Isotope Ratios and Abundance SensitivityObtained with an Inductively CoupledPlasma-Time-of-Flight Mass Spectrometer

D. P. Myers, P. P. Ma ho ney, G. Li, and G. M. Hi eftjeDepartm ent of Chern is trv . Ind iana L ruver- it v. Bloomi ng ton, Ind ia na . USA

Iso tope ratios and ab undance sensitivitics have been de termined wi th an ind uctive ly coupledplas ma -time-of-fligh t mass spec tro me te r (lCP-TO FMS). Abundance sensitivi ties are at leastin the 10" ran ge for low abundance ion s that pr ecede high ab un dance ions . Three method s ofdetection for isotope-ratio me asuremen t have been com pared. The three systems inv olvegated detection followed by ana log int egr a tion , ana log averaging , or ion co un ting . Ga ted ioncoun ting offers excellent p recision- between 0.64 and 1.000(' relative standard deviati on(RSD). Th ese va lues approach those p redicted from counti ng sta tistics and are comparable toth ose reported for othe r ind uct ively coupled pla sm a- mass spec trome try (ICP-MS) instru­ments. In add ition, a grea ter number of accum ulated coun ts or lon ger ana lys is tim es wo uldaffo rd precision s of 0.17r w ith stable ga ting electronics. The accuracy of the coun ting me tho dis in the 1- lOro;. ra nge if no correc tio n for mass bias is performed . However, th is ion co un tingme tho d suffers fro m a limited d ynamic range d ue to pulse pileup. Co ns tan t-frac tio n d iscr im­ination ga ted in teg ra tion and commercia l boxcar averager techniques offer a br oader d y­namic range beca use of thei r ana log na ture, but the attaina ble RSD va lues are limi ted by d riftin the detection systems and by the methods employed to calcu la te an accurate rati o. Overa ll,mass bias in the IC P-TOF:Y1S is more se ve re than previous work in ICP-MS due prim ar ily todetect ion sys tem bias. {j It/II S"l A"lll~~ Spcctrom 199!J , 6. 921! - 927J

I nd uctively co up led p lasma mass spec tromet ry(lCP-MS) w ith quadrupole ma ss analyzers has beenstudied an d tested extensive ly for isotope-ratio

an alysis in a variety of fields th at inc lude geochemica land environmen tal applica tions [I , 2]. Ho wever. recen twork er s have pointed out that its Widespread use hasbeen inhibi ted by its lack of precis ion [O.1 - 1.0C:! rela ­tive standard deviation ([~SD)] when compared totechniques such as thermal ioniza tion mass spec tro rn e­try (TIMS), which offers p recis ion levels better tha nO.005(i( RSD [3]. The main di sad vantage to TIMS is thelon g ana lys is tim es (u p to 1 day) requ ired to ac hievethe better precision [3]. Also, to obta in the h ighes tprecis ion W.I r;i·) on ICP-MS q uad ru pole instrumentsrequi res rapid peak-hoppi ng [1, -lL so the number o iiso to pes and eleme nts that can be measu red in a s ing ll'run of fixed tim e is limited . Obviouslv. it would beadvantageous to increase prec ision in rout ine ICI' -\ '1S;the sensi tivity o f the method unable-, shorter ana lys istimes and consequently higher sa mp le through pu t thanalte rna tive methods such as '["[1\.15 [ !1 ]. T ypica l l y theRSD values obtai ned in ICP -:Y1S have been 2- 3 time shigh e r than would be e xpected from co un ting sta tistics

A dd ress reprin t rL'qU l' ~t~ to Dr. l"H \ \ 1. f l l l' Hit' , I) l ' p ,u tm CI1I l dC hern istrv . Ind ia n a U n i\ "l'rs il} , Bl p PIl1in h t l )J1 , If'.,; -+ 7--+0 :;.

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[3]. Inst rument ins tability ha s been the ma jor ca use ofthe poorer precision [3, 6, i].

Recen t work has focused on ana lysis of the so urcesof noise in ICP-MS and subse que n t op timization of theinstrume nt to reduce or elimina te the noise compo­nents [3, 4]. For exa m ple, Furuta [4] has found th atred uc tion of the d well tim e between the measurementof individual isotope peak s can improve precision tova lues in the 0.1 -0.3 '/~ RSD range by elim ina tion ofm ultiplicative noise components that exist in rCP-MS.At the shor test dwell times, on the order of 10 j.Ls,p recis ion was limited by Poisson s ta tis tics in the ioncoun ts. Presumably, a larger number of sweeps co uldbe accum ulated to improve precisio n fu r ther. Begleyand Sha rp [3] used sim ilar techniques to attai n relat ivestandard devia tions of 0.05':';'. The ir stud ies showedalso that be tter p recis ion was possib le by red uction ofthe measurement in ter va l between isotop es . Th ey sug­gest tha t a multicollector mass spec tro me te r sho uldexhib it ev en better pr ecision than that see n with thesequen tia l qua d rupole mass ana lyze r, as lon g as thenoise compone n ts in each signa l remain multiplicat ive.In accorda nce with thi s exp lanation, rat ios ha ve bee nmea sured with a double-focu sing magne tic sector massana lyzer equipped wi th seven Faraday collectors forsim ultaneous detection of masses [8]. Th is mass ana­lyzer also ben efits from flat-topped sy mme tr ical pea ks,

Receive d November 11, 1994Revi sed Ma y 30, 1995Acce p ted Ju ne 1, 1995

Nl((ll'! J{ ,\ I IOS AUL'JD A:" C E S E!', S IT IVIIY IN ICI'-TOFMS 921

w hich allow mo re accurate deter mina tion of peakheights than is convenien t wi th the round ed peaksfrom qua d rupole filters l81. Isotope ra tios obtainedwith th is sector ins trumen t ha ve RSDs in the ra nge of0.014-0.022(1r and an error (accur acy ) o f 0.01(Ir [8-lU1.O f co u rse, the lim itat ion of suc h .1 multidetcctor s im u l­taneous inst rument is the numbe r o f isotopes a ndelements that can be ana lyzed in an avai lable measu re­ment period . Sma ll o r trans ien t sam ples cou ld no t beana lyzed for more th an a few isoto pe ra tios for a \'l' ryfew eleme n ts . C lea rly, a fas t ma ss ana lyzer tha t sa m­p les or measures all of the ato mi c ions si m u ltaneo us lyseems be tter su ited to generc11-p ur pose iso tope-rat ioanalyses,

1\ time-of-fl igh t mass spectrometer does not mea­sure ion signals at exa ctly the same tim e. Howeve r, allions ar e sampled s im ultaneous ly fro m tho ind uc ti vi-lvco u p led plasma ocr) in eac h cycle of ma ss ana lys is,Th er efo re, p rovided th at th e sa mp ling p rocess is repro­ducible, drift and m u ltip lica tive noise in th e sou rcecan he removed fro m the ind ividua l ion signals. Sig­nals also are measured a t the :--cl me detector, so itsnoise sho u ld be nea rly id en tica l for a ll ra tioed signills .In fact, iso tope-ra tio measuremen ts wi th time-o f-fligh tm ass spectro me ters a lready hav e show n prom ise wi thionizati on techniques such as resonance ionizati on[ll - I3], lase r ab latio n [14], and secondary ion massspectrome try [15). Measurem ent precisio n in thesestud ies was in the range of lU tll 1.O ~' ; relat ive s tan­dard deviat ion, lim ited ma inly by co un ting s ta tis tics a tthe best precision leve ls [11). Because the peak shapein time-of-flight mass spectrometry nOFMS) is a lsoro unded, jus t as w ith a quadrupole filter, a detect ionsystem tha t in teg ra tes the ion peaks would be ad va n­tilgeou s, as a lready has been demonstra ted wi th agated pulse coun te r [11].

Experimental

Th e ICP-TOFMS inst rumentat ion is iden tica l to th atdescribe d prev iously [16-18]. For a ll d at a p rese ntedhere th e ICP -TO FMS wa s opera ted in the reflectronmode because of th e high er reso lving pow er it offered .In addition, th e electrostatic quad ru pole lens was oper­ated in the pulsed ion-injection mod e [17], at an instr u­mental repetitio n ra te of 7,1 kH z. All other ins trumen tconditions are identical to tho se described prev iou sly[18]. In th is investiga tion we utilize and compilre th reesy stems for isotope- ratio measurement: (I ) commer cia lboxcar averagers, (2) cons tan t-frac tion di scriminato r(Cf'Dl-gated in teg ra tor [19], and (3) ga ted ion co un t­ing. All of these detection methods were described indetail previously [18]. A Tektron ix (Beaverton , O R)TDS 520 di gital oscilloscope, which is normall y used toacquire fu ll tim e-of-tligh t ma ss spectra, was ineffectiveto a tta in accep tab le iso tope-ratio p rec ision.

For th e for egoing methods, the tw o isotopes ofin terest were isolat ed by two sepa ra te timing ga tes oi30-50-n s duration each; optimizati on of gate wi dth

and delay time is cr itica l to ach ieve optima l perfor­man ce of all systems. In th e case of th e boxcar av er­age l's and CFD-ga ted integrat ors , the analog ou tp u ts ofth e two measu rement channels are ratioed. In theCFD-ga ted in teg ra tor th e rati o is perfo rmed by using a\Jationa l Instruments N B-MIO-16XL analog-to-d ig ita lco nverter board and LabV IEW II so ftwa re (Na tiona lInstruments, Austin, TX). With the boxcar averagersthe ou tp u t of bo th averagers is sent to th e ra tio func­tion of a Stanford Research Sys te ms Model SR235 ana ­log processor; the ou tpu t of thi s module is rea d out onthe Na tional Inst ruments bo ard .

When the bo xcar averagers we re used, bo th m od­ul es (Stan ford Resea rch Model SR2,')0) were se t to thesame se nsi tivi ty; however , to accoun t for gain differ­ence be tw een th e averagers, th e ga tes were switched intheir mass (time) af te r an ini tia l measuremen t and thera tio was determined again. The tw o measured ratioswere inserted into the following eq ua tion to obtain thega in -co rrected value:

ac tua l ratio = (ra tio 1 X rati o 2 ) l i2

(1)

The sam e method and ca lcu lation were em ployed wi ththe CFD-ga ted in tegra tor syste m, Anticipa ted uncer­ta inti es in th ese measurements w ere com p u ted ba sedon error p ropagation of the precision of the individualva lues ,

The ga ted coun ter method is sim ilar to a se tupdesc ribed by Green et. a l, [11). Two tim ing ga tes (op ti­m ized at widths betw een 30 and 50 ns) are used totr igger constan t-frac tion d iscrim inat ors (CF Ds : Ox fordInstrument s, Oak Rid ge, TN) th at co un t ion hits abovea certai n thresh old and outpu t nuclear ins tru men tmodule (NIM) pulses. The ou tp ut of the CFO is con­ne cted to a H ewlett-Packard (Avondale, FA; model5302; 50-MHz) uni ver sal counter. The ra tio m easure­ment s a re performed by the counter, which reads outthe co un ts of one cha nne l th at occ u r for a se lectednu mb er of co un ts ( I x 10 1- 1 X 101» in the other. Toprevent pulse pileup wi th th is method, the ra te of ioncounts in each cha nne l shou ld be maintained a t lessth,1I1 0.1 ions per repeller pulse . However, higher con­cen tra tions th at prod uce betw een 0.1 and 5 ions perrepelle r pulse were used in so me cases to reduce theanal vs is tim es. Accura te correc tions for pulse p ileupMe possible when s tanda rd samples of kn own iso topiccomposition ar e used in the SCIm e concen tration rangeas the unknown sam ples .

In the ra tio measurements here no cor rec tions weremade for ma ss bias in th e ins trument; however , in allof the methods suc h co rrections are possible. A ll ratiodete rmina tions were m ade on lead iso topic SRM 981(Natio na l Institute of Standards and Tec hn ology,Ga ithersbur g, tvID), which was di ssolve d in ni tric acidto produce a stoc k so lu tion {1309 ppm) for subseq uen tdi lution. The ce rtified va lues for this lead standard arelist ed in Ta ble 1. Th e concen tra tions ultimately usedfor iso tope-ra tio ana lyses w ere 1.31 ppm and 262, 196,an d 62 p pb ,

922 MYERS ET AL. J Am Soc Mass Spectrom 1995, 6, 920- 927

Table 1. Certified isotope ratios for SRM-981"

Z0 4 Pb/ z 0 6 Pb 0,059042 ± 0,000037Z07 Pb/Z06 Pb 0.91464 ± 0.00033

z 08 Pb/ z o 6 Pb 2.1681 ± 0.0008

aSource : Nation al In st itute of Sta nd ards and Technol og y .

To measure the abundance sensitivity in the ICP­TOFMS, an averaged spe ctru m, accumulated from1O,000individual scans, was obtained for both a solu­tion of 1000-ppm Bi (Johnson Matthey, Ward Hill , MA)and a blank solution. These spectra were recorded onthe TDS 520 digital oscilloscop e.

a 250

200

s 150-.§.OicDI 100iii

50-

0202 204 206 208

mlz

loA

210 212

204 205 206 207 206 209 210 211 212Ma•• (amu)

204 205 206 207 208 209 210 211 212Mee. (emu)

Figure 1. (a) ICP-TOFMS sp ectrum for a 4-ppm solu tion of bothPb and Bi by using the ion refJectron (average of 1000 shots). (b)Spec tru m of 1000-ppm Bi (ave rage of 10,000 sho ts) with nopreamplification. (c) Expanded scale of sp ectrum in (b).

state only that the lower limit of abundance sensitivityis at th is value (4 X 106

) , becau se the contrib utedamount of Bi at adjacent mass 208 is not detectable inour spectrum.

The expanded scale in Figure Ib reveals a problemin ICP-TOFMS : very intense ion signals can causesaturation of the MCP and ringing at the anode outputthat wo uld preclude the ana lysis of a less abundantion 1 mass unit heavier than the more abundant mass.Also, ther e is a differen ce in ba seline before and afterthe Bi peak due to saturation of the microchannel plate(MCP) detector. A similar problem was noted in a

0 .50

c 0.88

0.80

! 0 .72

"0~ 0.64Oic 0.56gUI

0 .48

b 3.0

2 .5

~ 2.0"0z,Oi 1.5e...iii

1.0

Results and Discussion

Abundance Sensitivity

Values for resolving power measu red with this ICP­TOFMS have ranged from 1400-2300 [full width athalf-maximum (fw hm r] [17], which would suggest highlevels of abundance sensitivity. However, abundancesensitivity depends not only on resolving power butalso on mass-spectral peak shape. Figure la shows aspectrum for a solution that contains an equal concen­tration of Pb and Bi (4 ppm) that exhibits a resolvingpower of - 1700 (fwhm), The peaks are generally wellshaped, but with obvious rin gin g on the high massside, and are baseline-resolved on the mass axis. In theflight-time domain the individual ion peaks range from20 to 40 ns at the base, which allows gated detection ofsing le masses.

The abundance sensitivity of a mass analyzer is animportant indicator of the ab ility to determine lowabundance isotopes that are adjacent to very abundantisotopes. Quadrupole mass analyzers possess abun­dance sensitivities of lO s-lOh. For the ICP-TOFMS,Figure Ia and b shows a spectrum obtained with theICP-TOFMS over the mass range 204-212 for a solu­tion of 1000-ppm Bi. Both spec tra we re acquired with­out preamplification because the large Bi signal wouldsa turate the ampli fier ou tput. A spe ctrum of the blan ksolution (5% HN03 ) also was obtained but is notd isplayed in Figure 1. At the adjacent mass M-l (208 u)no measurable difference was observed between the Bispectrum and the blank. However, the average back­ground level of 0.35901 V and its standard deviation of0.0013598 V can be used to es tima te the minimumdetectable concentration that would yield a signal of3a or 0.0041 V. The sensi tivity from Figure 1a for208 Pb is 30 mV /ppm; thus, this detectable level corre­sponds to a Pb concentra tion of approxima tely 140ppb. Use of the signal level for 1000-ppm Bi minus theblan k level and division by the minimum de tectablelevel of 0.0041 V, yields a ratio of 562.7. Wh en theconcentrati on differ ence between the I3i and Pb is takeninto account, an abundance sensitivity of 4 x 106 iscalculated . Based on the preced ing treatmen t we can

J Am Soc Mil" Spec trum 1'145, n, '120 YZ7 ISOTOPE RATIOS / ABUN DAN CE SENSITIV ITY IN ICP-TOFMS 923

laser microprobe mass analyzer, but was caused byasymmetry in the pea ks and was not as severe [12].The incorporation of a different detector and improvedimpedance-matching sys tem might alleviate the ring­ing; however, more than likely, a detector such as ad iscrete dynode electron multiplier might be necessaryto overcome the saturation problem.

Isotope-Ratio Analysis

Gated Ion Counting

With the ga ted counting method, isotop e ratios fromthe SRM-981 standard were determined for a range ofaccumula ted coun ts of the less abundant iso tope. Table2 lists the measured values for three Pb isotope ratios.As expected, at a greater total number of coun ts theprecision of the measurement improved due to count­ing statis tics . This tre nd is evid ent in Table 2, wherethe RSD for the me asured ratios clearly improves in allcases as the number of total coun ts increases. Ext rapo­lation to 1,000,000 counts and use of the experimen­tally verified de pendence of RSD on l/(nu mber ofcounts l v? ena bled the p recision to be computed as0.12%, approximate ly what would be expec ted fro mcounting sta tis tics <0 .1%). Of course, to acquire th isnumber of counts would require 3-8 h at th e signa llevels emp loyed in thi s experiment (recall th at signallevels are necessar ily low to av oid pulse pileup ).

A d isadvantage to counting measurements is thelong analys is time required to produce better preci-

sian. Moreover, for such extended analysis times, gatejitter and drift are likely to degra de the precisioneventually. A system with stable gates would allevi­a te this problem; however, truly excellent precision( < 0.01%) would still require a long measurementtime. One way to shorten the analysis tim e would beto increase the repetition rate of the TOFMS, whichcould approach 20 kHz with the current ar rangement[16, 18]. This modification would reduce analysis timesby a factor of ap proximately 3.

For both the 208/206 and 207/206 ratio measure­men ts the concentra tion used was 62 ppb except in thecase where 100,000 counts of the less abundant isotopewer e collec ted; in that situation a 196-ppb so lu tion wasemployed. The analysis time for the 196-ppb solutionwas approximately 2-5 min for each determination .Even though the ratio at this number of accumulatedcounts is more precise than when 10,000 counts wereaccumulated, accuracy is sacrificed somewhat because20HPb signals pile up ( > 2 ions per rep eller pulse countrate), whereas 200 Pb counts do not. As a consequence,the measured ratio of 2.0347 ± 0.0130 is lower than thecerti fied va lue of 2.1681 ± 0.0008. The 207/206 ratioexpe riences the same problem because buth iso topesapproach pileup condi tions (- 0.1 ion per rep ellerpulse). However, for values de termi ne d at 10,000counts, both rati os ag ree with the certified valuesw ith in expe rime ntal precision, as does the value forthe 204/206 ra tio. A more detailed discussion of preci­sion and mass bias will be offer ed in a later section.

The limi t of pr ecision for th e isotope rati o A lB fromcounting statistics can be determined fro m the follow-

Ta ble 2. Isotope ratio results for th e gated ion count ing system '!

RSDRat io (A / BI RSD Lim it "concentration Counts B Average (n = 24) (%) (%)

? 0 4 Pb/ 2 0 6 Pb

196-ppb SRM-981 100 0 .0672 ± 0 .005 1 7.6 10 .3

1,000 00663 ± 0.0021 3.2 3.3

10 ,000 0.0650 ± 0 .00 11 1.7 1.0

207 Pbl 206 Pb

62 -ppb SRM -98 1 100 0 .892 1 ± 0 1076 12 .1 14 .6

1,000 0 .9067 ± 0 0392 4 .3 4.6

10,000 0.9290 ± 0.021 7 2.3 1.4

196-ppb SRM -9 81' 100,000 0 .8998 ± 0 .0095 1.0 0.46

208 Pbl 206 Pb

62 -ppb SRM -981 100 1.8251 ± 0 .4 182 22 .9 16.5

1,000 2.1145 ± 0 .1221 58 5.5

10 ,000 2.0886 ± 0 .0387 18 1.8

196-ppb SRM -981' 100,000 2.0347 i 00130 064 0 .55

a CFD threshold = -- 50 mV. 50-ns gateb Counting stati st ics preclsion calculated from eq 2.cPulse pil eup cond.tio ns ( > 0 1 Ion per repell er pulse)

924 MYERS ET AL. J Am Soc Mass Spectrom 1995, 6, 920 -927

ing equation :

relative standard dev iation

= (A1/ 2/A + B I 2/B) I /2 x 100 (2)

Becau se of pulse pileup, gated ion counting wouldprobably be attractive over a range of only 2-3 ordersof ma gnitude in concentration with the current sys tem .

where A is the average number of counts in the moreabundant channel and B is the number of counts inthe less abundant channel 00-100,000 in these cases).Tabl e 2 shows the lim it of precision attainable in eachmeasurement based on the number of counts accumu­lated for the less abund ant isotope. Most measuredvalues approach precision lim its d ictated by countingstatistics and in some cases are better than those pre­dicted, due to the unc ert ainty in the small number ofmeasured values (Il = 24), In mo st cases, the largernumber of accumulated counts exh ibits precision lev­els poorer than the counting-statistics lim it. Long termtemporal drift in the gates contributes to thi s poorerprecision, but there might also be cros s ta lk betweenthe tw o gating channels. No te in Table 2 that themeasured RSD for the 207/ 206 rati o is tw o timeshigher than the RSD limi t, wh ereas the 208/206 rat io iscloser to the limit, even though the y both requiresim ilar analysis times. Th is obse rva tion may suggestthat there is more jitter or drift from interchannel crosstalk when the gate signals are close together in tim e.

Ov erall, the precision values are as close to count­ing- statistics limits as those obtained with qu ad rupolemass analyzers [3, 41. In general, the precision offeredby the gated ion -counting method is excellent, espe­ciall y when 100,000 or more counts of the less abun­dant isotope are accumulated . For analysis times re­stricted to less than 5 min for a sing le measurement ,pulse pileup mu st be end ured in the 208/206 and207/206 ratios if the best precision is to be obta ined .

Boxcar Averagers

Boxcar avera gers provide the ability to average up to10,000 shots of the TOFMS signal in a single channel.In addition , such analog measurements broaden thedynamic range and remove the problem of pulsepileup, both of which hinder the counting techniquedescribed earl ier . With the averaging capability theprecision of the ratio can be improved considerably.Tab le 3 lists the results ob tained with the averagers forlead 208/206 and 207/206 rati os. The sensitivity set­tings of both channe ls were identical for the ratios(207/206 and 208/206) mea su red here so that thepr ecision levels can be compared wi th confidence.

As d iscussed in the Experimen tal section the gain­corrected ratio is determined by sw itching the gates ofthe averag crs and usc of cq 1 to ca lculate the true ratiofrom tw o individual ratio measurements. The averagesof 100 measurements (each taking approximately 1.4 s)of each rati o (first directly and then with switchedcha nne ls) are listed in columns labeled Ratio 1 andRatio 2 of Table 3. The ratios calculated from eq 1 foreight determinations in the 208/206 case and fivedeterminations for 207/206 are listed in the third col­umn. These calculated ratios have associated with themthe propagated unc ertainties from the individual ratiomeasurements (columns Ratio 1 and Ratio 2). Theaverages of all the calculated ratios of both 208/206and 207/ 206 ar e shown at the bottom of column 3 inTabl e 3.

Table 3. Isotope ra tio results for the boxcar averagers:

Average

207 Pb / 20 6 Pb

Average

RSDRatio 1 Ratio 2 Calculated (%)

1.68 1 ± 0020 2631 ± 0 .042 2 .103 ±0.019 0 .90

1.689 ± 0 .027 2.634 ± 0 .055 2 .109 ± 0 .026 1.23

1.751 + 0 0 36 2533 ± 0 .059 2.106 ± 0.030 1.42

1.750 ±0.Q19 2.807 ± 0.047 2.216 ± 0 .021 0 .95

1.671 ± 0035 2706 ± 0 .038 2.126 ± 0 .023 1.08

1.651 ± 0 .059 2.684 ± 0 .058 2.10 5 ± 0 .037 1.75

1.782 ± 0021 2.660 ± 0 .062 2.177 ± 0 .027 1.24

1.788 ± 0 .016 2.350 ± 0076 2.050 ± 0 .034 1.66

2.124 ± 0 .062 2.92

1.28 9 1: 0 .025 0 77 8 1: 0 .0 15 1.001 ± 0 .014 1.37

1.402 ± 0 .034 0.784 ± 0 .017 1.048 ± 0.Q11 1.05

1.389 1: 0046 0 .572 ± 0 .012 0.891 ± 0 .010 1.12

1.235 :': 0 .021 0588 ± 0 .011 0 .852 ± 0 .008 0 .94

1.224 :::: 0024 0842 ± 0 .019 1.015 ± 0 .011 1.08

0.961 ± 0 .085 8 .82

a 1 .3 1- pp m SRM -981 . 50 -ns ga te ; 10 .00 0 aver ages. n o: 100 for eac h rati o.

ISOI ( )f' F RAT IOS / ABL ND A:--J Cf SEr\ SITIVlTY IN ICP -TOF MS 925

2.8

RSD = 1.18%

1.8 1 ,

\ Il\ ..•._· /·.I I' _.&_"'.~.••..I "......l _.~ ._ •••_ ..

Time (sec)

Figure 2. Effect of cha nn el sw itch ing on the precision of theisotope-ratio measurements. Each trace shows the time-depen­dent averaged ratios (lO.OOO av erages for eac h plotted point)produced by the boxcar averager immediately after the channelsw ere switched . The top tra ce was obtained first, the cha nnelsw ere sw itched, and th en th e bott om trace wa-. rerorded.

CFO-Gl1ted Integrators

The CFD-gated integrator system has producedpromising detection limit s th rough signal-to-noise ra-

precision values, especially in the 208/206 case, arequi te promisin g but are probably limited by indepen­dent drift in the two analog channels used to obtainthe rat io. In addition, the sw itch of channels duringratio analysis certainly degrades the run-to-run preci­sion.

Individual listed ratio values (first two columns inTable 3) rep resen t the means of 100 measurements,each of which is obtained from 10,000 boxcar averages.As would be expected, the precision of these determi­nations increases with the number of averages. Weobserved ea rlie r [18] that the RSD of the 208 Pb/u6Pbratio improves roughly as the square root of the num­ber of boxcar av erages. With th e number of box caraverages set to 10,000 (the limit of the SRS250 instru­ment), the precision can be as good as 0.57% [18]. Ifthe average of 10 of these 10,OOO-pulse averages(100,000 averages) is tak en, the RSD improves to 0.47%[18]. The lOO,OOO-averages value asymptotically seemsto app roac h a limit in precision for the boxcar-averagersystem. The limit at this point is dictated mainly bythe stability of the boxcar-averager output (0.01% / min)and by independent drift in the two boxcar channels.

In this earlier study, there was no attempt to correctfor mass bias in the two electronic measurement chan­nels, so there was no need to sw itch them. The onlyad jus tment was to optimize the boxcars and zero them.A comparison of these results with those of the presentstudy suggests that the channel-switching methodadopted here should be avoided.

In sum mary, the boxcar averagers offer an efficientand fast method of isotope-ratio analysis. In theseinv esti gations, each individual isotope ratio took about5-6 min to ascertain, a time limited mainly by thechannel sw itching used to compensate for differencesin averager response. The precision is fair, provideddrift from run to run can be eliminated. A possibleway to remove this drift would be to calibrate the ratiofrom the two channels based on a standard such asSRM-981, then to use these two channels exclusivelyfor the sa me isotopes. In this wa y, channel switchingcould be avoided.

The possibility exists with boxcar ave ragers (or anygated analog detection system) to monitor many ratiossimultaneously sim ply by us ing a greater number ofchannels. Such a setup also would permit normaliza­tion to othe r isotope rati os, possibly to improve preci­sion further, as has been demonstrated by others whoused a simultaneous multichannel mass spectrometer[8]. A multichannel gated analog system would takebest advantage of the sp eed of the TOFMS and shouldoffer precision at least as good as that for isotope-ratiomeasu rements made on typical lCP-MS quadrupolesystems.

1 5 0

RSD =1.60%

10050

1 .6o

2.4

2.6

.Q

~o 2.2"'-n~~ 2.0

for the 208/206 ratio the individual calculated val ­ues exh ibit precision in the range of 0.9 to 2.0W/( 160 ,which is about 10 times worse than the best isotope­ra tio precision rep orted for peak-hopping qu adrupolemass spectrometers [3, .1]. However, our measure­ments (eac h 10,000 averages ) we re obtained over int e­gration times of only 1.4 s, whereas the previous stud­ies [3, 7] used tim es close to 1 min. Moreover , thesystem used here should be able to produce any num­be r of isotope rati os sim ultaneous ly for a multielem entsolution. Because of this studvs shorter measurementtimes, the RSD values reported he re would be ex­pected to be higher than were reported from earlierstud ies with sim ultaneous ins truments such as thedouble-focusing multiple collector ICP-MS [8-10], withwhich measurement tim es were 5-'1 0 s [9, 10]. Therun-to-run variation in the present instrument is re­flected in the average 208/206 precision va lue ci ted inTable 3 (2.9200. This longer term instability probablyresults in pa rt fro m sw itching of the boxcar cha nne ls tocompensate for their gain difference. When the chan­nels are sw itched , there is a bri ef cha nge in the deoffset of the averagers . Evidence for this occurrencecan be seen in Figure 2, which shows the time-depen­dent variation in the two mea sured ratios immediatelyafter the channels are sw itched. Clearly, the first fewvalues at the beginning of each measurement are af­fected by thi s bri ef offset and serve to lower theprecision considerably. In the future, a better way tocompensate for in terchanne l gain will have to be found.

In the case of the lead 207/206 ratio the precision isapproximately of the same magnitude as that for the208/206 determinati on , but the var iation in all calcu­lated ratios is higher at 8.W:; , which indicates that therun-to-run precision is worse for this ratio . Reasons forthis disparity are unknown at this time. Both averagesof all calcu lated values agree with the certified values(see Table 1) within their precision. The individual

926 MY ERS ET AI J Am Suc Md" Spectrum 1995, 6, 920-927

the CFD-gated in tegrator. On e like ly source is timi ngjitter. The timing gates used in the gated integrator arehard ly sta te-of-the -art, whereas those in the aver agersare stable to tenths of nanoseconds.

ell 1 RSD =0.5%

....eh 2 RSO =0.9%

Ratio RSD = 0.72%...........-

. ···516 Hz• 760 Hz

- ~ . ' 2.1 kHz- ><- ' 7.1 kHz1 0,

. .................... ........._... .. . . . . . . RSD =0..4%

•••..•..• . .• . -e •• • . • . • • -• . -• .•. .•. .• ..•. .• RSD = 0.8%O.1+-.-r-r- ......,...........,...,..............................,..,r'"l""O......."T"I'..............,.....,...T"T'"1

o 100 200 300 400 500 600 700

Time (sec)

0.55 L~~~:;:;:~;:;:;~~~:;;:;;:;~....,..,...,o

~

~ *" *"" )of- *""*"" *""*"" X- X- X- X- X-X- X - X- X-X -'X-+< -+( RSD =0.3"0

'"iii"C.!~ <)-- o-<r-<>--o--o--o-o-o-o-o-o-o-o-o-o-o-o-o-o RSD =0.50/0!.E

b 0 .65

/" .0.8

~Ii 0 .75e'"iii ""--."C

0 .7..;;6. 0 .65..~

0.6

a

100 200 300 400 500 600 700Time (sec)

Figu re 4. Integra ted signa l output (20-s integ ration time) fromthe CFD-gated integrators for a dc signal of - 70 mV as a functionof tim e. (a) Integra ted signa ls for varied ga ting frequencies. (b)Integrated signals from both channe ls and their ratio for the2.1-kH z gating frequency .

tio enha ncement (18, 19]. As a result, the precision ofthis system was compared to that of the boxcar aver­agers and ga ted ion counting. However , we have ob­served that the precision of the CFD-gated integratorsis limited to about 1-2% RSD. Evidence for th is lim it. .. 208 00 0IS shown In Figure 3, where the RSD for the PbF Pbra tio is plotted as a function of in tegration time for c1

single ratio measurement (i.e., no switching of integra­tor channe ls). The pr ecision of the ratio does not im­prove appreciably beyond an integration time of 20 s.at w hich point it is ap proxi ma tely 1% RSD.

This limited precision can be traced to the stabilityof the two integrator channe ls. Figure 4a shows thesignal stability over a 5-min per iod produced by theCFD-ga ted int egrator (20-5 integration ) for a stable dcsignal (- 70 m v ) and at seve ral gating frequencies.Becau se the in tegrator gate is ordinarily opened by adi scriminator pulse only w hen a signa l is present (19],the gating operation necessarily bypassed the discrimi ­nator. Figu re 4b shows the signals from the tw o chan­nel s and their ratio at the 2.1-kHz gating frequency(w hich exhibited the poorest p recision in Figure 4a).Overall, the precision of each integrator is in the0.4-0.8% RSD range and for the ratio of the twochannels (Figure 4b) the RSD is 0.72 c,;c in the 2.1-kHzcase . The precision has a simi lar magni tude at all othe rtriggering frequencies.

If we assume the drift in the two channels is inde ­pendent, the precision for the ratio in Figure 4b shouldbe 1.0% ( 0.92 + 0.52 )1/ 2)]; however, as can be seen inFigure 4b, the drift in the two channels is somewhatcorrela ted, so the precision is better than th is pr edi c­tion. Regardless, based on the precision in Figure 4 forstable dc signals, it is unlikely tha t the gated in tegratorwill yield RSD values better than 10k in TOFMS. Be­cause this level of precision is considerably lower thanwas achi eved with the commercial boxcar avera gers,there must be additiona l sources of no ise and drift in

Integration Time (sec)

Figure 3. Effect of integrabon time on relative standard dcviation (RSO) of a sing le 20SPb / O" Pb ratio (SRM-98l standard )measurem ent <Colum ns Rat io 1 and Ratio 2 in Tab le 3) for theCFD-gated integrator system . '\in channe l sw itching w as em­p loyed .

Mass Bias in the Lead Isotope Ratios

( )omRtru c = Rmeas 1 + C

Mass discrimination in the ratio measurements can bees timated and, if necessar y, cor rected with the follow­ing equation (9]:

in whi ch Rtru c is the true ratio value, Rmcas is themeasured ratio value, C is the bias factor, and 8m isthe mass di fference. Mass bias was evalua ted in theICP-TOFMS for each detection system described previ­ous ly. Table 4 lists the calculated C values based onthe values in Tables 2 and 3. Clearly, mass bias isstrongly dependent on the chosen detection system.The worst mass bias occurs in the CFD-gated integra­tor, which is not sur pr ising because of its poor stabil­ity . Gated ion counting and the commercial boxcarave rager offer the lowest levels of mass bias.

70605040302010

5

2

3

2

J Am Soc Mass Spe ctr um 19'1\ 6, 9211 <.) 27 ISOT O PE RATIOS / ABUK DANCE SENSIT IVITY IN ICP-TOF MS 927

Table 4. Mass bias factors in the JCP- [OF'\1S tor themeasure ment systems employed

a Det erm in ed fr om valu es for 10,0 0 0 counts acc um u la te d (seeTable 2).

b Determined from av er ag e values of cal cu la te d ra t ios (se e Table3 ),

Gated ion counting"

204 Pb/206 Pb20 7 Pb/206 Pb2 0 8 Pb/ 20 6 Pb

Boxcar evereqers ?

208 Pb/206 Pb2 0 7 Pb / 2 06 Pb

c

0 ,0469

- 0 0 154

0 ,0 18 8

0,010

- 0 ,0 48

better precision than those obtained with sequentialmass spectrometers. Moreover, this improved preci­sion should be available for all ratios across the atomicmass rang e at once. Because the gated ion-countingmethod is at the limit of counting statistics, one wouldexpect that greater stability in the timing gates andintegrators used in analog methods would afford pre­cision levels better than those seen with scanning in­struments. As a consequence, more precise methods ofan alog detection will be inv est igated in the future.Finally, the ultimate isotope-ratio analysis method forICP-TOFMS would be one in which analog signals forall selected ions could be analyzed simultaneously in amultichannel system. Such a setup would take bestad vantage of the speed and signa l information gener­ated by the TOFMS.

Conclusions

The excellent resolution of the ICP·TOFMS instrumentallows gat ed detection of individual ion peaks in thetime-of-flight mass spectrum. Abundance sensitivity isat least 106 for less abundant ions that preced e abun­dant ones, but those that follow abundant peaks wouldbe d ifficult to detect becau se of ringing and satura tioneffects in the MCP detector. We have described threeelectronic detection method s to perform isotope-ratioanalyses via the ICP-TOFMS. Use of commercial box­car averagers. an in-house-built CFD-ga ted integrati onsystem, and gated ion counting offer precision forthese analyses of RSD values between 0.5 and 4':1, .Gated ion counting showed the best precision andaccuracy, but required long er analysis times. In addi­tion, the precision of the gated ion counter approachesthat based on counting statistics, which could be 0.1[ if

or below, provided long enough analysis times areused. This anticipated precision requires more stablegating electronics than are cur rently utili zed in thesystem. Unfortunately, the counting system has themost limited dynamic ran ge. Commercial boxcar ave r­agers offer acceptable precision; however, the tech­nique (channel switching) used initi ally to calcul ateaccurate isotope ratios compromises their precision.The CFD-gated inte grat or sys tem shows the grea tes tpromise for broad dynamic range and excellent preci­sion, althoug h the stability of the present integ ratorsand gates must be improved. Mass discrimination inthe ICP-TOFMS is severe compa red to wh at has beenreported for other simultaneous mass spectrometers[8], presumably becau se of detection-system bias.

In summary, the ICP-TOFMS offers the potential forisotope-rati o precision comparable to or better thanthat reported for quadrupole ICP-MS instruments.However, lim itations in the curre nt detection system spreclude very precise mea surements. Because theTOFMS extracts all ions simultaneously from the ICP,one would exp ect ra tioing techniques to exhibit even

Acknowledgments

This research wa s funded in part by the NationalInstitutes of Health through grant ROI GM48653.

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