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Title of the contribution prepared for the xxviii icpig31st ICPIG, July 14-19, 2013, Granada, Spain Molecular Beam Mass Spectrometry in Atmospheric Pressure Plasmas
Y. Aranda Gonzalvo and P. Hatton Plasma & Surface Analysis Division, Hiden Analytical Ltd., 420 Europa Boulevard, Warrington, WA5 7UN, UK There exist many plasma diagnostics for atmospheric plasma discharges but the most direct technique to measure fluxes of ions and neutral species is molecular beam mass spectrometry. It can be used to measure both negative and positive ions including short lived radicals species formed in the atmospheric pressure plasmas. An overview of different source configurations plasma discharges studied with a molecular beam mass spectrometer (MBMS) is presented. 1. Introduction
Atmospheric plasmas are becoming an important The studies presented by means of MBMS research filed for its interest in different kind of includes time- resolved measurements of the ionic applications from material processing up to species in the plasma plume ("bullet" formation) of biomedical treatment. Non-thermal plasmas can an atmospheric pressure helium microplasma jet , have additional benefits compared to conventional investigation of a capacitively coupled atmospheric methods for many applications inclining purification pressure RF excited glow discharge in He-water of water, the sterilization of surgical instruments, mixtures , reaction kinetics studies in a gas surface activation of plastics and polymers including composition corresponding to Titan's atmosphere in woven and unwoven textiles, removal of pollutant a corona discharge  and plasma-assisted desorption/ionization method (PADI) based in non- enhancement and treatment of biological samples. thermal plasma discharge (DBD) interacting with There exist many plasma diagnostics applied to the surface of the analyte to investigate the surface atmospheric plasmas but the most direct technique composition under ambient conditions [5-6]. to measure fluxes of ions and neutral species is molecular beam mass spectrometer. The main characteristic is that sampled species preserve their identity from being created at atmosphere pressure to vacuum monitoring by the rapid expansion of the formed molecular beam that "freezes out" any chemical reactions occurring. This is crucial especially for highly reactive species such as radicals and ions. It can be used to measure both negative and positive ions including short lived radical species formed in the atmospheric pressure An overview of different atmospheric pressure plasma sources studied with a molecular beam mass spectrometer (MBMS) will be presented. Fig. 1: Schematics of the MBMS Analyser. 2. Experimental setup
2.1. Microplasma jet
The MBMS used was a quadrupole-based Atmospheric pressure plasma jets normally are mass/energy resolved spectrometer (EQP) system, operated at an excitation frequency of several tens the HPR-60 MBMS (from Hiden Analytical Ltd) of kilohertz (ac or pulsed mode) or in having a three stage differentially pumped inlet radiofrequency (rf) range. Detailed measurements system separated by aligned skimmer cones and obtained with ICCD images revealed that these jets turbomolecular pumps as shown in figure 1. The are not continuous plasmas but consist of "plasma sampling orifices/skimmer cones are carefully bullets". Studies performed with emission aligned to produce a molecular beam which spectroscopy showed the evolution of the species minimises the collisions of the sampled particles along the trajectory of the bullets but it is the first with each other and with surfaces. time that measurements of positive and negative 31st ICPIG, July 14-19, 2013, Granada, Spain ions generated in the plasma jet is presented by for positive ions. At higher frequency the signal of negative ions tends to be constant in the active part spectrometry (MBMS) . of the applied voltage. That could be an indication The plasma jet used in this study was produced of the origin of negative ions and it shows clearly by a bipolar high voltage ac with a variable that the dynamics and chemistry associated with repetition rate of 5, 10 and 25 kHz. The recorded positive and negative ions are different. peak to peak voltage is generally 8kV and it is independent on the frequency applied. The plasma 2.2. Capacitively coupled atmospheric RF
generated is produced in a quartz glass tube of 1mm Atmospheric pressure glow discharges (APGDs) diameter with an electrode around the glass tube. have been extensively studied over the last decade The main gas used was He with a constant flow rate by means of electrical and optical measurements or of 1.34 slm. This produces a plasma plume of modeling. Mass spectrometry measurements of typically 12mm length. ionic species in capacitively coupled discharges are well known at low pressures up to a few Torr. Only few mass spectrometric studies have been performed in atmospheric pressure plasmas, mainly focusing on afterglows (plasma jets) and corona discharges (low thermal ion energies) as a consequence sampling did not occur in the sheath region of ionizing plasma. An RF excited APGD between two parallel bare metal plate electrodes in He-H2O mixtures has been investigated by molecular beam mass spectrometry . The ion sample is made in the sheath formed at the mass spectrometer plate with sampling orifice which acts as grounded electrode in the parallel plate electrode geometry, and not in the afterglow of jets (and corona) where the ions measured are typically thermalized with the neutrals due to the large amount of collisions before reaching the sampling orifice. The mean ion energies measured in these atmospheric pressure sheaths are also close to thermal which is expected as the mean free path at atmospheric pressure is less than 1 μm. Nonetheless, it is expected that there are different ionic species from the active/ionizing plasma zone compared with the ones produced in the afterglow Fig. 2: Time-resolved of positive and negative ion that are completely determined by charge exchange fluxes for selected species measured at a fix distance of 7 mm at 5kHz. The RF APGD (13.56MHz) is excited in parallel plate geometry between two bare metal electrodes. The time-resolved measurements by MBMS The electrode system consist of a water cooled reveal the evolution of the main species, positive circular copper electrode with a diameter of 20.5 and negatives, during the applied ac for the different mm interfaced to the inlet plate of the HPR-60 repetition rate applied. The main positive species molecular beam mass spectrometer (MBMS) with a 2 , O+, N2 , N+ and He+. These fixed inter-electrode distance of 0.5 mm. The power positive ions were found to follow the applied was kept constant at 20W for experiments voltage as shown in figure 2 and for higher conducted with different water concentrations. Two frequency or repetition rate we observe a decrease in mass flow controllers for introducing the helium- the number of species present in the plasma. For water mixture in the reactor were used. One flow is negative ions there is a strong presence of O−, OH−, going through a bubbling vessel to introduce water H2O−, H−, OH− (H2O) and the behavior of these ions in the helium flow, whereas the other flow contains with respect to the repetition rate is different than pure helium. The concentration of water in the 31st ICPIG, July 14-19, 2013, Granada, Spain reactor is adjusted by changing the ratio of helium most likely pathway for the formation of such flow through the water compared to the total helium molecular anions is H-loss dissociative electron flow and measured by residual gas analysis. attachment to HCN, H3CN and H5CN formed in the The dependence of the water concentration at discharge. These same anions have been detected in constant power of the ionic species for both positive Titan's atmosphere and the present experiments may and negative ions is investigated. For all the provide some novel insights into the chemical and investigated concentrations of He-H2O mixtures the dominant positive ions are H 2O+, OH+, O+, He2 , atmosphere and hence assist in the interpretation of 3 and H3 . Hydration of the ions increases results from the Cassini Huygens space mission. with increasing water vapour concentration and decreases with increasing discharge power. For concentrations of 900 ppm water in He and above negative ions can be detected as shown in figure 3. It was observed that the detected negative ion flux increases with increasing water concentration. The dominant ion is OH− and its clusters. With the emergence of the negative ions, there is a drop in positive ion flux to the mass spectrometer together with a significant increase in applied voltage indicating increasing electron loss by attachment and ion loss by mutual positive and negative ion recombination. Positive and negative ion cluster formation increases with decreasing discharge Fig. 4: Relative abundances of the most dominant power and increasing concentration of water. anions formed in negative corona discharge. 2.4. DBD for plasma assisted desorption
It has been developed a novel approach to ambient surface analysis, termed plasma assisted desorption ionization (PADI) [5-6] that coupled with atmospheric pressure sampling mass spectrometry provides a technique capable of rapid surface analysis without the requirement for sample preparation. The PADI source is a non-thermal atmospheric plasma jet which interacts directly with the surface of the analyte under study. Desorption and ionisation then occurs at the surface. As the analyte is positioned close to the entrance of an atmospheric sampling MBMS, ions (positive and Fig. 3: Variation of the Voltage discharge and the negative) from the analyte material are detected in ion flux in function of the H2O concentration in He. real time by the mass spectrometer. The PADI source is a non-thermal dielectric 2.3. Corona discharge
barrier discharge (DBD) 13.56 MHz RF plasma The formation of negative ions produced in a ‘needle' operating at atmospheric pressure. The negative point-to-plane corona discharge fed by a plasma is around 1mm in diameter and may extend N2(88%)/Ar(8%)/CH4(4%) gas mixture has been up to 10 mm from the tip. The plasma is generated studied using mass spectrometry . in flowing helium and operates in open air. It can be The measurements were carried out in flowing brought into direct contact with any of the surfaces regime at ambient temperature, at a current under study. The action of the plasma at the sample discharge of 0.8 mA and a reduced pressure of 460 surface produces ions from the surface material mbar. The CN− anion has been found to be the most which enter the gas phase and are readily detected dominant negative ion in the discharge and is by the molecular beam mass spectrometer. believed to be the precursor of heavier negative ions Typical data obtained for the detection of active such as C3N− and C5N− as shown in figure 4. The 31st ICPIG, July 14-19, 2013, Granada, Spain interfaced with the MBMS operated in positive ion  Jun-Seok Oh, Yolanda Aranda-Gonzalvo and mode are shown in figure 5. All the spectra were James W. Bradley, J .Phys. D: Appl. Phys, Vol. 44
acquired from approximately 1s of experimental data. Tablet and gel formulations were examined  P. Bruggeman, F. Iza, D. Lauwers and Y. including nonsterodial anti-inflamatory drugs such Gonzalvo Aranda, J.Phys.D: Appl. Phys. 43 (2010)
as mefenamic acid and Ibugel. For each of the compounds the expected protonated molecules were  G. Horvath, Y. Aranda-Gonzalvo, N.J. observed with high signal intensity. Mason, M. Zahoran and S. Matejcik, Eur. Phys. J. Appl. Phys. 49 (2010) 13105.
 Lucy V. et al., Anal. Chem. Vol. 79 (16)
 Alison J. Beck et al., Plasma Processes and Polymers 6, Issue8 (2009) 521.
Fig. 5: Positive ion mode PADI- MBMS spectra of
(a) a generic mefenamic tablet and (b) Ibugel (5%
It has been demonstrated that molecular beam mass spectrometry (MBMS) is the most direct technique to determine fluxes of ions and neutral species. atmospheric plasma discharges have been studied
with a molecular beam mass spectrometer and it is a
valuable diagnostic technique for the investigation
of atmospheric pressure plasma processes.
The authors would like to thank all the collaborators, in particular professors, Bradley, Bruggeman, Mason, Matthews and McCoustra for the joint work on the presented results. 5. References
 Y. Aranda Gonzalvo et al, J. Vac. Sci. Technol. A 24 (2006) 550.
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