Multipole Traps as Tools in Environmental Studies
Bogdan M. Mihalcea, Cristina Stan, Liviu C. Giurgiu, Andreea Groza, Agavni Surmeian, Mihai Ganciu, Vladimir Filinov, Dmitry Lapitsky, Lidiya Deputatova, Leonid Vasilyak, Vladimir Pecherkin, Vladimir Vladimirov, Roman Syrovatka
MMultipole Traps as Tools in Environmental Studies
Bogdan M. Mihalcea , Cristina Stan , Liviu C. Giurgiu ,Andreea Groza , Agavni Surmeian , Mihai Ganciu , VladimirE. Filinov , Dmitry Lapitsky , Lidiya Deputatova , LeonidVasilyak , Vladimir Pecherkin , Vladimir Vladimirov , RomanSyrovatka National Institute for Laser, Plasma and Radiation Physics (INFLPR), Atomi¸stilorStr. Nr. 409, 077125 M˘agurele, Romania Department of Physics,
Politehnica
University, 313 Splaiul Independent¸ei,RO-060042, Bucharest, Romania University of Bucharest, Faculty of Physics, Atomistilor Str. Nr. 405, 077125M˘agurele, Romania Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya Str.13, Bd. 2, 125412 Moscow, RussiaE-mail: [email protected]
E-mail: [email protected]
Abstract.
Trapping of microparticles, nanoparticles and aerosols is an issue of majorinterest for physics and chemistry. We present a setup intended for microparticletrapping in multipole linear Paul trap geometries, operating under Standard AmbientTemperature and Pressure (SATP) conditions. A 16-electrode linear trap geometryhas been designed and tested, with an aim to confine a larger number of particleswith respect to quadrupole traps and thus enhance the signal to noise ratio, as wellas to study microparticle dynamical stability in electrodynamic fields. Experimentaltests and numerical simulations suggest that multipole traps are very suited for highprecision mass spectrometry measurements in case of different microparticle speciesor to identify the presence of certain aerosols and polluting agents in the atmosphere.Particle traps represent versatile tools for environment monitoring or for the study ofmany-body Coulomb systems and dusty plasmas.PACS numbers: 37.10.Rs, 37.10.Ty, 52.27.Aj, 52.27.Jt, 92.60.Mt, 92.60.Sz
Keywords : microparticle, aerosols, linear Paul trap, dynamical stability, electrodynamicfields, many-body Coulomb systems, dusty plasmas a r X i v : . [ phy s i c s . p l a s m - ph ] D ec ultipole Traps as Tools in Environmental Studies
1. Introduction. Particle traps as tools for complex, one-componentplasmas (OCP)
Complex (dusty) plasmas represent a distinct type of low-temperature plasmas thatconsist of highly charged nano- or microparticles [1, 2, 3, 4]. Dusty plasmas areencountered in interstellar space and circumstellar clouds, as interplanetary dust oreven in the earth magnetosphere, atmosphere and mezosphere [5, 6, 7, 3, 8]. Collectiveprocesses occur in complex plasmas owing to the long range Coulomb interactionbetween particles characterized by large electrical charges, which leads to the occurrenceof strong coupling phenomena in the system [2]. Complex plasmas are intensivelyinvestigated in laboratories, as they are expected to shed new light on issues regardingfundamental physics such as phase transitions, self-organization, study of classical andquantum chaos, pattern formation and scaling [9, 10, 6]. Attention paid to the domainhas witnessed a spectacular increase after the discovery of plasma crystals [11, 12, 13, 14]and the detection of spokes in the rings of Saturn by the Voyager 2 mission in 1980 [5, 6].Present interest is focused on strongly coupled Coulomb systems of finite dimensions[15]. Particular examples of such systems would be electrons and excitons in quantumdots [3, 16, 17] or laser cooled ions confined in Paul or Penning type traps [18, 19, 20].First experimental observations of ordered structures consisting of charged ironand aluminium microparticles confined in a Paul trap were reported in 1959 [21]. Phasetransitions occurred, as an outcome of the dynamical equilibrium between the trappingpotential and the inter-particle Coulomb repulsion. In 1991, an experiment reportedthe storage of macroscopic dust particles (anthracene) in a Paul trap, operating inair [22]. Electrodynamic traps and ion trapping techniques combined with laser coolingmechanisms [18, 23, 17, 24] allow scientists to investigate the dynamics of small quantumsystems and prepare them in well-controlled quantum states [16, 20]. Trapped ions orparticles represent one-component plasmas (OCP). The OCP model is a reference onefor the study of strongly coupled Coulomb systems [13, 25, 26, 27]. Ion traps havealso opened new horizons towards performing investigations on the physics of few-bodyphase transitions [12, 5, 1, 2, 10]. Quantum engineering has opened new perpectives inquantum optics and quantum metrology [28, 24]. Applications of ion traps span massspectrometry, very high precision spectroscopy, quantum physics tests, study of non-neutral plasmas [25, 13], quantum information processing (QIP) and quantum metrology[29, 16, 30, 24, 20], use of optical transitions in highly charged ions for detection ofvariations in the fine structure constant [17, 20] or very accurate optical atomic clocks[31, 32, 33, 34, 35].In case of quadrupole traps, the second-order Doppler effect is the result of space-charge Coulomb repulsion forces acting between trapped ions of like electrical charges.The Coulombian forces are balanced by the ponderomotive forces produced by ionmotion in a highly non-uniform electric field. For large ion clouds most of the motionalenergy is found in the micromotion. Multipole ion trap geometries significantly reduceall ion number-dependent effects resulting through the second-order Doppler shift, as ultipole Traps as Tools in Environmental Studies
2. Aerosols. Micro and nanoparticles. General considerations
Atmospheric aerosols or astrophysical dusty plasmas are made out of microparticles[41]. Aerosols are presumed to have a larger impact on climate compared to greenhousegases, according to the IPCC WGI 4-th Assessment Report [42]. Nevertheless therestill is a high uncertainty about it, owing to the aerosol complex composition and astill incomplete picture needed to characterize the interactions between aerosols andglobal climate. We distinguish between two types of interaction: direct and indirectinteractions. By indirect effect, hydrophilic aerosols act as cloud condensation nuclei(CCN) affecting cloud cover and implicitly the radiation balance. Direct interactionsaccount for the light scattering mechanism on aerosols, resulting in cooling effects.On the other hand, aerosols containing black carbon (BC) or other substances absorbincoming light thus heating the atmosphere. According to measurements, the directradiative effect of BC would be the second-most important contributor to global ultipole Traps as Tools in Environmental Studies . There is a large interest towards minimizing theuncertainties associated with data collection when evaluating the impact of aerosols onglobal climate [43, 44, 45]. Different approaches and methods have been developed foranalyzing particles ranging from 10 nm to 10 m in diameter size, which consist of salts,soot, crustal matter, metals, and organic molecules, often mixed together [46, 47].Investigations on atmospheric aerosols, viruses, bacteria, and chemical agents, canbe performed using high precision mass measurements for micro and nanoparticles[48, 41, 49]. Study of such mesoscopic systems is of large interest, as mesoscopic physicsis linked to the fields of nanofabrication and nanotechnology. Late research indicatesthat nanoparticules are also associated with toxic effects on humans [50], as they arewidely used by the cosmetics industry. Many sunscreens contain nanoparticles of zincoxide or titanium dioxide. There are manufacturers that have added C Microparticles are known to inflict harmful effects on humans. There is a strongeffort towards limiting the maximum concentration and enforce safety levels for theatmospheric microparticles or dust most hazardous to human health. We distinguishbetween two categories: (i) fine particles with a diameter less than 2.5 microns (alsocalled fine particulate matter or PM . ), which are the most dangerous, and (ii) largerparticles with a diameter less than 10 microns but larger than 2.5 microns, namely theparticulate matter PM (also called coarse particles). Important steps are being takento reduce pollution due to PM . and PM microparticles, in order to minimise theirharmful effects on humans and biological tissue. In the EU directives are enforced whichregulate the PM . and PM microparticle levels. Member States must set up samplingpoints in urban and also in rural areas. Besides particulate matter, these samplingpoints must perform measurements on the concentration of sulphur dioxide, nitrogendioxide and oxides of nitrogen, lead, benzene and carbon monoxide.Strong evidence indicates that breathing in PM . over the course of hours to days(short-term exposure) and months to years (long-term exposure) can cause serious publichealth effects that include premature death, adverse cardiovascular and respiratoryeffects or even harmful developmental and reproductive effects [52]. Lung cancer isassociated with the emission of microparticles produced by diesel engines [43]. Scientificdata also indicates that breathing in larger sizes of particulate matter (coarse particlesor PM ), may also have public health consequences. In addition, particle pollutiondegrades public welfare by producing haze in cities or constantly increasing the rate ultipole Traps as Tools in Environmental Studies
3. Mass spectrometry using ion traps
A linear Paul trap uses a superposition of time varying, strongly inhomogeneous (a.c.)and d.c. electric potentials, to achieve a trapping field that dynamically confines ionsand other electrically charged particles [53, 40, 19, 16, 24]. When the a.c. trappingvoltage frequency lies in the few Hz up to MHz or even GHz range, electrons, molecularions or electrically charged nanoparticles with masses of more than 10 u (atomic massunits) are confined[54, 38, 39, 33]. Ion dynamics in Paul traps is described by a systemof linear, uncoupled equations of motion (Hill equations) that can be solved analytically[23, 40]. The linear form of the trap can be used as a selective mass filter or as anactual trap by creating a potential well for the ions along the z axis of the electrodes[53, 37, 55]. The linear trap design results in increased ion storage capacity, faster scantimes, and simplicity of construction [56, 57]. A Paul trap runs in the mass-selectiveaxial instability mode by scanning the frequency of the applied a.c. field [57, 58].Microparticle diagnosis can be achieved by operating the Quadrupole Ion Trap (QIT)as an electrodynamic balance, for low frequency values of the a.c. field applied to thetrap electrodes (typically less than 1 kHz).A specific charge m/z can be isolated in the ion trap by ejecting all other m/z particles (ions), by applying various resonant frequencies. Moreover, an ion trap canbe coupled to an Aerosol Mass Spectrometer (AMS) to investigate atmospheric aerosol(nano)particles. Experimental setups based on multipole Paul traps enable confinementas well as qualitative investigation of atmospheric particles and aerosols [45]. Usingsuch instruments, the chemical composition of the non-refractory component of aerosolparticles can be measured quantitatively. While the AMS uses either a linear quadrupolemass filter (Q-AMS) or a time-of-flight mass spectrometer (ToF-AMS) as a massanalyzer, the Ion Trap IT-AMS uses a 3D quadrupole ion trap. The main advantagesof an ion trap are the possibility of performing MS-experiments as well as ion/moleculereaction studies [59]. Experiments demonstrate that a mass resolving power larger than1500 can be achieved. This value is high enough to separate different organic speciesat m/z
43. Calibrations with laboratory-generated aerosol particles indicate a linearrelationship between signal response and aerosol mass concentration. These studies,together with estimates of the detection limits for particulate sulfate (0 . µ g/m ) andnitrate (0 . µ g/m ) demonstrate the ability of the IT-AMS to measure atmosphericaerosol particles [53, 60, 39, 61, 62, 58]. ultipole Traps as Tools in Environmental Studies
4. Experimental Setup
The 16-electrode trap geometry setup we have tested is shown in Fig. 1. Thetrap geometry consists of 16 brass electrodes of 60 mm length and 4 mm diameter,equidistantly spaced on a 46 mm diameter. If the AC potential is not too large( V ac > . cooling of the particleowing to friction in air [22, 63]. Such mechanism is similar with cooling of ions inultrahigh vacuum conditions by means of collisions with the buffer gas molecules. The16-electrode trap geometry we report is intended for investigating complex Coulombsystems (microplasmas), such as microscopic particles or aerosols in the atmosphere.We bring new evidence on microparticle trapping, while demonstrating that the stabilityregion for multipole traps is larger with respect to a quadrupole trap (electrodynamicbalance configuration) [64, 65]. Figure 1.
Photo of 16-electrode linear Paul trap
A radiofrequency (RF) voltage (typically between 1 ÷ . y − z plane, responsible for radial trapping ofparticles in a 2D potential [18, 23, 17]. A d.c. potential applied between the two endcap electrodes located along the trap x axis, helps achieving axial confinement forpositively charged particles. A harmonic secular potential results within the trap ifboth the RF and d.c. endcap potentials are quadratic, a condition difficult to achievefor the whole trap volume. In fact, it is assumed that the potential in the vicinity of thetrap axis is harmonic, which is a sufficiently accurate approximation. The 16-electrodePaul trap geometry is intended for investigating the appearance of stable and orderedpatterns for different charged microparticle species. Preliminary tests are performedusing alumina microparticles (with dimensions ranging from 60 up to 200 microns)in order to illustrate the trapping phenomenon, but other species can be considered.Specific charge measurements for trapped microparticle and nanoparticle species areexpected to result, as it is our intention to refine the setup. The trap we have designed ultipole Traps as Tools in Environmental Studies . ÷ . U x (0-1000 V), applied between the upper and lower multipole trap electrodes.The U x voltage (also called diagnose voltage) compensates the particle shift owing to thegravitational field and also enables performing a diagnose of the trapped microparticles.By using a precision microscope to ascertain the x -axis shift of the microparticle as afunction of the U x voltage variation, the specific charge of the microparticle species canbe determined. Ions confined in Paul traps that operate in ultrahigh vacuum arrangethemselves along the longitudinal z axis as they are contained within a large regionlocated around it, where the trapping potential is very weak. Things are different in caseof trapped microparticles, which we explain in Section 7. Another d.c. variable voltage U z is applied between the trap endcap electrodes, in order to achieve axial confinementand prevent particle loss near the trap ends. The polarity of the d.c. voltages can bereversed. Both a.c. and d.c. units of the electronic supply system will be driven by amicrocontroller unit.
5. Trap potential map for the 16 pole trap
The a.c. potential within the trap was mapped using an electrolytic tank filled withdedurized water. The trap was immersed within the water around 38 mm of its totallength. A needle electrode was used to measure the trap voltage, immersed at 16 mmbelow water level. The needle electrode can be displaced 36 mm both horizontallyand vertically, using a precision mechanism. The trap potential was mapped using atransversal (radial) section located at 22 mm with respect to the immersed end, for 2mm steps on both vertical and horizontal position. The experimental setup includingthe trap and the electrolytic tank is shown in Fig. 2.The Paul trap was supplied with a sine wave delivered by a function generator, ata 46.1 Hz frequency. An oscilloscope was used to monitor the sine wave. The rms valueof the a.c. voltage was measured using a precision voltmeter. We supply below mapsof the trap potential for a 0.5 V amplitude sine wave supplied to the trap electrodes.The sine wave was applied between even and respectively uneven electrodes, connectedtogether.We have also mapped the trap potential for a supply voltage of 1.5 V amplitude(1.037 V rms value) sine wave. ultipole Traps as Tools in Environmental Studies Figure 2.
Experimental setup photo showing the 16-electrode trap, the electrolytictank and the precision mechanism used to chart the electric potential
Figure 3.
Maps of the 16-electrode trap potential: contour plot, pseudocolor plot,surface plot and ribbon plot for an input sine wave of 0.5 V amplitude (0.335 V rms)
Data presented is preliminary, as it is our goal to demonstrate that such trapdesign is characterized by an extended region of lower field compared with 8-electrodeand 12-electrode geometries we have tested. Moreover, the 16-electrode trap design issuitable for various applications where larger signal-to-noise ratios are required. Theexperimental data and the numerical simulation results suggest that linear particle traps,such as the ones we have investigated [66, 67, 68, 69, 64, 70], are suited to use as Ion ultipole Traps as Tools in Environmental Studies Figure 4.
Maps of the 16-electrode trap potential: contour plot, pseudocolor plot,surface plot and ribbon plot for an input sine wave of 1.5 V amplitude (1.037 V rms)
Trap Aerosol Mass Spectrometers (IT-AMS) [60, 59]. Alumina (Al O ) microparticleshave been confined in our experiments using three dimensional electrodynamic multipolefields. The multipole trap geometries we report, have been investigated with an aimto levitate and study microscopic particles, aerosols and other constituents or pollutingagents that are present in the atmosphere. The research and simulations performed arebased on previous results and experience [66, 67, 69, 64, 70].
6. Physical modelling and computer simulation
Besides experimental investigations, we have performed numerical simulations in orderto illustrate particle dynamics in multipole traps. Our main concern was to chooseconditions as close as possible to the experiment, in an attempt to validate experimentaldata. Our simulations consider stochastic forces due to random collisions with neutralparticles, viscosity of the gas medium, regular forces produced by the a.c. trappingvoltage and the gravitational force. Thus, microparticle dynamics is described by astochastic Langevin differential equation, as follows [5, 71, 64, 72]: m p d rdt = F t ( r ) − πηr p drdt + F b + F g (1) ultipole Traps as Tools in Environmental Studies m and r p represent the microparticle mass and radius vector, η is the dynamicviscosity of the gas medium with η = 18 . µ Pa · s, and F t ( r ) is the ponderomotive trappingforce. The F b term stands for the stochastic delta-correlated forces accounting forstochastic collisions with neutral particles, while F g is the gravitational force. We haveconsidered a microparticle with mass density ρ p = 3700 kg/cm . The numerical methoddeveloped in [73] was used in order to solve the stochastic differential equation (1).The Coulomb force acting on the microparticle (expressed as the sum ofcontributions for each trap electrode) is the vector sum of forces of point-like chargesuniformly distributed along the electrodes, as shown in [64, 70]: | F t ( r ) | = (cid:88) s LV q N ln ( R R )( r s − r ) , (2)where L is the length of the trap electrodes, V is the trapping voltage: V ac sin(Ω t ) or V ac sin(Ω t + π ), q is the microparticle charge, N is the number of point-like chargesfor each trap electrode, R and R represent the radii of the grounded cylindrical shellsurrounding the trap and trap electrode respectively, while r and r s denote the vectorsfor microparticle and point-like charge positions respectively.For such model, the results of the computations depend on the following relationshipbetween the relevant trap parametersΦ p = V ac q R R ) . The following trap parameters were chosen: length of electrodes L = 6 . R = 25 cm, R = 3 mm, trap radius r t = 4 cm and V ac = 2kV. According to Eq. (2), theforce F t ( r ) describing microparticle interaction with the ponderomotive force dependson the electrical charge q , the trapping voltage V ac and the geometrical trap parameters.For such model the regions of microparticle confinement are mainly determined by theseforces. However, in reality the regions of microparticle stable confinement depend alsoon the trap volume, number of trapped microparticles, average interparticle distanceand, as a consequence, on repulsive forces of the interparticle interactions defined bythe particle charge q . To minimize the influence of these physical factors, we useda large value of the a.c. trapping voltage electrode V ac = 2 kV. In such case, thecaptured particles charge will be smaller and results of the simulations will be more orless universal.The equipotential surfaces in cross section of a linear trap with 16 electrodes isshown in Fig. 5. The equation describing the electric potential can be expressed as: U ( x, y ) = N el (cid:88) j =1 (cid:88) s ( − j LV ac N ln (cid:16) R R (cid:17) (cid:114)(cid:16) x − r t cos (cid:16) πjN el (cid:17)(cid:17) + (cid:16) y − r t sin (cid:16) πjN el (cid:17)(cid:17) + z s , (3)where N el is the number of trap electrodes and z s is the z axis coordinate of eachpoint-like charge for the trap electrodes. ultipole Traps as Tools in Environmental Studies Figure 5.
3D plots for the 16 electrodes trap.
Figure 6 presents the confinement region for the charged microparticle. Theconfinement region is located between black lines. Outside this region, the particleis not trapped. For small particle charge (at the left-hand of the confinement region),the a.c. trapping electric field cannot compensate the gravity force, and particles flowthrough the trap. At the right-hand of the confinement region when the electric chargevalue is large, the trap field is strong enough to push microparticles out of the trap.
Figure 6.
The regions of a single particle confinement as a function of the a.c. voltagefrequency f and particle charge. Numerical simulations were performed for an averageradius r p = 5 µ m of the microparticle and a particle charge value ranging between q = 3 · e to 5 · e . Vertical lines 1 − − q = 4 , , , · e thathave been used to estimate oscillation amplitudes within the regions. To study the influence of the frequency and particle charge on the behaviour ofparticles, we have investigated the average amplitude of particle oscillations within the ultipole Traps as Tools in Environmental Studies
Figure 7.
End view of the microparticle tracks in a 16-electrode trap at f = 60 Hz.Big black dots correspond to trap electrodes (not in scale). The microparticle radiuswas r p = 5 µ m and the electric charge value q = 8 · e . Particle oscillations are presented in Fig. 7. The dependences of average particleoscillation amplitude on the frequency and particle charge are shown in Fig. 8. Thephysical reason of the nonmonotonic decay of some dependences on frequency is possiblyrelated to their vecinity to the resonance frequencies.
Figure 8.
Dependencies of the average oscillation amplitude on the frequency and theparticle charge q . Numerical simulations were performed for parameters used in theexperiment such as: particle radius r p = 5 µ m and electric charge value ranging from q = 3 · e to 1 . · e . ultipole Traps as Tools in Environmental Studies
7. Conclusions
The trap design we have used is currently under test, but preliminary data suggest thatit can be used to levitate and detect different aerosol species. We consider using suchtrap geometry in ultrahigh vacuum conditions. The map of the trap potential coupledwith numerical simulations helped us identify extended regions of stable trapping, whichextend as far as near the vicinity of the trap electrodes. Such a trap design exhibits anextended region where the trapping field almost vanishes. Microparticle dynamics wasinvestigated by menas of mathematical simulations. Different programming codes aretested, based especially on C++ and Python. A trapped particle microplasma results(very similar to a dusty plasma, which is of great interest for astrophysics), consistingof tens up to thousands of particles. Such a setup is also suited to study and illustratethe appearance of ordered structures and crystal like formations.As revealed by the experimental data that map the a.c. field within the traps wehave investigated coupled with the numerical simulations we have performed, chargedparticles spend relatively little time in the high RF electric fields area, which resultsin longer confining times and higher stability. As experimental work with the trap iscurrently undergoing, we have used numerical simulations and maps of the trap potentialto compare the 16-pole trap with other designs we have tested, such as the 8-electrodeand the 12-electrode trap. Regions of stable confinement have been identified, dependingon the trap and experimental parameters. These traps are the subject of a differentpaper that will be published.Preliminary results show that thread-like formations of microparticles (dustyplasma) arranged both as planar and volume structures can be observed. These stablestructures tend to align with the z component of the radial field. These formations werenot disposed on the trap axis and many of them are located rather in the vicinity ofthe trap electrodes. We are able to report stable confinement for hours and even more.Experimental data shows good agreement with numerical simulation results.It is our intention to use such traps to confine aerosols and even nanoparticles and weconsider they are excellent tools to perform environment studies on presence of pollutingagents in the atmosphere and troposphere, coupled with other known techniques suchas Lidar.
8. Acknowledgements
The authors would like to acknowledge support provided by the Ministery ofEducation, Research and Inovation from Romania (ANCS-National Agency for ScientificResearch), contracts PN09.39.03.01 and Ideas Project PN-II-ID-PCE-2011-3-0958,Contract 90/2011.Mathematical model and the Brownian dynamics simulations have been carriedout in the Joint Institute for High Temperatures from Moscow, Russian Academy ofSciences (RAS), under financial support by the Russian Foundation for Basic Research ultipole Traps as Tools in Environmental Studies
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