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.

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...

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Since the inception of ambient desorption/ionization mass spectrometry, plasma ionization sources have played an increasing role in molecular mass spectrometry. Although a variety of discharge designs and geometries, along with a range of applications, have been introduced, very little published work has focused on the characterization and fundamental examination of these discharges, especially on the desorption/ionization processes they employ. In the present work, a simple yet effective ambient desorption/ionization source based on a dielectric-barrier discharge, the low-temperature plasma (LTP) probe, was optically characterized. By means of a spatially selective detection system, maps of reactive species created in both the plasma and the afterglow regions were recorded. From these maps, the origin of impurities important in mass spectrometric analyses, such as H 2 O, N 2 , and O 2 , was deduced. Electron number densities and rotational temperatures for the LTP were found to be similar to those reported for other dielectric-barrier discharges. Lastly, the effect of plasma parameters on emission spectra was correlated with mass spectral results previously reported for the same ionization source.

A novel vapor-generation technique is described for mercury determination in aqueous solutions. Without need for a chemical reducing agent, dissolved mercury species are converted to volatile Hg vapor in a solution cathode glow discharge. The generated Hg vapor is then transported to an inductively coupled plasma for determination by atomic emission spectrometry. Mercury vapor is readily generated from a background electrolyte containing 0.1 M HNO 3. Vapor generation efficiency was found to be higher by a factor of 2-3 in the presence of low molecular weight organic acids (formic or acetic acids) or alcohols (ethanol). Optimal conditions for discharge-induced vapor generation and reduced interference from concomitant inorganic ions were also identified. However, the presence of chloride ion reduces the efficiency of Hg-vapor generation. In the continuous sample introduction mode, the detection limit was found to be 0.7 microg L (-1), and repeatability was 1.2% RSD ( n = 11) for a 20 microg L (-1) standard. In comparison with other vapor generation methods, it offers several advantages: First, it is applicable to both inorganic and organic Hg determination; organic mercury (thiomersal) can be directly transformed into volatile Hg species without the need for prior oxidation. Second, the vapor-generation efficiency is high; the efficiency (with formic acid as a promoter) is superior to that of conventional SnCl 2-HCl reduction. Third, the vapor generation is extremely rapid and therefore is easy to couple with flow injection. The method is sensitive and simple in operation, requires no auxiliary reagents, and serves as a useful alternative to conventional vapor generation for ultratrace Hg determination.

A low-power, atmospheric-pressure microplasma source based on a dielectric barrier discharge (DBD) has been developed for use in atomic emission spectrometry. The small plasma (0.6 mm x 1 mm x 10 mm) is generated within a glass cell by using electrodes that do not contact the plasma. Powered by an inexpensive ozone generator, the discharge ignites spontaneously, can be easily sustained in Ar or He at gas flow rates ranging from 5 to 200 mL min(-1), and requires less than 1 W of power. The effect of operating parameters such as plasma gas identity, plasma gas flow rate, and residual water vapor on the DBD source performance has been investigated. The plasma can be operated without removal of residual water vapor, permitting it to be directly coupled with cold vapor generation sample introduction. The spectral background of the source is quite clean in the range from 200 to 260 nm with low continuum and structured components. The DBD source has been applied to the determination of Hg by continuous-flow, cold vapor generation and offers detection limits from 14 (He-DBD) to 43 pg mL(-1) (Ar-DBD) without removal of the residual moisture. The use of flow injection with the He-DBD permits measurement of Hg with a 7.2 pg limit of detection, and with repetitive injections having an RSD of <2% for a 10 ng mL(-1) standard.

The advent of ambient desorption/ionization mass spectrometry (ADI-MS) has led to the development of a large number of atmospheric-pressure ionization sources. The largest group of such sources is based on electrical discharges; yet, the desorption and ionization processes that they employ remain largely uncharacterized. Here, the atmospheric-pressure glow discharge (APGD) and afterglow of a helium flowing atmospheric-pressure afterglow (FAPA) ionization source were examined by optical emission spectroscopy. Spatial emission profiles of species created in the APGD and afterglow were recorded under a variety of operating conditions, including discharge current, electrode polarity, and plasma-gas flow rate. From these studies, it was found that an appreciable amount of atmospheric H(2)O vapor, N(2), and O(2) diffuses through the hole in the plate electrode into the discharge to become a major source of reagent ions in ADI-MS analyses. Spatially resolved plasma parameters, such as OH rotational temperature (T(rot)) and electron number density (n(e)), were also measured in the APGD. Maximum values for T(rot) and n(e) were found to be ~1100 K and ~4×10(19) m(-3), respectively, and were both located at the pin cathode. In the afterglow, rotational temperatures from OH and N(2)(+) yielded drastically different values, with OH temperatures matching those obtained from infrared thermography measurements. The higher N(2)(+) temperature is believed to be caused by charge-transfer ionization of N(2) by He(2)(+). These findings are discussed in the context of previously reported ADI-MS analyses with the FAPA source.

Laser Ablation Molecular Isotopic Spectrometry (LAMIS) was recently reported for rapid isotopic analysis by measuring molecular emission from laser-induced plasmas at atmospheric pressure. With 13 C-labelled benzoic acid as a model sample, this research utilized the LAMIS approach to clarify the formation mechanisms of C 2 and CN molecules during laser ablation of organic materials. Because the isotopic ratios in the molecular bands could deviate from statistical distribution depending on their formation pathways, the dominant mechanism can be identified through a comparison of the experimental observed isotopic patterns in the molecular emission with the theoretical statistical pattern. For C 2 formation, the experimental 12 C 12 C / 13 C 12 C ratios support a recombination mechanism through atomic carbon at early delay time but also indicate the presence of other operating mechanisms as the plasma evolves; it is proposed that some of the C 2 molecules are released directly from the aromatic ring of the sample as molecular fragments. In contrast, the temporal profiles in the 12 C / 13 C ratios derived from CN emission exhibited opposite behavior with those derived from C 2 emission, which unambiguously refutes mechanisms that require C 2 as a precursor for CN formation; CN formation likely involves atomic carbon or species with a single carbon atom.