Direct inlet probe mass spectrometry with wavelength selective resonance enhanced photo ionisation

We demonstrate a wavelength-selective atmospheric-pressure laser ionisation (APLI) workflow for direct inlet probe (DIP) evolved-gas analysis with ultra-high-resolution mass spectrometric detection. A tuneable optical parametric oscillator (213, 225, 248 and 266 nm) was integrated into a modified atmospheric-pressure ion source and coupled to Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to profile complex mixtures released directly from liquids and solids. Screening of over 70 aromatic standards revealed pronounced, compound-specific wavelength dependencies, with odd-electron molecular ions dominating and intensity maxima following known resonance-enhanced multiphoton ionisation (REMPI) band structures, enabling selectivity predictions based on the REMPI and APLI literature. Applied to fossil oils, oil blends, and asphaltenes, wavelength tuning systematically shifted molecular fingerprints while retaining aromatic selectivity in highly complex spectra (hundreds of assigned sum formulae per experiment). Shorter wavelengths (213/225 nm) emphasised naphthalene-type cores (double bond equivalent, DBE 7), whereas longer wavelengths (248/266 nm) enhanced phenanthrene/anthracene-type and more condensed aromatics (DBE 10-15). For polymers (polystyrene and crumb rubber), overall spectral patterns were less wavelength-dependent, but ion yields varied with wavelength, and additive signals (e.g., p-phenylenediamine antiozonants) were readily detected alongside characteristic pyrolysis series. This DIP-APLI-FT-ICR MS approach enables rapid analysis of insoluble materials with minimal sample preparation while providing the ionisation wavelength as a practical "dial" to tailor selectivity and reduce spectral congestion at the ionisation stage. Thus, this technique broadens the capabilities of common DIP-MS approaches, which are dominated by chemical ionisation and lamp-based methods (APCI/APPI).