sample analysis with miniaturized plasmas

80A Volume 60, Number 3, 2006

focal pointBY JOACHIM FRANZKE* AND MANUELA MICLEA

ISAS—INSTITUTE FOR ANALYTICAL SCIENCES

BUNSEN—KIRCHHOFF STR. 1144139 DORTMUND, GERMANY

Sample Analysis withMiniaturized Plasmas

INTRODUCTION

Microplasmas coupled withoptical or mass spectrom-etry are important tools for

the detection of molecular fragmentsand element analysis, and they havebeen discussed in a few reviews.1–3

Most miniaturized discharges, suchas dc plasmas, different capacitivelycoupled microplasmas, microstripplasmas (MSP), dielectric barrierdischarges, microstructured elec-trode (MSE) discharges, and eveninductively coupled plasmas (ICP)have been included in these reviewarticles. The present paper gives anupdated overview of the differentplasmas that have been developedand applied in analytical and molec-ular spectrometry, described in orderfrom high-frequency to direct-cur-rent plasmas. Their analytical capa-bilities are reported together withtheir real implementations in differ-ent analytical systems.

The second part of this FocalPoint contains a review of the micro-plasmas developed for the analysisof liquid samples. These can be ei-ther microplasmas coupled with dif-ferent sample injection devices orplasmas that use one electrode as aliquid or that are ignited directly inthe liquid. It must be stressed that alot of work has been done in this di-rection and further studies and de-

* E-mail: [email protected].

velopments will surely bring theirlevel of performance to that of theclassical analytical plasmas.

HIGH-FRQUENCY PLASMASMicrowave-Induced Plasmas,

900 MHz–2.45 GHz. All plasmasthat are created by the injection ofmicrowave power, i.e., electromag-netic radiation in the frequencyrange of 300 MHz to 10 GHz, canin principle be called ‘‘microwave-induced plasmas’’ (MIPs).4 This is,however, a general term encompass-ing several different plasma types,e.g., cavity-induced plasmas, free ex-panding atmospheric plasma torches,electron cyclotron resonance(ECRs), surface wave discharges(SWD), etc. These different plasmatypes operate over a wide range ofconditions, i.e., at a pressure rangingfrom less than 0.1 Pa to a few at-mospheres, from a power between afew W to several hundreds of kW,and are sustained in both noble gasesand molecular gases.

Following the current trends in an-alytical chemistry to miniaturizeparts or entire chemical analysis sys-tems, a miniaturized MIP using mi-crostrip technology has been devel-oped,5,6 and it is shown in Fig. 1.This microstrip plasma source(MSP) consists of a single 1.5 mmsapphire wafer with a straightgrown-in gas channel with a diame-ter of 0.9 mm and a length of 30mm. The electrode-less plasma is op-

erated at atmospheric pressure, witha microwave input power of 5–30 W.It has the potential to become veryuseful in analytical chemistry for theemission detection of non-metalssuch as halogens and chalcogens.5

Bilgic et al.6 described a plasmadevice constructed by gluing togeth-er two quartz plates with water glass,both of which have a groove madewith the aid of a diamond saw. Mi-crostrips were used for power trans-mission and produced by plasma va-por deposition with subsequent seal-ing by a galvanic covering with cop-per up to a thickness of about 1 mm.The latter passes the skin depth ofmicrowaves at a frequency of 2.45GHz, namely 2 mm. The underlyingcopper block has a thickness of upto 1 cm and acts both as a groundelectrode and as a cooling medium.The microstrip has a sidearm actingas an artificial load. Its length is se-lected such that fluctuations in theplasma load, e.g., when igniting orchanging the sample flows into theplasma, have the lowest possible in-fluence on the resonance conditions.

The device, the novel aspects ofwhich are described in Ref. 7, couldbe operated for hours without anydeterioration of the microstrips or ofthe channel in the quartz. It could beused with 0.5 L/min of argon and apower of 15 W in conjunction withthe mercury cold vapor technique forthe determination of mercury, withdetection limits down to 50 pg/mL

APPLIED SPECTROSCOPY 81A

FIG. 1. The microstrip plasma source operated at 2.45 GHz. The plasma confined inthe gas channel is shown in the photo at the top of the figure.

and excellent short- and long-termprecision.8 For leachates of soils, ac-curate determinations of traces ofmercury were shown to be possible.The chip could still be made smallerby using a 30 3 30 mm2 quartz plate1.5 mm thick in which a cylindricalchannel with a diameter of 0.9 mmis provided parallel to the surface.By adjusting the length of the match-ing element, also with helium, a sta-ble plasma discharge could be ob-tained at a power of 5–30 W and agas flow of 50–1000 mL/min.9 Thisplasma was shown to be able to dis-sociate halogenated hydrocarbons,and atomic emission signals for theCl(I) 912.1 nm line were obtained.

The plasma configuration couldalso be modified so that the plasmaexits from the chip; however, this re-quires an adaptation of the matchingelement length. Then, the space an-gle in the optical emission work isno longer limiting. At a power of 30W and with an argon flow of 0.5L/min, mercury concentrations in ar-gon below 10 ng/L can easily be de-tected when using the Hg(I) 253.65nm line.10

A stable microstrip microwaveplasma (MSP) operated at atmo-spheric pressure with a power of ap-proximately 10–20 W and at a gas

flow of 0.2–0.8 L/min of argon in aresonant structure produced with theaid of microstructuring technologyon a 5 3 5 cm2 quartz wafer provid-ed with a 0.6 mm diameter plasmachannel is also described. The deviceis shown to be useful for the exci-tation of atomic and molecular spe-cies and for the atomic emissionspectrometric determination of met-als and of nonmetals in gases at thetrace level, down to the ng/L level,as shown for the case of sulfur.11

The development of a new, verysmall coaxial plasma source basedon a microwave plasma torch (MPT)is described.12 It generates a plasmajet up to 4 mm long and can be op-erated with an argon gas flow rate ofless than 70 mL/min at down to 2 Wmicrowave power (2.45 GHz) at at-mospheric pressure. It also workswell with helium and does not showany wear during a test period of 30h of operation with argon. It is, inparticular, thought to be a source forthe atomic spectroscopy of gaseousspecies. The excitation temperaturefor this device is found to be about4700 K operating with helium and17 W microwave power. A detectionlimit for an example application inwhich Cl is detected from HCCl3 isfound to be below 66 ppb. The de-

sign considerations for the microstripcircuits are discussed and an approx-imated calculation for the layout ispresented. With the introduced pro-cedure it is possible to design evensmaller MPTs for special applica-tions.

The design and initial character-ization of a low-power microwaveplasma source based on a microstripsplit-ring resonator that is capable ofoperating at pressures from 6.7 Pa upto one atmosphere is described.13

The plasma device is shown sche-matically in Fig. 2. The plasmasource’s microstrip resonator oper-ates at 900 MHz and presents a qual-ity factor of Q 5 335. Argon and airdischarges can be self-started withless than 3 W of power in a relative-ly wide pressure range. An ion den-sity of 1.3 3 1011 cm23 in argon at53.3 Pa can be created using only 0.5W. Atmospheric discharges can besustained with 0.5 W in argon. Thislow power requirement allows forportable air-cooled operation. Con-tinuous operation at atmosphericpressure for 24 h in argon at 1 Wshows no measurable damage to thesource.

The application of microplasmasas sensors of industrial vacuum pro-cesses requires stable operation atgas pressures of less than 1 Pa.14 Inthis low-pressure regime, the addi-tion of a static magnetic field thatcauses electron cyclotron resonanceis shown to increase the emission in-tensity of the microplasma by 50%.Using atomic emission spectrometry,the detection of helium in air isfound to have a detection limit of1000 ppm, which is three orders ofmagnitude worse than the DL of SO2

in argon. The loss of sensitivity istraced to the high excitation energythreshold of He and to the poor ion-ization efficiency inherent in airplasmas. At atmospheric pressure, amicrodischarge is described that op-erates in a 25 mm wide gap in a mi-crostrip transmission line resonatoroperating at 900 MHz. The volumeof the discharge is similar to 1027

cm3, and this allows an atmosphericair discharge to be initiated and sus-tained using less than 3 W of power.


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FIG. 2. Low-power microwave plasma source based on a microstrip split-ring resona-tor operating at 900 MHz.

FIG. 3. The microfabricated inductively coupled plasma operated at 700 MHz.

The thermal characteristics of anatmospheric argon discharge gener-ated with a low-power microwaveplasma source have been investigat-ed to determine its possible integra-tion into portable systems. Rotation-al, vibrational, and excitation tem-peratures are measured by means ofoptical emission spectroscopy.15 It is

found that the discharge at atmo-spheric pressure presents a rotationaltemperature of 300 K, while the ex-citation temperature is similar to 0.3eV (similar to 3500 K). Therefore,the discharge is clearly not in ther-mal equilibrium. The low rotationaltemperature allows efficient air-cooled operation and makes this de-

vice suitable for portable applica-tions, including those with tight ther-mal specifications such as treatmentof biological materials.

Inductively Coupled Plasmas,13.56–900 MHz. Despite the hugesize, high weight, high power re-quirements of 1–2 kW, and the enor-mous Ar consumption of 12–20L/min of ICPs, they have wide ap-plicability and utility. Small micro-fabricated ICPs (mICP) have alsobeen developed in recent years.16–26

Whereas large ICPs are typically op-erated at a frequency of 13.56 MHz,it has been shown that the optimumfrequency for plasma generation in-creases to 460 MHz when the coildiameter is reduced to 5 mm.17 It wasfound that the electron density in-creases with the frequency and isabout an order of magnitude higherthan in a large scale ICP as a resultof the large surface-to-volume ratioof small discharges. The motivationfor the work was to develop portablemICPs that can be coupled to a mi-crofabricated Fabry–Perot interfer-ometer for measurement of gaseousanalytes in the field, for example,SO2.21–25 The first generation of m-ICPs was developed with load coildiameters of 5, 10, and 15 mm andoperated with powers of 0.5–20 W,frequencies between 100–460 MHz,and under low pressure. Theseproof-of-concept devices were de-veloped on printed circuit boards tostudy the effect of scaling laws onmICPs.17 Such studies formed thefoundation for further developments.For instance, for the next two gen-erations, photolithography and mi-cromachining technology were usedfor the fabrication of mICPs that hadplanar load coils, as shown in Fig. 3.A matching network was also micro-fabricated next to the load coil. Aone-mask fabrication process wasused for the matching network andthe mICP fabrication, thus reducingcost by alleviating the need for maskalignment as would be required ifmultiple masks were used.17–19 Theminiature ICP (mICP) was fabricatedby etching planar spiral inductors ina copper-clad epoxy board. Anotherpaper reports on the microfabricationand testing of monolithic mICP fab-


FIG. 4. Miniaturized inductively coupled plasma jet source operated at 144–146 MHz.

ricated on glass wafers using surfacemicromachining.18 The plasma issustained coupling a 450 MHz cur-rent into a low pressure gas. Ar aswell as air plasmas have been gen-erated in the range of 0.1–13 hPa.The operation power was 350 mW,although 1.5 W is required to initiatethe discharge. A new single-loopmICP source that is three times moreefficient than the former one hasbeen fabricated.20 In this case the coilwas situated closer to the plasma andwas operated with even higher fre-quencies up to 818 MHz. Ion den-sities of 1011/cm3 in Ar at 0.5 hPawere obtained with only 1 W ofpower.

The first calibration curves forSO2 were measured in an Ar mICPby optical emission spectrometry ofthe sulfur atomic 469.5 nm line. Theplasma chamber consisted of a cylin-drical hole 6 mm in diameter and 6mm in length. The pressure was 7.4hPa and the plasma power was 3.5W. The limit of detection of SO2 wasabout 190 ppb v/v.21

A miniaturized atmospheric-pres-sure thermal plasma jet sourceshown in Fig. 4 has been developedas a sensitive detector of a portableliquid analysis system that can fulfillvarious requirements of ‘‘on-site’’analysis.26 The plasma source designrequired for achieving higher powertransfer efficiency to the plasma hasbeen studied mainly so that it can beoperated with a commercially avail-

able compact VHF transmitter. Thedevice consists of two dielectricplates with an area of 15 3 30 mm2,an ICP discharge tube of 1 3 1 330 mm3 that was ‘‘mechanically en-graved on one side of the dielectricplate’’, and a planar antenna that wasphoto-lithographically fabricated onthe other side of the plate. The plas-ma was powered from a commer-cially available 144–146 MHz trans-mitter. The maximum output powerwas 50 W. With power losses to thematching network addressed, withselection of dielectric material sortedout, and with antenna thickness andnumber of turns for the load coil ex-amined, the authors studied the ef-fect of gas flow rate on plasma den-sity and measured excitation temper-atures using a Boltzmann plot of Arlines. Overall, plasma density wasreported to be about 8 3 1014 cm23

and excitation temperatures rangedbetween 4000 and 4500 K.

The spectrometer was a fiber op-tically coupled Czerny–Turnermonochromator equipped with aphotomultiplier tube detector. Thequestion of sample introduction wasalso addressed using a ‘‘miniaturizedpneumatic nebulizer’’ but that wasreported to be ‘‘quite difficult’’. Incontrast, electrospray (biased at 3kV) sample introduction was report-ed to be easier and emission signalsfrom a 100 ppm NaCl solution wereshown with a detection limit of 5ppm.

Capacitively Coupled Micro-plasmas 13.56 MHz. To sustain adirect-current (dc) glow discharge,the electrodes must be conducting.When one or both of the electrodesare nonconductive, e.g., when theglow discharge is used for the spec-trochemical analysis of nonconduct-ing materials or for the deposition ofdielectric films, where the electrodesbecome gradually covered with in-sulating material, the electrodes willbe charged up due to the accumula-tion of positive or negative charges,and the glow discharge will extin-guish. This problem is overcome byapplying an alternating voltage be-tween the two electrodes, so thateach electrode will act alternately asthe cathode and anode, and thecharge accumulated during one half-cycle will be at least partially neu-tralized by the opposite charge ac-cumulated during the next half-cy-cle.27 The frequencies generally usedfor these alternating voltages are typ-ically in the radio frequency (rf)range (1 kHz–103 MHz; with a mostcommon value of 13.56 MHz).Strictly speaking, capacitively cou-pled (cc) discharges can also be gen-erated by alternating voltages in an-other frequency range. Therefore, theterm alternating-current (ac) dis-charges, as opposed to dc discharges,might be more appropriate. On theother hand, the frequency should behigh enough so that half the periodof the alternating voltage is less thanthe time during which the insulatorwould charge up. Otherwise, therewill be a series of short-lived dis-charges with the electrodes succes-sively taking opposite polarities, in-stead of a quasi-continuous dis-charge. It can be calculated that thedischarge will continue when the ap-plied frequency is above 100 kHz. Inpractice, many rf processes operateat 13.56 MHz, because this is a fre-quency allotted by internationalcommunications authorities at whichone can radiate a certain amount ofenergy without interfering with com-munications.

A miniaturized, parallel-plate ca-pacitively coupled plasma (PP-CCP)at atmospheric pressure, first de-scribed by Liang and Blades,28 has


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FIG. 5. Parallel plate, capacitively coupled microplasma operated at 13.56 MHz.

been investigated for analyticalatomic spectrometry applications.29

Bass et al. described the implemen-tation of the PP-CCP on a 0.25 30.25 3 5 mm3 micro-machined fusedsilica chip, as shown in Fig. 5.30 Thismicroplasma has been developed ongrooved channels obtained with adicing saw. The channels were either200 or 500 mm deep. The channelswere covered with a quartz plate andwere glued together. The typicallength of the channel was 10 mm,although plasmas with differentlengths ‘‘could be fabricated easily’’.Helium was used as the carrier gas(flow rate 17–150 mL/min). The PP-CCP is sustained by application ofradio frequency power of 5–25 W toa pair of electrodes that are separatedby a quartz discharge tube, forminga capacitive, transverse discharge.The plasma is normally operated at13.56 MHz using He as a plasmagas. Water-cooling was required.Background spectra were collectedusing an Ocean Optics microspec-trometer. The emission lines of OH,NH, N2, N2

1, and He were measuredby applying a 20 W He plasma anda gas flow of 70 mL/min. Althoughthe PP-CCP has been investigatedfor application in analytical atomicspectrometry, no element-selectivemeasurements have been carried outso far by Bass et al. with the mini-aturized version.30

Yoshiki and co-workers developeda self-igniting, parallel-plate, atmo-spheric pressure micro-CCP.31–33 Al-

though different fabrication methodswere used, the above-described plas-ma is quite similar to that of Blades.Helium CCPs (flow rate up to about750 mL/min) were formed in quartzchannels (typically 5 mm long) withdepths varying between 65 and 500mm and widths varying between 500mm and 5 mm. For maximum powertransfer, a miniaturized (150 3 1003 40 mm3) matching network wasdesigned and utilized. Copper elec-trodes were placed on each side ofthe channels to bring power to theCCP (1–5 W, 13.56 MHz). Windowswere cut on the top electrode so thatoptical emission could be monitoredusing a fiber-optic spectrometer. He-lium background spectra of this self-igniting plasma and relatively lowexcitation temperatures (between1850 and 2300 K using He lines)were reported at atmospheric andsubatmospheric pressure operation.Cooling of the chip was reported tobe necessary at power levels above10 W (although typical power levelswere between 1 and 3 W). In addi-tion, a CCP was also used to im-prove (by plasma treatment) the sur-face of the inner wall of polymericcapillaries for use in capillary elec-trophoresis applications.32

LOW-FREQUENY PLASMASRadio Frequency Plasma at 350

kHz. In 1988 Skelton et al. devel-oped a 350 kHz radio frequencyplasma (RFP) that was used for sul-fur selective gas chromatographic

analysis by GC/OES.34–36 This detec-tor was found to have low limits ofdetection of sulfur (0.5 pg/s) and agood linear response (4 decades).The detector consists of a helium ra-dio frequency plasma sustained in-side of a 1 mm quartz tube. The gasflow was about 70 mL/min with anadditional oxygen flow of about 0.03mL/min. A macor ceramic cell bodywas used to provide thermal andelectrical insulation as well as tosupport the stainless steel electrode.The power consumption was about50 to 80 W.

The RFP was further miniaturizedby Pedersen-Bjergaard et al.37,38 andis shown in Fig 6. A 5 cm long pieceof polyimide coating and the station-ary phase were carefully burned offat the end of a fused silica GC col-umn. The last 2 cm of this uncoatedGC capillary served as the plasmatube and was placed inside a pieceof silica tube for protection. A steelwire placed at the column outletserved as the top electrode. The plas-ma was generated inside the end ofthe fused silica GC column betweenthe top electrode and the groundedreducing union by a radio frequencypower supply. Atomic emission wasmeasured side-on through the wall ofthe fused silica column and the pro-tecting silica tube. The detection lim-its for bromine and chlorine were 0.9and 1.1 pg/s, respectively.

Another paper reports on the ap-plication of this plasma device formass spectrometric detection in cap-illary gas chromatography.39 Theplasma was sustained at low pressurein the last 35 mm of a capillary GCcolumn (0.32 mm i.d.), which wasput inside of the ion source housingof a quadrupole mass spectrometer.This allowed direct introduction ofions from the plasma into the massanalyzer using only a repeller andelectrostatic lenses to focus the ions.The plasma was sustained with only25 mL/min of helium, which was ac-cepted by the mass spectrometervacuum system. This low gas flowalso served to enhance the energydensity of the discharge and to pro-duce a narrow spray of ions towardthe mass analyzer. Due to the mini-aturized nature of the plasma, it was


FIG. 6. Miniaturized radio frequency discharge operated at 350 kHz.

operated at a low power level (2.0W), and traces of oxygen were addedto avoid deposition of carbon on thecapillary wall. Chlorine was success-fully monitored down to the 2.2 pg/slevel without interference from ele-ments such as C, S, P, O, F, and N.

Radio Frequency Plasma at 20kHz. A similar atmospheric pressuremicroplasma cell also implementedon a fused silica capillary works at20 kV and 20 kHz and does not re-quire any ignition system or imped-ance matching circuit.40 Due to thevery low frequency, this plasma de-vice should be allotted to the dielec-tric barrier discharges. This micro-plasma is created with two annularelectrodes placed around the circum-ference of the capillary; thus, noneof the electrodes is in contact withthe plasma, which prevents deterio-ration of the electrodes and contam-ination of the plasma with metalsputtered from the electrode.

As it is assembled on a capillarycolumn, it does not depend on mi-cromachining and also has no deadvolume. It can be supported at a flowrate range corresponding to mostcommon capillary chromatographic

methods at atmospheric pressure,and therefore does not need a makeup gas. The detection of mercury in-troduced as vapor has been demon-strated, as well as the detection ofantimony and arsenic introduced astheir volatile hydrides. It was alsopossible to detect molecular emis-sion on the introduction of methaneor carbon dioxide, which could beused for quantitation. The applica-tion of this detector to a new groupof species, volatile organic com-pounds (VOCs), after separation bygas chromatography is demonstratedhere.

The excitation temperature of theplasma determined from heliumemission lines is about 4000 K. Itwas found possible to detect oxygenfrom its emission at 777 and 845 nm,hydrogen at 656 nm, and sulfur-con-taining species from emission at 923nm.41 The carbon-containing speciesCH4, CO, and CO2 could be deter-mined from an emission band at 385nm due to CN. Detection limits inthe range between about 1 and 10 ngwere obtained using a miniature di-ode array spectrometer.

The microplasma is implemented

as an on-column optical emission de-tector.42 The arrangement consists oftwo cylindrical electrodes placedside by side on the fused silica sep-aration capillary (250 mm i.d.) andan optical fiber for coupling emittedlight to a spectrometer. The plasmais generated with an ac voltage of 20kV and 20 kHz by capacitive cou-pling of the energy into the capillaryand can be sustained at flow ratesemployed in gas chromatography atatmospheric pressure. The detectionof volatile organic compounds waspossible via the emission from atom-ic carbon at 247.9 nm and from CNat 385.2 nm. Benzene was deter-mined with a detection limit of ap-proximately 80 pg of carbon.

The detection of Br, Cl, F, I, P, Se,and S in organic compounds sepa-rated in a helium carrier was possibleby monitoring emission lines at827.24 nm, 837.59 nm, 685.60 nm,804.37 nm, 956.40 nm, 888.50 nm,and 921.28 nm for the seven ele-ments, respectively.43 The systemshowed the following detection lim-its: Br, 0.3 pg s21; Cl, 0.1 pg s21; F,20 pg s21; I, 158 pg s21; P, 1805 pgs21; Se, 153 pg s21; and S, 6.6 pg s21.The determination of environmental-ly relevant halogenated volatile or-ganic compounds and pesticides wasdemonstrated.

Dielectric Barrier Discharge at20 kHz. The dielectric barrier dis-charge (DBD) or silent discharge isgenerally characterized by the pres-ence of at least one dielectric layer(barrier) in the discharge volume be-tween two metallic electrodes. An al-ternating rectangular high voltagewaveform in the range from a fewHz up to MHz is applied to sustainthe plasma. This type of discharge iswidely used, e.g., in flat-panel plas-ma displays, for industrial produc-tion of ozone or in surface treat-ment.44

Miclea et al. have developed a di-electric barrier discharge (shown inFig. 7) suitable for analytical appli-cations.45 The discharge chamberconsists of two glass plates coveredwith aluminum electrodes (50 mmlength, 1 mm width). The electrodesare covered with a 20 mm thickglass-type dielectric layer. The glass


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FIG. 7. Linear dielectric barrier discharge operated at 5–20 kHz.

spacers in between the glass platesdefine the distance of 1 mm betweenthe electrodes and form the plasmachannel of 1 3 1 3 50 mm3. Thedischarge works at reduced pressuresof 10–100 hPa in argon as well ashelium with a gas flow of 10–1000mL/min. The applied voltage has arectangular shape with a frequencyof 5–20 kHz and an amplitude of750 Vpp. The discharge displays atransient behavior with a half-widthof the current peak of about 10 ms.It is ignited over the whole length ofthe electrodes.

Plasma diagnostics reveals that thehot region of the plasma is constrict-ed for a short time during each dis-charge cycle close to the temporarycathode. In this thin layer the elec-tron density reaches a concentrationof 1015 cm23 and a gas temperatureof about 1000 K, while the rest ofthe discharge remains cold.46 Themean power consumption of the dis-charge is much smaller than 1 W.The slopes of the applied rectangularvoltage (e.g., 10 kHz) induce a shortcurrent pulse and therefore the half-width of the current pulse is only atenth of the whole plasma cycle. Fur-thermore, the highest atom density islocalized in about a tenth of the dis-charge volume. Consequently, thepeak power density of 1 kW/cm3 is

of the same order of magnitude asthe mean power density of an induc-tively coupled plasma. An ICP typ-ically has an input power of about 1kW, a conical plasma torch length ofabout 25 mm, and a diameter of 25mm, resulting in an average powerdensity of about 0.5 kW/cm3.

Measurements of halogenated hy-drocarbons by diode laser atomic ab-sorption spectrometry (DLAAS) inthe dielectric barrier discharge wereperformed by absorption of excitedatoms using a laser beam with a di-ameter of 1 mm along the dischargechannel. The absorption signal wasmeasured by a phase-sensitive detec-tion using twice the modulation fre-quency of the DBD. The limits ofdetection of CCl2F2, CClF3, orCHClF2 in the Ar DBD were about5 ppb v/v using the 837.824 nm Clabsorption line, which starts from ametastable level of Cl. In He the de-tection limits for CCl2F2 were 400ppt v/v and 2 ppb v/v using the Cl837.824 nm and the F 685.792 nmline, respectively. The authorsproved that the absorption signals ofCl and F correspond to the stoichio-metric ratios of theses elements inthe analyte molecules.

The capability of the small-sizeddielectric barrier discharge as an el-ement-selective diode laser atomic

absorption detector for gas chroma-tography is investigated. Detectionlimits have been determined for ha-logenated and sulfured hydrocarbonsin the low to the high pg/s range de-pendent on the element measured.Furthermore, the effect of dopinggas (oxygen) and make-up gas (ar-gon, helium) on the chromatogramswas studied.47

DIRECT-CURRENT PLASMAS

Glow Discharge. A molecularemission detector on a glass chipemploying a miniaturized direct cur-rent helium plasma for molecularfragmentation and excitation waspresented by Eijkel et al.48 The chan-nels and the plasma chamber are pro-duced by HF etching. The electrodesare formed by deposition of 50 nmof chromium and 250 nm of gold.Gas in- and outlet holes with diam-eters of 400 mm were drilled by ul-trasonic abrasion. The plasma wasgenerated in chambers of differentgeometrical dimensions, varying thechamber volume, the electrode dis-tance, and the in- and outlet channel,as well as the equivalent radius ofthe plasma chamber. In- and outletchannels were chosen to obtain aplasma chamber pressure of about100 hPa. A chamber with a volumeof 50 nL at a typical operating pres-sure of 170 hPa was used as the ex-citation source. Methane could bedetected with a detection limit of 33 10212 g/s (600 ppm v/v) by ob-serving the molecular emission ofthe CH radical. The lifetime of thedevice was limited to 2 h due tocathode sputtering.

A further paper by Eijkel et al. de-scribed an atmospheric pressure dcglow discharge on a microchip to beused as a molecular emission detec-tor.49 One manifestation of the plas-ma chip is shown in Fig. 8. The scal-ing theory for direct-current glowdischarges predicts that normal dis-charges can exist at atmosphericpressure in microscale dischargetubes. The validity of this theory wasdemonstrated by the creation of anatmospheric helium plasma in ananoliter-size discharge chamber ona microchip. Three chambers with


FIG. 8. DC microplasma on a chip.

volumes of 52.5 nL, 180 nL, and 50nL with flow rates of 5.75 nL/s, 500nL/s, and 50 nL/s, respectively, wereused in order to determine the limitof detection of methane in a heliumgas flow by measuring molecularbands of CH. The limits of detectionwere 7 ppm, 3 ppm, and 400 ppb,which correspond to 10214 g/s, 10212

g/s, and 10213 g/s, respectively. In thecase of low flow rates, the analyticalsignal was linear over two decades.It was shown that the microchipplasma could be successfully appliedfor molecular emission detection.The authors are claiming that thesimple instrumentation, the small de-tector size, and the good sensitivitymake the device highly suitable forintegration in microanalysis systems.

In a third paper, the same groupcoupled the plasma chip to a con-ventional gas chromatograph in or-der to investigate its performance asan optical emission detector.50 Al-though the lowest detection limit of10214 g/s was obtained earlier whenthe plasma volume was 52.5 nL andthe flow rate was 5.75 nL/s, a 180nL volume plasma with a higherflow rate of 320 nL/s was applied inorder to prevent broadening and tail-ing of the chromatographic peaks.The plasma was generated in heliumand the applied power was 9 mW(770 V, 12 mA). A number of carboncompounds were detected in the col-umn effluent recording the CO emis-

sion at 519 nm. For hexane, the de-tector showed a linear dynamicrange over two decades and a detec-tion limit of 10212 g/s (800 ppb).However, all components of thechromatogram showed considerablebroadening and tailing. The authorsproved that these effects were not re-lated to the performance of the plas-ma but to dead volumes in the con-nection tubes and the channels of theglass chip. It can be expected that theintegration of the chromatographiccolumn and the plasma detector on asingle chip reduces the dead volumesand improves the signals. The devicewas operated for more than 24 hwithout a significant change in per-formance. The operation is stableand instrumental requirements aresimple.

Atomic detection of bromine andchlorine was reported by Bessoth etal.51 A microplasma chip with a sizeof 2 3 0.07 3 0.07 mm3 and 125mW of power was coupled to a con-ventional, large gas chromatograph.Specific detection of elements andmolecular fragments in the elutedpeaks was demonstrated. Using the479.5 nm emission line a detectionlimit of 800 pg/s for chlorine wasfound, taking into account the in-jected amount of chlorinated com-pounds.

A direct-current, chip-based plas-ma has been used for gas sample in-jection in gas chromatography.52 A

second identical plasma chip hasbeen used as the excitation sourcefor an optical emission detector. Thefirst plasma is normally continuallysustained during operation, causingcontinuous ionization/fragmentationof the sample, while the second plas-ma records the optical emissiondownstream. For injection, the firstplasma is briefly interrupted, intro-ducing a ‘‘plug’’ of unmodified sam-ple into the system. Injection plugsizes of between 5 and 50 mL havebeen reproducibly obtained, al-though significantly smaller volumesmay be possible with the use ofsmaller cross-section columns, lowerflow rates, and/or shorter plasma in-terruption times.

Microhollow Cathode Dis-charge. The microhollow cathodedischarge (MHCD), also called mi-crostructured electrode discharge(MSE), shown in Fig. 9, is a multi-layer system consisting of two me-tallic foils of Cu, Ni, Pt, or W sep-arated by an insulator (e.g., Kapton,mica, or ceramic). The thickness ofthe layers is typically 30–150 mm. Abore with a diameter of 10–500 mmis drilled through the structure.53 Aplasma is produced between theelectrodes in noble gases, gas mix-tures, or air using ac or dc currentvoltage. A very high current densityis generated in the discharge due tothe hollow cathode geometry. Thereduced dimensions of the electrodesenable high-pressure operation ac-cording to the laws of similarity.54

The discharge can be operated in thepressure range from 10 mbar up to 2bar.

Recently, the plasma parametersof a particular Ar MSE discharge op-erated with a dc current voltage weremeasured by DLAAS.55 This MSEstructure was made of two Cu elec-trodes (120 mm thickness) separatedby a 50 mm Kapton insulator with abore diameter of 300 mm. It could beshown that the gas temperature in-creases linearly from near room tem-perature at 50 mbar to about 1200 Kat 400 mbar. The electron densityalso increases with pressure up to 53 1015/cm3 at 400 mbar and a con-stant gas flow of 100 mL/min. In afollowing publication,56 the authors


focal point

FIG. 9. DC microhollow cathode discharge.

extended the range of pressure up toatmospheric pressure in Ar as wellas in He for a microstrucure with Ptelectrodes and ceramic as the insu-lator. It was found that the gas tem-perature increased in Ar from 500 Kat 200 mbar to 2000 K at 1000 mbar.The electron density also increasedfrom 2 3 1015 to 9 3 1015/cm3 in thesame pressure range. In He, the gastemperature reached values up to800 K and the electron density in-creased up to 3 3 1014/cm3 at at-mospheric pressure.

The MSE discharge operates in astable mode for currents between 4and 8 mA in He as well as in Ar. Inthis range the gas voltage is constantand does not exceed 250 V. For thegiven geometric parameters and acurrent of 6 mA the current densityand the input power density areabout 60 A/cm2 and 1 MW/cm3, re-spectively. One can see that the pow-er density of an MSE discharge isthree orders of magnitude higherthan that of an ICP.57

Due to the high gas temperatureand high-energy ions the lifetime ofthese structures is only a few days.However, the lifetime can be im-proved by using ceramic as an in-sulator and metal electrodes withlow sputtering rates.

Atomic emission spectroscopy ofthe MSE discharge was applied tothe detection of chlorine and fluorine

resulting from the decomposition ofhalogenated molecules (CCl2F2,CHClF2) introduced into the Heplasma gas.58 The intensities are lin-ear over three orders of magnitude.The detection limits for CCl2F2 are20 ppb v/v using either Cl 912.114nm or the F 739.868 nm.

Besides coupling with the emis-sion spectrometry, the microhollowcathode discharge was investigatedas a promising miniaturized ioniza-tion source for mass spectrometry.57

The plasma produced in the bore ofthe structure was expanded in a low-pressure region, producing a super-sonic plasma jet. The plasma jetreached the skimmer of the interfaceand the ions were selected and ac-celerated towards the quadrupole de-tector. Different sample introductionsystems were used such as gas mix-tures, coupling to gas chromatogra-phy, or permeation bottles. The mi-crohollow cathode discharge–massspectrometry delivers detection lim-its in the pg/s range (correspondingto a few ppbv/v) for halogenated mol-ecules in gaseous or volatile sam-ples.

MICROPLASMAS FORAQUEOUS SAMPLES

Most of the microplasmas de-scribed above and developed for an-alytical purposes have so far concen-trated on gaseous samples, which

limits the potential applications inthe field of m-TAS and lab-on-a-chip. They are not universal devicesable to measure any kind of sampleand the principle reason for this isthe difficulty in achieving adequatesample transport from the liquid tothe gas phase and also the difficultyin increasing the coupled power intothe plasma over a certain limit. Forminiaturized plasmas the volumeand discharge power is such thateven small amounts of liquid caneasily extinguish the discharge. Thisis not surprising when even a 1 kWICP used with a dry sample intro-duction system is often extinguishedif a 5 mL sample is not fully driedprior to vaporization.3 Conventionalemission spectroscopy such as ICPor GD-AES achieve liquid sampleintroduction using pneumatic nebu-lization or through the drying of analiquot of sample onto one of thedischarge electrodes. Therefore, itwould also be necessary to developa suitable method of injection of liq-uid samples into microplasmas forthe establishment of a portable liquidsample analysis system. For exam-ple, in the case of conventional ICP-OES systems, pneumatic or ultrason-ic nebulizers are usually adopted foratomizing sample solutions.

An atmospheric pressure m-ICPsource on a ceramic substrate hasbeen presented in which detection ofsodium from an aqueous sample isperformed using both a miniaturizedpneumatic nebulizer and an electro-spray device.26 Prototypes of injectordevices were located at the down-stream position of the plasma sourcechip and sample injection into the ra-dial direction of the plasma jet wasattempted. In preliminary experi-ments it was found that better injec-tion performance was attainable forthe electrospray injection. Since theviscosity of the high-temperatureplasma jet is so high, sample injec-tion only by the gas flow of thepneumatic nebulizer seems to bequite difficult. In contrast, injectionassisted by the strong electrostaticforce is effective.

Based on these results, detectionof sodium from a standard NaCl so-lution (concentration 100 ppm) was


demonstrated using a miniaturizedplasma source, a multichannel spec-trometer, and the electrospray injec-tion method. Emission spectra fromthe Ar plasma jets with and withoutthe injection of a tiny amount of anaqueous solution of 100 ppm sodiumchloride were measured. Ar plasmajets were produced at a VHF powerof 50 W. The sample solution wasintroduced through the electrospraynozzle biased at 3 kV. A sharp emis-sion peak of Na(I) was clearly ob-served at 589 nm when the samplesolution was introduced, while inten-sities of Ar(I) emission peaks werehardly changed. A detection limit of5 ppm has been attained using amonochromator with a photomulti-plier as a detector.

Another attempt was to couple aso-called mini-in-torch vaporization(ITV) dry sample introduction sys-tem with an atmospheric pressureplanar geometry microplasma device(MPD).3 This mini-ITV consists of asample carrying probe formed byfive-turn-coiled filament made fromRe secured on a ceramic support anda small volume vaporizer chamberwith a carrier-gas inlet. In a typicalanalytical run, the tube joining theITV chamber to the MPD was dis-connected and the ceramic support/coiled filament combination was re-tracted from the vaporization cham-ber, and 5 mL sample was manuallypipetted onto the filament. The sup-port/coil filament was then reinsertedinto the vaporization chamber. Thesample was dried with a low powerand the outlet of the vaporizationchamber was connected to the inletof the MPD. A higher electricalpower was supplied in order to va-porize the residue that remains onthe coil. The vaporized sample wascarried to the MPD by Ar carrier gasand it was dissociated and detectedby optical emission spectrometry.The detection limit for Li is at leastbetter than 30 ppb.

Another discharge source, knownas the electrolyte as a cathode dis-charge (ELCAD), has received someattention and may have potential asa detector for m-TAS. The ELCADsource was first developed by Cser-falvi et al. and involves a glow dis-

charge being ignited between a metalanode and the surface of an electro-lyte sample solution that flows up-wards through a glass tube and over-flows into an open vessel. The sam-ple solution is electrically connectedto the negative electrode such thatthe overflowing liquid surface be-comes the discharge cathode.59

Emission spectroscopy of metal an-alytes in the electrolyte solution hasbeen performed in air at atmosphericpressure using this technique and theauthors suggest the cathodic sputter-ing as being the key mechanism ofsample transport from the liquid tothe gas phase. A relationship be-tween pH and spectral line intensitywas interpreted as being due to a re-action between hydroxonium ionsand solvated electrons, leading to anincrease in the secondary electronemission coefficient. This in turn re-duces the cathode fall potential,which otherwise inhibits transport ofsputtered ions into the negative glowregion of the discharge. Acidificationof the sample solution is thus appliedto most ELCAD sources to increasesample transport into the discharge.

A variation of the ELCAD source,known as liquid sampling atmo-spheric pressure glow discharge (LS-APGD), has been developed by Mar-cus and Davis, in which the electro-lyte sample flows through a stainlesssteel capillary connected to the neg-ative electrode and a glow dischargeis ignited between the capillary anda metal anode.60 In contrast to theELCAD source, most or all of thesample is consumed by the dis-charge. The authors cite thermal va-porization of the sample by Jouleheating as being the key mechanismof sample transport into the dis-charge.

A few attempts have been made tominiaturize an ELCAD source usingmicrofabrication techniques com-monly employed for m-TAS and lab-on-a-chip devices. The first attemptused a micro-fluidic device fabricat-ed in glass and ignited a glow dis-charge between a sample streamflowing within a micro-channel anda metal anode.61 Argon gas flowingfrom another micro-channel set up aflowing liquid–gas interface within

the device to allow ignition of thedischarge. The atomic emission de-tection of copper lines demonstratedthe feasibility of such a device witha discharge current of ø1–5 mA.However, discharge instabilities oc-curred due to the high gas tempera-ture.

An improved device has been pre-sented that goes some way towardaddressing the problems of dischargeinstabilities and device lifetime.62

Operation in air instead of argon hasalso been demonstrated. Furtherminiaturization of the inlet channeldimensions indicates the potentialfor coupling such a detection systemwith other m-TAS elements. A pre-liminary absolute detection limit of17 nmol s21 is obtained for sodiumwith a flow rate of 100 mL min21 andusing a 100 ms spectrometer integra-tion time.

Wilson and Gianchandani devel-oped an atmospheric pressure glowmicro-discharge fabricated on a glasssubstrate. This device, which drawsfrom the research reported by Cser-falvi, was referred to as a liquid elec-trode spectral emission chip LEd-SpEC.63 It was developed as a mon-itor of ‘‘water impurities’’. In thisdevice the liquid sample is confinedto an open reservoir connected to asemi-enclosed channel etched inglass. A glow discharge is ignitedbetween a patterned metal electrodeand the sample solution, which is incontact with a patterned metal cath-ode. Emission spectroscopy allowedthe detection of several metallic spe-cies, as with other ELCAD sources,but with a discharge current of only2.5 mA. However, the static natureof the sample droplet and large di-mensions would be problematic ifintegration with other m-TAS ele-ments is desired.

More recently a novel device waspresented that ignites a discharge ina liquid sample within a PDMS mi-cro-channel by passing a currentthrough the liquid sample such thatJoule heating generates a bubble ina constricted portion of the chan-nel.64 Once a gap has been estab-lished, the voltage applied is highenough to cause electrical break-down generating a discharge in the


focal point

sample vapor and allowing emissionspectroscopy to be performed. Thismethod differs from conventionalELCAD sources in that pure watervapor is effectively the carrier gasinstead of air (or in some cases noblegases) and as such the behavior iseven less well understood comparedto other ELCAD sources. The pre-liminary results obtained raise someinteresting questions about the op-erating mechanisms involved andwarrant further studies.

Some of these examples show thatsample introduction systems gener-ating vapors would not always be re-quired if microplasmas are to beused with liquids. Applications as adetector in a variety of liquid chro-matography and capillary electro-phoresis modes will require furthercharacterization of the role of elec-trolyte/mobile phase identity on sys-tem performance as well. Deviceslike these will have a place in thegeneral field of elemental speciation,where relatively simple devices oflow power consumption could bewidely applied.

ACKNOWLEDGMENT

The financial support by the Ministerium furInnovation, Wissenschaft, Forschung und Tech-nologie des Landes Nordrhein-Westfalen, by theBundesministerium fur Bildung und Forschung,and by the Deutsche Forschungsgemeinschaft(DFG) is gratefully acknowledged.

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sample analysis with miniaturized plasmas

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