RESUMO
A new method for arsenic detection by optical emission spectrometry (OES) is presented. Arsine (AsH3) is generated from liquid solutions by means of hydride generation (HG) and introduced into a capillary dielectric barrier discharge (DBD) where it is atomized and excited. A great challenge in OES is the reduction of the recorded background signal, because it negatively affects the limit of detection (LOD). In conventional DBD/OES methods, the signal intensity of the line of interest, in this case arsenic, is integrated over a long time scale. However, due to the pulsed character of the plasma, the plasma on-time is only a small fraction of the integration time. Therefore, a high amount of noise is added to the actual signal in each discharge cycle. To circumvent this, in the present study the emitted light from the DBD is collected by a fast gated iCCD camera, which is mounted on a modified monochromator. The experimental arrangement enables the recording of the emission signal of arsenic in the form of a monochromatic 2D-resolved picture. The temporal resolution of the iCCD camera in the nanosecond range provides the information at which point in time and how long arsenic is excited in the discharge. With use of this knowledge, it is possible to integrate only the arsenic emission by temporally isolating the signal from the background. With the presented method, the LOD for arsenic could be determined to 93 pg mL-1 with a calibration curve linear over 4 orders of magnitude. As a consequence, the developed experimental approach has a potential for both mechanistic studies of arsine atomization and excitation in DBD plasmas as well as routine applications, in which arsenic determination at ultratrace levels is required.
RESUMO
Dielectric barrier discharges (DBDs) have been used as soft ionization sources (DBDI) for organic mass spectrometry (DBDI-MS) for approximately ten years. Helium-based DBDI is often used because of its good ionization efficiency, low ignition voltage, and homogeneous plasma conditions. Argon needs much higher ignition voltages than helium when the same discharge geometry is used. A filamentary plasma, which is not suitable for soft ionization, may be produced instead of a homogeneous plasma. This difference results in N2, present in helium and argon as an impurity, being Penning-ionized by helium but not by metastable argon atoms. In this study, a mixture of argon and propane (C3H8) was used as an ignition aid to decrease the ignition and working voltages, because propane can be Penning-ionized by argon metastables. This approach leads to homogeneous argon-based DBDI. Furthermore, operating DBDI in an open environment assumes that many uncharged analyte molecules do not interact with the reactant ions. To overcome this disadvantage, we present a novel approach, where the analyte is introduced in an enclosed system through the discharge capillary itself. This nonambient DBDI-MS arrangement is presented and characterized and could advance the novel connection of DBDI with analytical separation techniques such as gas chromatography (GC) and high-pressure liquid chromatography (HPLC) in the near future.
RESUMO
Helium capillary dielectric barrier discharge driven by the square wave-shaped high voltage was investigated spatially and temporally by means of optical emission spectroscopy. The finding of the previous investigation conducted with the sinusoidal-like high voltage was confirmed, i.e., the plasma in the jet and the plasma in the capillary constitute two temporally separated events. The plasma in the jet occurs prior to the discharge in the capillary and exists only during the positive half period of the applied high voltage. The time delay of the capillary discharge with respect to the discharge in the jet depended on the high voltage, and it was between 2.4 and 8.4 µs for the voltage amplitude change in the range from 1.96 to 2.31 kV, respectively. It was found that, compared to sinusoidal-like voltage, application of the square wave high voltage results with stronger (~6 times) He line emission in the jet, which makes the latter more favorable for efficient soft ionization. The use of the square wave high voltage enabled comparison of the currents (~1 mA) flowing in the capillary during the positive and negative high voltage periods, which yielded the estimation for the charge dissipated in the atmosphere ((4 ± 20 %) × 10(-11) C) through the plasma jet.
RESUMO
A helium capillary dielectric barrier discharge was investigated by means of time-resolved optical emission spectroscopy with the aim of elucidating the process of the formation of the plasma jet. The helium emission line at 706 nm was utilized to monitor spatial and temporal propagation of the excitation of helium atoms. The discharge was sustained with quasi-sinusoidal high voltage, and the temporal evolution of the helium atomic emission was measured simultaneously with the discharge current. The spatial development of the plasma was investigated along the discharge axis in the whole region, which covers the positions in the capillary between the electrodes as well as the plasma jet outside the capillary. The high voltage electrode was placed 2 mm from the capillary orifice, and the distance between the ground and high voltage electrode was 10 mm. The complete spatiotemporal grid of the development of the helium excitation has shown that during the positive half-period of the applied voltage, two independent plasmas, separated in time, are formed. First, the early plasma that constitutes the plasma jet is formed, while the discharge in the capillary follows subsequently. In the early plasma, the helium atom excitation propagation starts in the vicinity of the high voltage electrode and departs from the capillary towards the ground electrode as well as several millimeters outside of the capillary in the form of the plasma jet. After relatively slow propagation of the early plasma in the capillary and the jet, the second plasma starts between the electrodes. During the negative voltage period, only the plasma in the capillary between the electrodes occurs.
RESUMO
The populations of the lowest excited helium states 2s 3S1, 2s 1S, 2p 3P0 J, and 2p 1P0 created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities n1 in the considered He states along the plasma length L. The field of the theoretical profiles for a series of the n1L parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, n1L corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of n1 could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: n1 av (2s 3S1) = (2.9 ± 1.1) × 1013 cm−3, n1 av (2s 1S) = (1.4 ± 0.5) × 1013 cm−3, n1 av (2p 3P0 J) = (1.1 ± 0.4) × 1013 cm−3, n1 av (2p 1P0) = (4.2 ± 1.6) × 1012 cm−3, and they represent the average populations along the plasma column in the capillary.
