors and a focusing lens. At the end of the probe, the energy from the laser pulses propagates through a thin metal foil as an acoustic wave, resulting in desorption of neutral molecules from the opposite side of the foil. Following desorption, the molecules can be ionized by electron impact or chemical ionization. Almost an order of magnitude greater power density (up to 5.0x10(9) W/cm2) is achievable on the backside of the foil with the high-power LIAD probe compared to the earlier LIAD probes (maximum power density approximately 9.0x10(8) W/cm2). The use of higher laser irradiances is demonstrated not to cause fragmentation of the analyte. The use of higher laser irradiances increases sensitivity since it results in the evaporation of a greater number of molecules per laser pulse. Measurement of the average velocities of LIAD-evaporated molecules demonstrates that higher laser irradiances do not correlate with higher velocities of the gaseous analyte molecules.

The internal energy of neutral gas-phase organic and biomolecules, evaporated by means of laser-induced acoustic desorption (LIAD) into a Fourier transform ion cyclotron resonance mass spectrometer, was investigated through several experimental approaches. The desorbed molecules were demonstrated not to undergo degradation during the desorption process by collecting LIAD-evaporated molecules and subjecting them to analysis by electrospray ionization/quadrupole ion trap mass spectrometry. Previously established gas-phase basicity values were remeasured for LIAD-evaporated organic molecules and biomolecules with the use of the bracketing method. No endothermic reactions were observed. The remeasured basicity values are in close agreement with the values reported in the literature. The amount of internal energy deposited during LIAD is concluded to be less than a few kilocalories per mole. Chemical ionization with a series of proton-transfer reagents was employed to obtain a breakdown curve for a protonated dipeptide, Val-Pro, evaporated by LIAD. Comparison of this breakdown curve with a previously published analogous curve obtained by using substrate-assisted laser desorption (SALD) to evaporate the peptide suggests that the molecules evaporated via LIAD have a similar internal energy as those evaporated via SALD.

Several experimental factors have been investigated that influence the efficiency of desorption and subsequent chemical ionization of nonvolatile, thermally labile molecules during laser-induced acoustic desorption/Fourier transform ion cyclotron resonance mass spectrometry (LIAD/FT-ICR) experiments. The experiments were performed by using two specially designed LIAD probes of different outer diameters (1/2 and 7/8 in.) and designs. Several improvements to the design of the "first generation" (1/2 in.) LIAD probe are presented. The larger diameter (7/8 in.) probe provides a larger surface area for desorption than the smaller diameter probe. Further, it was designed to desorb molecules on-axis with the magnetic field of the instrument. This is in contrast to the smaller probe for which desorption occurs 1.3 mm off-axis. This improved alignment, which provides better overlap between the desorbed molecules and trapped reagent ions, results in a substantial increase in the sensitivity of LIAD analyses. The thickness of the sample layer deposited on the irradiated metal foil and the number of laser shots fired on the backside of the foil were found to have a significant effect on the overall signal and the relative abundances of the ions formed in the experiment. Evaporation of a tetrapeptide, Val-Ala-Ala-Phe (VAAF), from Ag, Al, Au, Cu, Fe, and Ti foils, followed by protonation by protonated pyridine, revealed that the titanium foil provides the greatest signal. The importance of the laser power density was examined by desorbing a low MW polymer, polyisobutenyl succinic anhydride, at power densities ranging from 5.40 x 10(8) to 9.00 x 10(8) W/cm(2) at the backside of the foil. Higher laser power densities resulted in greater signals and an improved distribution for the higher molecular weight oligomers.

A mass spectrometric method is described for the identification and counting of hydroxyl groups in an analyte. Analytes introduced into a FT-ICR mass spectrometer and ionized by positive mode ESI were allowed to react with the neutral reagent diethylmethoxyborane. This results in derivatization of the hydroxyl groups of the analytes by replacement of a proton with a diethylborenium ion. Protonated polyols react by consecutive derivatization reactions, wherein all, or nearly all, of the hydroxyls are derivatized. The polyol derivatization products are separated by 68 mass units in the mass spectrum. This 68 Da mass shift, along with 30 Da mass shifts arising from intramolecular derivatization of the primary derivatization products, makes it easy to count the number of functional groups present in the analyte. The utility of this method for the analysis of polyols as single-component solutions, as mixtures, or in HPLC effluent (LC-MS analysis) is demonstrated.