Surface Ionization of Carbaryl, Papaverine, Cocaine, Morphine, and Triethylamine by Heated Surface-Oxidized Metal Filaments of W, Re, Pd, Mo, Ti, and SUS304 Under Atmospheric Pressure: Ionization Mechanism

Abstract

Rationale

The objective of the present study is to investigate the ionization mechanisms for atmospheric pressure surface ionization (APSI).

Methods

APSI of carbaryl, papaverine, cocaine, morphine, and triethylamine using heated surface-oxidized metal filaments of W, Re, Pd, Mo, Ti, and SUS304 were measured by mass spectrometry. Low-volatility analytes dissolved in methanol were desorbed by Leidenfrost phenomenon-assisted thermal desorption and introduced to the heated metal filament used as an emitter. Alkylamine benzene solutions were simply placed 10 mm below the filament.

Results

Carbaryl, papaverine, cocaine, and morphine (M) gave protonated molecules [M+H]⁺ with little fragment ions. Triethylamine (TEA) gave both [TEA+H]⁺ and [TEA−H]⁺.

Conclusion

Because [M+H]⁺ was detected with little [M−H]⁺, which is usually detected as a major ion by vacuum surface ionization (VSI), it was concluded that [M+H]⁺ is formed by the proton transfer reaction between the protonated filament surface and the gas-phase analyte molecules approaching the solid surface. Namely, the heated metal filament acts as a Brønsted acid. This idea is supported by the APSI for TEA.