tutorial 7

22 SPECTROSCOPY 16(11) NOVEMBER 2001 www.spec t roscopyon l i ne . com

Although quadrupole mass analyzers repre-sent more than 90% of all inductively cou-pled plasma mass spectrometry (ICP-MS)systems installed worldwide, limitations intheir resolving power has led to the develop-ment of high-resolution spectrometers basedon the double-focusing magnetic-sector de-sign. Part VII of this series on ICP-MStakes a detailed look at this very powerfulmass separation device, which has foundits niche in solving challenging applicationproblems that require excellent detectioncapability, exceptional resolving power, orvery high precision.

As discussed in Part VI of this series(1), a quadrupole-based ICP-MSsystem typically offers a resolutionof 0.7–1.0 amu. This is quite ade-quate for most routine applications,

but has proved to be inadequate for manyelements that are prone to argon-,solvent-, or sample-based spectral inter-ferences. These limitations inquadrupoles drove researchers in the di-rection of traditional high-resolution,magnetic-sector technology to improvequantitation by resolving the analytemass away from the spectral interference(2). These ICP-MS instruments, whichwere first commercialized in the late1980s, offered resolving power as high as10,000, compared with a quadrupole,which had a resolving power of approxi-mately 300. This dramatic improvementin resolving power allowed difficult ele-ments like Fe, K, As, V, and Cr to be de-termined with relative ease, even in com-plex sample matrices.

TRADITIONAL MAGNETIC-SECTORINSTRUMENTSThe magnetic-sector design was first usedin molecular spectroscopy for the struc-tural analysis of complex organic com-pounds. Unfortunately, it was initially

found to be unsuitable as a separation de-vice for an ICP system because it requireda few thousand volts of potential at theplasma interface area to accelerate theions into the mass analyzer. For this rea-son, basic changes had to be made to theion acceleration mechanism to optimize itas an ICP-MS separation device. This wasa significant challenge when magnetic-sector systems were first developed in thelate 1980s. However, by the early 1990s,instrument designers solved this problemby moving the high-voltage componentsaway from the plasma and interface andcloser to the mass spectrometer. Today’sinstrumentation is based on two differentapproaches, commonly referred to asstandard or reverse Nier-Johnson geome-try. Both these designs, which use thesame basic principles, consist of two ana-

lyzers — a traditional electromagnet andan electrostatic analyzer (ESA). In thestandard (sometimes called forward) de-sign, the ESA is positioned before themagnet, and in the reverse design it is po-sitioned after the magnet. A schematic ofa reverse Nier-Johnson spectrometer isshown in Figure 1.

PRINCIPLES OF OPERATION With this approach, ions are sampledfrom the plasma in a conventional man-ner and then accelerated in the ion opticregion to a few kilovolts before they enterthe mass analyzer. The magnetic field,which is dispersive with respect to ion en-ergy and mass, focuses all the ions withdiverging angles of motion from the en-trance slit. The ESA, which is only disper-sive with respect to ion energy, then fo-

TUTORIALTUTORIALS P E C T R O S C O P Y

A Beginner’s Guide to ICP-MSPart VII: Mass Separation Devices — Double-Focusing Magnetic-Sector Technology

R O B E R T T H O M A S

Electrostaticanalyzer

Electromagnet

Detector

Plasma torch

Entranceslit

Exit slit

Accelerationoptics

Focusingoptics

Figure 1. Schematic of a reverse Nier-Johnson double-focusing magnetic-sector mass spectrometer (Courtesy of Thermo Finnigan [San Jose, CA]).

cuses the ions onto the exit slit, wherethe detector is positioned. If the energydispersion of the magnet and ESA areequal in magnitude but opposite in direc-tion, they will focus both ion angles (firstfocusing) and ion energies (second ordouble focusing), when combined to-gether. Changing the electrical field inthe opposite direction during the cycletime of the magnet (in terms of the masspassing the exit slit) has the effect offreezing the mass for detection. Then assoon as a certain magnetic field strength

is passed, the electric field is set to itsoriginal value and the next mass isfrozen. The voltage is varied on a per-mass basis, allowing the operator to scanonly the mass peaks of interest ratherthan the full mass range (3, 4).

Because traditional magnetic-sectortechnology was initially developed for thestructural or qualitative identification oforganic compounds, there wasn’t a realnecessity for rapid quantitation of spectralpeaks required for trace element analysis.They functioned by scanning over a largemass range by varying the magnetic fieldover time with a fixed acceleration volt-age. During a small window in time,which was dependent on the resolutionchosen, ions of a particular mass-to-charge are swept past the exit slit to pro-duce the characteristic flat top peaks. Asthe resolution of a magnetic-sector instru-ment is independent of mass, ion signals,particularly at low mass, are far apart.The result was that a large amount oftime was spent scanning and settling the

magnet. This was not such a major prob-lem for qualitative analysis, but proved tobe impractical for routine trace elementanalysis. This concept is shown in greaterdetail in Figure 2, which is a plot of fourparameters — magnetic field strength,accelerating voltage, mass, and signal in-tensity — against time for four separatemasses (M1–M4). Scanning the magnetfrom point A to point B (accelerating volt-age is fixed) results in a scan across themass range, generating spectral peaks forthe four different masses. It can be seenthat this increased scanning and settlingoverhead time (often referred to as deadtime) would result in valuable measure-ment time being lost, particularly for highsample throughput that required ultra-trace detection levels.

Changing the electric field in the oppo-site direction to the field strength of themagnet during the cycle time of the mag-net has the effect of “stopping” the massthat passes through the analyzer. Then,as soon as the magnetic field strength ispassed, the electrical field is set to itsoriginal value and the next mass“stopped” in the same manner. The accel-erating voltage, as well as its rate ofchange, has to be varied depending onthe mass, but the benefit of this methodis that only the mass peaks of interest areregistered. This process is seen in Figure3, which shows the same four massesscanned. The only difference this time isthat as well as scanning the magnet frompoint A to point B, the accelerating volt-age is also changed, resulting in a step-wise jump from one mass to the next.This means that the full mass range iscovered much faster than just by scan-ning the magnet alone (because of the in-creased speed involved in electricallyjumping from one mass to another) (5).Once the magnet has been scanned to aparticular point, an electric scan is usedto cover an area of � 10–30% of the mass,either to measure the analyte peak ormonitor other masses of interest. Peakquantitation is typically performed by tak-ing multiple data points over a presetmass window and integrating over a fixedperiod of time.

It should be pointed out that althoughthis approach represents enormous timesavings over older, single-focusingmagnetic-sector technology, it is still sig-nificantly slower than quadrupole-basedinstruments. The inherent problem lies inthe fact that a quadrupole can be elec-tronically scanned much faster than a

S P E C T R O S C O P Y T U T O R I A L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A

Measurement timeScanning and

settling (dead) time

B

M1

M4

M2 M3

Inte

nsity

Time

Mas

sAc

cele

ratio

n po

tent

ial

Mag

netic

fiel

d

Figure 2. A plot of magnetic field strength,accelerating voltage (fixed), mass, and sig-nal intensity over time for four separatemasses (M1–M4). Note that only the mag-net is scanned, while the accelerating volt-age is fixed — resulting in long scan timesbetween the masses.

Changing the electric

field in the opposite

direction to the field

strength of the magnet

during the cycle time of

the magnet has the

effect of “stopping” the

mass that passes

through the analyzer.

A

Measurement time

Jump time

B

M1M4

M2M3

Inte

nsity

Time

Mas

sAc

cele

ratio

n po

tent

ial

Mag

netic

fiel

d

Figure 3. A plot of magnetic field strength,accelerating voltage (changed), mass, andsignal intensity over time for the same fourmasses (M1–M4). This time, in addition tothe magnet being scanned, the acceleratingvoltage is also changed, resulting in rapidelectric jumps between the masses.

NOVEMBER 2001 16(11) SPECTROSCOPY 23

magnet. Typical speeds for a full massscan (0–250 amu) of a magnet are in theorder of 400–500 ms, compared with 100ms for a quadrupole. In addition, it takesmuch longer for magnets to slow downand settle to take measurements — typi-cally 30–50 ms compared to 1–2 ms for aquadrupole. So, even though in practice,the electric scan dramatically reduces theoverall analysis time, modern double-focusing magnetic-sector ICP-MS sys-tems, especially when multiple resolutionsettings are used, are significantly slowerthan quadrupole instruments. Thismakes them less than ideal for routine,high-throughput applications or for sam-ples that require multielement determina-tions on rapid transient signals.

RESOLVING POWERAs mentioned previously, most commer-

cial magnetic-sector ICP-MS systems of-fer as much as 10,000 resolving power(5% peak height/10% valley definition),which is high enough to resolve the ma-jority of spectral interferences. It’s worthemphasizing that resolving power (R) isrepresented by the equation: R � m/�m,where m is the nominal mass at whichthe peak occurs and �m is the mass dif-ference between two resolved peaks (6).In a quadrupole, the resolution is se-lected by changing the ratio of the rf/dcvoltages on the quadrupole rods. How-ever, because a double-focusingmagnetic-sector instrument involves fo-cusing ion angles and ion energies, massresolution is achieved by using two me-chanical slits — one at the entrance tothe mass spectrometer and another at theexit, before the detector. Varying resolu-tion is achieved by scanning the magnetic

field under differ-ent entrance- andexit-slit width con-ditions. Similar tooptical systems,low resolution isachieved by usingwide slits,whereas high res-olution is achievedwith narrow slits.Varying the widthof both the en-trance and exitslits effectivelychanges the oper-ating resolution.This can be seen

in Figure 4, which shows two slit widthscenarios. Figure 4a shows an example ofa wide exit slit producing relatively lowresolution and a characteristic flat-toppedpeak. Figure 4b shows the same size en-trance slit, but a narrower exit slit, pro-ducing higher resolution with a charac-teristic triangular peak. The lowestpractical resolution achievable with a dou-ble-focusing magnetic-sector instrument,using the widest entrance and exit slits, isapproximately 300–400, whereas thehighest practical resolution, using thenarrowest entrance and exit slits, is ap-proximately 10,000. Most commercialsystems operate at fixed resolution set-tings — for example, low is typically300–400; medium is typically 3000–4000,and high is typically 8000–10,000 (thechoice of settings will vary depending onthe instrumentation).

However, it should be emphasized that,similar to optical spectrometry, as theresolution is increased, the transmissiondecreases. So even though extremelyhigh resolution is available, detection lim-its will be compromised under these con-ditions. This can be seen in Figure 5,which shows a plot of resolution againstion transmission. Figure 5 shows that aresolving power of 400 produces 100%transmission, but at a resolving power of10,000, only �2% is achievable. This dra-matic loss in sensitivity could be an issueif low detection limits are required inspectrally complex samples that requirethe highest possible resolution; however,spectral demands of this nature are notvery common. Table I shows the resolu-


Ion beam

Entrance slit Analyzer Exit slit

Wide slit Lower resolution

Higher resolution

Detector

Inte

nsity

(a)

(b)

Ion beam

Entrance slit Analyzer Exit slit

Narrow slit

Detector

Inte

nsity

Figure 4. Resolution obtained using (a) wide and (b) narrow exit slit widths as the magnetic field is scanned. The entrance slit widths are thesame in (a) and (b).

Table I. Resolution required to resolve some common polyatomic interferences from a selected group of isotopes.

Isotope Matrix Interference Resolution Transmission

39K H2O 38ArH 5570 6%40Ca H2O 40Ar 199,800 0%44Ca HNO3

14N14N 16O 970 80%56Fe H2O 40Ar16O 2504 18%31P H2O 15N16O 1460 53%34S H2O 16O18O 1300 65%

75As HCl 40Ar35Cl 7725 2%51V HCl 35Cl16O 2572 18%

64Zn H2SO432S16O16O 1950 42%

24Mg Organics 12C12C 1600 50%52Cr Organics 40Ar12C 2370 20%

55Mn HNO340Ar15N 2300 20%

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26 SPECTROSCOPY 16(11) NOVEMBER 2001

tion required to resolve fairly commonpolyatomic interferences from a selectedgroup of elemental isotopes, togetherwith the achievable ion transmission.

Figure 6 is a comparison between aquadrupole instrument and a magnetic-sector instrument with one of the mostcommon polyatomic interferences —40Ar16O on 56Fe, which requires a resolu-tion of 2504 to separate the peaks. Figure6a shows a spectral scan of 56Fe using aquadrupole instrument. What it doesn’tshow is the massive polyatomic interfer-ence 40Ar16O (produced by oxygen ionsfrom the water combining with argonions from the plasma) completely over-lapping the 56Fe. It shows very clearlythat these two masses are unresolvablewith a quadrupole. If that same spectralscan is performed on a magnetic-sectorinstrument, the result is the scan shownin Figure 6b. To see the spectral scan onthe same scale, it was necessary to exam-ine a much smaller range. For this rea-son, a 0.100-amu window was taken, as in-dicated by the dotted lines.

OTHER BENEFITSBesides high resolving power, another at-tractive feature of magnetic-sector instru-ments is their very high sensitivity com-bined with extremely low backgroundlevels. High ion transmission in low-resolution mode translates into sensitivityspecifications of typically 100–200 millioncounts per second (mcps) per ppm, whilebackground levels resulting from ex-tremely low dark current noise are typi-cally 0.1–0.2 cps. This compares with sen-sitivity of 10–50 mcps and backgroundlevels of �10 cps for a quadrupole instru-ment. For this reason detection limits, es-

pecially for high-mass elements like ura-nium where high resolution is generallynot required, are typically an order ofmagnitude better than those provided bya quadrupole-based instrument.

Besides good detection capability, an-other of the recognized benefits of themagnetic-sector approach is its ability toquantitate with excellent precision. Mea-surement of the characteristically flat-topped spectral peaks translates directlyinto high-precision data. As a result, inthe low-resolution mode, relative stan-dard deviation (RSD) values of0.01–0.05% are fairly common, whichmakes magnetic-sector instruments anideal tool for carrying out high-precision

isotope ratio work (7). Although preci-sion is usually degraded as resolution isincreased (because the peak shape getsworse), modern instrumentation withhigh-speed electronics and low mass biasis still capable of precision values of�0.1% RSD in medium- or high-resolutionmode (8).

The demand for ultrahigh-precisiondata, particularly in the field of geochem-istry, has led to the development of in-struments dedicated to isotope ratioanalysis. These are based on the double-focusing magnetic-sector design, but in-stead of using just one detector, these in-struments use multiple detectors. Oftenreferred to as multicollector systems,


100

80

60

40

20

0400 2000 4000 6000 8000 10,000

Resolution

Tran

smis

sion

(%)

Figure 5. Ion transmission with a magnetic-sector instrument decreases as the resolu-tion increases.

55 56 57

0.100 amu scan

56Fe 40Ar 16O

56Fe

(a)

m/z

Inte

nsity

55.935 55.957

40Ar 16O

(b)

m/z

Inte

nsity

Figure 6. Comparison of resolution between (a) a quadrupole and (b) a magnetic-sector in-strument for the polyatomic interference of 40Ar16O on 56Fe.

www.spec t roscopyon l i ne . com

NOVEMBER 2001 16(11) SPECTROSCOPY 27

they offer the capability of detecting andmeasuring multiple ion signals at exactlythe same time. As a result of this simulta-neous measurement approach, they arerecognized as producing the ultimate inisotope ratio precision (9).

There is no question that double-focusing magnetic-sector ICP-MS sys-tems are no longer a novel analyticaltechnique. They have proved themselvesto be a valuable addition to the trace ele-ment toolkit, particularly for challengingapplications that require good detectioncapability, exceptional resolving power,and very high precision. They do havetheir limitations, however, and perhapsshould not be considered a competitor forquadrupole instruments when it comes torapid, high-sample-throughput applica-tions or when performing multielementdeterminations on fast transient peaks,using sampling accessories such as elec-trothermal vaporization (10) or laserablation (11).

REFERENCES(1) R. Thomas, Spectroscopy 16(10), 44–48

(2001).(2) N. Bradshaw, E.F.H. Hall, and N.E.

Sanderson, J. Anal. At. Spectrom. 4,801–803 (1989).

(3) R. Hutton, A. Walsh, D. Milton, and J.Cantle, ChemSA 17, 213–215 (1991).

(4) U. Geismann and U. Greb, Fresnius’ J.Anal. Chem. 350, 186–193 (1994).

(5) U. Geismann and U. Greb, Poster Presen-tation at Second Regensburg Symposiumon Elemental Mass Spectrometry (1993).

(6) F. Adams, R. Gijbels, and R. Van Grieken,Inorganic Mass Spectrometry (John Wileyand Sons, New York, 1988).

(7) F. Vanhaecke, L. Moens, R. Dams, and R.Taylor, Anal. Chem. 68, 567 (1996).

(8) M. Hamester, D. Wiederin, J. Willis, W.Keri, and C.B. Douthitt, Fresnius’ J. Anal.Chem. 364, 495–497 (1999).

(9) J. Walder and P. A. Freeman, J. Anal. At.Spectrom. 7, 571 (1992).

(10) S. Beres, E. Denoyer, R. Thomas, and P.Bruckner, Spectroscopy 9(1), 20–26(1994).

(11) S. Shuttleworth and D. Kremser, J. Anal.At. Spectrom. 13, 697–699 (1998).

Robert Thomas has more than 30 years ofexperience in trace element analysis. He isthe principal of his own freelance writingand scientific consulting company, ScientificSolutions, based in Gaithersburg, MD. Hecan be contacted by e-mail at [email protected] or via his web site atwww.scientificsolutions1.com.�

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