Elucidation of Reaction Mechanisms Responsible for Afterglow and Reagent-Ion Formation in the Low-Temperature Plasma Probe Ambient Ionization Source

Abstract

The development of ambient desorption/ionization mass spectrometry has shown promising applicability for the direct analysis of complex samples in the open, ambient atmosphere. Although numerous plasma-based ambient desorption/ionization sources have been described in the literature, little research has been presented on experimentally validating or determining the desorption and ionization mechanisms that are responsible for their performance. In the present study, established spectrochemical and plasma physics diagnostics in combination with spatially resolved optical emission profiles were applied to reveal a set of reaction mechanisms responsible for afterglow and reagent-ion formation of the Low-Temperature Plasma (LTP) probe, which is a plasma-based ionization source used in the field of ambient mass spectrometry. Within the dielectric-barrier discharge of the LTP probe, He(2)(+) is the dominant positive ion when helium is used as the plasma supporting gas. This helium dimer ion (He(2)(+)) has two important roles: First, it serves to carry energy from the discharge into the afterglow region in the open atmosphere. Second, charge transfer between He(2)(+) and atmospheric nitrogen appears to be the primary mechanism in the sampling region for the formation of N(2)(+), which is an important reagent ion as well as the key reaction intermediate for the formation of other reagent ions, such as protonated water clusters, in plasma-based ambient ionization sources. In the afterglow region of the LTP, where the sample is usually placed, a strong mismatch in the rotational temperatures of N(2)(+) (B (2)Σ(u)(+)) and OH (A (2)Σ(+)) was found; the OH rotational temperature was statistically identical to the ambient gas temperature (~300 K) whereas the N(2)(+) temperature was found to rise to 550 K toward the tail of the afterglow region. This much higher N(2)(+) temperature is due to a charge-transfer reaction between He(2)(+) and N(2), which is known to produce rotationally hot N(2)(+) (B (2)Σ(u)(+)) ions. Furthermore, it was found that one origin of excited atomic helium in the afterglow region of the LTP is from dielectronic recombination of vibrationally excited He(2)(+) ions.

The Characterization of Self-Assembled Monolayers on Copper Surfaces by Low-Temperature Plasma Mass Spectrometry

“…Dimers from NDM SAMs have also been found with other mass spectrometric methods such as direct analysis in real time (DART) mass spectrometry [4] and two-laser mass spectrometry (L2MS) [29]. Scheme S-1 (in Supplementary Information) shows the proposed mechanism of the main produced ions based on Penning mechanism [30][31][32] + (m/z 403) from NDM SAMs is attributed to proton transfer and charge transfer [30,31]. More details can be seen in the Supplementary Information.…”

Surface analysis of lipids by mass spectrometry: More than just imaging

“…The plasma generated components exit the glass tube where they are directed at the sample and exposed to atmospheric components such as oxygen and water, ultimately leading to desorption and ionization of surface-bound analytes that then enter the mass spectrometer [249] (Fig. 10b).…”

Humidity Effects on Fragmentation in Plasma-Based Ambient Ionization Sources

“…All post-plasma ADI techniques that sample surfaces are thought to thermally desorb analytes. Several discharge gases have been tested and helium yielded good sensitivity for a range of analytes [14]. It is proposed that metastable helium from the plasma directly ionizes analytes via Penning ionization [6] and atmospheric gases to form reagent ions.…”

“…It is proposed that metastable helium from the plasma directly ionizes analytes via Penning ionization [6] and atmospheric gases to form reagent ions. Penning ionization and charge transfer reagents, chiefly protonated water, ionize analytes and yield a protonated water cluster background [14][15][16]. Commonly observed analyte species include the protonated molecule (MH + ), molecular ion (M +• ), and related ions.…”