Single photon ionization is used to softly ionize the constituents of soft materials for subsequent mass analysis. Soft materials under study include polymers, biological materials, and self-assembled monolayers (SAMs). The soft ionization process has been found to be very efficient, allowing for trace analysis of soft materials in micron scale surface regions. Soft ionization also substantially reduces fragmentation of the components of these materials, and has the added advantage of selectivity based on the ionization potential of the analyte molecule. Single photon ionization of molecules requires a laser operating in the vacuum ultraviolet (VUV) energy range. Reliable 157nm (7.9eV) lasers are now being used, and studies with tunable, powerful sources such as the new generation of free-electron lasers are being planned. The LEUTL at Argonne is expected to have 10-100 times the pulse energy of a conventional laser in the VUV.
In one recent study, a SAM was formed by adsorbing benzenethiol onto a gold surface. The surface was probed by laser desorption followed by single photon ionization, whereupon it was found that the molecules in the monolayer must form dimers with S-S bonds during desorption. This conclusion could not have been made with conventional ionization methods that cause significant fragmentation of molecules. With soft ionization, there was very little fragmentation in the spectra, allowing for easy identification of the molecular species (the parent ion was the largest high mass peak). The useful yield measured in this study was the highest ever reported for any molecular species (0.5%). The next generation instrument will improve this figure of merit by at least tenfold, approaching the theoretical limit of sensitivity for the technique.
Single-photon ionization allows for the first time a very sensitive probe of molecular surfaces by mass spectrometry. New studies on polymer surface migration, SAM growth for templating molecular materials, and trace analysis of DNA adducts are possible. For DNA adducts, the technique has potential to revolutionize cancer research. Many carcinogens bind directly to the DNA chain, forming an adduct and lowering the ionization potential (IP) of the DNA. With a wavelength just over the IP of the adduct but below that of the unmodified DNA, only the adduct is ionized. Detection of a trace amount of adduct in a large background of DNA is then possible, enabling cancer research on small carcinogenic loads in single subjects.
W.F. Calaway, J.F. Moore, M.J. Pellin, M.R. Savina, I.V. Veryovkin, Argonne National Laboratory
B.V. King, University of Newcastle, Australia