Characterization of direct-current atmospheric-pressure discharges useful for ambient desorption/ionization mass spectrometry

Abstract

Two relatively new ambient ionization sources, direct analysis in real time (DART) and the flowing atmospheric-pressure afterglow (FAPA), use direct current, atmospheric-pressure discharges to produce reagent ions for the direct ionization of a sample. Although at a first glance these two sources appear similar, a fundamental study reveals otherwise. Specifically, DART was found to operate with a corona-to-glow transition (C-G) discharge whereas the FAPA was found to operate with a glow-to-arc transition (G-A) discharge. The characteristics of both discharges were evaluated on the basis of four factors: reagent-ion production, response to a model analyte (ferrocene), infrared (IR) thermography of the gas used for desorption and ionization, and spatial emission characteristics. The G-A discharge produced a greater abundance and a wider variety of reagent ions than the C-G discharge. In addition, the discharges yielded different adducts and signal strengths for ferrocene. It was also found that the gas exiting the discharge chamber reached a maximum of 235°C and 55°C for the G-A and C-G discharges, respectively. Finally, spatially resolved emission maps of both discharges showed clear differences for N 2 ϩ and O(I). These findings demonstrate that the discharges used by FAPA and DART are fundamentally different and should have different optimal applications for ambient desorption/ionization mass spectrometry (ADI-MS). irect-current (DC) discharges have been widely used for elemental analyses since they were first introduced for alloy characterization [1]. When DC discharges were coupled with mass spectrometry, the result was a very sensitive and powerful tool for elemental [1] and molecular analyses [2,3]. Of the many electrical regimes of DC discharges, three forms have been found to have particular analytical merit: the arc, the glow, and the corona. Among these three types of discharges, the fundamental distinction is the operating current and voltage. The arc occurs at very high currents (hundreds of amperes) with a low voltage drop between electrodes (tens of volts). It also exhibits negative resistance; that is, the sustaining voltage drops as the current rises. The glow discharge (GD), which has conventionally been operated between 0.1 to 10 Torr, exists at much lower currents (tens of milliamperes) and a higher voltage drop (hundreds of volts). Lastly, the corona discharge operates with very low currents (a few microamperes) and a much higher voltage drop (several kilovolts).Corona discharges find their most common analytical application in atmospheric pressure chemical ionization (APCI) [4,5]. In conventional APCI, a corona discharge is formed by applying ϳ4 kV to a needle electrode in a selected atmosphere, to yield currents of ϳ5 A. After a series of reactions [5], reagent ions are produced that can then ionize a sample. Protonated water clusters are typically observed because of the presence of water vapor in the air. Such protonated clusters promote proton transfer ionization, resulting in mass spect...

What can we learn from ambient ionization techniques?

“…The primary ions (e.g., protonated reagent species) are generated using electrospray ionization (e.g., DESI, SESI, EESI), a corona discharge (e.g., DAPCI, DBDI), or a plasma torch (e.g., LTP). Metastable atoms created using discharge techniques (e.g., glowto-corona discharge) [49,50] are generally believed to be responsible for ionization in DART [3], being the species that initiate ion-molecule reactions and that trigger charge and proton transfer reactions. For most techniques, the reagent ion beam is restricted within a diameter less than 0.1 cm.…”

“…Shelley and coworkers have recently determined that according to the characteristic current/ voltage curves, the plasma in DART is a corona-to-glow transitional discharge [49]. One aspect that makes DART unique compared with other plasma based techniques is that the ionic species generated in the plasma source are filtered to prevent ion-ion recombination losses.…”

“…The reactive species in the gas stream coming out of the source are mainly highly excited neutrals, for example vibronically excited nitrogen for N 2 plasma gas or He(2 3 S) metastables for the more commonplace He plasma gas. The reported positive ion background mass spectra are dominated by protonated water clusters [3,49], which are proposed to form via Penning ionization according to [3]:…”

“…IR thermography measurements of a microscope slide exposed to the afterglow of a corona-glow transitional discharge (such as the one in DART but without any additional grids and electrodes) showed a peak temperature of 55°C [49] ( Figure 5). Thus, higher gas temperatures (up to 250°C) in DART are achieved only through additional heating of the source.…”

“…In FAPA [49], known formerly as the atmospheric pressure glow discharge (APGD) flowing afterglow [32,121], the recombining flow from a direct current discharge operated in the glow-to-arc transition phase [49,123], generally with He gas, is used for desorption/ ionization. The background spectra are dominated by water clusters, NO to water cluster formation, as in APCI, according to [124].…”