Pulsed laser desorption of biological molecules in supersonic beam mass spectrometry with resonant two-photon ionization detection

Abstract

Pulsed laser desorption Is used to volatllke small Momdecules for supersonlc Jet Injection Into a mass spectrometer. Following desorption of the molecular plume Into the Jet, the molecules are Ionized by resonant two-photon lonlzatlon (RPPI) In a tbne-of-flight mass spectrometer. Several ultraviolet wavelengths have been used to study the lonlzatlon properties of dlfferent groups of blomolecules lncludlng the aMllty to obtaln Son lonlzation and laser wavelength selective lonlzatlon. I n addition, fragmentation has been studied as a functlon of laser wavelength and power denstty. Laser RPPI Is shown to be a versatlle lonlzatlon source In whlch fragmentation can be convenlently controlled by adjustlng these two parameters. Those specles lnvestlgated Include several classes of clinkally Important molecules, In particular, catecholamines and thelr derlvatlves, Indoleamlnes, neuroleptic drugs, water-soluble vitamlns, and purlne bases.Resonant two-photon ionization (RPPI) has been shown to be an ionization method with unique properties for mass spectrometry (1-10). The RPPI process involves the absorption of two photons by a molecule where the first photon excites the species to an intermediate electronic state and absorption of a second photon results in ionization. The sum of the two photons must therefore exceed the ionization potential of the molecule. However, for most aromatics studied to date the excess energy deposited in the molecule in the ionization process remains low when near-ultraviolet photons are used. The result is that R2PI can produce relatively soft ionization where either only the molecular ion or minimal fragmentation is observed for identification. Although electron beam ionization can also produce soft ionization, it can do so only with a decrease in ionization efficiency of several orders of magnitude. The efficiency of R2PI is typically on the order of 0.1-1% with resulting production of molecular ions, although in some cases it is estimated to be as high as 25-100%(2, 4). In addition, with increase of the laser beam power density, extensive fragmentation may be induced for structural analysis. The fragmentation can be varied simply by changing the laser intensity, and a t high input energy, fragments as small as C+ have been observed (2, 7). Control over the fragmentation is therefore more versatile than that provided by other ionization methods.The feature that makes R2PI a unique ionization source for mass spectrometry is the potential for wavelength selectivity. Although ions are detected as the final product, the ionization signal reflects the absorption cross section of the resonant intermediate state. Thus, selectivity can be based on either the relative absorptions or ionization potentials of molecules using a single wavelength. In previous experiments increased selectivity has been achieved by tuning the wavelength to produce an optical-ionization spectrum characteristic of a species (49). The key point here is that RPPI can provide preselection of ions prior to mass analysis. At pre...

Resonance enhanced multiphoton ionization of nucleosides by using pulsed-laser desorption in supersonic beam mass spectrometry

“…As shown in previous studies [20,22], soft ionization mass spectra of adenine and cytosine were observed using laser desorption followed by R2PI at modest laser energy (-lo6 W cmm2). In addition, ionization of guanine was also observed at 266 nm due to the optimization of the desorption/entrainment of sample into the jet over that attained in previous work.…”

“…The supersonic beam time-of-flight mass spectrometer is similar to that used in previous experiments [22]. It consists of a differentially-pumped TOFMS mounted vertically in a stainless steel six-port cross pumped by a 6-in diffusion pump.…”

“…The desorption laser was an EXC-1 Quanta-Ray CO, laser. A Scientech 365 meter was used for measurement of the beam energy, and based upon the beam profile [22], the power density of the beam was estimated to be a maximum of 7 x lo6 W cm-* on the surface of the rod. The IR beam was softly focused with a lo-cm focal length biconvex germanium lens to a -2-3 mm spot for desorption, although this was adjusted for each case to optimize our results.…”

“…The advantage of R2PI is that, based on the wavelength chosen, one can produce relatively efficient soft ionization provided the excess energy of the two photons is relatively small compared to that required for ionization. This method has been used in previous work by our group [20][21][22]281 and that of Grotemeyer et al [25][26][27],311 using a high-resolution reflectron TOF device to demonstrate relatively soft ionization of peptides [21,, porphyrins [24], catecholamines [22] and other biological compounds [20-281 with either production of only the molecular ion or minimal fragmentation [20-271. However, by increasing the energy of the photon or by increasing the power of the laser, one can induce extensive fragmentation for structural analysis [29]. In the latter case the R2PI/MPI process may involve further absorption of many additional photons by the molecular ion, resulting in fragments as small as C+ as the power density is further increased [29].…”

Multi-photon ionization and fragmentation of uracil: Neutral excited-state ring opening and hydration effects

“…Previous single-color MPI experiments using ns laser pulses at 222 nm, 13 248 nm, 14 and 235-268 nm 12 did not produce any discernible uracil cation signals, whereas Brady et al 15 were able to record a REMPI spectrum with a 270-285 nm pump and a 193 nm probe (both ns-timescale pulses). Fig.…”