ABSTRACT
Laser-induced acoustic desorption coupled to microplasma-based atmospheric pressure photoionization (LIAD-APPI) using a nebulized sweep jet to aid in dopant introduction and ion transmission has been applied to the analysis of model, apolar lipid compounds. Specifically, several sterols, sterol esters, and triacylglycerols were detected using dopants such as anisole and toluene. Additionally, several triacylglycerols, sterols, carboxylic acids, and hopanoids were detected from complex mixtures of olive oil and Australian shale rock extract as a first demonstration of the applicability of LIAD-APPI on real-world samples. Detection limits using a sweep jet configuration for α-tocopherol and cholesterol were found to be 609 ± 61 and 292 ± 29 fmol, respectively. For sterol esters and triacylglycerols with a large number of double bonds in the fatty acid chain, LIAD-APPI was shown to yield greater molecular ion or [M+NH4]+ abundances than those with saturated fatty acid chains. Dopants such as anisole and toluene, with ionization potentials (IPs) of 8.2 and 8.8 eV, respectively, were tested. A greater degree of fragmentation with several of the more labile test compounds was observed using toluene. Overall, LIAD-APPI with a nebulized sweep jet requires minimal sample preparation and is a generally useful and sensitive analysis technique for low-polarity mixtures of relevance to biochemical assays and geochemical profiling. Graphical Abstract.
ABSTRACT
We demonstrate the first application of laser-induced acoustic desorption (LIAD) and atmospheric pressure photoionization (APPI) as a mass spectrometric method for detecting low-polarity organics. This was accomplished using a Lyman-α (10.2 eV) photon generating microhollow cathode discharge (MHCD) microplasma photon source in conjunction with the addition of a gas-phase molecular dopant. This combination provided a soft desorption and a relatively soft ionization technique. Selected compounds analyzed include α-tocopherol, perylene, cholesterol, phenanthrene, phylloquinone, and squalene. Detectable surface concentrations as low as a few pmol per spot sampled were achievable using test molecules. The combination of LIAD and APPI provided a soft desorption and ionization technique that can allow detection of labile, low-polarity, structurally complex molecules over a wide mass range with minimal fragmentation. Graphical Abstract á .
