novel atmospheric pressure plasma utilizing symmetric dielectric barrier discharge for mass...

3726 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 12, DECEMBER 2014

Novel Atmospheric Pressure Plasma UtilizingSymmetric Dielectric Barrier Discharge

for Mass Spectrometry ApplicationsChun-Yi Chen, Cheng-Hung Chiang, and Che-Hsin Lin

Abstract— This paper develops a novel symmetric dielectricbarrier discharge (DBD) plasma as an ion source for envi-ronmental mass spectrometry (MS) applications. The conven-tional linear-type DBD plasma generator suffers the drawbackof floating voltage at the plasma outlet. This paper developsan innovative symmetric T-shaped DBD plasma generator toproduce atmospheric plasma with zero-floating potential for high-sensitivity MS analysis. By changing the geometric configurationand the drive phase of the symmetric T-shaped DBD plasmagenerator, the resulting symmetric structural design can fullycancel the floating potential and noise signal. Therefore, themain objective of this paper is to compare the differencesbetween traditional linear-type DBD and the symmetric T-shapedDBD designs using MS, spectroscopy, and some basic electricalmeasurements. The most suitable parameters are determined bychanging the electrode design, voltage, temperature, gas flowrate, diameter, and other parameters of the plasma tube. Thesymmetric T-shaped design generator produces the zero-potentialplasma that generates fewer ambient gas molecules to form ozone,NOx, water clusters, and other strong oxidizing molecules suchthat less damage to the MS samples occurs. This in turn resultsin a less fragmented ion signal and higher sensing performancefor rapid MS applications. In addition, the proposed system candirectly ionize gas, liquid, and solid samples at more than107-cm−3 ion concentration. Results show that more information-rich spectra can be obtained with the developed symmetricT-shaped DBD plasma generator compared with the typicallinear-type DBD generator.

Index Terms— Atmospheric plasma, dielectric barrier dis-charge (DBD) plasma, floating voltage, mass spectrometry (MS).

I. INTRODUCTION

MASS spectrometry (MS) is known to be one of themost sensitive analytical methods. Traditional ioniza-

tion methods like electron impact ionization [1], chemical ion-ization (CI) [2], and fast atom bombardment [3] require samplepreparation/preseparation steps and operation in high vacuumchambers. Recently, several ambient desorption ionizationmethods have been developed, where samples can be directlyionized in open air without a high vacuum environment, andalso require minimal or no sample preparation. This allows

Manuscript received March 27, 2014; revised June 12, 2014; acceptedJuly 6, 2014. Date of publication August 11, 2014; date of current versionDecember 9, 2014. This work was supported by the National Science Councilof Taiwan under Grant 101-2221-E-110-002-MY3.

The authors are with the Department of Mechanical and Electro-MechanicalEngineering, National Sun Yat-sen University, Kaohsiung 804, Taiwan(e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2014.2341638

for rapid, real-time, and in situ analysis of chemicalcompounds. According to published literature, ambientdesorption ionization mass spectrometry (MS) can be clas-sified into two systems [4] with different primary ionizationmechanisms. One system is electrospray ionization (ESI)[5], [6], which forms the basis for desorption ESI [7],electrospray-assisted laser desorption/ionization [8], and laserablation ESI [9]. The other system is atmospheric pressureCI (APCI) [10], which is the basis for dielectric barrierdischarge ionization [11], atmospheric pressure thermal des-orption ionization [12], and desorption atmospheric pressurephoto ionization [13]. Plasma ionization methods are classifiedas APCI systems, and numerous helium-based plasma ioniza-tion studies have been reported since 2005.

Plasma-assisted desorption/ionization (PADI) was intro-duced in [14] and [15]. Unlike direct analysis in realtime (DART) [16], desorption APCI [17] and atmosphericsolids analysis probe [18]—methods which use high voltagecorona discharge to generate ions—PADI uses radio frequencyat 13.56 MHz to discharge samples. The driving voltage andpower are 300 V and 54 W. Due to the high frequency and lowvoltage properties, PADI can produce plasma at high densityand low temperatures. The analyte can also directly contactwith plasma to avoid the interference of discharging in air andthereby reduce undesired peaks in the MS spectra. In 2008, alow-temperature plasma (LTP) probe composed of a glass tubewith an internal electrode and an external electrode for plasmageneration was reported to generate plasma at a temperaturearound 30 °C [19], [20]. The detection limit of the LTP ionsource was reported to be as low as 1.0 ppb while analyzingatrazine.

However, these conventional linear-type plasma generatorsusually conduct the electric potential via the ionized gasmolecules and result in a floating potential at the output ofthe plasma generator. The floating potential at the output mayincrease the risk of short circuiting the MS machine andcausing damage to the MS system. In addition, high floatingvoltage may also discharge the sample molecules, break theweak chemical bonds, and form a number of oxidized com-pounds so that less information is obtained in the MS spectra.Therefore, it is of importance to develop a plasma generatorfor producing atmospheric pressure plasma with zero floatingpotential.

There are several ways to eliminate the output poten-tial of the plasma generators by changing the electrode

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CHEN et al.: NOVEL ATMOSPHERIC PRESSURE PLASMA UTILIZING SYMMETRIC DBD 3727

Fig. 1. Photos and schematics showing the two DBD plasma generators.(a) General linear type. (b) Symmetric T-shaped type.

structure and driving methods to produce plasma. One isremote-from-plasma DBD ion source, which was introducedin [21] and [22]. Through extending the inner groundingmetal electrode, high voltage can be grounded such that onlymetastable helium molecules remain at the exit of the plasmatube. This causes the ionization to progress more moderately,reducing the noise signal and improving the detection limit.For example, using this device to detect powerful explosiveslike hexamethylene triperoxide diamine, the limit of detectionis improved to 25 pg. However, this approach greatly reducesthe concentration of discharged ions and causes poor MSdetection performance.

The other method is pin-to-capillary flowing atmospheric-pressure afterglow source, described in [23] in 2011. Theirdesign uses an internal needle electrode to generate coronadischarge, and by placing a grounded metal capillary, thedevice is able to form only metastable helium gas moleculeswith an impressive detection limit (<100 fmol). However,it suffers from ineffective decoupling because of the highdielectric strength of the glass tube when using a groundedexternal electrode, and remaining discharge is often seen atthe outlet.

In this paper, a novel symmetric DBD atmospheric pressureplasma ion source for MS applications has been developed.By changing the general linear-type DBD design, a symmetricT-shaped DBD design is introduced to successfully eliminatethe high floating voltage of the plasma tube without reducingion intensity. A comparison of traditional linear-type DBDdesign to this symmetric T-shaped DBD design is performedthrough MS, spectroscopy, and some basic electrical measure-ments. This system can indeed improve the detection limitand has a good signal-to-noise ratio (SNR). In addition, thedeveloped plasma system can directly ionize samples in thesolid, solution, and gas phases for rapid MS analysis.

II. DESIGN AND FABRICATION

A. Configuration of Novel Symmetric DBD AtmosphericPressure Plasma Ion Source

Both general types of DBD plasma generators and symmet-ric T-shaped DBD plasma generators consist of a glass tube(o.d. 3 mm and i.d. 1.5 mm) with two outer electrodes (coppertape) surrounding the glass tube, as shown in Fig. 1. The wallof the glass tube was used as the dielectric barrier for gener-ating DBD plasma. The experimental setup shown in Fig. 2consists of two different types of plasma generator, the helium

Fig. 2. Experimental setup for using the developed linear and symmetricT-shaped plasma generators as the ion sources for MS detection.

carrier gas, a homemade low cost high voltage pulse driver,and a Bruker Esquire 3000 Plus mass spectrometer (Bruker,United States).

The high voltage driver can provide a pair of alternatinghigh voltages at the same amplitude (1–5 kV) and frequency(20–200 kHz), but at the opposite phase. Note that the entiresetup of this driver costs less than US$20, as well as beingvery small and portable. The homemade driver uses flybackdesign, where the switching of the transistor produces a squarewave to drive the high voltage transformer. The driver circuitwas composed only of one timing IC and one power FETtransistor which consumed less than 100 mW but at gooddriving efficiency. An NE555 IC was used to generate a squarewave to drive the N-channel MOSFET transistor (IRF730,Fairchild Semiconductor, USA) and produced a high currentpulse signal for driving the flyback transformers.

B. Parameters

The entrance voltage of the mass spectrometer will affectthe ion intensity. This parameter is set as capillary voltagein the MS software (−5 to +5 kV), which was originallyto provide a reference voltage for ESI. However, it is notnecessary for plasma because the vacuum inside the MS itselfcan bring ions in. Nevertheless, this voltage value is still animportant parameter in this experiment. Since the plasma jet ofa general linear-type DBD plasma generator is about 20 mm,the distance between the device and MS is 30 mm. The heliumcarrier gas flow rate is 1 SLM. Our experiments determined thebest entrance voltage of the MS to be −2000 V. This valuechanges with different entrance design, as well as distanceand angle between generator and MS inlet. For another MS(Agilent 6410) in this experiment, the best entrance voltagewas found to be about 1–1.5 kV.

III. EXPERIMENTAL RESULTS

A. Plasma Optical Emission Spectrum Measurement

Plasma is a group of charged gas molecules, so the typeof gas molecules, energy level, and intensity can be easily


Fig. 3. Measured optical emission spectra of the discharged gas using(a) general linear-type and (b) symmetric T-shaped type plasma generator.

observed on the spectrum. However, since the mean freepath of atmospheric pressure plasma is very short, the lightintensity of plasma is insufficient and requires placement ofa collimator in front of the fiber to receive light. Fig. 3(a) is thespectrum of a traditional linear-type DBD plasma generator.Since this device will directly discharge into open air, it willproduce strongly excited light throughout the 300–450-nmwavelength. These spectrum signal can be confirmed to bemostly generated by nitrogen according to the NIST atomicspectra database [24]. However, the most important spectrumsignal, that of metastable helium for ionization at 707 nm,is not observed in the spectrum. The produced He+ ionswill transfer 90% of the energy to ambient gas moleculeswithin a distance of less than 2 mm after leaving the outletof the plasma generator [16]. The helium ions may collide airmolecules and formed significant amounts of nitrogen, oxygen,and water ions. The produced side products, including N+

4 ,ozone, H3O+, and water cluster ions, are unstable and highlyreactive [25], [26]. Therefore, the sample molecules may reactwith these side products and formed some unwanted derivates.In contrast, Fig. 3(b) shows the spectrum of the symmetricT-shaped generator. Since the plasma of this device does notdirectly discharge into air, there is a strong peak only at707 nm, which is the characteristic spectrum of metastablehelium (He I) that can exist for about 8000 s. This indicatesthat the ionization mechanism is relatively stable and does lessdamage to weak samples.

B. Electrical Measurements

Since the high voltage conducts through internal heliumto the exit of the tube, it preferentially discharges to thelowest impedance grounding point, such as aqueous samples,metal chips, or metal sample plates. A small piece of lowcapacitance wire connected to the tip of an HVP 39pro highvoltage probe (Pintek Electronics Co., Taiwan) can simulatesamples discharged by high voltage in front of the plasmatube at 1 cm, and the oscilloscope waveform can be recorded.The result shown in Fig. 4 demonstrates that the symmetricT-shaped DBD design does indeed fully eliminate the highfloating voltage at the exit of the plasma tube. The maximum

Fig. 4. Measured floating voltages at the tube outlet under different appliedpowers for the two DBD plasma generators.

Fig. 5. Relationship between the measured ion intensity and the applieddischarged voltage.

output voltage is about 20 mV, which is extremely low whencompared with the traditional DBD plasma generator whichreaches 700 V maximum output voltage. Clearly, the generalDBD plasma itself will do more damage to some weakersamples and produce a more fragmented signal.

In general, plasma generated with higher driving voltage andpower results in higher ion concentration. The temperature ofplasma increases when the driving power of plasma increases,which leads to better ionization efficiency. However, since themean free path of atmospheric plasma is relatively short, freeelectron will immediately collide with other particles, turninginto heat. Even if the power is raised, the increase of ionintensity is still small. Therefore, it is generally difficult forthe ion intensity of atmospheric pressure plasma ion sourcesto reach as large a concentration as 109 ions · cm−3.

Fig. 5 is the impact of driving voltage from 1.5 to 3.5 kV onion intensity for the two kinds of electrodes. Results show thatfor the conventional linear-type of DBD plasma, the value ofion intensity reaches its maximum value at the lowest drivingvoltage, and decreases with increasing voltage. This is becausethe high voltage will interfere with the ions reaching the MS.On the contrary, since there is no plasma jet at the output ofthe symmetric T-shaped tube, the ion intensity increase up tomore than 3 × 107 ions · cm−3 with increasing voltage.


Fig. 6. MS spectra of detecting DIBP for the two different plasma sources.(a) General linear-type DBD. (b) Symmetric T-shaped DBD designs.

C. MS Measurements

Water molecules exist not only as a single molecule in theair, but also in water cluster molecules [27] such that themolecular formula can be written as (H2O)n . The peaks forwater cluster molecules will combine with H3O+(m/z 19),NH+

4 (m/z 18), and NH3(m/z 17) due to the formation ofhydrogen bonds. These four molecules in combination witheach other will form a variety of ions like H3O+(H2O)n

(m/z 18n+19), H2O+ (H2O)+n (m/z 18n+18), as well assomewhat less NH+

4 (H2O)n(m/z 18n+17). When DART waspublished in 2005, a significant amount of water cluster peakshad already been observed in the background signal in theform of (H2O)nH+ or (H2O)n+.

A water cluster is a discrete hydrogen bonded assemblyor cluster of molecules of water which often appears inthe mass spectra and interferes with the major signal peaks.Since water cluster molecules are very unstable and easilyaffected by ultraviolet light, temperature, air pressure, andair flow rate, they produce many different signals dependingon MS brand, entrance structure, and MS internal structure.Also, because the water cluster molecules easily combine withcertain samples, their combination is extremely complicatedand leads to complex peak formations in MS results.

In order to demonstrate that the symmetric T-shaped plasmagenerator can produce fewer water cluster molecules thanthe linear one, plasma is directly used to ionize diisobutylphthalate (DIBP) in open air. Fig. 6(a) and (b) shows theMS spectra for detecting DIBP using the two different plasmasources. Note that the DIBP peak obtained using the symmet-ric T-shaped plasma generator exhibits lower signal intensityfor water clusters and results in stronger peak intensity. Theintensity of water cluster generated by the symmetric T-shapedDBD structure is dramatically reduced after n = 6 (m/z108), whereas the intensity of water cluster generated by thelinear-type DBD plasma generator does not decrease untiln = 18. Further, the intensity of the major ions peak (DIBP)for the symmetric T-shaped DBD type is higher than for thelinear one. These results show that a reduction in charginginto open air can indeed reduce the generation of water clustermolecules and improve the SNR of MS measurement.

Fig. 7. Measured MS spectra for detecting oleic acid using (a) generallinear-type and (b) symmetric T-shaped DBD plasma generators.

Due to the strong oxidizing property of ozone, it willbreak the double bonds in normal hydrocarbon molecules andform numerous fragments [28]. For example, a double bondbetween the ninth and tenth carbon in oleic acid (C18H34O2,m/z 282.46) will be broken by ozone throughout the progressof plasma ionization and form aldehyde oxidation fragments(m/z 173). Although this mechanism can be used to confirmdouble bonds in fatty acids, it will difficult to interpretthe results when analyze several samples at a time or when theconcentration of analyte is quite low. In this paper, using thesymmetric T-shaped plasma generator to ionize oleic acid,shown in Fig. 7(a), results in a fragment ion intensity eighttimes lower than for the linear-type, as in Fig. 7(b). Becauseof the higher boiling point of oleic acid, helium gas is heatedto 150 °C to increase desorption efficiency. Results show thatfewer fragments were produced and the detection limit wasgreatly improved while using the developed T-shaped designto analyze samples like oleic acid.

Similar tests using a saturated fatty acid (C17H34O2,m/z 270.45) as samples shows similar improvements broughtabout by the symmetric T-shaped design. The signal peaksshown in Fig. 8(a) and (b) are significantly different. Themolecular ion peak [M]+ (m/z 270) and [M+H]+ (m/z 271)did not appear in the MS spectra for either structure becausethe carboxyl group could not be ionized and charged byplasma. Only adduct ions (m/z 297.2) were observed due tothe ozone and nitrogen oxides generated by the plasma jetwhich oxidized the saturated fatty acids. Therefore, the higherthe adduct ion peak, the higher the oxidative capacity of the ionsource. As shown in Fig. 8(b), due to the elimination of outputpotential, the symmetric T-shaped structure only generatesaround 9000 counts of m/z 297.2 ions. However, as shownin Fig. 8(a), using the general linear-type DBD type withoutsuppression of floating voltage resulted in the m/z 297.2 ionsrising to 40 000 counts. The symmetric T-shaped DBD plasmagenerator can actually improve the detection limits and reduceoxidation of samples.

Fig. 3 presents spectra results confirming that the plasma jetat the tube outlet will excite nitrogen, oxygen, and water in


Fig. 8. MS spectra for detecting saturated fatty acid of heptadecanoic acidusing (a) linear type and (b) symmetric T-shaped DBD plasma generators.

Fig. 9. MS spectra for detecting diphenylamine using (a) general linear typeand (b) symmetric T-shaped type plasma sources.

the open air and form numerous high energy ions which willproduce much more complicated compound signals. In orderto demonstrate that the symmetric T-shaped plasma generatorcan produce fewer oxidized products from the sample thanthe typical linear one, two compounds, including one amineand one alkane series molecule, were used to characterizethe detection performance of the two plasma generator types.Fig. 9 shows the MS spectra for detecting an antioxidationagent of diphenylamine ((C6H5)2NH, m/z = 169.23) obtainedusing the typical linear DBD plasma [Fig. 9(a)] and thedeveloped symmetric T-shaped plasma generator [Fig. 9(b)].Results show that the symmetric T-shaped plasma ion sourceprovides a clear spectrum since only molecular ion peak[M+H]+ appears and fewer undesired products were produced

Fig. 10. MS spectra for detecting hexadecane using (a) linear-type and(b) symmetric T-shaped DBD plasma sources.

in the spectra, resulting in a better detection performance.In addition, the intensity of the molecular ion peak of [M+H]+(3 × 106) of the symmetric T-shaped plasma generator is10 times higher than that obtained using the linear DBDgenerator (3.5 × 105).

Fig. 10 shows the MS spectra for detecting hexadecane(C16H34, m/z 226.44) obtained using the typical linear DBDplasma source [Fig. 10(a)] and the developed T-shaped plasmagenerator [Fig. 10(b)]. Adduct ions [M+N]-H+ (m/z 239)and [M+N+O]-H+ (m/z 255) were observed when usingatmospheric pressure plasma as an ion source to analyzealkane, rather than molecular ion peak [M]+ (m/z 226) or[M+H]+ (m/z 227). Fig. 10(b) shows that the spectra ofthe T-shaped plasma generator has only two major peaks atm/z 239 and m/z 255. In stark contrast, Fig. 10(a) showsnot only two major peaks, but also several undesired peaksappearing in the spectra of the linear DBD plasma generator.These are due to the strong oxide the plasma jet produces, andthey introduce some difficulty in interpretation.

IV. CONCLUSION

This paper demonstrates a novel symmetric T-shapedplasma generator design which is driven by a successfully-developed high-performance high voltage driver for generatingplasma. The driver can provide a pair of pulse alternatingvoltages at both high voltage and frequency and costs lessthan US$20, as well as being very small, portable, andhighly efficient. Only 2 W of power can produce more than107-ion·cm−3 ions. The novel symmetric T-shaped plasmagenerator design, used to analyze an unsaturated fatty acid,can reduce the fragment signals by 20% when compared withthe general linear type. Results of detecting diphenylamineshow that the intensity of molecular ion peak [M+H]+ is10 times higher than that of the linear without increasingthe intensity of oxide signal [M+O2]H+. Measurements offloating voltage show that the symmetric T-shaped structureremains at 20 mV, unlike that of the linear structure, whichreaches voltages up to 700 V. This novel symmetric T-shapedDBD plasma design, therefore, has been demonstrated to trulyeliminate the output high voltage and do less damage toweak samples. Though simple, this symmetric T-shaped DBDplasma generator provides high performance ion generationfor rapid MS applications.


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Chun-Yi Chen was born in Kaohsiung, Taiwan, in1990. He received the B.S. degree in mechanical andelectromechanical engineering from National SunYat-sen University, Kaohsiung, in 2012, where heis currently pursuing the M.S. degree.

His current research interests include the applica-tions of atmospheric plasma system and the designof ion sources for mass spectrometry analysis.

Cheng-Hung Chiang was born in Kaohsiung, Tai-wan, in 1984. He received the B.S. degree inmechanical engineering from National Cheng KungUniversity, Tainan, Taiwan, in 2008, and the M.S.degree in mechanical and electromechanical engi-neering from National Sun Yat-sen University, Kaoh-siung, in 2012.

His current research interests include electronicinstrumentation, the applications of atmosphericplasma system, and the development of ion sourcesfor mass spectrometry.

Che-Hsin Lin received the B.S. degree in chem-ical engineering from National Taiwan University,Taipei, Taiwan, in 1994, and the M.S. and Ph.D.degrees in biomedical engineering from NationalCheng Kung University, Tainan, Taiwan, in 1996and 2002, respectively. His master’s study focusedon bioceramics and biomechanics, and then involvedMEMS for bioanalytical applications in the Ph.D.study.

He is currently a Full Professor and the Chair-man of the Department of Mechanical and Electro-

mechanical Engineering with National Sun Yat-sen University, Kaohsiung,Taiwan. His current research interests include MEMS fabrication technolo-gies, bioMEMS, microfluidic systems, biophotonics, and the applications ofatmospheric plasma.

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