Matrix-Assisted Infrared-Laser Desorption-Ionization Mass Spectrometry of Organic Molecules on NaNO 3
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R. F. HAGLUND, JR. Department of Physics and Astronomy and W. M. Keck Foundation Free-Electron Laser Center, Vanderbilt University, Nashville TN 37235 [email protected] *Physics Department, Reed College, 3203 SE Woodstock Boulevard, Portland, OR 97202-8199
ABSTRACT We demonstrate sensitive mass identification of model organic compounds directly from sodium nitrate by infrared matrix-assisted laser desorption and ionization mass spectrometry. Sensitivity limits of order 10' are achieved for resonant desorption, with a linear calibration verified for crown ether molecules over almost three decades in concentration. We also have observed that crown ether molecules can abstract atomic sodium from the sample when irradiated at the resonant frequency of the NO 3 stretching vibrations around 7.1 Pm. A model for this process is proposed which suggests great promise for this technique on untreated tank waste.
INTRODUCTION High-sensitivity, rapid and economical characterization of high-level tank wastes is a critical unmet need in the Department of Energy's environmental management program [I]. We are developing mass spectrometry protocols for molecular constituents of high-level tank wastes in solid, liquid and slurry forms using tunable, ultrashort-pulse, infrared lasers by investigating: the effects of laser wavelength, intensity and fluence on desorption and ionization of atomic and molecular species from nitrate, phosphate, sulfate, carbonate and ferrocyanide samples; the correlation between extinction coefficient, deposited laser energy and desorption-ionization thresholds in solid, liquid and slurry materials using photoacoustic, surface-plasmon and FTIR spectrometry; and the systematic effects of laser parameters and chemical reactivity between matrix and analyte on our ability to quantitate the desorption and ionization of desorbed species, including the formation of adduct complexes on analyte ions. In this work, we particularly seek to capitalize on the high efficiency of desorption, bond-breaking and ionization in solids, liquids, and slurries when initiated by resonant vibrational excitation with picosecond laser pulses. In this paper, we present some of the principal findings emerging from our initial studies of desorption of organic molecules from sodium nitrate, including: (1) the capacity of a tunable infrared laser, in conjunction with time-of-flight mass spectrometry, to identify organics on solid and water slurries of sodium nitrate; (2) initial calibration curves for ion yields of eighteen-crown ether (specifically, 18C6) as a function of concentration down to I ppm; and (3) demonstration that Nae adduct ions are formed during in the desorption of 18C6 from sodium nitrate. The fact that the most efficient desorption occurs at the resonant frequency for the NO3 stretching vibration permits us to develop a tentative model for this adduct formation. This work suggests that IR-MALDI can be a useful quantitative analytical tool in three areas where current methods are either insufficient to mee
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