100 Years of Photoionization; Desorption Photoionization for Minimally-invasive Cultural Heritage Analysis

As an outgrowth of studies of the photoelectric effect in the 1920s, F. L. Mohler developed a photoionization (PI) source for mass spectrometry – most particularly with three publications in 1926 – that preceded the electron impact ionization method that dominated MS analysis of organic molecules for the next sixty years. In the USSR in 1986, atmospheric pressure photoionization (APPI) was published for the first time, demonstrating the direct analysis of multiple classes of organic compound and presaging the integration of the technique with liquid chromatography (LC). Interest in APPI grew in the 1990s with multiple commercial patents filed for various forms of photoionization lamp and the publication of APPI-LCMS. The early 2000s further saw the development of photoionization for ambient sampling. 2026 marks the 100th anniversary of photoionization, the 40th anniversary of APPI, and the 25th anniversary of the first commercially-available APPI source.

In original research, a novel sampling system is constructed for minimally-invasive sampling of large object surfaces located meters away from a mass spectrometer: non-proximate desorption photoionization. Cultural heritage objects that are too rare, precious or culturally sensitive to accommodate invasive microsectioning – and too large for typical ambient sampling methods – are positioned and sampled with a heated jet of neutral nitrogen without leaving a visible mark. Desorbed analytes are transported through a custom-engineered line and patented in-line dopant permeator to a photoionization lamp and high-resolution MS to observe dyes, plasticizers, and other small organic molecules. A portable probe version of the methodology is also created to sample in real time not just from large objects, but from concave surfaces and other difficult-to-access geometries.