The de Vries lab has helped develop the use of an atomic force microscope (AFM) to sample material at submicron resolution, well below the optical diffraction limit. This technique is designed with the goal of unambiguously identifying organic componenents in cultural heritage objects. The sampled material is later analyzed by a combination of high resolution laser spectroscopy and mass spectrometry. This powerful combination provides detailed information in three dimensions: spatial mapping, high resolution spectroscopy and mass spectrometry. Other analytical methods are currently being explored.
This method retains the bulk of the sample, which may be retained and/or used for analysis by other complementary techniques. By separating the microscopy and the chemical analysis steps it becomes possible to apply other analytical techniques at the submicron scale. We employ LMS as the second step, which combines multiphoton ionization and double resonance spectroscopy with mass spectrometry. This approach collects simultaneously both vibronically resolved optical spectra and fragment-free mass spectra for molecules of interest. The high resolution wavelength selectivity in our ionization step (a) allows for detection of a specific analyte in a mixture, eliminating the need for sample preparation, (b) provides the necessary high sensitivity, and (c) provides unique selectivity in the identification of the compound.
Atomic force microscopy enables much higher spatial resolution but lacks the chemical information that light based techniques can provide. Our new combination of laser MS with the AFM-TD sampling now makes it possible to perform spectroscopic analysis with submicron spatial resolution. We have shown that this approach allows R2PI analysis on small samples such as painting cross-sections typically found in cultural heritage. This method is applicable to the cross section samples typically used in conservation science research, with the spatial resolution necessary to isolate small quantities of organic material in discrete layers of a cross section, and the sensitivity to provide molecularly specific identification of materials present in very low concentrations.