Tutorial 9

small number ofelements are recog-nized as having poordetection limits by

inductively coupled plasmamass spectrometry (ICP-MS).These elements are predomi-nantly ones that suffer frommajor spectral interferencesgenerated by ions derived fromthe plasma gas, matrix compo-nents, or the solvent–acid usedto get the sample into solution.Examples of these interferencesinclude:• 40Ar16O on the determination

of 56Fe • 38ArH on the determination

of 39K • 40Ar on the determination of

40Ca • 40Ar40Ar on the determination

of 80Se • 40Ar35Cl on the determination

of 75As • 40Ar12C on the determination

of 52Cr • 35Cl16O on the determination

of 51V.

AA The cold/cool plasma ap-proach, which uses a lower tem-perature to reduce the forma-tion of the argon-basedinterferences, is a very effectiveway to get around some of theseproblems (1); however, it issometimes difficult to optimize,it is only suitable for a few of theinterferences, it is susceptible tomore severe matrix effects, andit can be time consuming tochange back and forth betweennormal- and cool-plasma con-ditions. These limitations andthe desire to improve perform-ance led to the development ofcollision/reaction cells in thelate 1990s. Designed originallyfor organic MS to generatedaughter species to confirmidentification of the structure ofthe parent molecule (2), theywere used in ICP-MS to stop theappearance of many argon-based spectral interferences.

Basic Principles of Collision/Reaction CellsWith this approach, ions enter

the interface in the normalmanner, where they are ex-tracted under vacuum into acollision/reaction cell that is po-sitioned before the analyzerquadrupole. A collision/reactiongas such as hydrogen or heliumis then bled into the cell, whichconsists of a multipole (aquadrupole, hexapole, or octa-pole), usually operated in theradio frequency (rf)-only mode.The rf-only field does not sepa-rate the masses like a traditionalquadrupole, but instead has theeffect of focusing the ions,which then collide and reactwith molecules of the collision/reaction gas. By a number ofdifferent ion-molecule collisionand reaction mechanisms, poly-atomic interfering ions like 40Ar,40Ar16O, and 38ArH, will either beconverted to harmless noninter-fering species, or the analyte willbe converted to another ionwhich is not interfered with.This is exemplified by the reac-tion [1], which shows the use ofhydrogen gas to reduce the38ArH polyatomic interference inthe determination of 39K. Hy-drogen gas converts 38ArH to theharmless H3

� ion and atomicargon, but does not react withthe potassium. The 39K analyteions, free of the interference,then emerge from thecollision/reaction cell, wherethey are directed toward thequadrupole analyzer for normalmass separation.

www.spectroscopyonl ine.com42 Spectroscopy 17(2) February 2002

A Beginner‘s Guide to ICP-MSPart IX — Mass Analyzers: Collision/Reaction Cell TechnologyRobert Thomas

T U T O R I A LT U T O R I A L

RobertThomashas more than30 years ofexperience intrace elementanalysis. He isthe principal ofhis ownfreelance writingand consultingcompany,ScientificSolutions, basedin Gaithersburg,MD. He can becontacted by e-mail [email protected] via his website at www.scientificsolutions1.com.

The detection capability of traditional quadrupole mass analyzers forsome critical elements is severely compromised by the formation ofpolyatomic spectral interferences generated by either argon, solvent, orsample-based ionic species. Although there are ways to minimize theseinterferences — including correction equations, cool plasma technology,and matrix separation — they cannot be completely eliminated. How-ever, a new approach called collision/reaction cell technology has re-cently been developed that virtually stops the formation of many ofthese harmful species before they enter the mass analyzer. Part IX ofthis series takes a detailed look at this innovative new technique andthe exciting potential it has to offer.

February 2002 17(2) Spectroscopy 43

Tutorial

38ArH� � H2 � H3� � Ar

39K� � H2 � 39K� � H2 (no reaction) [1]

The layout of a typical collision/reaction cell instrument is shown inFigure 1.

Different Collision/Reaction ApproachesThe previous example is a very simplis-tic explanation of how a collision/reaction cell works. In practice, com-plex secondary reactions and collisionstake place, which generate many unde-sirable interfering species. If thesespecies were not eliminated or rejected,they could potentially lead to additionalspectral interferences. Basically two ap-proaches are used to reject the productsof these unwanted interactions:• Discrimination by kinetic energy • Discrimination by mass.

The major differences between thetwo approaches are in the types of mul-tipoles used and their basic mechanismfor rejection of the interferences. Let’stake a closer look at how they differ.

Discrimination by Kinetic Energy The first commercial collision cells forICP-MS were based on hexapole tech-nology (3), which was originally de-signed for the study of organic mole-cules using tandem MS. The morecollision-induced daughter species thatwere generated, the better the chance ofidentifying the structure of the parentmolecule; however, this very desirablecharacteristic for liquid chromatogra-

phy or electrospray MS studies was adisadvantage in inorganic MS, wheresecondary reaction–product ions aresomething to be avoided. There wereways to minimize this problem, butthey were still limited by the type ofcollision gas that could be used. Unfor-tunately, highly reactive gases — such asammonia and methane, which are moreefficient at interference reduction —could not be used because of the limita-tions of a non-scan-ning hexapole (in rf-only mode) toadequately control thesecondary reactions.The fundamental rea-son is that hexapolesdo not provide ade-quate mass discrimi-nation capabilities tosuppress the un-wanted secondary re-actions, which necessi-tates the need forkinetic energy dis-crimination to distin-guish the collisionproduct ions from theanalyte ions. This istypically achieved bysetting the collisioncell bias slightly lesspositive than the massfilter bias. This meansthat the collision-product ions, whichhave the same energyas the cell bias, are dis-criminated against

and rejected, while the analyte ions,which have a higher energy than the cellbias, are transmitted.

The inability to adequately controlthe secondary reactions meant that lowreactivity gases like He, H2, and Xe werethe only option. The result was thation-molecule collisional fragmentation(and not reactions) was thought to bethe dominant mechanism of interfer-ence reduction. So even though the iontransmission characteristics of a hexa-pole were considered very good (withrespect to the range of energies andmasses transmitted), background levelswere still relatively high because the in-terference rejection process was notvery efficient. For this reason, its detec-tion capability — particularly for someof the more difficult elements, like Fe,K, and Ca — offered little improvementover the cool plasma approach. Table Ishows some typical detection limits inppb achievable with a hexapole-basedcollision cell ICP-MS system (4).

Recent modifications to the hexapoledesign have significantly improved its

Analyzingquadrupole

Multipolereaction–

collision cell

Ion lenssystem

ICP-MSinterface

ICP

Detector

Gas inlet

Figure 1. Layout of a typical collision/reaction cell instrument.

Table I. Typical detection limits (in ppb) achievable with a hexapole-based collision cell ICP-MS system (4).

Elemental DetectionSensitivity Limit

Element Isotope (cps/[�g/mL]) (ppb)Be 9 6.9 � 107 0.0077Mg 24 1.3 � 108 0.028Ca 40 2.8 � 108 0.07V 51 1.7 � 108 0.0009Cr 52 2.4 � 108 0.0007Mn 55 3.4 � 108 0.0017Fe 56 3.0 � 108 0.017Co 59 2.7 � 108 0.0007Ni 60 2.1 � 108 0.016Cu 63 1.9 � 108 0.003Zn 68 1.1 � 108 0.008Sr 88 4.9 � 108 0.0003Ag 107 3.5 � 108 0.0003Cd 114 2.4 � 108 0.0004Te 128 1.3 � 108 0.009Ba 138 5.9 � 108 0.0002Tl 205 4.0 � 108 0.0002Pb 208 3.7 � 108 0.0007Bi 209 3.4 � 108 0.0005U 238 2.3 � 108 0.0001


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collision/reaction characteristics. In ad-dition to offering good transmissioncharacteristics and kinetic energy dis-crimination, they now appear to offerbasic mass-dependent discriminationcapabilities. This means that the kineticenergy discrimination barrier can beadjusted with analytical mass, which of-fers the capability of using smallamounts of highly reactive gases. Figure2 shows an example of the reduction ofboth 40Ar12C and 37Cl16O using heliumwith a small amount of ammonia, inthe isotopic ratio determination of

52Cr/53Cr (52Cr is 83.789% and 53Cr is9.401% abundant). It can be seen thatthe 52Cr/53Cr ratio is virtually the samein the chloride and carbon matrices asit is with no matrix present when theoptimum flow of collision/reaction gasis used (5).

Another way to discriminate by ki-netic energy is to use an octapole in thecollision/reaction cell instead of a hexa-pole. The benefit of using a higherorder design is that its transmissioncharacteristics, particularly at the lowmass end, are slightly higher than lower

order multipoles. Similar in design tothe hexapole, collisional fragmentationand energy discrimination are the pre-dominant mechanisms for interferencereduction, which means that lower re-activity gases like hydrogen and heliumare preferred. By careful design of theinterface and the entrance to the cell,the collision/reaction capabilities can beimproved, by reducing the number ofsample/solvent/plasma-based ions en-tering the cell. This enables the collisiongas to be more effective at reducing theinterferences. An example of this is theuse of H2 as the cell gas to reduce theargon dimer (40Ar2

�) interference in thedetermination of the major isotope ofselenium at mass 80 (80Se). Figure 3shows an example of a dramatic reduc-tion in the 40Ar2

� background at mass 80using an ICP-MS fitted with an octa-pole reaction cell. By using the opti-mum flow of H2, the spectral back-ground is reduced by about six ordersof magnitude, from 10,000,000 cps to10 cps, producing a background equiva-lent concentration of approximately 1 ppt for 80Se (6).

Discrimination by MassThe other way to reject the products ofthe secondary reactions/collisions is todiscriminate them by mass. Unfortu-nately, higher order multipoles cannotbe used for efficient mass discrimina-tion because the stability boundariesare diffuse, and sequential secondary

14

12

10

8

6

4

20 1 2 3 4 5 6

Collision gas flow rate (mL/min)

52Cr

/53Cr

No matrix0.2% Cl matrix0.06% C matrix

Figure 2. The use of a helium/ammonia mixture with a hexapole-basedcollision cell for the successful determination of 52Cr/53Cr isotopic ratios(courtesy of Thermo Elemental, Franklin, MA).

0 0.7 1.1 1.5 1.9 2.3 2.7 3.1 3.5 3.9 4.3 4.7 5.0

Hydrogen flow (mL/min)

Sign

al a

t mas

s 80

(cps

)

BEC

(ppb

)

107

106

105

104

103

102

101

100

104

103

102

101

100

10�1

10�2

10�3

1ppb Se (cps)

Blank (cps)

BEC (ppb)

Figure 3. Background reduction of the argon dimer (40Ar2�) with

hydrogen gas using an octapole reaction cell (Courtesy of AgilentTechnologies, Wilmington, DE).

Quadrupolemass filter

Analyzerquadrupole

Ion–moleculereactions and collisions

Gas inlet

Reactive gas (NH3)

++ ++

++

++++

++ ++++

++++++++++

++ ++

++

++++

++

40Ar16O

ArO� � NH3 O � Ar � NH3�

56Fe

56Fe

NH3�

O

Ar

Figure 4. Elimination of the ArO interference with a dynamic reaction cell.


pole electrical fields, unwanted reac-tions between the gas and the samplematrix or solvent (which could poten-tially lead to new interferences) are pre-vented. Therefore, every time an analyteand interfering ions enter the dynamicreaction cell, the bandpass of thequadrupole can be optimized for thatspecific problem and then changed on-the-fly for the next one. Figure 4 showsa schematic of an analyte ion 56Fe andan isobaric interference 40Ar16O enteringthe dynamic reaction cell. The reactiongas NH3 reacts with the ArO� to formatomic oxygen and argon together witha positive NH3 ion. The quadrupole’selectrical field is then set to allow thetransmission of the analyte ion 56Fe tothe analyzer quadrupole, free of theproblematic isobaric interference,40Ar16O. In addition, the NH3

� is pre-

vented from reacting further to producea new interfering ion. The advantage ofthis approach is that highly reactivegases can be used, which increases thenumber of ion–molecule reactions tak-ing place and therefore more efficientremoval of the interfering species. Ofcourse, this also potentially generatesmore side reactions between the gasand the sample matrix and solvent;however, by dynamically scanning thebandpass of the quadrupole in the reac-tion cell, these reaction by-products arerejected before they can react to formnew interfering ions.

The benefit of the dynamic reactioncell is that by careful selection of the re-action gas, the user takes advantage ofthe different rates of reaction of the an-alyte and the interfering species. Thisprocess is exemplified by the elimina-tion of 40Ar� by NH3 gas in the determi-nation of 40Ca. The reaction between

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reactions cannot be easily intercepted.The way around this problem is to use aquadrupole (instead of a hexapole oroctapole) inside the reaction/collisioncell, and use it as a selective bandpassfilter. The benefit of this approach isthat highly reactive gases can be used,which tend to be more efficient at inter-ference reduction. One such develop-ment that uses this approach is calleddynamic reaction cell technology (7, 8).Similar in appearance to the hexapoleand octapole collision/reaction cells, the

dynamic reaction cell is a pressurizedmultipole positioned before the ana-lyzer quadrupole. However, the similar-ity ends there. In dynamic reaction celltechnology, a quadrupole is used in-stead of a hexapole or octapole. Ahighly reactive gas, such as ammonia ormethane, is bled into the cell, which is acatalyst for ion molecule chemistry totake place. By a number of different re-action mechanisms, the gaseous mole-cules react with the interfering ions toconvert them either into an innocuous

species different from theanalyte mass or a harmlessneutral species. The analytemass then emerges from thedynamic reaction cell free ofits interference and steeredinto the analyzer quadru-pole for conventional massseparation. The advantagesof using a quadrupole inthe reaction cell is that thestability regions are muchbetter defined than a hexa-pole or an octapole, so it isrelatively straightforward tooperate the quadrupole in-side the reaction cell as amass or bandpass filter, andnot just an ion-focusingguide. Therefore, by carefuloptimization of the quadru-

��

�

�

�

�

Ca�

Ar�

Ar� � NH3

(Exothermic reaction)

NH3� � Ar

Ca� � NH3

(Endothermic reaction)

no reaction

NH3

Figure 5. The reaction between NH3 and Ar� is exothermic and fast, while there is no reactionbetween NH3 and Ca� in the dynamic reaction cell.

Table II. Typical detection limits in ppt of an ICP-MS system fitted with a dynamicreaction cell (9).

Detection DetectionAnalyte Limit (ppt) Analyte Limit (ppt)

Li 0.08 Co 0.07Be 0.6 60Ni 0.4B 1.1 Zn 1Na 0.3 As 1.2Mg 0.6 Se* 5Al 0.07 Sr 0.02K* 1 Rh 0.0140Ca* 1 In 0.01V* 0.3 Sb 0.06Cr* 0.25 Cs 0.03Mn* 0.09 Pb 0.0356Fe* 0.15 U 0.01

* Indicates elements determined in dynamic reaction cell mode.

Collision/reaction cells

have given a new lease on

life to quadrupole mass

analyzers used in ICP-MS.


Tutorial

NH3 gas and the 40Ar� interference,which is predominantly a charge ex-change, occurs because the ionizationpotential of NH3 (10.2 eV) is low com-pared with that of Ar (15.8 eV). There-fore, the reaction is exothermic and fast;

however, the ionizationpotential of Ca (6.1 eV)is significantly less thanthat of NH3, so the reac-tion, which is endother-mic, is not allowed toproceed (8). Figure 5shows this process ingreater detail.

This highly efficientreaction mechanismtranslates into a dra-matic reduction of thespectral background atmass 40, which isshown graphically inFigure 6. At the opti-mum NH3 flow, a re-duction in the 40Ar

background signal of about eight ordersof magnitude is achieved, resulting in adetection limit of 0.5–1.0 ppt for 40Ca.

Table II shows some typical detectionlimits in parts per trillion (ppt) of anICP-MS system fitted with a dynamic

reaction cell. The elements with an as-terisk were determined using ammoniaor methane as the reaction gas, whilethe other elements were determined inthe standard mode (no reaction gas).

Collision/reaction cells have given anew lease on life to quadrupole massanalyzers used in ICP-MS. They haveenhanced its performance and flexibil-ity, and most definitely opened up thetechnique to more-demanding applica-tions that were previously beyond itscapabilities. However, it must be em-phasized that even though differencesexist between commercially availableinstruments, they all perform very well.The intent of this tutorial is to presentthe benefits of the technology to begin-ners and give an overview of the differ-ent approaches available. If it has cre-ated an interest, I strongly suggest that aperformance evaluation is made basedon your own sample matrices.

References1. K. Sakata and K. Kawabata, Spec-

trochimica Acta 49B, 1027 (1994).2. B.A. Thomson, D.J. Douglas, J.J. Corr,

J.W. Hager, and C.A. Joliffe, Anal. Chem.67, 1696–1704 (1995).

3. P. Turner, T. Merren, J. Speakman, andC. Haines, in Plasma Source MassSpectrometry: Developments and Ap-plications, eds. G. Holland and S. Tan-ner (Royal Society of Chemistry, Cam-bridge, UK, 1996).

4. I. Feldmann, N. Jakubowski, C. Thomas,and D. Stuewer, Fresenius‘ J. Anal.Chem. 365, 422–428 (1999).

5. ”Collision Cell Technology with EnergyDiscrimination,“ Thermo Elemental Ap-plication Note (Thermo Elemental,Franklin, MA, September 2001).

6. E. McCurdy and D. Potter, Agilent Tech-nologies ICP-MS Journal 10, October2001.

7. Covered by U.S. Patent No. 6140638.8. S.D. Tanner and V.I. Baranov, At. Spectr.

20(2), 45–52, (1999).9. U. Voellkopf, K. Klemm, and M. Pfluger,

At. Spectr. 20(2), 53–59, (1999). �

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

NH3 flow (mL/min)

Ion

sign

al (c

ps)

109

108

107

106

105

104

103

102

101

100 ppt Ca

Water blank

Figure 6. A reduction of eight orders of magnitude in the 40Arbackground signal is achievable with the dynamic reaction cell —resulting in �1 ppt detection limit for 40Ca.

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Tutorial 9

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