A system combining laser desorption, jet-cooling, resonant 2-photon ionization (R2PI) spectroscopy, and time-of-flight mass spectrometry (TOF-MS) generates exceptionally well resolved vibronic spectroscopy and fragment free mass spectrometry for molecules within a complex matrix. Following laser desorption, gas-phase molecules are entrained in a supersonic molecular beam of argon. This causes extremely efficient jet-cooling of the internal degrees of freedom within the molecules and makes it possible to perform high resolution spectroscopy. The cold molecule is resonantly excited to the first electronic excited state by a tunable dye laser, and subsequently ionized by another photon before being detected in the TOF-MS. The result is the ability to selectively ionize a single target molecule within an extremely complex matrix.
The advantages of R2PI spectroscopy have been extended to include nanoscale proximal probe thermal desorption, allowing detection of organic paint pigments in situ within microscopic paint cross sections. These organic paints are often present as part of complex mixtures within thin layers, well below the spatial resolution of many instruments. We also report the first high resolution spectroscopy of chromophores within madder dye, an historically significant organic colorant, allowing for the unambiguous identification of the molecule within the intrinsically complex nature of ancient paintings.
Additionally, pottery sherds from Maya vessels (600-900 CE) and Mississippian vessels (1100-1200 CE) were examined for three molecular markers (theobromine, theophylline and caffeine) using R2PI spectroscopic techniques. The data obtained supports the existence of a previously unknown trade network between Mesoamerican and North American civilizations.
