CA2241320C - A method for reduction of selected ion intensities in confined ion beams - Google Patents

Abstract

A method for producing an ion beam having an increased proportion of analyte ions compared to carrier gas ions is disclosed. Specifically, the method has the step of addition of a charge transfer gas to the carrier analyte combination that accepts charge from the carrier gas ions yet minimally accepts charge from the analyte ions thereby selectively neutralizing the carrier gas ions. Also disclosed is the method as employed in various analytical instruments including an inductively coupled plasma mass spectrometer.

Description

P~T/~ U ;i 2 3 IPE~ Q 6 AUG 1997 A METHOD FOR REDUCTION OF SE~ ECTED ION INTENSITIES IN
SCONFINED ION BEAMS

FIELD OF THE INVENTION

The present invention relates generally to a method for producing an ion 10 beam having an increased plupollion of analyte ions con~pa,~,d to carrier gas ions.
More specifically, the method has steps resulting in selectively neutralizing carrier gas ions. Yet more s~cirlcally, the method has the step of ~dAitic!n of a charge~la~l gas to the carrier analyte combination that accepts charge from the carrier gas ions yet minimqlly accepts charge from the analyte thereby sel~li~ly 15 neutralizing the carrier gas ions.

BACKGROUND OF THE INVENTION

Many analytical or i~hl~hi~l processes require the generation of beams of 20 ions of particular ~b~t~nces or analytes. For e~ le, ion beams are used in ion guns, ion ill pl~t~ ,~, ion thrusters for attitude control of satellites, laser ablation plumes, and various mass ~holl-et~l~ (MS), inrlu~Aing linear quadmpole MS, ion trap quadrupole MS, ion cyclotron I~SOn~'~r~ MS, time of flight MS, and electric and/or magnPtil~ sector MS. Scveral s k~ are known in the art for 25 genelaling such ion beams in~ nAing el~holl impact, laser irradiation, io Lst,lay, ele~;hv~ldy, th~mospray, ~ ,ly coupled plasma sources, glow dischalges and hollow cathode discha,ges. Typical arlA~ colllbi~ the analyte with a carrier or support gas w~ elJy the carrier gas is utilized to aid in h~olling, ionizing, or both hA~lh~g and ioni7ing, the analyte.
For example, in a typical al,A~&.-.. ~ an analyte is coll~biLx:l with the carrier gas in an elc~hical field, ~I~.~ poll the analyte and the carrier gas are ionized in a strong electric or magn~tic field and later used in an analytical or industrial process. In another typical allA~e~ the carrier gas is first ionized ~IE~ S~

PCT/US 9 7 / O ~ 3 2 3 IPE~ O6 AU~i1997 in a strong electric or magnetic field whereupon the analyte is then introduced into the ionized carrier gas. Electric fields are ge~ ted by a variety of m~thodc well known in the art including, but not limited to, capacilive and inductive coupling.
In an inductive coupling arrangement, a radio frequency (RF) voltage is 5 applied to a coil of a con~lucting material, typically brass. In the interior of the coil, one or more tubes supply a carrier gas, such as argon, and an analyte, which may be any substance. The analyte may be supplied in a variety of forms inrlulling but not limited to a gaseous form, as a liquid, as a droplet form as in an aerosol, or as a laser ablated aerosol. A large el~;~.ical field is ge~.at~d within 10 the coil. Within this field, any free el~llons will initiate a chain reaction in the analyte and the carrier gas causing a loss of el~llons and thus ioni7~tion of the carrier gas and the analyte. Several ...- Ih~c well known in the art, inrhl~ing but not limited to the introduction of a Tesla coil, the introduction of a g~ rod, or thermal emission of ele~lons, will provide free el~l,ons causing initiation of a 15 chain ~ io" The result is a weakly ioni_cd gas or plasrna con~; ~;ng of both free cle~LIons and charged and uncha~g~ species of the carrier gas and the analyte. The species of both the carrier gas and the analyte in the plasrna may be - in the form of particles, atoms or molx~ s, or a ll~ e of particles, atorns and molecules, :le~ ~ing on the particular species s~l~t~l for use as the carrier gas 20 and analyte.
The carrier gas and the analyte may be combined by a wide variety of methods well known in the art. For example, as described above, the analyte and the carrier gas in an aerosol form are coll,~ii~d and are then dh~x~l to the interior of a coil in an inductively coupled plasma. Another typical a~,~nge~elll 25 employs a needle which l~Ci~,~,S a liquid sample of analyte from a source such as a liquid chrollla~ograph. Su~ u~iing thc needle is a tube which supplies a carrier gas such as argon as a high velocity atomi7.ing carrier gas. Both the needle andthe tube empty into a chall,bcr. Upon disch~ge from the needle, the analyte liquid is evaporated and atomi_ed in the argon carrier gas. Ions of both the 30 evaporated liquid analyte and the argon carrier gas are produced by cleaLing an ~CT/US ~ - f ~ V ~ 2 3 IPE~J O 6 hl~G 1997 electric field within the chamber. The electric field may be produced by creating a voltage dirr.,le.lce between the needle and the chamber. A voltage dirr~lel~cemay be created by applying a voltage to the needle and grounding the chamber.
The resultant plasma genel~ted by any of the foregoing methods is typically 5 directed towards either an analytical appal~lus or towards a reaction zone wheleill the carrier gas and analyte ions are analyzed or otherwise reacted or utilized in some fashion. The resultant plasma is typically directed by means of an electricor m~gn~tir field, or by means of a plcs~ule dirr~elllial~ or both. As the plasma is directed, the plasma is converted from a plasma to an ion beam. As used 10 herein, the term "ion beam" re&rs to a stream co~i~li..g primarily of positively charged and neutral species. The bulk of the ~eg~ ly charged species in the plasma are typically electrons, which are rapidly di~ ed as the plasma is directed by either electric or magnPtic fields or by a pleS~ dirr~lenlial.
However, even after sigl-iri~ di,~ dl of the ion beam, the ion beam may not 15 be co-l,lctcly void of negatively charged species. As the plasma progresses folw~d, the free clecllulls, due to their low mass relative to the po~ ely charged ions, tend to disperse from the plasma, thus CG~ g the plasma to an ion beam.
Also, the ion beam itself will tend to dis~.~ due to several effects. Most plUIIIi~l~lll among these effects is the repulsive forces of ch~,~ species within the ion beam. The beam is also dis~l~ through free jet eAl~n~ion. The effect of dispersion of the con~tituent species in the ion beam is charge separation amongthose species and is well known in the art. The reSlllt~nt ion beam is thus typically chal~t~ ed by high net positive charge density which is primarily alllibulable to the relatively high abu~ re of posilivcly charged carrier gas ions.
In many applications, the ab~ln~l~nre of positively charged carrier gas ions and/or the resultant high charge density may be ullde6irdble. For e~ le, it is often desildble that the ion beam be focused through a small apellure, for example, if the analyte ions were to be analyzed in a mass ~ ter. In such an arrang~ll.ent, where the ion beam is dh~ted through an apcllul~, the high charge density will p,esclibe a space charge limit to the amount of the ion beam 7~6 hU~(i319~97 that may be passed through a given apcllulc. When the space charge lirnit is reached, the rernqin~r of the beam is unable to pass through the a~l~w~ and is thus lost. In many applications, the portion of the beam which is lost includes analyte ions. Indeed, a loss of a portion of the beam may result in a S disproportionate loss of some or all of the analyte ions because the analyte ions may not be evenly disllilwLed throughout the ion beam or may not respond to the various dis~rsil g forces in the same manner as the carrier gas ions.
Another example where the ~r~scnce of carrier gas ions is unde.,ildblc is in an ion trap mass s~ ,Llul.l~,t~,. where the ion trap has a limited ion storage 10 capacily. In an ion beam dil~t~d at an ion trap, the carrier gas ions cull.~L~. with analyte ions for the limited storage capacity of the ion trap. Thus, to the extent that carrier gas ions can be sele~Li~ eliminated from the ion beam, the storage capacity for analyte ions~ in the ion trap is thereby IllCl~dSOd.
A third example where the pl~ ce of carrier gas ions is undesirable is any 15 application where the analyte ions are to be used in a process or l.,a_Lion where the carrier gas ions might interfere with such process. By way of further example, in many integratcd circuit and chip "~ -r;~ g plocesses, ion bcams may be direct~d tow~ds a targeted material such as a silicon wafer to impart elecl,ical or physical p.~pe.lies to the material. The desired prope.lics are typically highly20 ~ fk..L on thc specific ions di.~xt~ at such materials. Thus, carrier gas ions may cause ul~desilablc effects in the ~2ct~ materials.
Thus, in an ion beam having a carrier gas and an analyte, there exists a need for a method of selectively el;...;n ~ing carrier gas ions without eli...;n~;ng or neutralizing the analyte ions.
SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention in one of its aspects to provide amethod for producing an ion beam with i~ eased propollion of analyte ions and a 30 co~res~nding dc~ scd number of carrier gas ions by neutralizing carrier gas ~CT/US 9 ' / o o o 2 3 UEAI~ O 6 AUG 1997 ions while minim~lly removing or neutralizing the analyte ions. This is accomplished by providing the ion beam at a desired kinetic energy and dil~ling the ion beam through a volume of a reagent gas thereby allowing the carrier gas to selectively ~ rer charge to the reagent gas rendering the reagent gas a charged 5 species and the carrier gas a neutral species. As used herein, "selectively" means that the transfer of charge from the carrier gas ions to the reagent gas proceeds at a rate at least ten times, and preferably over one thousand times, the rate of the transfer of charge from the analyte gas ions to the reagent gas. After this charge lr~f~,l, the charged reagent gas is then sclecli~ly di~l~d, leaving an ion beam 10 having a greater fraction of analyte ions to total ions. As used hcrein, charge l-~L,r~,. refers to any pa~ a~ wh~.~m thc net effect is that charge is exc~
bet..xn a charged species and a neutral species. The pathway may involve step which are not charge L,ar~r~,l reactions. Steps within the pàlll~.a~ may include but are not limited to rh~olnir~l reaction(s), alone or in series, such as ,~so~t charge 15 ~.~r~,l(s), cl~n llal~Ç~l, proton ll~f,l, and Auger neutralization. As used herein, analyte ions refers to any ions gen~ldt~d by any means in-~lu-lir~ but not limited to thermal ionization, ion beams, el~l-~n impact ionization, laser ~- irradiation, ions~ y, cle.,lr~ o~l~, inductively coupled pl~"~c, microwave plas~ds, glow discharges, arc/spark dischalE,~s and hollow cathode 20 discharges. As used herein, reagent gas refers to any gas suitable for ar~t;.~g charge ll~Ç,l provided by any means i~ li~ but not limited to coll.l...,lcially available ~vb~ s provided in gaseous form and ~lules thereof and gases generated by evaporation of con~ nced ~ es or laser ablation of con~
substances. Further, reagent gas as used herein may include neutral species of 25 analyte ions gel~l~t_d by any of the folegohlg ~ sdc. Also, as will be appa,~to those skilled in the art, the method of the present invention is not limited to ,llls co.~ g a carrier gas per se. Typically, the two gas species are an analyte and a carrier gas. However, the method of the present invention will work equally well in any system having two or more ion speciPs, even if none of the 30 species were provided as a carrier gas. For example, in applications where ~E~ ~

~US ~ ~ 0 3 ~ 2 3 IPE~ 06 AU~1997 ghter ions gen~ tcd by tble dissociation of any charged species are undesirable,suitable reagents may be selected to remove or neutralize those ~ gb-- ions by charge ~ fer. Similarly, a particular analyte may contain a substance of interest in mixture with a separate inte.r~.ing subst~n~e. Suitable leage.l~ may be se!ected S to remove or neutralize the s~l~araLc hltc.~ g ~lb~l~,~e by charge ~ r~,r.
In a plcr~ d embodiment of the invention, the carrier gas selected is argon and the reagent gas selected is hydrogen. Accoldi~gly, it is an object of the invention in one of its aspects to provide a method for selectively ~educillg the charge density of an ion beam by neutralizing the ions of an argon carrier gas, 10 without çl;...in~;n~ or neutralizing the analyte ions. This is accolllJlished by dhe~,~ing the ion beam Lllro~gh a volume of hydrogen at kinetic ene.gies ..L.,.cm the argon ions selectively ll~r~r charge to the L~dro6en. In this manner, it is theorized that the bulk of the ion beam is sel~li~ely shifted from a mass to charge ratio (m/z) of 40 (Ar+) to m/z 3 (H3+) and m/z 2 (H2+). It is Il~ .,fol., a 15 further object of the invention in one of its aspects to provide a method allowing the sele~ c transfer of charge from Ar+ to H2. Due to l~dlo~n's lower molecular weight, in many applications it is possible to rapidly and selectively- eject H3+ and H2+ from an ion beam without eje.,~ing analyte ions where it would have been ~liffi~l-lt or hJI~ssible to sele~ ,ly eject Ar+ ions from the ion 20 beam without also eje~ g or removing analyte ions. Thus, it is ll-cl~r~, a further object of the invention in one of its aspects to provide a method for rapidly eje~ling H3+ and H2+ from an ion beam, yet minim~lly re(hlcing or ejcc~ g analyte ions.
Thus, it is a further object of the invention in one of its aspects to provide a25 method for providing a beam selectively d~pleted in Ar+, and thel~rol~ having a much lower total ion density, yet minim~lly reduced ion density of analyte.
The subject matter of the present il.~e.-tion is particularly pointed out and distinctly cl~im~d in the conrluding portion of this specffication. However, both the orga~ ion and method of operation, together with further advantages and 30 objects thereof, may best be u~del~lood by ~fc.e~e to the following de~li~)tion ~r ~

PN/tJS ~ / 0 ~ 0 2 3 !P~ G 1997 taken in connection with accompanying drawings wherein like reference charactersrefer to like elemPntc.

BRIEF DESCRIPIION OF THE DRAWINGS

FIG. 1 is a sch~nnArir drawing of the apparatus used in the first plcfcl~d embodiment of the present invention.
FIG. 2 contains two mass spectra from e~ elll~ p~ .rollllcd in the ap~ us used in the first p,~ f .,ed embodiment of the present i.l~enl,on.
10 FIG. 3 is a s~ ic drawing of the app~atlls used in the second pl.f~ d e.llbo~l;...- -n of the present invention.
FIG. 4 is a scl-f-..At;~ drawing of the _ppaldlus used in the third p-~fe.,~
embodiment of the present invention.
FIG. 5 contains two mass spectra from e~ ~ellts pe.rollllcd in the dppallllUS
15 used in the third 1"~ f~ .nho~ of the present invendon.
FIG. 6 co,l~ins two mass spectra from e~.~nls ~lro""ed in the ap~,~dlus used in the third pl. f~ .lcd e-nho~ n of the present invention.
~- FIG. 7 is a schematic dlawil)g of the apparatus used in the fourth p~fe"~d embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S) The method of lr~r. .ling charge from s~ec~xl ions in an ion beam having more than one species of ions to a reagent gas and thereafter pl._fe.e.lLially 25 di~ g the charged reagent gas was demol~tlat~d in inductively coupled plasma mass s~~ te.~ ..art~. called ICP/MS). An ICP/MS is a device wh~ a plasma co~ g of a carrier gas (typically argon) and an analyte is generated in an inductively coupled plasma (ICP) and a mass s~ u,,,~t, . is employed to Sepa~alt~ and di~ ;uish con~ atoms and isotopes. For both co~ ience of 30 operation and to m~int~in a desirable temperature in the plasma, the ICP is ~CT/IIS ~ ~ / O ~ ~ 2 3 IP~ 6 Al~G 1997 typically operated at atmospheric p~s~ur~. In order to transfer ions from the plasma to a mass s~llo~lRter, the plasma is directed through two a~.lul~,s and then through a lens stack. The plasma is thereby converted into an ion beam cont~ining analyte ions and carrier gas ions. A lens stack typically consists of a 5 series of metal pieces, typically plates and/or cylindrical tubes which have potentials applied to them and which have apclluies through which the ion beam is directed. The ion beam is dil~.,t~ through these charged plates which focus the ion beam into a narrow stream which is directed to a ion discrimin~ing unit, typically a linear quadrupole. As used herein, ion discrimin~ti~ unit refers to 10 any ~)pala~us which se~,a,dt~,s charged species accol~g to their m/z and/or kinetic energy. Ion discl.ll~ating units include but are not limited to a linear4ua~ole, an ion trap, a time-of-flight tube, a maenPti~ sector, an electric sector, a combination of a ma~tir sector and an electric sector, a lens stack, a DC
voltage plate, an rf/dc multipole ion guide, and an rf multipole ion guide.
15 Modified ICP/MS systems have been built which use a three ~ cionql RF
quadrupole ion trap, either alone or in combi~tion with a linear RF ~lua~pole asan ion dis~ ating unit. Upon exiting the lens stack, the ion beam is dil~;t~
~- into the ion di~fl-.~ e unit. Ions are scl~li~ly emitted from the ion discrimin~i~ unit according to their mass to charge ratio (m/z) and/or kinetic 20 energy. These selectively emitted ions are then dil~ct~d to a ch~g~i particle~etec~or. In this manner, the ICP/MS is able to ~le~ the pl~se~e of select~
ions in an analyte according to their (m/z) and/or kinetic energy. It is critical to mq-intqin the ion discli...i.~lin~ unit in a V~;uulll because collisions or reactions between the ions and any gases present in the ion discl;.~ g unit will tend to 25 deflect ions away from the charged particle detector or neutralize the ions of analyte. It is critical to mqintqin the chal~ particle d&t~lor in a ~CUU1l~
because the high potential across the detector will cause an el~l,icdl di~ch&~ in any gas present in sufficient pressure, typically above 104 Torr. One or more pumps are thus typically utilized to evacuate a series of ch~mbers in ~t~.~n the30 ICP and the charged particle d-,t~lor. The chambers are scp~dt~d by one or ~C~US ~ ~ ~ r,~ 3 lPE~ O 6 AllG 1997 more a~.lules to achieve the transition from atmospheric pr~s~ure at the ICP to high vacuum at the charged particle detector (typically between about 10-7 and 10-4 torr). To effect the large dirf~ lial in plCS~ul~" ICPIMS systems typically employ apc~ s between approximately 0.5 mm to approximately 2 mm.
In operation, the reagent gas is introduced within an ion beam having a carrier gas and an analyte to allow the charge of the carrier gas ions to be transferred to the reagent gas, wl~..,u~ll the now charged reagent gas may be -- selectively di~l~d from the ion beam. The extent of reaction or compl~t~--fc.c of this charge Ll~rcl will be driven by at least four factors. First, any two 10 species s~lecte ;1 will have an i~lel rate of reaction which will affect the compll t~ ~ss of charge ll~f~r over a ghen period of time, all other things heldconstant. Second, lower velocities of the carrier gas ions will provide a longerresidence time for carrier gas ions in the reaction zone and thereby provide a greater extent of reaction. Third, there is a velocity depen~len~ for the reaction cross section which is in general dirr.,.c;~ for any given l~eacling species so tha for any given reaction the OptilllU L~ veloc;l~l~ may be low or high. Thus, the compl~ t~"nfSS of charge l~f~-~ in a given time period is incl~d as the probability of a collision ~h.~n carrier gas ions and reagent gas species is ,ased. Thc~fole, the co~ ,lPt~ s of charge ~r~. is dcpf.~..~1 upon the 20 prcs;.~u~, of the reagent gas and the time that the two gases are in contact. If the reagent gas species is present at low concc.~ ,lion or pll,S;~ " the carrier gas ions must have s ~rl;~i- .1 oppollul~i~s to come into contact with the reagcnt gas, i.e., a long l~ r~ time must be employed.
As will be ap~ to those skilled in the art, although the present ill~ lion 25 has been desclil,cd as employed in an ICP/MS, the method of the present invention may be adv~nt~ol~cly applied in any system having a carrier gas and ananalyte gas where it is desired to remove or neutralize the carrier gas ions. The ICP/MS system, as well as the ~hulllents de~libcd in the p~f~ ,d embo~l;...P-~
which follow, both practice and are demo~llali~ of the present invention bccause ~AE~ S~

PCT/IIS ~ ' ~ O ~ 0 2 3 IPEA/~ O 6 AUG 1997 they contain detection methods to verify the selective neutralization or removal of carrier gas ions.

THE FIRST PREFERRED EMBODIMENT

In a first p~cfell~d embodiment shown in ~G. 1, a conventional ICP/MS
mqn-lfa~hlred by VG Flem~nt-q-l, now Fisons (Winsford, Cheshire, F.nglqn l; model PQ-I) was modified by l~lac~g the linear quadrupole and its associated eleckonics (not shown) with n RF 4ua.11u~Jole ion trap 10 and its qes~iqt~d 10 electronics (not shown). The ion trap 10 was inet~qllPd with the ion input and output ends reversed to maximize the ion L~f~.r ~rr~-~iC n~ from the lens stack 60 into the ion trap 10. The ion trap 10 used was removed from an ion trap mass S~;IIUI11C~I mqmlf~ctllred by Finnigan MAT (San Jo~, California). The CI~IIOn gun (not shown) and injection gate electrode as~mbly (not shown) were removed 15 to allow lla~Çe~ of ions from the lens stack 60 into the ion trap 10. The ~aCUUlll system was m~ifi~ from a standard Fisons vacuum system and consisted of three vacuum regions separated by two d~.lules. The~ vacuum regions are evacuated - by ~ti~ vacuum pumps (not shown). The first va UUIll region 15 is co.. ~i.~l in between a first a~ ule 20 and ~cond a~.lul~ 30 and is typically opc._~ed at 0.1 to 10 Torr. The second vacuum region 25 is cc~ en the second a~x.lule 30 and a third ape.l~ 40 and is typically opelat~ at 10-5 to 10-3 Torr.The third apellule 40 is located within the lens stack 60 at ~llb~ lly the same position as employed in the ~da.d Fisons ICP/MS. The third vacuum region 35 is ~p&l~t~ from the second vacuum region 25 by the third ape,lule 40. The third vacuum region 35 contains a portion of the lens stack 60, the ion trap 10 and a charged pallicle detector 50. The third vacuum region 35 is typically O~la~d at 10-8 to 10-3 Torr.

~E~

~ S ~ J ~J 2 P~A/~JO6 AU~1997 A series of e~ye~ le.ll~ was pclrolnlcd utili7ing the apparatus described in the first plefe..~ embodiment. The configuration of the various components is shown in ~IG. 1. The vacuu_ regions 15,25,3S were O~.dt~ under conventional conditions as desc~il~ above. The potentials applied to the lens stack 60 were within the ranges reco.. P~YIP~l by the m~nllf~ rer of the ICP/MS
(Fisons). The first and second ape.lulc~ 20,30 were both grounded. The third apellule 40 was biased at a DC potential of about -120 V. The potentials on the 10 lens stack plates 70,80 were opth.,i~ for ~..~ a~r~. errlcie~ of ions into the ion trap 10 and were di~.."lt than the potentials used in con~.,lional ICP/MS ic~ e.lt~. Ions are gated into the ion trap 10 by switching the po~tial on plate 80 in the lens stack 60. The pbt~,~lials on plate 80, de~lil~d as lens element L3 by the ,.U..~ra~ el (Fisons), were ~ hed ~h.~en a ~ati~e value 15 used to adrnit ions into the ion trap 10, in the range ~h.~n about -10 V to about -500 V, prefelably -35 V, and a positive value used to prevent ions from ~.ltl ~ ;i~g the ion trap 10, in the range bet~.~n about +10 V to about +500 V, pl~,f~.ably ~- above +10 V, or the kinetic energy of the ions. The ele.,llol~ie gating control (not shown) used for ~.i~hing the voltage on plate 80 was provided by h.~e.lhlg the 20 s~ddl~ signal provided by the ~h ~ gan MAT ITMS to gate elccllo~s. This inversion was accoll.plished using an extra hl~e.~r (not shown) on the printed circuit board (not shown) that pe.ru~ s the gating.
The ion trap 10 is rnanufactured with a port 90 typically used for introduction of a buffer gas such as helium. Reagent gases were introduced into 25 the ion trap 10 by adding the reagent gases to the helium. Typical helium buffer gas pres~ .,s were in the range ~h.~n about 10-5 and 10-3 Torr. Reagent to buffer gas pl~,s~.l.e ratios ranged ~t~ .. about 0.01% to 100~. EA~Ii-le~t~
were performed in this h~ cn~ wL~,.eil~ Ar, H2, Xe, or Kr were introduced as reagent gases into the ion trap 10.

~/US 9 ~ / ~ O 0 2 i3 IPEAll~S O 6 AUG 1997 The effect of these reagent gases on the analyte and ion signals were observed by recording the ion trap mass ~c~ l. Repl~,se.lLali~e mass spectra showing the effects of added H2 are shown in ~IG. 2. The upper trace 100 in FIG. 2 was obtained using pure helium buffer gas and is offset from ~ro for the sake of clarity in FIG. 2. The lower trace 110 in F~G. 2 was obtained using about 5% H2 and about 95% helium. The upper trace 100 shows the inle~ily of various peaks, most notably, H20+ at m/z 1~ 102, H30+ at m/z 19 104, Ar+ at m/z 40106, ArH+ at m/z 41 108. With the addition of H2 as a reagent gas, Ar+, H20+, ArH+, and H30+ are ~ ~-ir~lly reduced as indicated by the 10 reduction of peak i.~ n~;l ;es at the a~pl~liate mtz in the lower trace 110, in.1i~ the near or total elimination of these ch~,~ species.
In addition to the elimination of these charged species, one must also be co~llled with the effect of any added reagent gases on the analyte ions. The following CIP~ ; were tèsted as analyte ions for reaction with H2 in the lS appd~alus of the first plefe,lcd embodiment as desc~il~ above using argon as carrier gas: Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Ag, Cd, In, Xe, Cs, Ba, Tl, Pb, Bi, and U. In all of the eA~ c~t~, the red~cti-n in Ar+ il~te~ily was at least 100,000 times greater than the reduction in any of the ;.-r~-. l;Ps of those analyte ions.
THE SECOND PREFERRED EMBODIMENT

In a second ~lefe~l~d embodiment as shown in ~IG. 3, a con~_~tional ICPtMS m~nllfactllred by VG Fl~mPnt~l, now Fisons (Winsford, Cheshire, 25 F.l~l~rl-~ model PQ-I) was m~ifiP~ by i~.~osing an RF quadmpole ion trap 210 ~t~.~n the linear quadrupole 200 and the charged particle detector SO. ~ltho.lgh, the electrodes (not shown) used in the ion trap 210 were custom built to be scaled versions of the IWS electrodes ~ J I~ Jred by Finnigan MAT (San Jose, California), standard ion trap electrodes would work equally well. The electrodes 30 of the custom built ion trap 210 were 44% larger than the electrodes of the ~E~ SH~

~/Us97/Qo02 3 IPEAIUS O 6 AUG t997 Finnigan MAT ITMS and were assembled in a pure quadrupole, or un~ ,t~hed geometry. The standard ITMS elu;llonics package (not shown) mqnllfa~tllred by Finnigan MAT was used with the mo~ifi~vqtions as described in the first plefell~d embo~im~nt using the voltages as desclibcd below.
The ~nd~d lens stack 240 is operated at potentials reco~ .ended by the mqnllfa~ rer. In addition to the standard lens stack 240, a second lens stack 250 is interpo~d ~h.een the third apellw~ 220 and the ion trap 210 in the fourth vacuum region 230. The second lens stack 250 consisted of three plates 252,254,256 taken from ~l~d~-i Fisons lens stacks, specifically two L3 plates and an L4 plate. The second lens stack 250 was fabricated to provide high ion transport err~ .~n the linear quadrupole 200 and the ion trap 210. A
potential of ~h..xn about -10 V and about -300 V, pl~fe.lably about -30 V were applied to plates 252 ~r~ at each end of the second lens stack 250. The center plate 254 was used to gate ions into the ion trap 210 and the potc.llial applied was 15 varied ~eh.~n about -180 V for the open potential and about +180 volts for the closed potential. The ele~;L,on~c gating control (not shown) used for the centerplate 254 of the second lens stack 250 was provided by inverting the s~d '- signal provided by the Finnigan MAT ITMS to gate ele~;L.ol~s. This hl~ ion was acco,.lplislled using an extra h~ (not shown) on the printed circuit board (not 20 shown) that p-,.Ç~lll.s the gating.
The vacuum system was the ~ Fisons system co~is~ing of four vacuum regions separated by three ape.lul~s with an additional pump on the fourth vacuum region 230. These vacuum regions are evacuated by standard ~ ;UW~l pumps (not shown). The first vacuum region 15 is col~ ed in ~h.een a first apcl~ul~; 20 and second a~.lul~ 30 and is typically operated at 0.1 to 10 Torr.
The second Va~;UWll region 25 is contained ~h..,en the second a~.t~e 30 and a third apellul~, 40 and is typically operated at 10-5 to 10-3 Torr. The third aperture 40 is located within the lens stack 240. The third vacuum region 215 is contained ~h.~n the third a~.lule 40 and the fourth aperturc 220 and is 30 typically ope.dt~ at 10-8 to 10-4 Torr. The third vacuum region 215 contains the ~CT/IIS~ ~ / OO02 3 IP~AIU~ O 6 A.JG 1997 linear quadrupole 200. The fourth vacuum region 230 is separated from the third vacuum region 215 by the fourth apellu~ 220. The fourth vacuum region 230 contains the ion trap 210 and a charged particle ~le~;lor 50. The fourth vacuum region 230 is typically operated at 10-8 to 10-3 Torr.
S As illustrated in FIG. 3, a 1/16" ~i~m~ter metal tube 260 was provided toallow the introduction of reagent gases into the second vacuum region 25 throughtwo ports 280 provided in the housing 270 ~lvu~dillg the first vacuum region 15.~ The tube 260 was fashioned into a shape so as to avoid electrical contact with the lens stack 240 and to position the end of the tube 260 approximately 1 cm behindthe base of the second ape.nl.~, 30 and appro~ 1 cm from the central axis defined by the four dpC~ S 20,30,40,220. In this way, reagent gases are introduced into the second vacuum region 2S as close to the second a~.t~ 30 as possible without i,ltelre~g with the gas dynamics of the s~p'~~ plasma and with minimql distortion of the electric field ge~lat~d by the lens stack 240.

~--- A series of eA~ .,nls was pe.rO,."ed u~ ing various reagent gases and an argon carrier gas in the above ~ d apparatus shown in ~IG, 3. Reagent gases, H2, Ar, Xe, Kr and an Ar/Xe/Kr ~iAlul~, were i~ ~ced vh tube 260 into the second vacuum region 25. Mass spectra were obtained for reagent gas partial ple~;,~.,s in vacuum region 25 ~t~.een zero and about 1 mTorr to about 10 mTorr. Table I lists relative Mtes of reaction for the carrier gas and analyte ions shown in the first column with i~leashlg p~s~e of the reagent gases listed at the top of the remqining columns. Thus, by way of example, the values in the second column under the h~A~ H2" show that as the H2 pl_~e is i~.~, the Ar+ ion intensity falls about 10 times faster than the In+ ion intensity, co.~r~ g the selective removal of carrier gas ions.

~CT/ls~l/Qoo23 I?L~ G ~ Y9~' Table I. Relative Reaction Rates of Carrier Gas Ions and Analyte Ions with Reagent Gases Ions ¦ H2 Ar¦ Ar/Xe/Kr Ar+ 0.1 0.6 --ArH + non-linear 0.35 0.25 Sc+ 0.017 0.23 0. 18 ~4Kr+ 0.06 -- 0.26 , 115In+ 0.01 0.24 0.14 lZ~xe+ 0.01 -- 0.15 THE THIRD PREFERRED EMBODIMENT

In a third plcr~ d e.llboiil.l.,n~ shown in FIG. 4, a conventional ICP/MS
15 miqmlfs~ lred by VG F.lPmPntql, now Fisons (Winsford, Cheshire, F.r~g]qn l model PQ-II+) was mo~ifiP~I by pl~vidillg a 1/16" .li~.... t~,l metal tube 260 to allow the ud~l~;Lion of reagents into the second vacuum region 25 in a manner identi~ql tothe second ~l~,f.,~l~l embo~imPnt As shown in Fig. 4, the remqin~lP~r of the ICP/MS WAS not mo~ifi-P~ from that provided by the mqmlfactllrer. A series of 20 e~ ,~ was p~lÇol..led utili7ir~g an argon carrier gas and H2 as a reagent gasinL.ud~d via tube 260 into the seicond ~acuuJJl region 25. Mass spectra were obtained for H2 p~ ule in the second vacuum region 25 ~h.~n zcro and about 2 mTorr and are ~ .. q.iLed below.

The effect of H2 p~s~c on the analyte and ion signals were ob~.~d by recording the mass S~,LIUII1 in both the Analog and pulse cc~unt.lg modes of operation of the ICP/MS as provided by the m~mlf.~elllrer. Two mass spectrA
30 recorded will~oul addition of H2 intû the second ~A~;Uulll region 25 are shown in T/US 9 7 ~ 00 0 2 3 FIG. 5. The upper trace 500 in FIG. 5 was obtained using the analog mode of operation. The lower trace 510 in FIG. 5 was obtained using the pulse counting mode of operation. The upper trace 500 shows the il,t. n~i~ of various peaks, most notably, N+ at m/z 14 502, O+ at m/z 16 504, OH+ at m/z 17 506, H2O+ at m/z 18 508., Ar+ at m/z 40 512, ArH+ at m/z 41 514,H2+ at m/z 2 516, and H3+ at m/z 3 518. Two mass spectra recorded with addition of a pres~ule of about 2 mTorr H2 into the second vacuum region 25 are shown in FIG. 6. The upper trace 600 in ~IG. 6 was obtained using the analog mode of operation. The lower trace 610 in FIG. 6 was obtained using the pulse Cuul~
mode of opcldlion. The vertical and ho.;~ t.l scales of FIG. S and ~IG. 6 are the same. The same ion peaks arc labeled in ~IG. C as in FIG. 5, namely, N+ at m/z 14 602, O+ at m/z 16 604, OH+ at m/z 17 60C, H20+ at m/z 18 608., Ar+ at m/z 40 612, ArH+ at m/z 41 614,H2+ at m/z 2 616, and H3+ at m/z 3 618.
As the mass spectra in FIG. S and ~IG. 6 show, this method of il.lpl~ on allows the direct d~;o~- of H3+ produced in the r~c~ll of ,. ~ Ar+ with H2. The fo~ ;o-- of this ion is strongly i~f~ d from the e~
performed in the appdlalu~s of the first two embo~imPntc~ but H3+ could not be detec~d using the Finnigan MAT ion trap mass ~pe~,l,.,l.le~.~. Tn~ .h as this mPth~ ploduces a mass ~ in the same way as a convçntion~l ICP/MS
u~ , polyatomic ions which are commonly observed in conventional ICP/MS, but not by using the meth~ of the first and second plefc,,l~
embo~ , may also be obs~.~ed here. Thus, for example, the effect of elevated H2 p,~s~ s in Va~;UWll region 25 on Ar+ may be ob~.~d along with the effects on ArO+ and Ar2+.
The most d~ ic effect of added H2 is an approximately 200 fold iLu~lease in the h~.lsil~ of the H3 + peak 618. Addition of H2 also causes an approximately 10-fold decrease in the hlt~nsily of the Ar+ peak 612 and an approximately 2-fold increase in the intensity of the ArH+ peak 614. These mass spectra show minimql reduction (less than 10%) in the hltensily of the peaks for O O o 2 3 other analytes (not shown). These mass spectra thus show a selective removal of Ar+ and an il~lease in H3+ thereby confirming the mPch~nicm of charge in the reaction of H2 with Ar+.

A series of eA~line.lls was also pe.ro~ ed utili7ing the ICP/MS with no mo~lifir~ti-~ns other than adjusting the potenlials in the lens stack 240 to reduce the kinetic energy of the ions from typical values under normal OpC.aLiilg conditions.
10 H2 was introduced as a reagent gas into the second vacuum region 25 via the vacuum port 400 provided by the ~ f* ~.el for pie;.~ lea~.lll~.lt~. H2 pres~.lres ranged from about 0.1 mTorr to about 1 mTorr. The ~.leas~d Ar+
hllensily was reduced by a factor of two with the introduction of the H2 reagentgas, d~ O~llati~g that introduction of H2 into the second vacuum region 25 of an15 llnm~lifiPd ICPIMS can be used to reduce the Ar+ ion intensity. We further ob~e,~.,d an hlclease of about a factor of 10 in signal at m/z 41, indicating formation of ArH+ consistent with the eA~.h~ tal observations from the ' - appal.. lus of the first embo~iimpnt Table II cor~t~inc ~k~t~J data from the e~ rolled using the 20 apparatus of the first, second, and third plef,ll~d c..lbod;.-.~ deselibed herein.
Each row of the table gives reduction factors for Ar+ and an analyte ion as wellas the ratio of these reduction factors. The ratio is the sel~tivily with which the Ar+ i~t~.~ily in the mass ~ is reduced relative to the intensity of the analyte ion. The entries in the first column in Table II lists the p.~fel.ed 25 embo~ ..Pn~ used to obtain the data given in each row. The second column in Table II lists the reagent gas used. The reagent gas was i ll.~hlc~d into the ion trap 20 for the results shown in Table II for the first ~lef~ ll. d emboAimPnt above.
The reagent gas was introduced in vacuum region 25 for the results shown in Table II for the second and third embo~lim~ntc. Thus, by way of e~cample, the 30 third row in Table II shows that the reaction of the carrier gas ion (Ar+) leads to ~/IIS'3 ~ ~ Q~ 0 2 3 IPEAll~S 0 6 AUG 19~-a 30-fold reduction in Ar+ illt~,l~ily under conditions that reduce the int~sily of Sc+ by a factor of two.

Table II. Selectivity of Ar+ Removal Embodiment Reagent Reduction Factors Ar+ Analyte Ion Ratio Re~lloti~n Re~luction First H2100,000 (In+)<5% 1,000,000 Second Ar 300 (Sc+) 7 45 Second H2 30 (Sc+) 2 15 Third H2 10 (In+) <10% 100 THE FOURTH PREFERRED EMBODIMENT

~...
In a fourth pl~f~ ed embodiment as shown in FIG. 7, carrier gas ions and analyte ions ge~.dted from an ion source 700 are dil~t.,d through a first aperture 710 to a cell 720 where the ions are allowed to react with a reagent gas. S~ le 20 ion sources include, but are not limited to thermal ionization sources, electron impact, laser irradiation, ion spray, ele.;~ , thPrmospray, inductively coupled plasma sources, arc/spark disch~ges, glow disch~es, hollow cathode dischdrg,es and microwave plasma sources. While the fourth p-~f~ ,d embo~1imPnt as described herein is limited to what are considelcd its es~Pntiql CGlllpO~t~, it will 25 be app~ l to those skilled in the art that the fourth plefe,l~,d emb~;..~ could readily be constructed using conventional ICP/MS colllpon~l~ as des~,lil.ed in prior plefell~,d embodiments. The cell is coll~iL~ed within a first vacuum region 730. The cell 720 confines ions in a region close to the apellu e 710 through which the ions are introduced into the first vacuum region 730. In this manner, r ~ a7 ions are directed from tke ion source 700 to the cell 720 with minimllm opportunity for ion dispersion. The first vacuum region 730 is made to contain the optimal pressure of reagent gas which allows both ion transport through the cell 720 and sufficient charge transfer between the carrier gas ions and the reagent 5 gas.
The cell 720 also can be made to control the kinetic energy of the ions.
Thus, the cell 720 can be used to il~clease the resi~n~e time the carrier gas ions - are in contact with the reagent gas and thus to il~clcase the extent of charge ll~r~,l. Also, the cell 720 can be made to discl lui~dle ag~in~t i.e., not 10 transmit, slow ions by application of velocity or kinetic energy discl;,..i,u~ g methods, such as the application of suitable DC electric fields. In this manner,charge excll~n~e between fast carrier gas ions and slow reagent gas neutrals can be used to remove sf~ t~fd carrier gas ions from the ion beam. The kinetic energy of the ions in the cell 720 is ~ ;n~ as high as possible so as to ~ni--;~ , space 15 charge e~ n~Qn of the ions, but low enough for a given ples~ of reagent gas to allow ~urr~if-~- charge l,~r~,l. The optimal pl~ of the reagent gas will be limited by ~e~ept~ble analyte ion scattering losses in the cell and pl~;lical ~~ considerations such as l,~up~g l~uile u~llt~.
As an e~uple, the fourth pl~,f .-~,d embodiment may be operated using 20 argon as the carrier gas. The cell 720 may be provided as any app~atl,s suitable for courn~g the ions in the first vacuum region 730, including but not limited to, an ion trap, a long flight tube, a lens stack or an R~ multipole ion guide. For example, by SClf~ g the cell 720 as an RF multipole ion guide, the cell 720 may be operated to selectively di~l~ reagcnt gas ions from the ion bea~n. By 25 self~ling a reagent gas having a low mass, such as H2, the RF mllltipole ion guide may be op_.ated with a low mass cut-off greater than mlz 3. In this manner, H2+ and H3+, which are formed as charge lr~r~,l products, are sele~ ly di~ ed from the ion beam by virtue of their low m/z.
The reslllt~nt ion beam may then bc utilized as one of any uulubel of end 30 uses in~ ing but not limited to an ion gun or an ion implanter. Further, the resultant beam may be analyzed in various apparalus including but not limited toan optical ~c~ollleter, mass sp~ ull.eters (MS), including linear quadrupole MS, ion trap quadrupole MS, ion cyclotron l~,so~lce MS, time of flight MS, and magnetic and/or electric sector MS. Finally, the resultant ion beam may be 5 directed through any electrical or m~gn~tir ion focusing or ion dhe~,ling appalalus, including but not limited to, a lens stack, an RF multipole ion guide, an ele~,l,o~tic sector, or a magn~.tir. sector.
The resultant ion beam thus has an hlcleas~d luropollion of analyte ions comp~d to carrier gas ions. Thus, in any of the s~lggested uses ~L~.~,.n the 10 res~lt~nt ion beam is dhe~t~d through an dp~ e at the space charge current limit, the hh;lcased ~lol)ullion of analyte ions cOlllpal~d to carrier gas ions directed into the a~.~ul., will create an inc~ in the rate at which the analyte ions pass through the ap.,llule.
While a plefell~d embo~im~nt of the present invention has been shown and 1~ described, it will be app&c.ll to those skilled in the art that many changes and mc-lif1r ~ions _ay be made without del)a,ling from the invention in its broader aspects. The alp~n-lPd claims are Ih.,.efol~ t~-~ed to cover all such chq~ees and modifications as fall within the true spirit and scope of the in~e.llion.

~JIEt~ S~

Claims (14)

We claim:

1. An improved method of providing an ion beam in a system where a mixture of carrier gas ions and analyte ions is provided, wherein the improvement comprises:

a) exposing said mixture to a reagent gas, and b) selectively transferring charge from the carrier gas ions to the reagent gas, thereby neutralizing the carrier gas ions and forming a charged reagent gas.

2. The method of Claim 1 further comprising the step of selectively removing the charged reagent gas from the ion beam.

3. The method of Claim 2 further comprising the step of providing an ion discriminating unit for selectively removing the charged reagent gas from the ion beam.

4. The method of Claim 3 wherein the ion discriminating unit provided is selected from the group comprising a linear quadrupole, an ion trap, a time-of-flight tube, a magnetic sector, an electric sector, a combination of a magnetic sector and an electric sector, a lens stack, a DC voltage plate, a rf multipole ion guide, and a rf/dc multipole ion guide.

5. The method of Claim 1 wherein the carrier gas is selected from the group consisting of He, Ne, Ar, Kr, Xe and combinations thereof.

6. The method of Claim 1 wherein the reagent gas is selected from the group consisting of H2, D2, HD, N2, He, Ne, Ar, Kr, Xe and combinations thereof.

7. The method of Claim 1 wherein the analyte ions are provided by a method selected from the group consisting of thermal ionization, ion beams, electron impact ionization, laser irradiation, ionspray, electrospray, thermospray, inductively coupled plasmas, microwave plasmas, glow discharges, arc/spark discharges, hollow cathode discharges, gases generated by evaporation of condensed substances, laser ablation of condensed substances and mixtures thereof.

8. In an inductively coupled plasma mass spectrometer comprising an ion beam having a mixture of analyte gas ions and carrier gas ions, a method of increasing the ratio of the analyte gas ions to the carrier gas ions comprising the steps of:

a) exposing said mixture to a reagent gas, and b) selectively transferring charge from the carrier gas ions to the reagent gas, thereby neutralizing the carrier gas ions and forming a charged reagent gas.

9. The method of Claim 8 further comprising the step of selectively removing the charged reagent gas from the ion beam.

10. The method of Claim 9 further comprising the step of providing an ion discriminating unit for selectively removing the charged reagent gas from the ion beam.

11. The method of Claim 9 wherein the ion discriminating unit provided is selected from the group comprising a linear quadrupole, an ion trap, a time-of-flight tube, a magnetic sector, an electric sector, a 22a combination of a magnetic sector and an electric sector, a lens stack, a DC voltage plate, a rf multipole ion guide, and a rf/dc multipole ion guide.

12. The method of Claim 9 wherein the carrier gas is selected from the group consisting of He, Ne, Ar, Kr, Xe and combinations thereof.

13. The method of Claim 9 wherein the reagent gas is selected from the group consisting of H2, D2, HD, N2, He, Ne, Ar, Kr, Xe and combinations thereof.

14. In an inductively coupled plasma mass spectrometer having a mixture of analyte gas ions and argon carrier gas ions, a method of increasing the ratio of the analyte gas ions to the carrier gas ions comprising the steps of:

a) exposing said mixture to a reagent gas containing hydrogen in a cell, and b) selectively transferring charge from the carrier gas ions to the hydrogen, thereby neutralizing the carrier gas ions and transferring charge to the hydrogen.