more secondary electrons form.The potential gradient insidethe tube varies based on posi-tion, so the secondary electronsmove farther down the tube. Asthese electrons strike new areasof the coating, more secondaryelectrons are emitted. Thisprocess is repeated many times.The result is a discrete pulse thatcontains many millions of elec-trons generated from an ionthat first hits the cone of the de-tector (1). This process is shownsimplistically in Figure 1.

This pulse isthen sensedand detectedby a very fastpreamplifier.The outputpulse from thepreamplifierthen goes to adigital dis-criminator andcounting cir-cuitry, whichcounts onlypulses above a

certain threshold value. Thisthreshold level needs to be highenough to discriminate againstpulses caused by spurious emis-sion inside the tube, stray pho-tons from the plasma itself, orphotons generated from fastmoving ions striking thequadrupole rods.

Sometimes the rate of ionshitting the detector is too highfor the measurement circuitryto handle in an efficient man-

emission spectroscopy (ICP-OES); however, instead of usingindividual dynodes to convertphotons to electrons, the chan-neltron is an open glass cone —

coated with a semiconductor-type material — that generateselectrons from ions impingingon its surface. For the detectionof positive ions, the front of thecone is biased at a negative po-tential and the far end, nearestthe collector, is kept at ground.When the ion emerges from thequadrupole mass analyzer, it isattracted to the high negativepotential of the cone. When theion hits this surface, one or

For some applications where

ultratrace detection limits are

not required, the ion beam from

the mass analyzer is directed

into a simple metal electrode,

or Faraday cup.

www.spectroscopyonl ine.com34 Spectroscopy 17(4) April 2002

ince inductively coupledplasma–mass spectrom-etry (ICP-MS) was firstintroduced in the early

1980s, a number of different iondetection designs have beenused, the most popular beingelectron multipliers for low ion-count rates, and Faraday collec-tors for high-count rates. Today,the majority of ICP-MS systemsused for ultratrace analysis usedetectors that are based on theactive film or discrete dynodeelectron multiplier. They arevery sophisticated pieces ofequipment compared with ear-lier designs and are very effi-cient at converting ion currentsinto electrical signals. Before wedescribe these detectors ingreater detail, it is worth lookingat two of the earlier designs —the channel electron multiplier(channeltron) and the Faradaycup — to get a basic under-standing of how the ICP-MS iondetection process works.

Channel electron multiplier. Theoperating principles of thechannel electron multiplier aresimilar to those of a photomul-tiplier tube used in ICP–optical

A Beginner‘s Guide to ICP-MSPart X — DetectorsRobert 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 atthomasrj@bellatlantic.netor via his website at www.scientificsolutions1.com.

Part X of this series on ICP-MS discusses the detection system — animportant area of the mass spectrometer that counts the number of ionsemerging from the mass analyzer. The detector converts the ions intoelectrical pulses, which are then counted by its integrated measurementcircuitry. The magnitude of the electrical pulses corresponds to thenumber of analyte ions present in the sample. Trace element quantita-tion in an unknown sample is then carried out by comparing the ionsignal with known calibration or reference standards.

SSFor some applications where

ultratrace detection limits are

not required, the ion beam from

the mass analyzer is directed

into a simple metal electrode,

or Faraday cup.

way the measurement circuitry handleslow and high ion-count rates. WhenICP-MS was first commercialized, itcould only handle as many as five or-ders of dynamic range; however, whenattempts were made to extend the dy-namic range, certain problems were en-countered. Before we discuss how mod-ern detectors deal with this issue, let’sfirst take a look at how it was addressedin earlier instrumentation.

Extending the Dynamic Range Traditionally, ICP-MS using the pulsecounting measurement is capable ofabout five orders of linear dynamicrange. This means that ICP-MS calibra-tion curves, generally speaking, are lin-ear from ppt levels to as much as a fewhundred parts-per-billion. However, anumber of ways exist to extend the dy-namic range of ICP-MS another threeto four orders of magnitude to workfrom sub-part-per-trillion levels, to asmuch as 100 ppm. Following is a briefoverview of some of the different ap-proaches that have been used.

Filtering the ion beam. One of the firstapproaches to extend the dynamicrange in ICP-MS was to filter the ionbeam by putting a non-optimum volt-age on one of the ion lens componentsor the quadrupole itself to limit thenumber of ions reaching the detector.This voltage offset, which was set on anindividual mass basis, acted as an en-ergy filter to electronically screen theion beam and reduce the subsequention signal to within a range covered bypulse-counting ion detection. The maindisadvantage with this approach wasthat the operator had to have priorknowledge of the sample to know whatvoltage to the apply to the high concen-tration masses.

in the early 1990s to develop an ICP-MS system using a Faraday cup detectorfor environmental applications, but itssensitivity was compromised and, as aresult, it was considered more suitablefor applications requiring ICP-OES de-tection capability. However, Faradaycup technology is still used in somemagnetic sector instruments, particu-larly where high ion signals areencountered in the determination of

high-precision isotope ratios using amulticollector detection system.

Discrete dynode electron multiplier. Thesedetectors, which are often called activefilm multipliers, work in a similar wayto the channeltron, but use discretedynodes to carry out the electron mul-tiplication (2). Figure 2 illustrates theprinciples of operation of this device.The detector is usually positioned off-axis to minimize the background fromstray radiation and neutral species com-ing from the ion source. When an ionemerges from the quadrupole, it sweepsthrough a curved path before it strikesthe first dynode. On striking the firstdynode, it liberates secondary electrons.The electron-optic design of thedynode produces acceleration of thesesecondary electrons to the next dynode,where they generate more electrons.This process is repeated at each dynode,generating a pulse of electrons that is fi-nally captured by the multiplier collec-tor or anode. Because of the materialsused in the discrete dynode detectorand the difference in the way electronsare generated, it is typically more sensi-tive than channeltron technology.

Although most discrete dynode de-tectors are very similar in the way theywork, there are subtle differences in the

When ICP-MS was first commercialized, it could only

handle as many as five orders of dynamic range;

when attempts were made to extend the dynamic

range, certain problems were encountered.

April 2002 17(4) Spectroscopy 35

Tutorial

ner. This situation is caused by ions ar-riving at the detector during the outputpulse of the preceding ion and notbeing detected by the counting system.This “dead time,” as it is known, is afundamental limitation of the multi-plier detector and is typically 30–50 ns,depending on the detection system.Compensation in the measurement cir-cuitry has to be made for this dead timein order to count the maximum num-ber of ions hitting the detector.

Faraday cup. For some applicationswhere ultratrace detection limits arenot required, the ion beam from themass analyzer is directed into a simplemetal electrode, or Faraday cup (1).With this approach, there is no controlover the applied voltage (gain), so aFaraday cup can only be used for highion currents. Their lower working rangeis in the order of 104 counts/s, whichmeans that if a Faraday cup is to beused as the only detector, the sensitivityof the ICP mass spectrometer will beseverely compromised. For this reason,Faraday cups are normally used in con-junction with a channeltron or discretedynode detector to extend the dynamicrange of the instrument. An additionalproblem with the Faraday cup is that,because of the time constant used in thedc amplification process to measure theion current, it is limited to relatively lowscan rates. This limitation makes it un-suitable for the rapid scan rates re-quired for traditional pulse countingused in ICP-MS and also limits its abil-ity to handle fast transient peaks.

The Faraday cup never became pop-ular with quadrupole ICP-MS systemsbecause it wasn’t suitable for very lowion-count rates. An attempt was made

Preamplifier

�3 kV

( )

Secondaryelectrons

Ions frommass analyzer

Figure 1. The path of an ion through a channelelectron multiplier.

When ICP-MS was first commercialized, it could only

handle as many as five orders of dynamic range;

when attempts were made to extend the dynamic

range, certain problems were encountered.

Figure 3 (above). Dual-stage discrete dynode detector measurement circuitry. (Figures 3, 4, and 5are courtesy of PerkinElmer Instruments, Shelton, CT.)

www.spectroscopyonl ine.com36 Spectroscopy 17(4) April 2002

Tutorial

Using two detectors. Another techniquethat was implemented in some of theearly quadrupole ICP-MS instrumenta-tion was to use two different detectors,such as a channel electron multiplier tomeasure low current signals, and aFaraday cup to measure high ion cur-rents. This process worked reasonablywell, but struggled with some applica-tions because it required rapid switch-ing between the two detectors. Theproblem was that the ion beam had tobe physically deflected to select the op-timum detector. Not only did this de-grade the measurement duty cycle, butdetector switching and stabilizationtimes of several seconds also precludedfast transient signal detection.

The more modern approach is to usejust one detector to extend the dynamic

range. By using the detector in both thepulse-counting and analog modes, highand low concentrations can be deter-mined in the same sample. Three ap-proaches use this type of detection sys-tem; two of them involve carrying outtwo scans of the sample, while the thirduses only one scan.

Using two scans with one detector. Thefirst approach uses an electron multi-plier operated in both digital and ana-log modes (3). Digital counting pro-vides the highest sensitivity, whileoperation in the analog mode (achievedby reducing the high voltage applied tothe detector) is used to reduce the sen-sitivity of the detector, thus extendingthe concentration range for which ionsignals can be measured. The system isimplemented by scanning the spec-

trometer twice for each sample. A firstscan, in which the detector is operatedin the analog mode, provides signals forelements present at high concentra-tions. A second scan, in which the de-tector voltage is switched to digital-pulse counting mode, provides highsensitivity detection for elements pres-ent at low levels. A major advantage ofthis technology is that users do notneed to know in advance whether to useanalog or digital detection because thesystem automatically scans all elementsin both modes. However, its disadvan-tage is that two independent mass scansare required to gather data across an ex-tended signal range. This not only re-sults in degraded measurement effi-ciency and slower analyses, but it alsomeans that the system cannot be usedfor fast transient signal analysis of un-known samples because mode switch-ing is generally too slow.

The second way of extending the dy-namic range is similar to the first ap-proach, except that the first scan is usedas an investigative tool to examine thesample spectrum before analysis (4).This first prescan establishes the masspositions at which the analog and pulsemodes will be used for subsequently col-lecting the spectral signal. The secondanalytical scan is then used for data col-lection; the system switches the detectorback and forth rapidly between pulseand analog mode depending on theconcentration of each analytical mass.

The main disadvantage of these twoapproaches is that two separate scansare required to measure high and lowlevels. With conventional nebulization,this isn't such a major problem exceptthat it can impact sample throughput.However, it does become a concernwhen it comes to working with tran-sient peaks found in electrothermal va-porization, flow injection, or laser sam-pling ICP-MS. Because these transientpeaks often last only a few seconds, allthe available time must be spent meas-uring the masses of interest to get thebest detection limits. When two scanshave to be made, valuable time is

Ion path

Noise

Individual dynodesQuadrupole rods

Generationof electrons

Figure 2 (left). Schematic of a discrete dynodeelectron multiplier.

Incoming ion

DetectorMCA

Analog signal

Pulse signal

Counter1

Counter2

To quadrupole

Scancontroller

Midpointdynode

Data system

Figure 4. The pulse-counting mode covers rates as high as 106 counts/s, and the analog circuitry issuitable from 104 to 109 counts/s with a dual-mode discrete dynode detector.

www.spectroscopyonl ine.com38 Spectroscopy 17(4) April 2002

Tutorial

wasted, which is not contributing toquality of the analytical signal.

Using one scan with one detector. Theselimitations of using two scans led tothe development of a third approachusing a dual-stage discrete dynode de-tector (5). This technology uses meas-urement circuitry that allows bothhigh and low concentrations to be de-termined in one scan. This is achievedby measuring the ion signal as an ana-log signal at the midpoint dynode.When more than a threshold numberof ions are detected, the signal isprocessed through the analog cir-cuitry. When fewer than the thresholdnumber of ions are detected, the signalcascades through the rest of the dyn-odes and is measured as a pulse signalin the conventional way. This process,

which is shown in Figure 3, is com-pletely automatic and means that boththe analog and the pulse signals are col-lected simultaneously in one scan (6).

The pulse-counting mode is typicallylinear from zero to about 106 counts/s,while the analog circuitry is suitablefrom 104 to 109 counts/s. To normalizeboth ranges, a cross calibration is per-formed to cover concentration levels,which could generate a pulse and ananalog signal. This is possible becausethe analog and pulse outputs can be de-fined in identical terms of incomingpulse counts per second, based onknowing the voltage at the first analogstage, the output current, and a conver-sion factor defined by the detection cir-cuitry electronics. By performing across calibration across the mass range,a dual-mode detector of this type is ca-pable of achieving approximately eightto nine orders of dynamic range in onesimultaneous scan. Figure 4 shows thepulse-counting calibration curve (yel-low) is linear up to 106 cps, and the ana-log calibration curve (blue) is linearfrom 104 to 109 cps. Figure 5 shows thatafter cross calibration, the two curvesare normalized, which means the detec-tor is suitable for concentration levelsbetween 0.1 ppt and 100 ppm — typi-cally eight to nine orders of magnitudefor most elements.

There are subtle variations of thistype of detection system, but its majorbenefit is that it requires only one scan

106

00.1 ppt 100 ppm

Analyte concentration

Pulse

(cps

)

109

104

Anal

og (c

ps)

Circle 20

Figure 5. Using cross calibration of the pulse and analog modes, quantitation from sub-part-per-trillion to high parts-per-million levels is possible.

April 2002 17(4) Spectroscopy 39

Tutorial

109

00.1 ppt 100 ppm

Analyte concentration

Cros

s-ca

libra

ted

coun

ts (c

ps)

Circle 21 Circle 22

to determine both high and low con-centrations. Therefore, it not only offersthe potential to improve samplethroughput, it also means that the max-imum data can be collected on a tran-sient signal that only lasts a few sec-onds. This process will be described ingreater detail in the next installment ofthis series, in which I will discuss differ-ent measurement protocols and peakintegration routines.

References1. Inductively Coupled Plasma Mass

Spectrometry, Ed. A. Montasser (Wiley-VCH, Berlin, 1998).

2. K. Hunter, Atomic Spectroscopy 15(1),17–20 (1994).

3. R.C. Hutton, A.N. Eaton, and R.M.Gosland, Applied Spectroscopy 44(2),238–242 (1990).

4. Y. Kishi, Agilent Technologies Applica-tion Journal (August 1997).

5. E.R. Denoyer, R.J. Thomas, and L.Cousins, Spectroscopy 12(2), 56–61,(1997).

6. Covered by U.S. Patent No. 5,463,219. �