Mass spectra of diverse organic species utilizing the liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma ionization source

Abstract

This laboratory has demonstrated the analytical versatility of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source. Demonstrations have included elemental analysis of solutions and laser ablation (LA)-produced aerosols, ambient desorption of small organic molecules on substrates, and direct speciation of organometallic complexes in solution. Here we present initial efforts in mapping the operation space of the device as it applies to obtaining molecular mass spectra from diverse organic molecules. An initial parametric evaluation is performed using caffeine as the test molecule, with a preliminary limit of detection of 5.9 × 10⁻⁹ M (43 pg) for 20 μL injections. Conditions optimum for that molecule are applied in assessing the qualitative nature of spectra from caffeine, sinapinic acid, daidzin (a flavonoid), a FITC-labeled lipid tethered ligand, the mass marker Ultramark 1621, and the heme-protein myoglobin. The product spectra for the small molecules are very much like what would be obtained from electrospray and atmospheric pressure chemical ionization sources, predominately the production of the protonated pseudomolecular ion of the form (M + H)⁺. The mid-range molecules show ion signatures of the thermally stable fragments. Finally, the macromolecules yield mass spectra that are very much like those from electrospray, producing a range of protonated charged states. While much optimization and understanding are needed to best apply the source to such solutes, the fact that the same ion source can be used across the spectrum from elemental/isotopic analysis to perhaps proteomics is totally unique and is worthy of continued development.