Development of a graphite low-temperature plasma source with dual-mode in-source fragmentation for ambient mass spectrometry

A new low-temperature plasma (LTP), based on dielectric barrier discharge (DBD), has been developed as an alternative ionization source for ambient mass spectrometry. For organic samples, the source is able to produce two different fragmentation patterns which are selectable by an electrical switch. The two source modes are different only in the second electrodes: in configuration (A), bar-plate and in configuration (B), coaxial bar-cylinder shapes are used. A disposable graphite probe is used as the first electrode, the same in both configurations, and a copper foil is used as the second electrode. The ionization source is applicable to gas and liquid samples, without any change being necessary in its design. Under optimal conditions, to take ethylbenzene as an example, a detection limit of less than 25 ng was obtained and a relative standard deviation (RSD) of 13.36% has been demonstrated for 50 ng of ethylbenzene (n = 11). We have found several interesting differences in the mass spectra of the tested volatile organic compounds (VOCs) in the two modes, which might be applicable in identification studies. We have investigated the effect of variation of the first electrode material and the second electrode length in mode B. Moreover, in this design the graphite electrode is capable of acting as a sample adsorbent, which is a new sampling method for LTP mass spectrometry (MS). This capability was investigated by adsorption of the selected VOCs onto the surface of the graphite electrode in a headspace solid-phase microextraction (SPME) system, and direct desorption and ionization of the samples by LTPMS.