Flexible ion conduit for use under rarefied atmospheric conditionsGyorgy Hars and Henk LC. Meuzelaar Citation: Review of Scientific Instruments 68, 3351 (1997); doi: 10.1063/1.1148296 View online: http://dx.doi.org/10.1063/1.1148296 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/68/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Suppression of unimolecular decay of laser desorbed peptide and protein ions by entrainment in rarefiedsupersonic gas jets under weak electric fields J. Chem. Phys. 140, 034201 (2014); 10.1063/1.4861040 Kinetics of Suprathermal Atoms and Molecules in the Rarefied Planetary Atmospheres AIP Conf. Proc. 1084, 1047 (2008); 10.1063/1.3076436 Hypersonic Rarefied Flow Simulation Using 2D Unstructured DSMC with Free Stream Condition AIP Conf. Proc. 762, 1223 (2005); 10.1063/1.1941700 Description of a dust particle detection system and measurements of particulate contamination from shock, gatevalve, and ion pump under ultrahigh vacuum conditions Rev. Sci. Instrum. 69, 3818 (1998); 10.1063/1.1149184 In-series combination of a magnetic-sector mass spectrometer with a time-of-flight quadratic-field ion mirror Rev. Sci. Instrum. 69, 1650 (1998); 10.1063/1.1148822

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Flexible ion conduit for use under rarefied atmospheric conditionsGyorgy HarsTechnical University of Budapest, 1111 Budapest, Budafoki ut 8, Hungary

Henk LC. MeuzelaarUniversity of Utah, Center for MARC, Salt Lake City, Utah 84112

~Received 12 March 1997; accepted for publication 30 May 1997!

A tubular ion conduit has been constructed, which transports ions by convection by means of acarrier gas. Typical inlet pressures are in the 10–100 Torr range, with outlet pressures as low as1023 Torr. The 20–30 cm, 1–2-mm-i.d., capillary tube, made of an electrically insulating material,is surrounded by a specifically configured pair of helical electrodes~‘‘helical dipole’’ !, which aresupplied with symmetrical voltages in the tens of volt amplitude and in 1 MHz frequency range. Thevibrational average force field generated reduces the tendency of ions to hit the inner wall of thetube. This way ions can be transported with minimal loss. Previously, known ion guides areoperated under molecular flow~high vacuum! conditions only, as opposed to the method describedhere, where the carrier gas enters under viscous flow conditions and exits as molecular flow. Inaddition, existing ion guides are stiff in contrast to the flexible construction described here, whichcan be easily and inexpensively manufactured. The ion conduit is expected to have importantapplications in connecting ambient or near-ambient pressure electrospray ionization or atmosphericpressure ionization type ion sources to mass spectrometers, while reducing pumping requirements,e.g., field portable equipment. Furthermore, the device may provide a means for connecting electronmultiplier detectors to near ambient pressure analyzers such as ion mobility spectrometers. ©1997American Institute of Physics.@S0034-6748~97!00709-0#

I. INTRODUCTION

Capillary tubes provide an effective and convenient wayof introducing neutral, i.e., nonionized gas and vapor speciesinto the high vacuum system of a mass spectrometer becauseof the inherently low carrier gas flow conductance. Further-more, many different tubing materials, ranging from metals,ceramics, and glasses to plastics, rubbers, and other poly-meric materials are available to help satisfy a broad range ofmechanical, electrical, and thermal physicochemical designrequirements. This has encouraged several researchers to in-vestigate the possible use of capillary tubes for transportingionized molecular or atomic species into vacuum systems aswell. Compared to the usual nozzle and skimmer arrange-ments, for introducing ionic species into the massspectrometer1 a high efficiency capillary ion guide wouldhave obvious advantages in terms of pumping requirements,mechanical flexibility and simplicity of design.

Baiwei et al.2 reported a successful ion transport througha 25-cm-long, 1.47-mm-i.d. glass capillary tube using a flowin the range of severall atm/min of air, while keeping thetube entrance at ambient pressure and the tube exit at a pres-sure from several Torr to almost atmosphere. However, iontransport efficiencies were low with losses estimated at morethan 99%. They concluded that most ions must be lost at thelow pressure end of the tube, as the average path length ofthermal motion starts to approach or exceed the radius of thetube with resultant increase in ion losses due to the randomwall collisions. Perhaps more important are the effects ofradially oriented space charge forces, which can be expectedto become more effective in driving ions towards the innerwall of the tube as pressure drops into the molecular flowrange. To minimize losses when transporting ions under high

vacuum conditions Bahret al.3 developed a rf driven stackedring ion guide, whereas Shenhenget al.4 and Yasuakiet al.5

described an electrostatic ion guide. However, these ionguide configurations were not designed for use in rarefiedgaseous, i.e., viscous flow dominated environments.

The work reported here investigates the use of a simplehelical dipole ~i.e., double stranded electrical coil! aroundelectrically nonconductive capillary tubing in combinationwith suitable rf fields, and an inert carrier gas as a means ofimproving ion transport efficiencies, particularly during thetransition from viscous to molecular flow conditions.6

II. THEORETICAL DISCUSSION

First, the vibrational average force~VAF! will be treatedin presence of drag. Similar calculation was made byDehmelt7 for the drag-free situation called as the pseudo po-tential model.

Let us consider a time variant electric force fieldE(r ) cosVt in general. This kind of electric field is created ifthe electrodes in the space are at rest, and the driving voltageis alternating according to cosVt law. The equation of mo-tion is as follows:

md2r

dt21k

dr

dt2QE~r ! cosVt50, ~1!

wherem andQ are the mass and the charge of the particle,ris the position vector,E„r … is the electric vector field,k is thedrag coefficient andV is the angular frequency of the drivingvoltage. Switching from time to phase variablew5Vt theequation can be transformed.

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d2r

dw2 1k

mV

dr

dw2

Q

mV2 E~r ! cosw50. ~2!

Here the drag parameterD will be used with the followingdefinition: D5k/mV. The solution is regarded as a smallamplitudep oscillation around a working pointr 0.

r5r 01p~w!. ~3!

Let us substitute Eq.~3! into Eq. ~2!.

d2p

dw2 1Ddp

dw2

Q

mV2 E~r01p! cosw50. ~4!

The first and second terms of the Taylor series ofE„r …

aroundr 0 will be used.

E~r 01p!5E~r 0!1dE

dr Urop

. ~5!

Here dE/dr ur0denotes the derived tensor in the working

point. Formula~5! is substituted to Eq.~4!

d2p

dw2 1Ddp

dw2

Q

mV2 E~r0! cosw2Q

mV2

dE

dr U r0

p cosw

50. ~6!

In this approximation the last term of Eq.~6! can be ignoredsinceupu is assumed to be small. Equation~6! transformed asfollows:

d2p

dw2 1Ddp

dw5

Q

mV2 E~r0! cosw. ~7!

The solution is looked for in the following form:

p5E~r0!~A cosw1B sinw!. ~8!

Here A and B are identified upon substitution into Eq.~7!.After doing so function of the oscillationr (w) can be writ-ten:

r ~w!5r01Q

mV2 E~r0!S D

11D2 sinw21

11D2 cosw D .

~9!

On the other hand the forceF exerted to the particle is theresultant of the drag and the electric field as can be seen inEq. ~2!.

F52kVdr

dw1QE~r ! cosw50. ~10!

Equation~9! is substituted into Eq.~10! which will result inthe F(w) function ~11! below:

F~w!52kVQ

mV2

E~r0!

11D2 ~D cosw1sin w!

1QEF r01Q

mV2

E~r0!

11D2 ~D sinw2cosw!G3cosw. ~11!

Here the parentheses@ # indicates the argument of theEfunction. Now only this part is transformed:

EF r01Q

mV2

E~r0!

11D2 ~D sinw2cosw!G5E~r0!1

dE

dr Ur0

Q

mV2

E~r0!

11D2 ~D sinw2cosw!.

~12!

So all togetherF(w) results in Eq.~13!:

F~w!52kVQ

mV2

E~r0!

11D2 ~D cosw1sin w!

1QS E~r0!1dE

dr Ur0

Q

mV2

E~r0!

11D2

3~D sinw2cosw!D cosw. ~13!

The average ofF(w) is denotedF with the following defi-nition:

F51

2pE0

2p

F~w!dw. ~14!

There is only one term with nonzero average in Eq.~13!,which results from the cos2 w.

F52dE

dr Ur0

E~r0!Q2

mV2

1

11D2

1

2pE0

2p

cos2 wdw. ~15!

Here an equivalency of the vector calculus has to be used:

dE

dr Ur0

E~r 0!5 12gradE2~r 0!. ~16!

In addition the average of cos2 w is one half is also consid-ered, so Eq.~15! is transformed:

F~r !52Q2

4mV2

1

11D2 gradE2~r !. ~17!

Here r 0 has been switched tor since there is no more needfor the distinction.

The VAF electric fielde can now be defined:

e~r !5F

Q52gradS Q

4mV2

1

11D2 E2~r ! D . ~18!

The VAF electric fielde results as a negative gradient of ascalar field, which is now obviously the VAF scalar potentialU.

U~r !5Q

4mV2

1

11D2 E2~r !. ~19!

Formula~19! shows, that in the case of fast alternating volt-ages, the motion of the particle is predominantly determinedby the VAF potential, which is a time invariant field. Thereal alternating voltage results in some kind of fine structureon the top of the motion determined by VAF potential.

Let’s take a closer look at the average vibrational kineticenergyWvib of the particle. The velocity of vibrationvvib(w)can be calculated from Eq.~9!

vvib~w!5Vdr

dw5

Q

mQ

E~r0!

11D2 ~D cosw1sin w!, ~20!

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Wvib51

2pE0

2p1

2mvvib

2 ~w!dw5Q2

4mV2

1

11D2 E2~r !.

~21!

Now, by comparing Eq.~19! to Eq. ~21! the following rela-tionship can be observed:

Wvib5QU. ~22!

This relationship shows, that the potential energy of VAFfield is being stored in the form of vibrational kinetic energy.On the other hand the system is dissipative, because of thepresence of drag. The dissipated average power can readilybe calculated:

Pdis51

2pE0

2p

QE~r ! coswvvib~w!dw. ~23!

By means of Eq.~20! the dissipated power results:

Pdis~r !5Q2

2mV2

D

11D2 E2~r !. ~24!

In the formulas above, the presence of the drag parameterDis essential. Let’s try to calculate it from some other physicalparameters. SinceD5k/mV, the drag coefficientk shouldbe determined first, which is defined by the following equa-tion:

F5kvdrift . ~25!

To find out drag coefficientk assume that an ion is acceler-ated by the forceF between two collisions with the gasparticles. Then the drift velocity of the ion in average will bethe half of the maximum velocity reached between two col-lisions

F

2mZ5Vdrift , ~26!

whereZ is the collision frequency, well-known in the kineti-cal theory of gases. By comparing Eqs.~25! and ~26! k canbe identified.

k52mZ. ~27!

Substituting Eq.~27! to the definition of the drag parameter(D5k/mV), Eq. ~28! results:

D52Z

V. ~28!

According to the kinetical theory of gases the collision fre-quency is determined:

Z5A2pd2nvaverage, ~29!

where d is the kinetical diameter of the gas molecules inmeters,n is the volume density of the gas particles in 1/m3

and vaverageis the average velocity of the gas molecules inthermal motion in m/s units. Heren can be expressed fromthe equation of ideal gases:

n5pL

RT, ~30!

wherep andT are the pressure and temperature of the gas inPa and K units,L is the Avogadro number 631023 1/moland R58.3 J/molK is the ideal gas constant. The averagevelocity can also be expressed:

Vaverage5A2RT

M, ~31!

whereM is the molar mass of the gas in kg/mol units. Sub-stituting Eqs.~31! and ~30! to Eq. ~29! the collision fre-quency results:

Z52pLd2p

AMRT. ~32!

So by means of Eqs.~28! and~32! the drag parameterD canbe readily expressed.

In order to have an impression on the distribution of theVAF filed in the conduit a computer simulation was made.The program used for this is calledSIMION. This can handlepotential arrays up to 16 000 points. The program was up-graded by the authors with the VAF option to calculate thesquare of the gradient of the specified potential array accord-ing to the formula~19!. After this, the newly generated po-tential array can be displayed and contoured in the samemanner as any other original potential array. The distributionof the VAF field in the helical dipole conduit is shown inFig. 1. The helical dipole conduit transports charged particlesvia convection. Since this process is associated with a netspace charge in the tube, the knowledge of the radial poten-tial difference between the wall and the center is essential.Namely, this potential must be compensated by the VAFfield in order to enhance the transmission of the conduit.

In our experiment the flowF is measured on the exhaustof the pump at atmospheric pressure. In the conduit, how-ever, the pressurep is much smaller. So the law of continuityis the following:

p0F5pAv trans, ~33!

wherep05105 Pa,F is the flow in m3/s, p is the pressure inthe conduit in Pa,v trans is the transportation velocity in thetube in m/s andA is the inner cross section of the tube inm2 units.

On the other hand the conduit transports a convectiveelectric currentI too.

I 5rAv trans, ~34!

wherer is the volume density of the electric charge in theconduit in As/m3 units. Combining these last two formulasrcan be expressed:

r5Ip

Fp0. ~35!

The potential differenceU rad between the wall and the centercan be expressed by Gauss’s law.

U rad5r

4e0R2, ~36!

wheree058.86310212 As/Vm andR is the inner radius ofthe conduit tubing in meters.

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III. EXPERIMENT

Two different experimental setups were used. Figure 2shows an earlier version~I! with a wire grid current detectorat the end of the conduit. A more sophisticated configurationversion ~II ! is shown in Fig. 3, with a secondary electronmultiplier facing the end of the conduit in the UHV chamber.The experimental assembly consists of three major parts,namely ion source, flexible conduit with helical dipole elec-trodes, and an UHV chamber with ion detector.

IV. ION SOURCE

A corona type ion source was designed. The needle isgrounded through a 10 kV resistor, which is used to monitorthe source current~0.5–1 mA!. The concentric cylinder sur-rounding the source is;5 cm in diameter, and connected toa positive voltage of about 1 kV. The chamber of the ion

source is pumped by means of a 5 m3/h rotary vane pump.A needle valve is used to introduce either helium or air. Thebackground pressure is ca. 1021 Torr, and can be monitoredwith a Priani gauge as well as a Convectron gauge. Theentrance of the conduit is introduced into the ion source con-centrically, thus facing the tip of the needle.

V. FLEXIBLE CONDUIT WITH HELICAL DIPOLEELECTRODES

A 25-cm-long flexible Teflon tube~1.73-mm-i.d.! isused. A slightly smaller diameter Teflon tube~1.8-mm-o.d.!in 20-mm-length is inserted at the entrance side as a restric-tion at version~II ! only. Two identical lengths of copper wire~0.538-mm-o.d. varnish insulation! are used to wind the he-lical dipole electrodes around the tube. This pair of elec-

FIG. 3. Setup of the helical dipole conduit experiment version~II !. Thetransmitted current is detected by a SEM placed into the vacuum chamberfacing the end of the conduit.

FIG. 1. Computer calculation of the equipotential VAF lines in a helical dipole conduit. The inner wall of the conduit is indicated. The VAF potential contoursrange from 3 V to 3 mV in three orders of magnitude. The equidistant structure of the contours shows an exponential-like behavior of the VAF potential asthe function of radius. The slight asymmetry is due to computer artifacts. The image was generated bySIMION program upgraded by the authors.

FIG. 2. Setup of the helical dipole conduit experiment version~I!. Thetransmitted current is detected by a wire net detector, placed into the flow ofthe carrier gas.

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trodes has a measured capacitance of;600 pF. The elec-trodes are driven with a symmetrical sinusoidal voltage witha frequency around 1 MHz, and with a linear amplitude rampfrom zero to about 120 Vrf amplitude. One such ramp lasts 0.5s.

VI. UHV CHAMBER WITH ION DETECTOR

The chamber is pumped by a TPU 170l /s turbo pumpusing a rotary vane pump as a forepump. The pressure ismonitored by means of a Baratron gauge, a Pirani gauge anda Penning type cold cathode gauge. The background pressureis around 1025 Torr. In version~I! a wire net detector isplaced in the carrier gas flow, thus filtering out the chargescarried by the ions. The wire net detector is hooked up di-rectly to the electrometer which is normally operated in the 1V/100 pA or V/1 nA range. The electrometer output signal iscoupled to the vertical deflection of the oscilloscope. Theoscilloscope is synchronized to the analog control voltage ofthe rf amplitude ramp. The ultimate sensitivity of the elec-trometer is 1 V/1 pA. However, using this highly sensitiverange increases the noise and slows the electrometer down.The wire net detector is constructed in such a way, that itdoes not restrict the flow significantly. This is important,since it has been shown, that any restriction at the exit of theconduit flaws the experiment. In version~I!, the ion source isoperated with helium or air for which the pressure ranges are15–160 Torr and 3.8–53 Torr, respectively. The flow in theconduit is roughly around 1l atm min21, depending on thepressure.

In version~II !, the secondary electron multiplier~SEM!is facing in the direction of the opening of the capillarythrough which the gas and the ions enter. The SEM ishooked up to a negative~1600 V/dc! power supply. Thecollector electrode is connected to an electrometer which isnormally in the 1 V/1 nA position. The electrometer outputsignal is coupled to the vertical deflection input of the oscil-loscope. The oscilloscope is synchronized to the analog con-trol voltage of the rf amplitude ramp. In version~II ! the ionsource is normally operated with air at 10–20 Torr. The highvoltage of the source is around 900 V whereas the sourcecurrent is 0.7 mA. The measured air flow on the conduit was40 ml atm min21. The pressure in the SEM chamber is about231023 Torr.

VII. RESULTS AND DISCUSSION

A. Version (I) (wire grid detector version)

Helium and air were used at various pressures to deter-mine the dark current~i.e., the current without the rf voltage!and the VAF current~i.e., the total transmitted current in thepresence of the rf voltage!. Whenever it was possible the rfvoltage amplitudes were determined which resulted in themaximum VAF current~last column of the tables!. Data ofversion~I! experiments are presented in Tables I and II.

In Table I data are summarized of the experiment withthe following conditions: Carrier gas is helium, the sourcecurrent is 0.62 mA. The 250-mm-long Teflon tube is 1.73mm/2.3-mm-i.d./o.d. The tube was free of any restriction. Itcan be seen, that between source pressures 16 and 43 Torr

the VAF current exceeds the dark current by more than threeorders of magnitude. In Table II data are summarized of theexperiment with the following conditions: Carrier gas is air,the source current is 0.62 mA. The 250-mm-long Teflon tubeis 1.73-mm/2.3 mm-i.d./o.d. The tube was free of any restric-tion. It can be seen, that between source pressures 14 and19.4 Torr the VAF current exceeds the dark current by morethan two orders of magnitude. In order to carry out the ac-curate VAF current measurements, the rf amplitude was ad-justed to the optimum value in static mode, rather than usingrf ramp modulation. This measurement has also shown, thatthe detected VAF current is a stationary phenomenon as op-posed to a transient current in the case of a, e.g., chargingcapacitance. Data of Tables I and II show, that the ions aretransported with minimal loss in the tube between the saidpressure values by means of rf field switched on. Let’s cal-culate some fundamental formulas numerically of Sec. II in

TABLE I. Effect of rf field on ion current transmitted by the helical dipoleconduit in helium carrier gas flow with wire net detector.

Sourcepressure~Torr!

Detectorpressure~Torr!

Flow( l atm min21!

Darkcurrent~pA!

VAFcurrent~nA!

rf voltage(Vrf amplitude)

15 0.12 0.6 n.d. 0.01 4216 0.26 0.8 n.d. 1.1 4320 0.5 0.9 n.d. 2.5 3922 0.8 1.0 n.d. 4.5 3923 0.91 1.1 n.d. 5.0 4028 2.0 1.3 0.1 14.0 3831 2.1 1.4 0.2 13.0 4038 3.4 1.6 0.2 13.0 4343 4.7 1.7 0.2 14.0 4447 5.3 1.8 0.3 0.23 120a

68 10.4 2.2 10 0.42 120a

76 11.7 2.5 15 0.42 120a

95 ••• 2.8 30 0.4 120a

128 ••• 3.5 100 1.1 120a

160 ••• 4.2 350 1.1 120a

n.d.5not detectable i.e.,,0.1 pA.aNo Gaussian shaped peak detectable.~120 V corresponds to the saturationvalue of the rf supply.!

TABLE II. Effect of rf field on ion current transmitted by the helical dipoleconduit in air flow with wire net detector.

Sourcepressure~Torr!

Detectorpressure

~1023 Torr!Flow

( l atm min21!

Darkcurrent~pA!

VAFcurrent~nA!

rf voltage(Vrf amplitude)

3.8 7 ••• n.d. n.d. •••4.9 7 ••• n.d. 0.0001 656.3 8 ••• n.d. 0.0008 639 9 ••• n.d. 0.0025 64

11.7 11 ••• n.d. 0.004 6014 14 0.9 n.d. 0.1 6215.6 28 0.9 3 2.2 5618.2 85 1 5 2.4 5819.4 140 1 10 1.0 6220.5 220 1.1 10 0.28 5924.6 650 1.1 40 0.19 5730 1000 1.2 50 0.15 120a

53 3600 2 300 0.3 120a

n.d.5not detectable~i.e., ,0.1 pA!.aNo Gaussian shaped peak detectable.~120 V corresponds to the saturationvalue of the rf supply.!

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order to substantiate the claim that the physical quantitiesinvolved are in realistic order of magnitude. Line 6 inTable I is chosen for interpretation. First, collision frequencyis calculated for helium based on formula~32!, with the nu-merical values: d52.2310210 m, p53724 Pa, M5431023 kg/mol andT5300 K. With these valuesZ52.163108 1/s results. Now the drag parameterD is easy to cal-culate according to Eq.~28! with V52p106 1/s. FinallyD568.7 results. Once the drag parameter is known theSI-

MION VAF simulation ~Fig. 1! can be quantified. The VAFpotential difference between the inner wall of the tube andthe center isUVAF5592 mV.

On the other hand, the volume density of the electriccharger in the conduit can also be calculated according toEq. ~35!, based on the following numerical values:I 514 nA,F51.3 l /min52.1731025 m3/s, p528 Torr53724 Paandp05105 Pa. So,r52.431025 As/m3 results. By meansof this value, the radial electrostatic voltage@Eq. ~36!# can benumerically evaluatedU rad5506 mV. SinceUVAF and U rad

are of the same order of magnitude, and the VAF trappingvoltage somewhat exceeds the radial electrostatic repulsion,the final conclusion can be drawn: The suggested physicalmodel of the helical dipole conduit adequately explains, andinterprets the phenomenon of the minimum loss transporta-tion of ions in the convective flow of carrier gas.

B. Version (II) (electron multiplier version)

Version ~II ! is characterized by a restriction piece in-serted into the tube on the entrance side, made of a slightlysmaller diameter Teflon tube~1.8-mm-o.d., 20-mm-long!.An optimum position of the restriction sleeve, has thor-oughly been experimented. Either end of the tube has beentried, and it has been shown that any restriction at the exit ofthe conduit flaws the experiment. To apply the restrictionwas imperative, since those pressures which were measuredin the detector chamber in version~I! experiments, are toohigh to allow the use of a SEM. The pressure around themultiplier must not exceed the glow discharge limit~around1022 Torr depending on the gas!. Otherwise, gas dischargemay occur which could result in damage to the SEM. Heliumwas omitted in version~II ! experiments because of its highsusceptibility for glow discharge formation. Air was used atvarious pressures to determine the VAF current~i.e., the totaltransmitted current in the presence of the rf voltage!, and thehexane current~i.e., that specific VAF current which is mea-sured, while the air suction hose is dipped into the headspaceof hexane!. The rf voltage amplitudes correspond to maxi-mum VAF current were also determined. Data of version~II !experiments are presented in Table III with the experimentalconditions below: Carrier gas was air, the source current andvoltage values were 0.72 mA and 890 V, respectively. TheSEM was supplied with negative 1600 V. A Teflon restric-tion piece~1.8-mm-o.d., 20-mm-long! was inserted into the250-mm-long main Teflon conduit tube~1.73-mm/2.3-mm-i.d./o.d.! at its entrance side.

When the rf voltage ramp was on, a symmetrical peakappeared on the screen of the oscilloscope, indicating thatthe loss of the ions was greatly diminished over a specificvoltage range. The exact rf voltage amplitude corresponding

to the apex of the VAF current peak was determined in staticmeasurement~last column of the Table III!, similarly to thewire net detector experiment.

The SEM has a gain around 104 at 1600 V. Dividing thecurrent values in Table III with the gain of the SEM, resultsin the real current transmitted by the conduit. It is interestingto observe, that when increasing from 8 to 14 Torr, the cur-rent increases to;3003 higher value in the presence ofhexane headspace vapor. This effect might perhaps be uti-lized in a new kind of vapor detector device.

VIII. POTENTIAL APPLICATIONS

Ions produced in atmospheric pressure ionization~API!or electrospray ionization~ESI! sources could be transported,by the helical dipole ion conduit from viscous flow to highvacuum~molecular flow! environments. Because of the re-striction to the carrier gas and relatively enhanced conduc-tivity for ionic species, vacuum pumping requirements couldbe greatly reduced, thus paving the way for portable appli-cations of mass spectrometers equipped with API and ESIsources. Another potential application is replacement of Far-aday cup detectors with the highly sensitive SEM detectorsin applications, such as ion mobility spectrometry, where theanalyzer is at near ambient pressure. It should be pointed outthat the ionic content of the carrier gas could be enriched byusing porous tubing or a jet separator, and by pumping awaythe carrier gas while retaining the ions with a helical dipoleion conduit. The cause of the large increase in ion current inthe presence of hexane vapor during the experiments re-ported here, is being investigated further with regard to thepossible use as a new type of specific or universal vapordetector. Finally, the method reported here may pave the wayfor eventual transport of ions in flexible tubing similar totransport of light by an optical cable.

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2B. Lin and J. Sunner, J. Am. Soc. Mass Spectrom.5, 8732~1994!.3R. Bahr, D. Gerlich, and E. Teloy, Verh. Dtsch. Phys. Ges.4, 343~1969!.4S. Guan and A. G. Marshall, J. Am. Soc. Mass Spectrom.7, 101 ~1996!.5Y. Takada, M. Sakairi, and Y. Ose, Rev. Sci. Instrum.67, 2139~1996!.6A patent disclosure has been registered by the University of Utah Tech-nology Transfer office and patent applications are currently pending.

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TABLE III. Effect of rf field on ion current transmitted by the helical dipoleconduit in air flow with SEM detector.

Sourcepressure~Torr!

Detectorpressure

(1023 Torr!Flow

( l atm min21)

VAFcurrent~nA!

Hexanecurrent~nA!

rfvoltage

(Vrf amplitude)

8 1.1 0.013 0.002 0.6 639 1.4 ••• 0.040 14 639.5 1.9 ••• 0.3 110 64

11 2.9 3 280 6314 5.4 0.040 35 700 6217 16.0 ••• 250 250 a

aIndicates that measured values were fluctuating intensively, due to glowdischarge formation around the SEM.

3356 Rev. Sci. Instrum., Vol. 68, No. 9, September 1997 Ion conduit This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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