A novel chemical ionization source for organic mass spectrometry is introduced. This new source uses a glow discharge in the flowing afterglow mode for the generation of excited species and ions. The direct-current gas discharge is operated in helium at atmospheric pressure; typical operating voltages and currents are around 500 V and 25 mA, respectively. The species generated by this atmospheric pressure glow discharge are mixed with ambient air to generate reagent ions (mostly ionized water clusters and NO+), which are then used for the ionization of gaseous organic compounds. A wide variety of substances, both polar and nonpolar, can be ionized. The resulting mass spectra generally show the parent molecular ion (M+ or MH+) with little or no fragmentation. Proton transfer from ionized water clusters has been identified as the main ionization pathway. However, the presence of radical molecular ions (M+) for some compounds indicates that other ionization mechanisms are also involved. The analytical capabilities of this source were evaluated with a time-of-flight mass spectrometer, and preliminary characterization shows very good stability, linearity, and sensitivity. Limits of detection in the single to tens of femtomole range are reported for selected compounds.

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 flowing afterglow-atmospheric pressure glow discharge (APGD) ionization source described in part 1 of this study (in this issue) is applied to the direct analysis of condensed-phase samples. When either liquids or solids are exposed to the ionizing beam of the APGD, strong signals for the molecular ions of substances present on their surfaces can be detected without compromising the integrity of the solid sample structure or sample substrate. As was observed for gas-phase compounds in part 1 of this study, both polar and nonpolar substances can be ionized and detected by mass spectrometry. The parent molecular ion (or its protonated counterpart) is usually the main spectral feature, with little or no fragmentation in evidence. Preliminary quantitative results show that this approach offers very good sensitivity (detection limits in the picogram regime are reported for several test compounds in part 1 of this study) and linear response to the analyte concentration. Examples of the application of this strategy to the analysis of real-world samples, such as the direct analysis of pharmaceutical compounds or foods is provided. The ability of this source to perform spatially resolved analysis is also demonstrated. Preliminary studies of the mechanisms of the reactions involved are described.

The advent of ambient desorption/ionization mass spectrometry has resulted in a strong interest in ionization sources that are capable of direct analyte sampling and ionization. One source that has enjoyed increasing interest is the Flowing Atmospheric-Pressure Afterglow (FAPA). FAPA has been proven capable of directly desorbing/ionizing samples in any phase (solid, liquid, or gas) and with impressive limits of detection (<100 fmol). The FAPA was also shown to be less affected by competitive-ionization matrix effects than other plasma-based sources. However, the original FAPA design exhibited substantial background levels, cluttered background spectra in the negative-ion mode, and significant oxidation of aromatic analytes, which ultimately compromised analyte identification and quantification. In the present study, a change in the FAPA configuration from a pin-to-plate to a pin-to-capillary geometry was found to vastly improve performance. Background signals in positive- and negative-ionization modes were reduced by 89% and 99%, respectively. Additionally, the capillary anode strongly reduced the amount of atomic oxygen that could cause oxidation of analytes. Temperatures of the gas stream that interacts with the sample, which heavily influences desorption capabilities, were compared between the two sources by means of IR thermography. The performance of the new FAPA configuration is evaluated through the determination of a variety of compounds in positive- and negative-ion mode, including agrochemicals and explosives. A detection limit of 4 amol was found for the direct determination of the agrochemical ametryn, and appears to be spectrometer-limited. The ability to quickly screen for analytes in bulk liquid samples with the pin-to-capillary FAPA is also shown.

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...