Becker & Wu, On the photoionization of large molecules, J. Am. Soc. Mass Spectrom. 6 (1995) 883–888. doi:10.1016/1044-0305(95)00472-P.
ABSTRACT: There is no apparent limit to the size of a molecule for which photoionization can occur. It is argued that it is difficult to obtain useful photoionization mass spectra of peptides (above ∼ 2000 u), proteins, and oligonucleotides, because of the high internal energy of these polar molecules as a result of the desorption event and because vibrationally excited radical cations readily fragment. Evidence to support this hypothesis is presented from the 118-nm single-photon ionization (SPI) mass spectra of the cyclic decapeptide gramicidin S and of fullerenes, from null SPI results with the linear peptides substance P and gramicidin D and oligonucleotides, and from a variety of data found in the literature. The literature data include mass spectra from jet-cooled peptides, perfluorinated polyethers, collisional ionization of small neutral peptides, and the ultraviolet photoelectron spectroscopy of polymeric solids.
Soltwisch, Kettling, Vens-Cappell, Wiegelmann, Müthing, & Dreisewerd, Mass spectrometry imaging with laser-induced postionization, Science. 348 (2015) 211–215. doi:10.1126/science.aaa1051.
ABSTRACT: Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) can simultaneously record the lateral distribution of numerous biomolecules in tissue slices, but its sensitivity is restricted by limited ionization. We used a wavelength-tunable postionization laser to initiate secondary MALDI-like ionization processes in the gas phase. In this way, we could increase the ion yields for numerous lipid classes, liposoluble vitamins, and saccharides, imaged in animal and plant tissue with a 5-micrometer-wide laser spot, by up to two orders of magnitude. Critical parameters for initiation of the secondary ionization processes are pressure of the cooling gas in the ion source, laser wavelength, pulse energy, and delay between the two laser pulses. The technology could enable sensitive MALDI-MS imaging with a lateral resolution in the low micrometer range.
Huang, Fan, & Murray, Matrix-assisted laser desorption ionization of infrared laser ablated particles, Int. J. Mass Spectrom. 274 (2008) 21–24. doi:10.1016/j.ijms.2008.04.006.
ABSTRACT: An infrared (IR) laser was used to ablate particles that were subsequently ionized by matrix-assisted laser desorption ionization (MALDI). Infrared light from a pulsed optical parametric oscillator (OPO) laser system was directed at a solid sample under vacuum containing a 2,5-dihydroxybenzoic acid (DHB) matrix and peptide or protein analyte. A pulsed 351 nm ultraviolet (UV) excimer laser that was directed 1.4 mm above and parallel to the sample surface was used to irradiate the ablated material in the desorption plume. Ions created by post-ablation ionization were detected with a linear time-of-flight (TOF) mass spectrometer. Mass spectra of the peptide bradykinin and proteins bovine insulin and cytochrome c, were recorded. Under these conditions, two simultaneous mass spectra were generated: an IR–MALDI mass spectrum from the OPO and a UV post-ablation spectrum generated by irradiating material in the plume. Factors affecting the two-laser ion yield were studied, including the delay time between the laser pulses and the fluence of the IR and UV laser.
Leisner, Rohlfing, Berkenkamp, Hillenkamp, & Dreisewerd, Infrared laser post-ionization of large biomolecules from an IR-MALD(I) plume, J. Am. Soc. Mass Spectrom. 15 (2004) 934–941. doi:10.1016/j.jasms.2004.03.010.
ABSTRACT: A two-infrared laser desorption/ionization method is described. A first laser, which was either an Er:YAG laser or an optical parametric oscillator (OPO), served for ablation/vaporization of small volumes of analyte/matrix sample at fluences below the ion detection threshold for direct matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). A second IR-laser, whose beam intersected the expanding ablation plume at a variable distance and time delay, was used to generate biomolecular ions out of the matrix-assisted laser desorption (MALD) plume. Either one of the two above lasers or an Er:YSGG laser was used for post-ionization. Glycerol was used as IR-MALDI matrix, and mass spectra of peptides, proteins, as well as nucleic acids, some of which in excess of 10(5) u in molecular weight, were recorded with a time-of-flight mass spectrometer. A mass spectrum of cytochrome c from a water ice matrix is also presented. The MALD plume expansion was investigated by varying the position of the post-ionization laser beam above the glycerol sample surface and its delay time relative to the desorption laser. Comparison between the OPO (pulse duration, tau(L) = 6 ns) and the Er:YAG laser (tau(L) approximately 120 ns) as primary excitation laser demonstrates a significant effect of the laser pulse duration on the MALD process.