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

Abstract

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.

Rapid quantification of acetaminophen in plasma using solid‐phase microextraction coupled with thermal desorption electrospray ionization mass spectrometry

“…After the desorption the analytes are ionized via interactions with charged species generated by electrospray ionization (ESI), [5][6][7][8][9][10][11]18 atmospheric pressure chemical ionization (APCI), 12,13,15,16 atmospheric pressure photoionization (APPI), 17 or low-temperature plasma (LTP). 19 All these techniques enable rapid and highly sensitive analysis, even if the desorption and ionization are separate events.…”

Characterization of two modes in a dielectric barrier discharge probe by optical emission spectroscopy and time-of-flight mass spectrometry

“…12 Optical spectroscopy experiments 13,14 showed the state distributions inside the plasma and, thus, also the ionization efficiency in MS-experiments to be strongly dependent on external parameters of the plasma source. Over the years the main inuences could be identied to be briey: (1) the torch geometry, including the material and arrangement of the electrodes and dielectric, 9,[15][16][17][18] (2) type and ow rate of the discharge gas, 8,14 as well as the use of additives 19,20 and (3) the high voltage pulse characteristics. 21 Based on a comprehensive study on the inuence of the applied voltage, Franzke and co-workers postulated two individual operational modes in which current ows: a homogeneous moiety and a lamentary moiety 22 that can be selectively discriminated against by altering the driving voltage.…”

Dielectric barrier discharges in analytical chemistry

“…This proved that the ionization process with a dielectric barrier plasma, the LTP probe, is a simple approach for reaction monitoring with least requirements for experimental devices and procedures. 53 Almasian et al developed a graphite LTP probe with a dual mode fragmentation selectable by an electrical switch for ambient MS. 54 The setup is similar to the previously described ones whereas the axially centered electrode connected to the ac voltage is made out of graphite. It is operating in two modes, in mode A a copper electrode underneath the glass slide serves as second electrode and in mode B a copper foil around the capillary acts as second electrode alternatively.…”

“…Both modes show different fragmentation patterns, which might be applicable in identification studies according to the authors. 54 Recently, the same group demonstrated the application of LTP to elemental analysis of solids. Depth profiling experiments of nanometre coating by LTP and elemental analysis with ICP-MS were proven in ref.…”