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Mass spectrometry of atmospheric pressure plasmas

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2015 Plasma Sources Sci. Technol. 24 044008

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1. Introduction

The plasma chemistry of cold atmospheric pressure plasmas (APPs) is rich in neutral and charged species as for example demonstrated by recent simulations [1, 2]. The resulting high reactivity of these cold APPs can be used in many applications such as ozone production, destruction of volatile organic com-pounds, activation of polymer surfaces, treatment of living tissues for decontamination or acceleration of wound healing and in deposition of thin films or nano-structured materials. However, the complexity of plasma-chemical processes in the discharge requires a combined experimental and theoretical approach in plasma analysis, where quantitative and qualita-tive plasma diagnostics are compared with plasma simulations. Experimental diagnostics have to be applied to validate these models and to provide better insight into plasma chemistry

processes. A large variety of diagnostics can be applied to measure densities of relevant species starting with simulation based diagnostics combining optical emission spectroscopy data with simulations [3, 4], broadband or laser absorption spectroscopy [5–7], laser induced fluorescence (LIF) [8], two photon absorption LIF (TALIF) [9], Rayleigh scattering [10] or calorimetric measurements [11] to name some of the methods recently applied for the analysis of APPs. However, especially optical diagnostics require knowledge about rel-evant quenching rates and are often limited to the detection of one specific molecular or electronic state depending on the wavelength of the laser.

An alternative diagnostic method with a broad range of detectable species is mass spectrometry (MS). With MS it is possible to measure absolute densities of neutrals and relative ion fluxes. In general the species are measured in their ground

Plasma Sources Science and Technology

Mass spectrometry of atmospheric pressure plasmas

S Große-Kreul1, S Hübner1, S. Schneider1, D Ellerweg1, A von Keudell1, S Matejčík2 and J Benedikt1

1 Research Group Reactive Plasmas, Institute for Experimental Physics II, Ruhr-Universität Bochum, 44780 Bochum, Germany2 Department of Plasma Physics, Comenius University, 84248 Bratislava, Slovakia

E-mail: simon.grosse-kreul@rub.de

Received 11 March 2015, revised 11 May 2015Accepted for publication 26 May 2015Published 15 July 2015

AbstractAtmospheric pressure non-equilibrium plasmas (APPs) are effective source of radicals, metastables and a variety of ions and photons, ranging into the vacuum UV spectral region. A detailed study of these species is important to understand and tune desired effects during the interaction of APPs with solid or liquid materials in industrial or medical applications. In this contribution, the opportunities and challenges of mass spectrometry for detection of neutrals and ions from APPs, fundamental physical phenomena related to the sampling process and their impact on the measured densities of neutrals and fluxes of ions, will be discussed. It is shown that the measurement of stable neutrals and radicals requires a proper experimental design to reduce the beam-to-background ratio, to have little beam distortion during expansion into vacuum and to carefully set the electron energy in the ionizer to avoid radical formation through dissociative ionization. The measured ion composition depends sensitively on the degree of impurities present in the feed gas as well as on the setting of the ion optics used for extraction of ions from the expanding neutral-ion mixture. The determination of the ion energy is presented as a method to show that the analyzed ions are originating from the atmospheric pressure plasma.

Keywords: atmospheric pressure plasmas, mass spectrometry, plasma jets, microplasmas

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S Große-Kreul et al

Mass spectrometry of atmospheric pressure plasmas

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state, but in some cases even information about excited states can be obtained. It is not limited by existence or non-existence of accessible optical transitions and can therefore be used to detect most of the species generated in plasmas, especially in case of plasmas with molecular gases with a complex chem-istry such as C2H2 [12]. The basic idea of MS is simple: The gas mixture is sampled through a small sampling orifice into a differentially pumped sampling system, where a mass spec-trometer is located. Neutral species can be detected by ion-izing them in the ionizer of the mass spectrometer and positive or negative ions can be focused into the mass spectrometer by a set of ion optic lenses. The mass spectrometer then fil-ters ions according to their mass (most common mass filter in the MS plasma analysis is a quadrupole mass filter) and alternatively also according to their energy (filtered by means of sector field or Bessel-box energy analyzer). The schemes of the measurement geometries are shown in figure 1. However, also in the case of MS there are challenges that have to be addressed: the detection limit for neutrals is usually worse than in the case of laser diagnostics mainly due to low ioniza-tion efficiency in the ionizer. Very careful design of the sam-pling and differential pumping stages with proper background signal correction have to be performed in order to obtain reli-able absolute data without systematic errors in the calibra-tion. Additionally, the reactions of molecular species on the hot filament used to generate electrons in the ionizer are a possible source of uncertainty during analysis of neutrals by MS. Radicals such as atomic oxygen [13] and nitrogen [7] produced by a micro atmospheric pressure jet (μ-APPJ) have been analyzed by MBMS revealing densities in the effluent of the order of 1014–1015 cm−3 depending on the gas mixture and the operating conditions.

The relative ratios of the measured ion signals are sensi-tively depending on the tuning of the ion lenses and on the

differences of the kinetic energy of by gas expansion accel-erated ions. Possible sources of systematic errors are shown in figure  1 as well and will be addressed in this article. Absolute ion flux measurements are not possible at atmos-pheric pressure since a suitable calibrated ion source does not exist. Therefore, only relative intensities and signal trends depending on external parameters may be compared.

The general principles of quadrupole MS for analysis of mainly low pressure reactive plasmas have been provided in a recent review [14], which we recommend to any reader not familiar with MS of plasmas as a good starting point. Here, the focus is put on analysis of APPs, where we will demonstrate the MS principles on the analysis of a micro-APP jet (μ-APPJ) plasma source operated at a gas flow rate of typically 1.4 slm He with an admixture of less than 1% O2 or N2. This device is described in great detail elsewhere [15].

First, the basic issues related to collisional sampling with formation of a supersonic free jet, including the role of the composition distortion, the effect of too high background pres-sure and the geometry of the sampling orifice will be discussed. Then, the detection of the neutral species and the calibration of the corresponding densities will be introduced. Finally, the detection of ions and their chemistry will be treated.

2. Sampling from atmospheric pressure

The most distinctive difference compared to MS analysis of low pressure plasmas is the collisional sampling of the gas into the mass spectrometer. The Knudsen number λ=Kn L/ at the sampling orifice, defined as the ratio of mean free path (λ ∼ 0.1 μm at atmospheric pressure), to sampling orifice diameter ( ∼L 20 to 300 μm), is much smaller than 1. This leads to an acceleration of the gas mixture to supersonic velocities, when the pressure drop is larger than the critical pressure ratio P0/Pb:

⎜ ⎟⎛⎝

⎞⎠

γ⩾ = + γ γ( − )P

PG

1

2.

b

0/ 1

(1)

The critical pressure ratio G is less than 2.1 for all gases. For a pressure ratio lower than the critical ratio G, (super)sonic speed is never reached and the exit pressure at the orifice is the background pressure Pb. No further expansion of the gas takes place. However, if the pressure condition (equation (1)) is fulfilled, sonic speed is reached at the throat and the exit pressure is equal to P0/G. Since the exit pressure is higher than the background pressure, the gas flow is underexpanded and consequently has to expand into the low-pressure region. This type of expansion is called ‘free’-jet, in contrast to nozzle jets, where the expansion is confined by a nozzle [16]. The under-expanded gas continues to expand behind the orifice, and the pressure in the free-jet decreases further. A supersonic flow is not able to ‘sense’ downstream boundary conditions because boundary information cannot propagate with a velocity faster than the speed of sound. Consequently, the free-jet continues to expand even when its pressure falls below the background pressure Pb and the gas becomes overexpanded. Two scenarios are possible depending on the value of Pb.

Figure 1. Generic schemes of the overall concept of (A) molecular beam and (B) ion mass spectrometry for atmospheric pressure plasma analysis, highlighting the experimental challenges in the determination of ion fluxes and absolute densities of neutral species.

B) Ions

atm. pressure plasma

ion conversion

lens tuning and ion accaleration relative fluxes

energy filter

A) Neutrals

atm. pressure plasma

interference mass spectrometer

plasma-

composition distortion

absolute density calbration

background subtractionbeam chopper

ionizer- fragmentation- threshold ionization

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2.1. Expansion into high pressure

At high Pb the overexpanded free-jet is re-compressed by a system of shock fronts (localized isentropic zones of large den-sity, pressure, temperature and velocity gradients) to adjust to the downstream boundary conditions. The formation of such shock fronts is demonstrated in figure 2, where a fluid simulation of the expansion of argon into the 100 Pa background pressure vacuum chamber through a sampling orifice with a diameter of 100 μm and a length of 250 μm is shown. Compressible Navier–Stokes equations are used and solved in a time domain to obtain a convergence for the solution. The selected back-ground pressure of 100 Pa is still high enough so that the mean free path in the low pressure region (about 100 μm at 100 Pa) is still short to justify the fluid simulation approach.

The region with overexpanded gas and the so called Mach disk shock in the downstream region are clearly visible. Shock fronts are also present at the sides of the free-jet. The thick-ness of a shock front is of the order of the mean free path and the location of the Mach disk xm (in units of the orifice diam-eter d) was empirically found to be at [17]:

=x

d

P

P0.67 .m

b

0 (2)

The calculated position of the Mach disk for the simulation

conditions from figure 2 is 2.1 mm according to equation (2), in perfect agreement with the simulation.

A background pressure in the range of tens of Pascals up to hundreds of Pascals is often reported in the literature as a pressure in the first pumping stage. This large value is a result of the high gas flux through the sampling orifice (depending on the gas, temperature and geometry in the range of few tens of sccm to few hundreds of sccm) and reduced pumping effi-ciency due to limited gas conductivity of the first pumping stage. The shock equilibration with the background gas inside the Mach disk can result into a loss of reactive species and formation of new ones, especially in the case of ions or highly reactive radicals. The presence of a Mach disk is one of the sources of possible composition distortion mechanisms which

occur during the sampling process from free jets, and should be avoided or minimized. This can be achieved by using a skimmer at the position of the orifice connecting the first and second pumping stage, which is inserted into the zone of silence of a free jet [18]. For the experimental conditions dis-cussed here, a well designed skimmer has to be placed closer than 2.1 mm away from the sampling orifice.

A smaller sampling orifice will result in a lower back-ground pressure in the first stage and lead to an increase of Kn. In such rarefied gases (Kn > 0.01) conventional Navier–Stokes CFD methods are not applicable anymore and kinetic methods have to be used to simulate the gas expansion in and after the sampling orifice. The Direct Simulation Monte Carlo (DSMC) method is a suitable numerical technique to obtain reliable information in very low pressure systems [19].

2.2. Expansion into collisionless vacuum

If the background pressure is reduced to a level, where the mean free path is larger than the characteristic dimension of the first pumping stage (usually distance between the sam-pling orifice and the orifice into the 2nd stage), the free-jet expands into a collisionless ‘vacuum’. It transforms under these conditions into a collisionless molecular beam (MB) at a so-called quitting surface, which is located at position, where the mean free path of particles in the MB gets larger than its diameter (see figure 3). Since the mean free path is a function of the cross-section for collisions with background particles, the exact location of the quitting surface can be different for different species. This may cause problems during the sam-pling of ions, as discussed in section 4.

A possible way to realize a very low background pressure in the situation where an intense MB is sampled, is the use of a beam chopper with a rotating skimmer (see figure 4 and [21]). In this sampling approach, the MB is formed only for 1% of the rotating period and the background pressure is reduced in the remaining 99% resulting in the effective background pressure of 10−2 Pa or less. This system is used for most of the neutral particle measurements discussed in this article.

Figure 2. Fluid simulation of the expansion of argon gas into the 100 Pa background pressure of the first pumping stage of the mass spectrometer. The pressure (logarithmic color scheme) and the gas streamlines are shown. The steep pressure drop behind the orifice, the zone of silence (region with supersonic gas velocities visible as a dark area with the pressure lower than the background pressure) and the Mach disc shock are visible.

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2.3. Composition distortion in MBs

One of the characteristic features of the free-jet expansion is that the gas composition at atmospheric pressure will be changed during the process of sampling and formation of a MB. This inherent effect plays a major role independently on the pressure difference under operation of a specific molec-ular beam MS (MBMS) setup. Different processes that have an influence on this so-called composition distortion are dis-cussed in the literature in great detail and will be briefly sum-marized here [22, 23]. The sources of distortion are discussed in the order of appearance from atmospheric pressure region to free molecular flow region and they are also indicated in figure 3.

(i) Radical recombination on surfaces: During sampling from a plasma, reactive species such as radicals will interact and eventually recombine due to collisions with the surrounding walls. Besides the sampling orifice holder the cylindrical walls of the orifice itself may have an influence in dependence on its diameter. By comparing the characteristic time for diffusion from the wall to the centerline of the beam to the residence time in the orifice it can be shown that this effect is negligible for large ori-fices (order of 100 μm) but becomes relevant for smaller orifices (order of 10 μm). In general, the aspect ratio of the sampling orifice (2.5 for orifice in figure  2) is less critical by collisional sampling, because mainly the gas following the streamlines on and close to the orifice axis will arrive at the mass spectrometer. It is different to the sampling of low pressure gases, where the aspect ratio of the sampling orifice has to be as small as possible.

(ii) Acceleration into sampling orifice: Particles from the plasma will be accelerated towards the sampling orifice due to high pressure gradients. This acceleration results in an apparent shift of the axial concentration profile for a given species. Experimental studies based on the comparison of Laser-induced fluorescence (LIF) and MBMS profiles lead to the conclusion that the profile is shifted between 2 and 5 times the orifice diameter. In other words, the measured gas mixture originates from the region 2 to 5 orifice diameters above the surface.

(iii) Chemical relaxation: Fundamental gas properties like pressure and temperature are rapidly changing as a function of distance to orifice during the expansion of the gas. This results in a distortion of the chemical con-centration of reactive species. This effect depends on the ratio between residence time in the sampling orifice and relaxation time of the species. The effect is negligible for a residence time smaller than the relaxation time. However, it becomes important for orifice length larger than 300 μm [22].

(iv) Radial diffusion: Besides the axial pressure gradient strong radial pressure gradients are characteristic for a free jet expansion. As a consequence, diffusion leads to radial mass separation downstream from the sampling orifice, which discriminates lighter species. This unavoid-able effect is strongly depending on the gas composition and is most severe for species with a strong deviation in molecular weight compared to the major-species present in the gas.

(v) Skimmer interference: In typical MBMS setups skimmers are used for the connection of the first and the second pumping stage. Distortion is caused by beam molecule collisions in close vicinity of the skimmer and with the skimmer walls. The axial position of the skimmer is cru-cial due to the presence of shock fronts, especially when sampling through rather large orifices.

Figure 3. Schematic illustration of expansion of a free-jet into chamber with negligible background density and continuous transition into the molecular flow regime (after [20]). The different effects leading to composition distortion of the sampled gas are indicated and explained in detail in section 2.3.

p0

p∆

molecular flow

region

collisional region

quitting surface

p << pBG 0

radial diffusion+ speed-ratiofocusing

radical recombination,acceleration andchemical relaxation

hypothetical source

Figure 4. Schematic view of the beam chopper experiment with rotating skimmer, which is positioned in the firststage of the differential pumping system and maintains two roles: (i) chopping the molecular beam for background correction and (ii) serve as a valve, which can effectively stop the gas flow into the 2nd and 3rd stage when closed.

rotatingskimmer

st 1 s

tage

rd 3 s

tage

-5

p=10

Pa

ionizer

photodiode

free jetexpansion

molecularbeam

sampling orifice

pum

p

pum

p

nd 2 s

tage

-2p=

10 P

a

pum

p

µ-APPJ

p=1 atm

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(vi) Speed-ratio focusing: In a free jet large differences of velocities perpendicular ( ⊥v ) and parallel ( ∥v ) to the beam are present. The final beam velocity ∥v is independent on the molecular weight and therefore equal for all species as pointed out in detail below. However, ⊥v remains a strong function of the molecular weight. This causes a difference in speed-ratio ∥ ⊥v v/ of particles with different mass and leads to a distortion of number densities of the corresponding species at the position of the skimmer. This effect is also known under the name Mach-number focusing.

During the process of supersonic expansion the enthalpy in the gas is converted into directed kinetic energy. The velocity of the final beam is dependent on pressure and temperature of the gas mixture as well as on the molecular weight of the main constituent and is equal independent on the mass of a given particle [24]. Therefore, the kinetic energy of particles extracted from the quitting surface has a linear dependence on the mass. For example, ambient nitrogen molecules with a mass of 28 amu, seeded into a helium beam, will have a final kinetic energy of more than 0.3 eV [17], a much higher energy than the helium atoms. We will clearly demonstrate this effect in the section describing measurements of ions.

2.4. Sampling orifice in contact with APPs

The collisional sampling with its properties can also influence the plasma behavior in case the plasma is in direct contact with the sampling orifice. As mentioned above, the pressure gradient in front of the orifice is influencing the region within the 2 to 5 times the sampling hole diameter. Therefore, the sampling orifce should be at least 2 times smaller than the plasma sheath thickness to minimize the possible effects.

It is also crucial to avoid the penetration of the plasma into the low pressure region of the first pumping stage, because it will inherently lead to a significant distortion of the meas-ured ion composition and also ion energies. The ion energy distribution function (IEDF) serves as a good indicator for such an unwanted penetration of the plasma into the sampling system. In some early studies about ion mass spectrometry of cold APPs ion energies of more than 20 eV have been reported [25]. Such high values are not possible since the ions get thermalised in the electric-field-free collisional expan-sion. The correct measured ion energy is determined by the final velocity of the particles during the process of supersonic expansion. Depending on the operating conditions (stagnation pressure, orifice size, pumping speed) it should be of the order or less than 1 eV [17, 26, 27]. Generally a sampling orifice with a diameter that is smaller than the typical dimension of the plasma sheath should be used [28]. Bruggeman et al used an orifice with a diameter of 20 μm to avoid successfully such effects in the case of a radio-frequency glow discharge [28]. However, in some studies much larger orifices with diameters of 100 μm and even larger have been used without discussing the plasma sheath thickness and without checking the energy of the analyzed ions [29, 30].

Additionally, it should not been forgotten that the metal sampling orifice functions as an additional grounded electrode,

which can in the case of plasma jets change the plasma proper-ties significantly compared to the case, where a dielectric sub-strate is treated. Using a ceramic sampling orifice or floating metal orifice is tricky, especially in case of pulsed plasmas or plasmas driven with an alternating voltage. The electric field appears in both cases also on the low pressure side and can therefore influence the ion trajectories or even generate a para-sitic plasma.

3. Measurement of neutral species

3.1. Molecular beam mass spectrometry

The absolute densities of neutral (reactive) species are mea-sured with the help of MBMS. The main idea is very simple: The MB with sampled particles of interest is line of sight with the ionizer of the MS. The resulting background corrected ion current (see below) as function of mass-to-charge ratio or elec-tron energy in the ionizer (threshold ionization MS: TIMS) is recorded and compared (considering the corresponding ionization cross-sections and mass-dependent transmission function of the MS device) to the background corrected ion current of some calibration species with known density [14]. The density ratio in the MB in the ionizer between species of interest and calibration species is determined like this and since the ionizer is in line-of-sight with the sampling orifice, even highly reactive species, which are usually lost when col-liding with surfaces, can be measured. This ratio is the same as the density ratio at the sampling orifice for collisionless sampling (e.g. low pressure plasmas analysis), because the density at a given position in the MB scales linearly with the initial density and quadratically with the ratio of the sampling orifice diameter over the distance from it in this case. The latter dependence is just a geometric factor, which is the same for all species and cancels therefore in the MB density ratio [14, 31]. Knowing the density of the calibration species at the sampling orifice, the density of the species of interest can be determined.

3.2. MBMS with collisional sampling

The situation gets more complicated with collisional sam-pling from atmospheric pressure due to composition dis-tortion effects discussed in the previous section. The beam density depends strongly on the mass of the main collisional partner(s), weakly on the collisional cross sections and is also influenced by the geometry of the whole sampling system due to speed-ratio focusing and also skimmer interference effects. In general, the light species are discriminated com-pared to heavier species. Figure 5 illustrates this behavior on the example of relative signal of Ne and N2O. Their concen-trations (1% and 0.5% respectively) are kept constant in the gas mixture of He and air (realized as a mixture of N2/O2) with air concentration varying from 0% to 100%. The Ne and N2O signals are the largest in pure Helium, since helium is very light and diffuses very quickly out from the beam axis leaving heavier Ne and N2O behind. Both signal intensities are, how-ever, clearly decreasing with increasing air fraction, which is

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the consequence of the presence of heavier collisional part-ners in the sampled gas mixture. Both signals show the same trend with respect to the air fraction and are decreasing to 25% and 20% relative intensities in air compared to helium for Ne and N2O respectively. The slight difference between these sig-nals is a consequence of different masses and different cross sections  for elastic collisions of species involved. This data clearly demonstrates that the same concentration can result in significantly different ion currents easily with differences of a factor five.

Typical example of such changing gas mixtures is the ema-nation of a helium gas jet into the surrounding air atmosphere, where the gas mixture is changing from helium dominated into air dominated. Any measurement of neutral densities as function of distance from the jet has to take this change of gas composition into account. It is achieved by measuring, next to the signal of the species of interest, the gas mixture composi-tion at each measuring position by following the signals of helium and N2 or O2. The density calibration with help of the measurement of the signal of the calibration species has to be then performed at all particular gas compositions, at which the measurements have been performed. We applied this approach successfully by analysis of O atom and O3 molecule densities in the effluent of a microscale atmospheric pressure plasma jet (μ-APPJ) with He/O2 gas mixture operated in ambient air [13]. It should be noted at this position that the density in the MB drops with the square of the distance from the sampling ori-fice in both collisionless and collisional sampling cases. The ionizer of the mass spectrometer should be, therefore, placed as close as possible to the sampling orifice to obtained the highest detection sensitivity [14]. This can be achieved easily with the chopper system from figure 4, because no additional place for a beam chopper in front of the ionizer is needed.

3.3. Background subtraction

It is very important to consider ion current resulting only from the ionization of particles in the MB. The ion current due to ionization of particles in the gas background or after some sur-face reflection in the mass spectrometer has to be subtracted from the data, which is usually done by chopping the MB and

measuring the background signal only and background plus beam signal [14, 31]. We have demonstrated just recently that the proper open design of the ionizer plays an important role. Highly reactive species such as oxygen atoms are effec-tively lost at most of the surfaces and the signal measured is almost solely due to the beam particles. On the contrary, stable particles that are used for calibration have a substan-tial background pressure in the MS pumping stage, which is superimposed on the MB density and has to be mesured sepa-rately and subtracted from the ion current. Moreover, the MB particles can result in ‘filling’ of the ionizer, especially in the case where a closed ionizer geometry is used. The beam chop-ping for the background signal measurement does not help in this case because this additional beam-related background in the ionizer is present only when the beam is passing the ion-izer and disappears when the beam is blocked by the chopper [32]. As a consequence, a mass spectrometer with a closed ionizer geometry cannot be used accurately to measure the ion current corresponding only to the beam density of stable (calibration) particles. This effect explains the discrepancy observed between experimental data and plasma-chemistry simulations in the low-pressure C2H2 discharge [32, 33].

The difference between the measurements with open ion-izer and closed ionizer is demonstrated in figure 6. A pulsed MB is generated for 200 μs by the sampling system shown in figure 4. Air is sampled and the signal at mass-to-charge ratio of 28 (N2 molecule) is recorded. Measurements performed with a mass spectrometer with an open ionizer (figure 6(c)), cross-beam ion source of HiQuad QMG 700 mass spectrom-eter) shows a clear narrow peak corresponding to MB parti-cles. These particles can leave easily the ionizer after colliding with its parts and don’t build up any significant background signal. The same measurement with a closed beam ionizer (figure 6(a)), PSM mass spectrometer) shows a steep rise of

Figure 5. Relative intensities of signals of constant concentrations of Ne and N2O as measured in the He/air gas mixture with variation of the mixing ratio.

0 10 20 30 40 50 60 70 80 90 1000.0

0.2

0.4

0.6

0.8

1.0

norm

aliz

ed s

igna

l (a.

u.)

Air content (%)

admixed: Ne (1%) N

2O (0.5%)

100 90 80 70 60 50 40 30 20 10 0He content (%)

Figure 6. Time resolved normalized MS signals at mass 28 (nitrogen molecules) of a pulsed MB with pulse length of 200 μs as measured with an ‘open’ and ‘closed’ ionizer, after [32]. Additionally, the signal obtained with modified ionizer, where 40% of the surface area of the closed ionizer has been replaced by a mesh. The beam is generated with the chopper shown in figure 4.

0.0

0.5

1.0

0.0

0.5

7.0 7.5 8.0 8.5 9.0 9.5 10.00.0

0.5

"closed" ionizer

norm

aliz

ed s

igna

l of N

2 (a

.u.)

"open" ionizer

~ 200 µs pulse of beam particles

time (ms)

open cross-beam ionizer

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the signal within the pulse as the ionizer is filled with beam species. Additionally, a long tale is observed after the pulse since the particles can leave the ionizer only very slowly through the relatively small beam entrance and exit holes. The dashed line indicates the probable level of the background signal, which is much higher than the beam component above it. The situation can be improved by replacing the part of the solid ionizer housing wall by a metal mesh, see figure 6(b)). The steep rise of the signal in the pulse is reduced and the signal decay after the pulse is accelerated. However, an even more open geometry is needed to achieve the optimal situation with the almost fully opened cross-beam ionizer.

Additional complications during analysis of neutral species arise due to the fact that the gas flux sampled through the ori-fice is very large due to atmospheric pressure conditions. This worsens background pressure related problems. The large background pressure of molecular species can lead to signifi-cant formation of radicals or new molecules on the surface of the hot filaments used in the ionizer as a hot cathode electron source. For example, we have observed a large background signal corresponding to OH radicals produced by thermal decomposition of water on filaments, which prevented OH measurements in a He/H2O vapor plasma. Similarly, high background signal of NO species is also observed, very prob-ably due to reaction of H2O and N2 molecules on the hot fil-aments. It should be mentioned here that the problem with ‘filling’ of the closed-design ionizer with beam particles will also result in modulation of the production of species on the hot filament which can lead to false interpretation of the meas-ured ion currents.

3.4. Threshold ionization mass spectrometry

Threshold ionization mass spectrometry (TIMS) is used for the measurement of transient reactive species. The TIMS technique utilizes the difference of the electron impact ioniza-tion threshold of a given radical (e.g. O atom) and the elec-tron impact dissociative ionization (DI) threshold of molecule (e.g. O2), which is typically several electron volts larger. The radical signal (O+ from direct ionization of O) can be detected by lowering the electron energy below the threshold for DI of the molecule, which otherwise also result in the formation of O+ ions [34, 35]:

+ → + ++e eO O O 22 (3)

When the measurements are performed with varying elec-tron energy at fixed mass, both the ionization potential of the radical as well as the appearance potential of the ions from DI of stable atoms or molecules can be determined and used for identification of species. TIMS can also be used for identi-fying and measuring the density of excited species in an APP. Agarwal et al. reported TIMS measurement of absolute densi-ties of electronically excited N2 in an inductively coupled N2 plasma [36]. Figure 7 shows similar measurements of vibra-tionally excited N2 performed in the effluent of a He/0.25% N2 plasma [7]. The signal is measured at mass 28 amu (N+

2) for plasma off and plasma on conditions. It appears at plasma off conditions solely due to direct ionization of the ground state

N2 molecules with a threshold energy of 15.58 eV. The signal is detectable down to ∼13.5 eV due to a finite electron energy distribution. An additional signal appears at lower electron energy when the plasma is turned on. This lowering of the ionization threshold can be explained by vibrationally excited N2 molecules formed in the APP as indicated by the dashed curve corresponding to the ionization of N2 in the seventh vibrational level.

Stable neutral species can also be measured with higher electron energy, typically 70 eV, using the fact that the elec-tron impact ionization cross section has usually its maximum around this value. Direct ionization is however accompanied by DI in this case and a so-called cracking pattern is obtained for each molecular specie with signals at the masses of the parent ion and of the fragment ions. The final mass spectrum of a measured gas mixture is a linear combination of cracking patterns of all constituents of the mixture. Consequently, 70 eV cannot be used for analysis of low density reactive radicals, which are fragments of the dominant stable molecules. Still, there are unfortunately studies confusingly identifying the fragment ions of molecular species as signal corresponding to radicals, see for example [30].

4. Measurement of ions

Along with the fast development of mass spectrometry in the ’50s there has been an increasing interest in analysis of ions originating from various kinds of APPs. The investigated plasmas vary strongly in temperature and ionization degree from relatively hot hydrogen flames over inductively cou-pled plasmas to colder radio-frequency driven microplasmas [23, 37–39].

Figure 7. MS signal at mass-to-charge ratio of 28 amu (N2) as function of electron energy for plasma on and off condition. The APP was produced by a microscale atmospheric pressure plasma jet (μ-APPJ) operated 1.4 slm He and an admixture of 0.25% N2. The dashed curve indicate the possible signal due to ionization of N2 in its seventh vibrational level. The inset image with potential energy curves of N2 molecule demonstrates this process.

12 13 14 15 16 17 1810-4

10-3

10-2

10-1

100

plasma off plasma on additional signal due to N

2(v>0)

sign

al a

t m/z

= 2

8 (a

.u.)

electron energy (eV)

N2+

N2

vib. excited state

ground state

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Previous studies on ion mass spectrometry of APPs are per-formed with two main objectives. First, the development of analytical instruments, e.g. for detection of trace elements in environmental or forensic applications [37, 40] and second, the analysis of the ion chemistry in cold APPs. The goal for a proper analytical device is usually a high sensitivity together with a linear dependence of ion signal on analyte concen-tration over several orders of magnitude [37]. In the case of cold APPs, the investigation of the plasma chemistry plays a major role in order to unravel the relevant processes taking place in the discharge and at the surface. Sampling and meas-uring ions from a gas-ion mixture at atmospheric pressure is rather straightforward while the physical processes during the expansion are quite complex. The influence of the sampling and ion manipulation with ion lenses on the measured ion composition is often neglected [17] and will be in the focus of this section.

4.1. Ion mass spectrometer setup

A typical mass spectrometric analysis of ions originating from APPs consists of three steps. First, ions are produced either by an exothermal reaction due to high temperatures [23] or with electric fields by ignition of an APP [37, 41]. Second, these ions are extracted through a sampling orifice (usually 20–300 μm) into a multi-stage pressure reduction system containing ion lenses in order to collect and focus ions from the expanding ion-gas mixture into the entrance of the mass spectrometer. Third, a mass filter (sometimes combined also with an energy filter) and an ion detection device (Faraday cup or secondary electron multiplier) is used to measure the mass (and energy) dependent ion current. Examples of posi-tive and negative ion spectra obtained from a variety of APPs were measured by the commercially available HPR-60 mass spectrometer (Hiden Analytical Ltd.) [28, 29, 38, 42–44]. A scheme of the setup used in this work is shown in figure 8.

In one of the first studies on mass spectrometry of ions from APPs by Knewstubb and Sugden [23] it is emphasized to clearly distinguish between ions originating from the APP itself and ions which are formed inside the sampling device or the spectrometer. Main mechanisms that may have an influence

on the measured ionic composition are secondary ionization, charge transfer, dissociation and ion clustering. In addition to these effects, photo-ionization via plasma emitted VUV pho-tons may play a role depending on experimental conditions.

In the setup in figure 8, ions are sampled through an orifice with a diameter of 20 μm into the first pumping stage, where the a pressure of ⋅ −5 10 5 mbar is maintained by a turbomo-lecular pump. As soon as the ions pass the quitting surface during the expansion they are extracted and focused by an ion lens into the second pumping stage. The ion lens consist of four cylindrical elements that can be biased between +250 V and −250 V (voltages U1, U2, U3 and U4 in figure  8). The first lens element is made of a stainless steel mesh to pro-vide an efficient pumping of the neutral species from the ion collection region [37]. The second pumping stage is housing a quadrupole mass spectrometer (PSM, Hiden Analytical). The system is simpler in comparison to the neutral MBMS, because no ionizer is needed and there is no ‘ion-background’ signal. Therefore, a beam chopper is not needed. The entrance into the mass spectrometer can also be at much larger dis-tance from the sampling orifice compared to the measurement of neutrals, because the beam of sampled ions is focused by the ion optics. In fact, additional skimmers like those in the MBMS setup can hinder an effective ion collection, which supports the idea of two separate setups for neutral and ion mass spectrometry from APPs.

4.2. Tuning of ion optics

The performance of the ion lens is optimized by scanning the voltages U1−4 and maximizing the ion signal at a specific mass-to-charge ratio. Here we want to demonstrate that this optimization is mass dependent due to the above-discussed effects during gas expansion and that the relative intensities of ion currents for different ions should be taken with care. In figure 9 a voltage scan for the first lens element voltage U1 is

Figure 8. Schematic diagram of the experimental setup used for the detection of ions originating from APPs.

Figure 9. Signal of selected ions in dependence of the first lens element voltage U1. Ions are created in the μ-APPJ operated at a gas flow rate of 1.4 slm He and an admixture of 0.3% N2 at a distance of 2 mm from the sampling orifice.

0 -50 -100 -150 -200 -2500

1

2

3

4

5

6

7

8

sign

al (

105 c/

s)

lens voltage U1 (V)

H3O+ (19)

N+

2 (28)

NO+ (30) N+

3 (42)

N+

4 (56)

U2 = -45 V

U3 = -175 V

U4 = -250 V

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presented, while the other voltages are set to a constant value, as indicated in the graph. The ion current at different masses are changing as the set U1 is varied, revealing two maxima typical for the single lens. However, a strong dependence of the curve shapes on ion mass can be observed.

The maximum signal for each mass is reached at a different U1 voltage which will result in a strong variation of the rela-tive signal intensities depending on which ion will be used for tuning. This is again a consequence of the mass dependent gas sampling. The position of the quitting surface as well as the angular distribution will vary with the ion mass, resulting in different optimal voltage settings for the ion optics.

Summarizing, it is emphasized that great care has to be taken when comparing relative ion signals of mass spectra obtained from APPs, since the individual ion signals have a significant mass dependence due to the physics of the super-sonic expansion. To illustrate that: in figure 9 when shifting the voltage U1 by only 10 V the ratio of m/z = 28 to m/z = 42 can vary from 1.5 to 0.8.

4.3. Ion energy and sampling

As already explained above, detected ions are all thermalized after the sampling with energies at or below 1 eV and, in prin-ciple, no energy filter is needed. Still, mass spectrometers with energy filter are used in many cases, because they are avail-able from previous studies or because the energy filter such as a Bessel box is used to block the line-of-sight from the ion detector to the plasma, which avoids parasitic signals caused by plasma-emitted UV and VUV photons. The great advan-tage of an built-in energy filter is that it can be used to verify that the plasma is not penetrating into the low pressure sam-pling stage. The ion energies have to be in the range of 1 eV or below, because the ion energy is defined by the terminal axial velocity of the gas mixture in the supersonic expansion. Any ion energies larger than these values are an indication for the existing problem of plasma penetration.

The PSM mass spectrometer used here has a built-in Bessel box energy filter and its schematic diagram is shown as an inset in figure 10(b). The pass energy of ions that are transmitted through the energy filter depends on four parameters, namely the angle of incidence, under which ions enter the Bessel-box, and the three tunable potentials called ‘energy’, ‘endcap’ and ‘cylinder’. In the following, these voltages will be referred to as Uene, Uend and Ucyl, respectively. The IEDFs of four ions originating from a micro-APP jet (μ-APPJ) operated at a gas flow rate of 1.4 slm He with an admixture of 0.3% N2 [7, 15] are presented in figure 10(a). The IEDFs are obtained as scans of the potential Uene, the other two voltages Ucyl and Uend are set constant to 1.2 V and −17 V, respectively. The ion current maxima are at different values of Uene and in figure 10(b) it is shown that their positions depend linearly on the ion mass as expected for the supersonic expansion, where all species have the same terminal velocity, not the energy. The ion current is measured at an unphysical negative ‘energy’, which is appar-ently due to an offset between real energy and the Uene poten-tial. The Bessel box energy filter is designed for determination of an IEDF typical for a low pressure plasma with a mean ion

energy of the order of 10 eV or more and small shift of few eV of the energy scale is possible. The energy scale can be absolutely calibrated by extrapolating the mass dependence energy of maxima positions to the mass zero, see figure 10(b). The offset of the energy scale is about −1.8 V. This results in real peak ion energies in the range of 0.2 eV to 0.65 eV. The linearity allows to express the terminal mean ion energy as a function of ion mass by the following equation:

[ ]( [ ]) = ( ± ) ⋅ [ ]E m meV amu 0.012 0.001 amukin (4)

Data in figure  10(a) points at one very important issue related to energy resolved measurements. The mass spectra are usually recorded at one ion energy. However, this energy is optimal just for species with mass corresponding to this energy and all other species are discriminated. Imagine the mass spectrum would be recorded at = −U 1.15ene V (posi-tion of IEDF maximum for N+

4). The water ion H2O+ will be hardly observable in the spectrum in this case and also the other ions will have lower signals than at their energy maxima. Measuring at the energy maximum of the H2O+ ion will on the other hand discriminate the other heavier ions. To correct for this effect, the energy setting of the Bessel-box should be varied parallel to the mass setting or, alternatively, the IEDF should be measured at each mass of interest and the maximum values should be compared. This observation points again to

Figure 10. (a) Dependence of ion current (normalized to unity) on Uene (energy) for four ion masses. (b) Plot of peak position versus ion mass for determination of the energy scale offset. The inset shows a cross-sectional view of the the cylinder-symmetric Bessel box energy analyzer. Ions are created in the μ-APPJ operated at a gas flow rate of 1.4 slm He and an admixture of 0.3% N2 at a distance of 2 mm from the sampling orifice.

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.00.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40 45 50 55 60-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

(a)

Ucyl

= 1.2 V

Uend

= -17 V

norm

aliz

ed s

igna

l (a.

u.)

Uene

(V)

H2O+ (18)

N+2 (28)

N+3 (42)

N+4 (56)

peak positionlinear fit

Uen

e(V

)

mass (amu)

(b)

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.00.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40 45 50 55 60-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

(a)

Ucyl

= 1.2 V

Uend

= -17 V

norm

aliz

edsi

gnal

(a.

u.)

Uene

(V)

H2O+ (18)

N+2 (28)

N+3 (42)

N+4 (56)

peak positionlinear fit

Uen

e(V

)

mass (amu)

(b)

Uend Ucyl Uend

Uene

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the conclusion that the relative intensities of ion peaks in the mass spectrum should be taken with care.

In addition to the experimental determination of the ion energy described above, ion trajectories are simulated using SIMION 8.1 software. The path of the ions from the quitting surface of the expanding gas jet through the custom-made ion optics as well as the MS ion optics together with the Bessel box energy filter is simulated and the resulting ion trajecto-ries are shown in figure 11. The electric field is determined by solving numerically the Laplace equation and ion trajecto-ries are calculated based on initial conditions of the particles (starting position, angular distribution, energy, charge, mass).

Experimentally obtained potentials for the different lens elements were set in the simulation and the ion energy has been scanned to obtain maximum transmission. The resulting ion energies are in the range 0.2 to 0.8 eV, in a very good agreement with expectations from the literature for a supersonic expansion under the experimental condi-tions used here [17, 26] and also with our experimental values determined above and summarized in equation  (4). It demonstrates that the simulation of ion trajectories is a powerful tool for the development and verification of ion sampling systems.

Special operating condition are mandatory when analyzing negative ions from electronegative discharges (for example the μ-APPJ operated with He/O2 gas mixture). Theoretical simulations of this gas mixture excited by RF voltage pre-dict the presence of O−, O−

2 and O−3 ions [47]. However, the

detection of negative ions from the μ-APPJ was not pos-sible as readily as the detection of positive ions, even though large fluxes (∼105 cs−1) of negative ions could be detected in negative corona discharges. For the detection of negative ions from the μ-APPJ operated in a He/O2 mixture it is neces-sary to place the plasma jet in close vicinity (< 2 mm) to the sampling orifice. This results in the formation of a plasma in direct contact with the sampling orifice as was reported in  [28]. Only under such conditions negative ions fiom the μ-APPJ are observable.

Negative ions are easily lost by ion–ion recombination reactions in the effluent when the plasma is not in contact

with the sampling orifice which prohibits detection of those from the effluent at large distances. Increasing the electric field in the first pumping stage at the position of the quitting surface did not solve the problem which shows that recom-bination takes places predominantly in the high pressure region of the system. The measured negative ion spectrum shown in figure 12 is in agreement with the plasma model results [47].

5. Ion chemistry at atmospheric pressure

The discussion in this section is started with an example of ion mass spectra measured from the μ-APPJ source operated in ‘pure’ helium. In figure 13 mass spectra measured one minute and 1 h after ignition are shown. The μ-APPJ has been oper-ated using a gas flow rate of 1.4 slm He without admixing additional reactive gases. Impurities within the feed gas as well as adsorbed species on the wall of the APP device have a significant effect on the measured ion composition especially during the initial phase after ignition. The most abundant ion signals were obtained for water cluster ions of the form H ( )+ H O n2 up to n = 5 in agreement with previous studies on

Figure 11. Cross-sectional view of trajectories of ions passing through the potential field created by the custom-made ion optics and the first part of the mass spectrometer. The simulation reveals that the ions have an energy of 0.2 to 0.8 eV depending on the potentials set at the Bessel box energy filter.

Figure 12. Negative ions measured from the μ-APPJ operated in a He/O2 mixture. Negative ion detection requires operation of the μ-APPJ in close vicinity (< 2 mm) to the sampling orifice.

0 5 10 15 20 25 30 35 40 45 50 55 60101

102

103

104

105

106

H2O-

2

OH-

O-3

O-2

sign

al(c

/s)

m/z

O-

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several APP sources [29, 39, 45, 46], where the formation of water cluster ions of the form H ( )+ H O n2 is a common striking feature.

Significant changes of the relative ion signal can be observed during the first hour of operation which are related to the desorption of water from the walls of the whole system. Once a steady state water concentration is reached, the spec-trum is stable and the most abundant ion has changed from H ( )+ H O2 4 to H ( )+ H O2 due to a smaller water content in the gas. Here, for both spectra the energy filter has been optimized for m/z = 28 so we should keep in mind that ions lighter or heavier than 28 amu are discriminated. However, the relative changes between both spectra at each mass are still relevant and can be compared.

Besides H ( )+ H O n2 one can observe ions that are the product of other impurities like N2 or O2 or dissociated H2O as marked in figure 13. The appearance of HHe+

2 is an indica-tion for a high degree of purification since an ion containing helium may only reach the detector when the probability of charge transfer in the effluent and during the expansion is reduced. The water concentration in the feed gas is clearly decreasing with increasing operation time of the plasma which becomes visible as a shift from heavy to light cluster ions. The spectra shown in figure 13 demonstrate the fact that most ions measured by MS from APPs are not primary ions generated directly in plasma by electron impact or Penning ionization, but mostly secondary ions originating from ion-neutral reactions. A detailed description of the ion chemistry is in the focus of this section.

Formation of positive or negative ions in electric discharges is a very complex process including a large number of dif-ferent reactions. This complexity grows rapidly with introduc-tion of molecular gases. The gas pressure, gas temperature, purity of the supplied gases and the humidity are additional important factors which have an influence on the formation of ions in the discharge. With increasing discharge pressure the importance of three body reactions increases and more com-plex ions are formed via associative reactions.

In addition, neutral species generated in the discharge have a significant influence on the resulting ion mass spectrum due to ion-molecule reactions. In the case of atmospheric pres-sure plasmas in rare gases it is important whether a molecular admixture is added to the rare gas prior or posterior to the dis-charge. Different ions are observed whether they are formed via direct plasma-admixture interaction or via interaction of rare gas primary ions with the admixed species.

At atmospheric pressure the formation of cluster ions is dominant. The inherent presence of water as a polar trace gas leads to the observation that the majority of detected ions in both polarities appear in the form of clusters as shown in figure 13. Furthermore, the gas temperature has a significant influence on kinetics of ion processes in plasmas, opening new reaction channels not accessible at room temperature or determine the size distribution of the cluster ions.

Under our conditions the ions are sampled from the effluent of the μ-APPJ, where a distance between the active plasma region and the sampling orifice is at least 1 mm. Typical gas drift velocities are of the order 10 m s−1 depending on the

gas flow rate and the geometry. This results in ∼105 collisions of primary plasma ions with neutrals in the effluent before reaching the sampling orifice. As was mentioned, association reactions are very important, and for example the association rate frequency of N+

2 into N+4 in air is orders of magnitude

higher than the transport frequency to the mass spectrom-eter with the above given velocity and distance. But even for plasmas in the direct contact with the sampling orifice, the ions will undergo hundreds or thousands of collisions in the plasma sheath and in the expansion in and after the sampling orifice. These collisions and the ion-molecule reactions alters significantly the sampled ion mass spectrum compared to the primary ions in the plasma with strong dependence on their transport time in the high pressure region. To conclude, the composition of ions in an APP is very dynamic and gradu-ally develops with time. For analysis of ion mass spectra three different phases that influence the ion composition shall be considered:

(i) Initial phase—formation of primary ions (both polari-ties), electrons, excited and neutral species

(ii) Conversion phase—the primary ions are converted into thermochemically more stable ions via a chain of ion-molecule reactions, Penning ionization, three body electron attachment and dissociative electron attachment of thermal electrons to molecules and subsequent conver-sion of the negative ions

(iii) Clustering—the stable ions undergo clustering reactions

5.1. Initial phase (plasma phase)

In the initial phase we consider the processes directly respon-sible for the ionization event that results in formation of positive or negative ions by electron impact and photo- or

Figure 13. Stackplot of mass spectra measured at t = 1 min and t = 60 min after ignition of the μ-APPJ operated at 1.4 slm He in a controlled atmosphere at a distance of 2 mm from the sampling orifice.

0 10 20 30 40 50 60 70 80 90 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

H+(H2O)

3

H+(H2O)

2

O+

2

NO+

N+

2

H2O+

OH+

O+

H+(H2O)

sign

al o

ffset

(10

5 c/s

)

m/z

60 min after ignition 1 min after ignition

HHe+

2

H+(H2O)

4

H+(H2O)

5

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field-ionization. These processes form the primary ions and new neutral particles (radicals, molecules and excited states).

(a) Positive ions The spectrum of primary positive ions reflects directly the

initial composition of the gas. In air at atmospheric pres-sure mainly molecular ions such as N+

2 or O+2 (denoted

below as A+2) are formed, whereas in rare gases at atmos-

pheric pressure readily molecular rare gas ions such as He+

2 (denoted as R+2) are formed.

+ → ++e eA A 22 2 (5)

+ → ++e eR R 2 (6)

+ → ++ +R 2R R R2 (7)

Depending on the discharge power, dissociative ioniza-tion of molecular ions may occur. This results in the formation of atomic ions such as N+ and O+.

+ → + ++e eN N N 22 (8)

+ → + ++e eO O O 22 (9)

Free electrons formed in the course of ionization reactions are thermalized quickly by elastic and inelastic collisions with atoms and molecules. In the conversion phase they can interact via thermal electron attachment reactions with electronegative molecules, or via recombination reactions with positive ions. In [47] a simulation predicts already the full electron attachment at 1 mm downstream from an atmospheric pressure μ-APPJ with He/O2 gas flow.

(b) Negative ions In this first phase the primary negative ions are formed via

dissociative electron attachment (DEA) with non-thermal electrons. In air the main channel in this phase is the DEA to O2 responsible for formation of the O− negative ions.

+ → +−e O O O2 (10)

Negative ions are exclusively formed when one of the fragments has positive electron affinity (EA). For this reason electron interactions with N2 or rare gases do not result in formation of negative ions.

(c) Neutral species Reactive atomic and molecular species (O, N, OH, CO,

NH3) are formed through dissociative processes, i.e. dissociative ionization (equation (8)), direct dissociation (equation (11)), dissociative excitation (equation (12)) and DEA (equation (10)).

+ → + +e eA A A2 (11)

+ → * + +e eA A A2 (12)

In gas mixtures containing rare gases, metastable states of rare gases Rm play an important role as sources of ions and electrons via the process of Penning ionization.

+ → + ++ eR A R Am2 2 (13)

→ + + ++ eR A A (14)

5.2. Conversion phase (plasma + early afterglow)

The primary ions and electrons formed in the initial phase sur-vive at atmospheric pressure only for a very short time since they are rapidly converted in a chain of different reactions into secondary ions due to the high collision frequency.

(a) Positive ions At elevated pressures the conversion usually occurs via

associative reactions. For example in ambient air N+ and N+

2 ions are rapidly converted into N+3 and N+

4 ions via three body association

+ → ++ +N N N N2 2 3 (15)

+ + → ++ +N N M N M2 2 4 (16)

+ + → ++ +N N M N M2 3 (17)

where M denotes a third body. Similar processes are responsible for the loss of other primary ions. Additionally, charge transfer of primary ions to molecules with a lower ionization energy (IE) plays an important role. H2O+ cannot react with the main constituents of air or rare gases due to the relatively low IE. Therefore, the only loss channel is the proton transfer reaction

+ → ++ +H O H O H O OH2 2 3 (18)

H3O+ is one of the terminal positive ions. This results in the observation that even trace amounts of water in a plasma of low ionization degree will lead to a high abun-dance of water cluster ions [29, 45, 46]. More complex reaction chains may lead to the conversion of N+

x, O+2 ,

CO+2 and NO+

2 ions to H3O+ ions. One important excep-tion is NO+. As its ionization energy (IE(NO)= 9.26 eV) is below that of H2O (IE(H2O)=12.62 eV) no conversion occurs and NO+ remains during the sampling process and is a candidate for soft ionization [48]. In figure 14 the positive ion mass composition from the effluent of a μ-APPJ plasma is measured as a function of the distance. Primary ions such as He+

2 are not observable due to the reaction pathways discussed above. With increasing dis-tance to orifice more terminal and cluster ions appear in the mass spectrum. In addition, proton transfer reactions towards other species can be found. One example is that with the admixture of nitrogen to the feed gas, protonated nitrogen species such as N2H+ are detected.

(b) Negative ions The free electrons are thermalized by elastic and inelastic

processes with molecules. Once the electron energy is sufficiently low, the collision with electronegative par-ticles (positive EA) results in the formation of negative

Plasma Sources Sci. Technol. 24 (2015) 044008

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13

ions. The negative ions are generated either via three body electron attachment or dissociative electron attach-ment, e.g. with oxygen species:

+ + → +−e O M O M2 2 (19)

+ → +−e O O O3 2 (20)

The EA of a molecule is a quantity, which describes the energy necessary for removal of an electron from the negative ion. This quantity is very important in the case of charge transfer reactions. The electrons jump in the direction of formation of more stable ions, as for example expressed by the reaction

+ → +− −O O O O2 3 3 2 (21)

where the EA(O2) = 0.451 eV and EA(O3) = 2.103 eV [49]. This leads to the observation of Skalny et al [43] that few O−

2 ions where found in the cluster ions.

5.3. Clustering phase (late afterglow)

The high abundance of water cluster ions in the literature about ion mass spectrometry of APPs is striking [28, 38, 43, 50]. The main reason for the predominant formation of such ions lies in the peculiar strength of hydrogen bonds. In general, clusters can occur with any hydrogen present in a polar group. The total energy of the cluster ion is continuously reduced with increasing size [51]. Therefore, depending on the plasma and sampling conditions such clusters may include hundreds of water molecules [29, 45, 46] and can be analyzed with respect to their structure [52]. The only variable is the nature of the parent ion, which depends on the conversion phase. For added water impurities mostly protonated or hydroxide water cluster ions H+(H20)n or OH−(H20)n occur, see also figure 14. At ambient air however, also other parent ions are found in the mass spectra, such as CO−

3, HCO−3, NO−

n, NO+ and O−n among

others [43]. As was outlined, complex reactions in the conver-sion phase determine the terminal ion composition, and con-sequently the composition of parent ions is a strong function of the residence time [50]. It is emphasized that water cluster ions may be destroyed by neutral impact. In the following X denotes the parent ion, which can be also negative.

( ) + → ( ) + ++ +−X H O M X H O H O Mn n2 2 1 2 (22)

Clustering is not only an issue for plasmas at ambient air or with added humidity. Even when high purity rare gases (5.0) are used, 10 ppm of trace components including water are pre-sent, which is more than the ionization fraction in a typical low temperature APP. Additional sources of trace impurities are due to contamination from walls of the vacuum system or through diffusion from the surrounding environment. The latter is especially important for open air plasma sources. In addition, usage of permeable tubes made from polymers lead to a very high level of impurities. At atmospheric pressure these impurities of molecular gases determine the final com-position of the positive or negative mass spectrum to a large extend (see figure 13).

For plasma sources at atmospheric pressure with low drift velocities such as corona discharges, the residence time until sampling is large. That means the conversion but also the clustering phase is inherently connected to the sampling pro-cedure and water cluster ions will be always detected. The sampling from plasma sources with high flow velocities such as plasma jets can be different. However, as explained above, a distance of plasma to sampling orifice of 1 mm results in a large number of possible secondary conversion reactions.

6. Conclusion

Practical and theoretical aspects connected with mass spectrom-etry (MS) measurements of absolute densities of neutral species and relative signals of positive and negative ions originating in atmospheric pressure plasmas (APPs) have been discussed. MS is a powerful technique for analysis of APPs and, if designed and operated correctly, can provide irreplaceable information about many species and many plasma-chemical processes. Fundamental and inherent physical processes during sampling of ions and neutrals from APPs have a significant influence on the measured composition as shown above. The key issues con-nected with MS used for analysis of APPs and important mes-sages for its correct design and operation are summarized here:

6.1. Collisional gas sampling

• The gas sampling in MBMS of APPs is collisional with a supersonic gas expansion in a so-called free-jet. This is a distinctive difference compared to the analysis of low pressure plasmas, where the sampling is in the collision-less molecular flow regime and is the same for all species.

• The gas composition in the molecular beam (MB) is always distorted compared to the composition of the sampled gas. Lighter species are usually discriminated compared to heavier species with strong dependence on

Figure 14. Positive ion mass spectrum obtained by the system described in section 4 in a He/N2 mixture at different distances from a RF plasma jet. The full numbers denote the ratio of m/z in [u]. For simplicity not all detected ions are shown.

0 1 2 3 4 5 6 7 8 9 10 11 12103

104

105

106

29: N2H+

17: HO+

48: NO+(H2O)

He (5.0) + 0.2 % N2

sign

al (

c/s)

distance (mm)

37: H+(H2O)2

19: H3O+

28: N+2

56: N+4

30: NO+

55: H+(H2O)3

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the mass and weak dependence on the collision cross sec-tion.

• Too high background pressure in the first pumping stage will result in the formation a shock front (Mach disk), which can additionally influence the composition of the MB. This should be considered by designing a skimmer to the second pumping stage, which samples the gas in front of the shock front, or by avoiding the shock front by keeping the background pressure very low (expansion into ‘vacuum’). One possible way to realize this low background pressure is by using a special beam chopper with a rotating skimmer installed directly behind the sampling orifice.

• The sampled gas is accelerated into the sampling orifice. The measured gas originates therefore from the region 2 to 5 diameters above the sampling orifice.

• Using a too large orifice can influence the plasma (pres-sure drop) and can allow the plasma to penetrate into the low pressure region at and behind the sampling orifice. The orifice diameter should be smaller than the the plasma sheath thickness to avoid these problems.

6.2. MS of neutrals

• The ion current measured in MBMS is proportional to the species density in the ionizer. The background com-ponent of this density has to be subtracted from the signal in order to obtain the density in the MB in ionizer. This background density in the ionizer is due to background species in the pumping stage of the mass spectrometer, but it can also be locally and temporally enhanced by beam particles when closed geometry of the ionizer is used.

• The measurement of calibration species has to be per-formed at the gas mixture with the same concentrations of main components (for example He-air mixture) to account for the composition distortion effects.

• The measurement of radicals or excited species requires careful setting of the electron energy in the ionizer to avoid formation of the same ions by dissociative ionization of larger molecules at electron impact energies beyond a characteristic threshold.

• The density in the MB and with that also the measured signal intensity drops with the square of the distance between the sampling orifice and the ionizer. Therefore, this distance should be kept as small as possible.

6.3. MS of ions

• The sampled ions are accelerated and focused into the MS by an ion optics system. No ionization source and no ion correction is needed.

• Ions can convert very fast in ion-molecule reactions at atmospheric and sub-atmospheric pressures. The sampled ion composition always differs from the primary ion dis-tribution in the plasma.

• The position of quitting surface and the angular velocity distribution (the radial velocity components) differs for

ions with different masses. As a consequence, there is not a unique set of parameters of the ion optics valid for all ions and the relative intensities have to be interpreted with care.

• All particles including ions leave the expansion with the same axial velocity component. This results in a linear dendence of the kinetic ion energy on the ion mass. The ion energies are in the range or below of 1 eV for typical conditions in APPs.

• The relative signal intensities for ions with different masses depend on the ion energy selected in the energy filter, if installed. The ion energy should be scanned par-allel to the mass scan to ensure that the measurement is performed at the maximum of the IEDF.

• Relative intensities among different masses should be in any case analyzed and considered with care. On the other hand, the relative changes at one single mass, when other parameters of the mass spectrometer are kept the same, provide reliable information about relative behavior of the particular ion.

As this list shows, mass spectrometry needs a lot of atten-tion during its design and operation to avoid systematic errors and misinterpretation of the measured data. This is especially dangerous here, because it is relatively easy to obtain some ion signals and to perform the analysis even with a wrongly designed and not-well-tuned mass spectrometer. Still, we are convinced that, when aware of the discussed difficulties, a properly designed and correctly operated mass spectrometry setup is a very powerful diagnostic tool for analysis of APPs.

Acknowledgments

This work has been funded by the German Research Founda-tion (DFG, grant PlasmaDecon PAK 728 to J Benedikt (BE 4349/2-1)), as well as FOR 1123 and the Research Depart-ment Plasmas with Complex Interactions of the Ruhr-Univer-sity Bochum (RUB) and the Marie Curie international training network ‘RAPID’. The authors would like to thank Norbert Grabkowski for his technical support and Hiden Analytical Ltd. for the helpful discussions.

References

[1] van Gaens W and Bogaerts A 2014 J. Phys. D: Appl. Phys. 47 079502

[2] Murakami T, Niemi K, Gans T, O’Connell D and Graham W G 2013 Plasma Sources Sci. Technol. 22 015003

[3] Waskoenig J, Niemi K, Knake N, Graham L M, Reuter S, Schulz-von der Gathen V and Gans T 2010 Plasma Sources Sci. Technol. 19 045018

[4] Pothiraja R, Ruhrmann C, Engelhardt M, Bibinov N and Awakowicz P 2013 J. Phys. D: Appl. Phys. 46 464012

[5] Niermann B, Böke M, Sadeghi N and Winter J 2010 Eur. Phys. J. D 60 489–95

[6] Spiekermeier S, Schröder D, Schulz-von der Gathen V, Böke M and Winter J 2015 J. Phys. D: Appl. Phys. 48 035203

[7] Schneider S, Dünnbier M, Hübner S, Reuter S and Benedikt J 2014 J. Phys. D: Appl. Phys. 47 505203

Plasma Sources Sci. Technol. 24 (2015) 044008

S Große-Kreul et al

15

[8] Verreycken T, van der Horst R M, Baede A H F M, van Veldhuizen E M and Bruggeman P J 2012 J. Phys. D: Appl. Phys. 45 045205

[9] Niemi K, Schulz-von der Gathen V and Döbele H F 2005 Plasma Sources Sci. Technol. 14 375–86

[10] Verreycken T, van der Horst R M, Sadeghi N and Bruggeman P J 2013 J. Phys. D: Appl. Phys. 46 464004

[11] Bornholdt S, Wolter M and Kersten H 2010 Eur. Phys. J. D 60 653–60

[12] Benedikt J, Consoli A, Schulze M and von Keudell A 2007 J. Phys. Chem. A 111 10453–9

[13] Ellerweg D, von Keudell A and Benedikt J 2012 Plasma Sources Sci. Technol. 21 034019

[14] Benedikt J, Hecimovic A, Ellerweg D and von Keudell A 2012 J. Phys. D: Appl. Phys. 45 403001

[15] Schulz-von der Gathen V, Schaper L, Knake N, Reuter S, Niemi K, Gans T and Winter J 2008 J. Phys. D: Appl. Phys. 41 194004

[16] Scoles G 1988 Atomic and Molecular Beams (Oxford: Oxford University Press)

[17] Fenn J B 2000 Int. J. Mass Spectrom. 200 459–78 [18] Luria K, Christen W and Even U 2011 J. Phys. Chem. A

115 7362–7 [19] Scanlon T J, Roohi E, White C, Darbandi M and Reese J M

2010 Comput. Fluids 39 2078–89 [20] McCay T D and Price L L 1983 Phys. Fluids 26 2115–9 [21] Benedikt J, Ellerweg D and von Keudell A 2009 Rev. Sci.

Instrum. 80 055107 [22] Knuth E L 1995 Combust. Flame 103 171 [23] Knewstubb P F and Sugden T M 1960 Proc. R. Soc.

255 520–37 [24] van de Meerakker S Y T 2008 Nature Phys. 4 595–602 [25] Stoffels E, Sakiyama Y and Graves D B 2008 IEEE Trans.

Plasma Sci. 36 1441–57 [26] Christen W and Rademann K 2008 Phys. Rev. A

77 012702 [27] Anderson J B and Fenn J B 1965 Phys. Fluids 8 780 [28] Bruggeman P, Iza F, Lauwers D and Gonzalvo Y A 2010

J. Phys. D: Appl. Phys. 43 012003 [29] McKay K, Oh J-S, Walsh J L and Bradley J W 2013 J. Phys.

D: Appl. Phys. 46 464018 [30] Malović G, Puač N, Lazović S and Petrović Z 2010 Plasma

Sources Sci. Technol. 19 034014 [31] Singh H, Coburn J W and Graves D B 1999 J. Vac. Sci.

Technol. 17 2447

[32] Krähling T, Ellerweg D and Benedikt J 2012 Rev. Sci. Instrum. 83 045114

[33] Benedikt J, Agarwal S, Eijkman D, Vandamme W, Creatore M and van de Sanden M C M 2005 J. Vac. Sci. Technol. 23 1400

[34] Agarwal S, Quax G W W, van de Sanden M C M, Maroudas D and Aydil E S 2004 J. Vac. Sci. Technol. 22 71

[35] Ellerweg D, Benedikt J, von Keudell A, Knake N and Schulz-von der Gathen V 2010 New J. Phys. 12 013021

[36] Agarwal S, Hoex B, van de Sanden M C M, Maroudas D and Aydil E S 2003 Appl. Phys. Lett. 83 4918

[37] Houk R S, Fassel V A, Flesch G D and Svec H J 1980 Anal. Chem. 52 2283–9

[38] Oh J-S, Aranda-Gonzalvo Y and Bradley J W 2011 J. Phys. D: Appl. Phys. 44 365202

[39] Benedikt J, Ellerweg D, Schneider S, Rügner K R R, Kersten H and Benter T 2013 J. Phys. D: Appl. Phys. 46 464017

[40] Huang E C, Wachs T, Conboy J J and Henion J D 1990 Anal. Chem. 62 713–25

[41] Miclea M, Kunze K, Franzke J and Niemax K 2004 J. Anal. At. Spectrom. 19 990

[42] Beck A J, Aranda Gonzalvo Y, Pilkington A, Yerokhin A and Matthews A 2009 Plasma Process. Polym. 6 521–9

[43] Skalny J D, Orszagh J, Mason N J, Rees J A, Aranda-Gonzalvo Y and Whitmore T D 2008 Int. J. Mass Spectrom. 272 12–21

[44] McKay K, Walsh J L and Bradley J W 2013 Plasma Sources Sci. Technol. 22 035005

[45] Cheng H-P 1998 J. Phys. Chem. A 102 6201–4 [46] Searcy J Q 1974 J. Chem. Phys. 61 5282 [47] Hemke T, Wollny A, Gebhardt M, Brinkmann R P and

Mussenbrock T 2011 J. Phys. D: Appl. Phys. 44 285206

[48] Sabo M and Matejčík S 2013 Analyst 138 6907 [49] Rienstra-Kiracofe J C, Tschumper G S, Schaefer H F, Nandi S

and Ellison G B 2002 Chem. Rev. 102 231–82 [50] Abd-Allah, Z, Sawtell D A G, McKay K, West G T, Kelly P J

and Bradley J W 2015 J. Phys. D: Appl. Phys. 48 085202

[51] Wròblewski T, Ziemczonek L and Karwasz G P 2004 Czech. J. Phys. 54 C747–52

[52] Shin J-W, Hammer N I, Diken E G, Johnson M A, Walters R S, Jaeger T D, Duncan M A, Christie R A and Jordan K D 2004 Science 304 1137–40

Plasma Sources Sci. Technol. 24 (2015) 044008