Towards nanoscale molecular analysis at atmospheric pressure by a near-field laser ablation ion trap/time-of-flight mass spectrometer.

An instrument that combines near-field laser ablation at atmospheric pressure with an ion trap/time-of-flight mass spectrometer was developed. By coupling a UV laser into a fiber tip of a scanning near-field optical microscope, ablation craters much smaller than achievable with conventional laser optics can in principle be obtained. Laser ablation was performed on samples such as DHB, anthracene, and pyrene. Desorbed neutral analytes are transferred from atmospheric pressure to an ion trap, ionized, and stored. After 10 ms, the ions are extracted into a sensitive time-of-flight spectrometer. We demonstrate the feasibility of this unique SNOM-MS instrument for chemical analysis with unprecedented lateral resolution at atmospheric pressure. Spatially resolved molecular analysis with a lateral resolution of 5 microm (fwhm) and a sensitivity of approximately 60 fmol of solid anthracene is demonstrated, along with topographical analysis with the same instrument. No other technique available today offers this lateral resolution in combination with soft mass spectrometry and the capability of sampling fragile specimens at atmospheric pressure.

Exciton mobility and trapping in a MALDI matrix.

Energy transfer (ET) from excited matrix to fluorescent traps is used to probe the mobility of excitations in the matrix-assisted laser desorption/ionization (MALDI) matrix material 2,5-dihydroxybenzoic acid. The dependence of host and guest fluorescence on excitation density (laser intensity) and trap concentration gives clear evidence for long-range energy transport in this matrix. This conclusion is further supported by time-resolved emission data showing a 2 ns delay between matrix and trap emission. Rate equation and random walker models give good agreement with the data, allowing determination of hopping, collision, and trapping parameters. Long-range energy transfer contributes to the pooling reactions which can lead to primary ions in MALDI. The results validate the pooling aspect of the prior quantitative MALDI ionization model (J. Mass Spectrom. 2002, 37, 867-877). It is shown that exciton trapping can decrease MALDI ion yield, even at low trap concentration.

Ionization energy reductions in small 2,5-dihydroxybenzoic acid-proline clusters.

The photoionization of (pro)(n)DHB (pro = proline, DHB = 2,5-dihydroxybenzoic acid, n = 0, 1, 2 or 4) clusters was studied both experimentally and computationally. Experimentally the (pro)(n)DHB clusters are generated in the gas phase by laser desorption and supersonic jet entrainment. The photoionization thresholds are then determined by the mass-selective measurement of both one- and two-color photoionization efficiency curves. These experiments demonstrate that the ionization energies (IEs) of the (pro)(n)DHB clusters are substantially reduced in comparison with the IE of free DHB. Computational studies of the (pro)(n)DHB clusters provide insights into the mechanism of IE reduction. For the (pro)DHB system the IE reduction results from spin delocalization in the ion state of the cluster. In contrast, for the (pro)(2)DHB and (pro)(4)DHB clusters the IE reduction results from an inductive delocalization of electron density from pro to DHB in the ground state of the cluster. This latter effect, which is a result of the specific hydrogen-bonding interactions occurring in the mixed clusters, leads to IE reductions of >1 eV. Finally, determination of the energetics of the (pro)(2)DHB radical cation demonstrate that the DHB-to-proline proton transfer reaction is a barrierless, exoergic process in the ion state and that energetic demands for cluster dissociation to protonated (pro)(2) plus a deprotonated DHB radical are substantially lower than those for cluster dissociation to (pro)(2) plus DHB(+*). Cumulatively, these studies provide new energetic and mechanistic insights into both primary and secondary MALDI ionization processes.