Comparison of Mass Discrimination Correction Methods and Sample

ac research

Anal. Chem. 2008, 80, 8355–8363

Articles Comparison of Mass Discrimination Correction Methods and Sample Introduction Systems for the Determination of Lead Isotopic Composition Using a Multicollector Inductively Coupled Plasma Mass Spectrometer Ce´line Gallon,*,† Jugdeep Aggarwal,‡ and A. Russell Flegal† Environmental Toxicology Department, University of California Santa Cruz, Santa Cruz, California 95064 The influence of sample introduction system on Neptune MC-ICPMS lead isotopic ratio measurements was tested on dilute solutions of the lead certified material NIST SRM 981 ([Pb] ) 0.2-170 ng g-1) using (1) a SIS spray chamber, (2) a MCN 6000 desolvating system, or (3) an Apex inlet system. The impact of using a high-efficiency X-cone in place of a standard H-cone with the MCN and Apex was also investigated. Performance of the sample introduction systems varied with lead concentration. Over 10 ng g-1, no system was significantly more precise or accurate. As lead concentrations decreased, both accuracy and precision diminished, and below 1 ng g-1, use of an X-cone in combination with the Apex and particularly with the MCN system notably improved the quality of the measurements. Various mathematical methods of mass bias correction using thallium additions were tested. Selection of 205Tl/203Tl for NIST SRM 997 to optimize data (1) daily, (2) for each introduction system, and (3) over all sessions significantly improved the data, with no major difference in the output between the three methods. Consistency of the 205Tl/203Tl ratio (2.3888) optimized over all data with previous observations by others supports the use of this value for future measurements. For the last six decades, lead isotopes have been used as environmental tracers in a variety of media. While most of those * To whom correspondence should be addressed. E-mail: gallon@ etox.ucsc.edu. † Environmental Toxicology Department. ‡ W.M. Keck Isotope Laboratory, Earth and Planetary Sciences Department, University of California Santa Cruz, CA 95064. Current address: Institute of Geophysics and Planetary Physics, University of California Santa Cruz, CA 95064. 10.1021/ac800554k CCC: $40.75  2008 American Chemical Society Published on Web 10/21/2008

measurements have been made by thermal ionization mass spectrometry (TIMS), this technique requires extensive measurement time. The development of multicollector inductively coupled plasma mass spectrometers (MC-ICPMS), with a higher ionization efficiency of the source, has made it possible to perform highly precise and accurate measurements with considerably shorter measurement time. Furthermore, mass bias correction using neither double- nor triple-spiked techniques yields more precise and accurate Pb isotope measurements by MC-ICPMS than has been possible by TIMS. Additionally, various types of introduction systems have been developed to improve sensitivity by increasing transport efficiency and to reduce matrix interferences in quadrupole-, high-resolution-, and MC-ICPMS instruments. High-efficiency sample introduction systems such as the ESI Apex or the CETAC MCN-6000 were developed to enhance analyte transport efficiency and to reduce the solvent loading to the plasma along with the amount of oxides and hydrides by isolating the analyte of interest from volatile sample components (acids and organic solvents) prior to their introduction into the plasma source.1 Combined with an MC-ICPMS, these systems raise the possibility of producing highly accurate data for samples with concentrations of lead on the order of nanograms per gram. However, concerns have been raised about the accuracy of measurements provided by some of these introduction systems.2 Here, we present a side-by-side comparison of these different systems and their influence on the precision and accuracy of isotopic measurements of lead. (1) Allen, L. B.; Siitonen, P. H.; Thompson, H. C.,., Jr. J. Anal. At. Spectrom. 1996, 11, 529–532. (2) Kamenov, G. D.; Mueller, P. A.; Perfit, M. R. J. Anal. At. Spectrom. 2004, 19, 1262–1267.

Analytical Chemistry, Vol. 80, No. 22, November 15, 2008

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Figure 1. Average values ((SD, 1σ) of 208Pb/206Pb, 208Pb/204Pb, 207Pb/206Pb, and 206Pb/204Pb obtained for repeated measurements of NIST SRM 981 solutions with [Pb] > 10 ng g-1, using exponential law correction of mass bias with (1) 205Tl/203Tl ) 2.3871 (gray), (2) 205Tl/203Tl optimized for each session (blue), (3) 205Tl/203Tl optimized for all sessions using a same introduction system (yellow), and (4) 205Tl/203Tl ) 2.3888 (red). Circles refer to the SIS/H-cone setup, squares to the Apex/H-cone, triangles to the MCN/H-cone, diamonds to the Apex/X-cone, and inverted triangles to the MCN/X-cone combination. Double- and triple-spike values8,18-22 (black) and MC-ICPMS data (white) are shown for comparison. White triangles correspond to data corrected using modified power law,23 circles with exponential law,3,4,6,7,22 and diamonds with an empirical external correction.9,10 The dotted line represents the average of literature double- and triple-spike values8,18,20-22 used as a reference.

Because isotopes going through a mass spectrometer experience instrumentally produced mass bias, mathematical correction is necessary to obtain accurate isotopic compositions. Since lead does not have a naturally occurring invariant isotopic ratio that would allow an internal correction of mass bias, the addition of double- and triple-spike lead solutions to samples has been used with both TIMS and ICPMS. These methods produce highly accurate data but are relatively complicated and costly. As an alternative, an external correction can be performed by adding thallium with a known isotopic composition, because its mass range overlaps with that of lead. This is usually done using an exponential function; however, this method introduces discrepancies with results obtained using spikes, and corrections for these are still subject to debate.3-10 In this report, we describe the quality of lead isotopic ratio measurements performed on a Thermo Finnigan Neptune MCICPMS using three sample introduction systems, with a range of lead concentrations covering 3 orders of magnitude. The three systems tested were as follows: (1) a stable introduction system (SIS) spray chamber, (2) an Apex HF inlet system, and (3) an MCN 6000 desolvating system. In order to further enhance the intensity of the ion beam, we also investigated the use of a highefficiency cone (X-cone) in place of the standard (H-cone) skimmer cone, in addition to the MCN and Apex. For each system, we tested and assessed the validity of various mass discrimination corrections methods using thallium additions.

MATERIAL AND METHODS Materials and Reagents. Acids used were purified by subboiling distillation, and dilute solutions were prepared with highpurity (18.3 Ω cm) water (Milli-Q). A set of mixed 2:1 Pb-Tl solutions was prepared in 2% HNO3 from stock solutions of standard reference materials (SRM) from the National Institute of Standards and Technology (NIST) SRM 981 for lead and SRM 997 for thallium. Lead concentrations in these solutions ranged from 170 to 80, 40, 20, 10, 8, 4, 2, 1, 0.8, 0.4, and 0.2 ng g-1. Instrumentation. Samples were introduced into the system with a PFA 50 nebulizer (Elemental Scientific, Inc.) free aspirating at a nominal flow rate of 50 µL/min. Three sample introduction systems were used: (1) a SIS, or cyclone double-pass spray chamber (Thermo Finnigan), (2) an Apex HF inlet system (Elemental Scientific Inc.), (3) and a MCN 6000 (CETAC Technologies Inc.). The SIS spray chamber is glass, consisting of a small-volume cyclonic spray chamber followed by a second homogenization chamber for signal stability. The Apex contains a heated (100 °C) Teflon cyclonic spray chamber followed by a multipass Peltier cooled (2 °C) condenser. The MCN-6000 has a heated PTFE spray chamber (85 °C) followed by a heated membrane desolvation system (160 °C), where volatile solvent components diffuse through a porous Teflon membrane and are removed by a counterflow of argon. The resulting dry aerosol formed by the analyte particles remains in the channel and,

(3) Belshaw, N. S.; Freedman, P. A.; O’Nions, R. K.; Frank, M.; Guo, T. Int. J. Mass Spectrom. 1998, 181, 51–58. (4) Collerson, K. D.; Kamber, B. S.; Schonberg, R. Chem. Geol. 2002, 188, 65–83. (5) Mare´chal, C. N.; Te´louk, P.; Albare`de, F. Chem. Geol. 1999, 156, 251– 273.

¨ ; amper, M.; Halliday, A. N. Int. J. Mass Spectrom. 1998, 181, 123– (6) Rehk 133. (7) Rehka¨mper, M.; Mezger, K. J. Anal. At. Spectrom. 2000, 15, 1451–1460. (8) Thirwall, M. F. Chem. Geol. 2002, 184, 255–279. (9) White, W. M.; Albare`de, F.; Te´louk, P. Chem. Geol. 2000, 167, 257–270. (10) Woodhead, J. J. Anal. At. Spectrom. 2002, 17, 1381–1385.

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combined with nitrogen, is then transported to the plasma.1,11,12 A small amount of nitrogen gas is added to the argon flow after both desolvation units to increase the sensitivity and stability of the signal. In both configurations, argon and nitrogen flow are adjusted to attain optimum sensitivity. Analyses were performed on a Thermo Finnigan Neptune double-focusing MC-ICPMS housed in a Class 1000 clean laboratory kept at constant temperature (20 °C) and humidity (∼65%). The instrument was equipped with nine Faraday collectors and an ion counter located behind a retardation potential quadrupole (RPQ) filter to give high-abundance sensitivity. The accelerating voltage was set to ∼10 000 V, and the rf power to 1200 W. Coolant and auxiliary argon flows were set to 15 and 0.8 L min-1, respectively. Both the sampler and skimmer cones were made of nickel. Two types of skimmer cones were used. In a first set of measurements, the standard skimmer cone designed for the Neptune (H-cone, Thermo Finnigan) was used in conjunction with the three types of introduction systems. In a second set of experiments, a high-efficiency cone (X-cone, Thermo Finnigan) replaced the standard cone and was used in combination with the MCN and the Apex. The back geometry of the X-cone, different from than in a conventional H-cone, allows a better transmission of ions and increases the sensitivity. Measurement Procedure. Ion beams were measured in static mode on seven Faraday collectors in the following positions: 202Hg in L3, 203Tl in L2, 204Pb in L1, 205Tl in the center cup, 206Pb in H1, 207 Pb in H2, and 208Pb in H3. The instrument parameters were optimized on a daily basis for maximum intensity of the signal with a Pb/Tl solution (170 ng g-1 Pb/85 ng g-1 Tl). Data collection was performed over 100 1-s cycles, with a 90-s sample uptake time. A 2σ rejection of the outliers was performed automatically. A blank (2% HNO3) was measured before each sample over 20 1-s cycles to correct for the baseline, and the introduction system was washed for 90-200 s with 2% HNO3 after each analysis. Periodically, a mixture of 2% HNO3 and 0.1% HF was used with the APEX and the MCN to prevent the buildup of thallium in the system. Blank values were stable over the course of each session, with 208Pb signals of ∼1 mV for the SIS/H-cone setup, 2 mV for the Apex/H-cone, 3 mV for the MCN/H-cone, 9 mV for the Apex/H-cone, and 6 mV for the MCN/X-cone combination (∼10-20 pg g-1 Pb). Counts of 204Pb were corrected for isobaric interferences from 204 Hg by monitoring 202Hg and assuming natural abundances of mercury isotopes (204Hg/202Hg ) 0.2298). The magnitude of the tail of 205Tl at mass 204 represented only ∼0.02% of the signal intensity. This number was lower than the minimum of 0.03% RSD measured for 208Pb and represented a low end for the lowabundance 204Pb. This suggests that the influence of the 205Tl tail on the 204Pb signal was negligible and did not require a mathematical correction or the use of the RPQ. The correction of the measured isotopic ratios for mass bias was made with (11) Coedo, A. G.; Dorado, T.; Padilla, I. Appl. Spectrosc. 1999, 53 (8), 974– 978. (12) Martin, M.; Volmer, D. A. Rapid Commun. Mass Spectrom. 1999, 13, 84– 86.

thallium using the exponential function and a reference ratio 205Tl/ 203 Tl ) 2.3871.13 RESULTS AND DISCUSSION Sensitivity. Sensitivity of lead measurements increased when switching the sample introduction interface from the conventional SIS glassware to higher efficiency systems, as well as upon replacement of the conventional H-cone with an X-cone. When using the H-cone, the intensity of the 208Pb signal reached 35 ± 4 V/ppm (n ) 4) for the SIS system and increased to 70 ± 4 (n ) 3) and 158 ± 15 V/ppm (n ) 5) with the Apex and the MCN, respectively. When using the X-cone in conjunction with the Apex or the MCN, the sensitivity increased by a factor of 2.6 relative to the Apex alone (200 ± 44 V/ppm; n ) 4) and 2.8 relative to the MCN (417 ± 26 V/ppm; n ) 4). These observations were in the range of other reported values.8,14-17 Stability. Overall, the MC-ICPMS showed good signal stability for all tested introduction systems. As lead concentrations decreased, there was an increasing instability in association with a decrease in signal intensity. With the standard H-cone configuration, the SIS presents a better stability than the other systems, especially at lower signal intensities, with 208Pb RSD 10 ng g-1 were in the order of 650-1000 ppm for 208Pb/206Pb, 300-450 ppm for 207Pb/206Pb, 550-900 ppm for 206Pb/204Pb, 800-1300 ppm for 207 Pb/204Pb, 1200-1900 ppm for 208Pb/204Pb, and 300-550 ppm for 208Pb/207Pb. Discrepancies between values obtained with thallium correction and with spiking techniques suggest that thallium and lead have different fractionation factors.9 Two different approaches have been used to correct for this apparent dissimilarity in mass bias between thallium and lead. The first is an empirical method that corrects directly for the difference,5,9 by determining the fractionation factor of lead isotopes for each sample from the slope of a regression line between ln(208Pb/206Pb) and ln(205Tl/203Tl). Further improvement involved determination of a specific factor for each ratio.10 We attempted to correct our data with this method using a fractionation factor of lead determined for each analytical session. The adjustment was performed with each isotopic ratio measured for concentrations of >10 ng g-1, to avoid interferences due to a lower stability at lower concentrations. Isotopic ratios obtained with this method were, for the most part, higher than doubleand triple-spike values obtained by others8,18-22 and showed a dramatic decrease in stability and accuracy compared to the initial exponential law correction (Table 1). Overall accuracy values typically ranged from 10 to 100%. Poor accuracies resulting from this empirical method of mass bias correction have been attributed to a good stability of mass discrimination that resulted in data clusters in the ln(rPb) versus ln(rTl) plot, preventing the calculation of accurate correlations.7 Evidence of this relative stability is given by mass bias values of thallium, defined as

( )

RTl RTl ) rTl

(

1 M205-M203

)-1

(2)

(16) Nowell, G. M.; Pearson D. G.; Ottley, C. J.; Schwietzers J., Dowall, D. In Plasma Source Mass Spectrometry: applications and emerging technologies; Holland, G., Tanner, S. D., Eds.; Royal Society of Chemistry: London, 2003; p 307 (International Conference on Plasma Source Mass Spectrometry). (17) Krachler, M.; Zheng, J.; Fisher, D.; Shotyk, W. Anal. Chem. 2004, 76, 5510–5517. (18) Thirwall, M. F.; Anczkiewicz, R. Int. J. Mass Spectrom. 2004, 235, 59–81. (19) Todt, W.; Cliff, R. A.; Hanser, A.; Hofmann, A. W. In Earth Processes: Reading the Isotope Code; Hart, S. R., Basu, A., Eds.; Kluwer Academic Publishers: Dordrecht, 1996; Vol. 95, pp 429-437. (20) Galer, S. J. G.; Abouchami, W. Mineral. Mag. 1998, 62A, 491–492. (21) Thirwall, M. F. Chem. Geol. 2000, 163, 299–322. (22) Baker, J.; Peate, D.; Waight, T.; Meyzen, C. Chem. Geol. 2004, 211, 275– 303. (23) Hirata, T. Analyst 1996, 121, 1407–1411.

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where RTl and rTl represent the true and measured 205Tl/203Tl ratios, respectively. White et al.21 observed a mass bias of ∼1% amu-1 on a VG Plasma 54 that varied by 4-7% during an analytical session, while Rehka¨mper and Mezger7 reported ∼0.7% amu-1 ± 1-3% on a Micromass IsoProbe. In comparison, we observed very stable mass bias of 0.5-0.9% amu-1 ± 0.2-3.1% during the course of each analytical session. Therefore, the poor accuracy noted using the empirical method of mass bias correction can be attributed to the very good instrument stability experienced during the course of this study that did not allow the calculation of suitable correlations. Woodhead10 solved the problem of the small spread of data by constructing a calibration curve with samples having variable amounts of matrix. Alternatively, Thirwall8 used data from several days to compute the slope of ln(208Pb/206Pb) versus ln(205Tl/203Tl) with sufficient precision. When we attempted to combine runs for each sample introduction system, although the spread of data was increased (up to 3-9%), it did not result in an improvement in either accuracy or precision (Table 1). A second correction method for the apparent difference in mass bias between thallium and lead consists of an adjustment of the 205 Tl/203Tl ratio to provide a value of 208Pb/206Pb that is the closest to a reference value.3,6 Further improvement includes adjusting the thallium ratio to best agree with each lead isotope ratio of interest.7,10 We adopted the latter approach and adjusted the value of 205Tl/203Tl through a least-squares calculation on a daily basis in order to provide the best fit for 208Pb/206Pb, 207Pb/206Pb, 206Pb/ 204 Pb, 207Pb/204Pb, 208Pb/204Pb, and 208Pb/207Pb ratios relative to the average spike value. Although this method may seem circular, Rehka¨mper and Mezger7 argued that it is only apparent since the adjusted thallium ratio is ultimately used to correct the mass bias of unknown samples run during the same analytical session. The average results obtained for concentrations of >10 ng g-1 using each analytical setup are shown in Table 1. Accuracies were in the order of 50-100 ppm for 208Pb/206Pb, 1-110 ppm for 207Pb/206Pb, 3-100 ppm for 206Pb/204Pb, 10-90 ppm for 207Pb/204Pb, 4-50 ppm for 208Pb/204Pb, and 15-100 ppm for 208Pb/207Pb. These values were consistently better than those obtained for the two other procedures. The 205Tl/203Tl ratio used for our calculations was changed on a daily basis and ranged from 2.3883 to 2.3897 (Table 1). These values are higher than the certified reference composition of NIST SRM 997 thallium (2.3871 ± 0.0010) but remain for the most part within the range (2.3865-2.3889) reported by others using a similar correction method (Table 1). These measurements were conducted over periods of two months for the SIS, Apex, and MCN/H-cone systems and 1 year for the Apex and MCN/X-cone configurations. During that period, the thallium ratio optimized for each system was relatively stable and varied by e0.0007 for each of them. Collerson et al.4 questioned the daily adjustment of the 205Tl/ 203 Tl ratio for two reasons. First, they noted that the range of 205Tl/ 203 Tl values (377 ppm) used by Rehka¨mper and Mezger7 was noticeably larger than the external range of the 206Pb/204Pb ratio (172 ppm), which hinders the validity of an “optimized” ratio over an entire measurement session. However, in our case, for each configuration the variation of the adjusted 205Tl/203Tl ratio was e300 ppm, while the reproducibility of 205Tl/203Tl over the


8359

Modified exponential law

(c) This study Standard exponential law

Baker et al.22 (AXIOM-double focusing) Kamenov et al.2 (Nu Plasma)

Woodhead10 (Nu Plasma)

Thirwall and Anczkiewicz18 (Micromass IsoProbe) Baker et al.22 (AXIOM-double focusing) Average DS/TS valuesb (b) Hirata23 (Plasma 54) Belshaw et al.3 (Nu Plasma) Rehkamper and Halliday6 (VG Elemental Plasma 54) Rehkhamper and Mezger7 (Micromass IsoProbe) White et al.9 (VG Elemental Plasma 54) Collerson et al.4 (Micromass IsoProbe) Thirwall8 (Micromass IsoProbe)

(a) Certified values Todt et al.19 (TIMS) Galer and Abouchami20 (TIMS) Thirwall21 (TIMS) Thirwall8 (Micromass IsoProbe)

introduction system

Pb-205Pb DS Pb-204Pb-206Pb TS Pb-204Pb DS Pb-204Pb DS

207

SIS/H-cone Apex/H-cone Cetac MCN 6000/H-cone Apex/X-cone Cetac MCN 6000/X-cone SIS/H-cone Apex/H-cone

2.3889

exponential law

law law law law law law law

2.3871 2.3871 2.3871 2.3871 2.3871 2.3885-2.3888 2.3887-2.3888

2.3875 3.3875

exponential law exponential law

exponential exponential exponential exponential exponential exponential exponential

2.3889

exponential law

empirical external

2.3889

exponential law

2.38869

2.3871

Micromist nebulizer + Cinnabar spray chamber Cetac Aridus desolvating nebulizer Cetac Aridus desolvating nebulizer Cetac Aridus desolvating nebulizer spray chamber Nu instruments DSN 100 desolvating nebulizer

empirical external

Glass expansion nebulizer

2.3865-2.3874

2.388808

exponential law

exponential law

Cetac MCN 6000

Tl/203Tl

2.3871 2.3875

205

Cetac MCN 6000

exponential law

Cetac MCN 6000

Pb-204Pb DS

207

modified power law exponential law

Pb-204Pb DS

207

207

207

202

normalization procedure

Glass expansion nebulizer Cetac MCN 6000

-Micromist nebulizer Cinnabar spray chamber - Cetac Aridus desolvating nebulizer Cetac Aridus desolvating nebulizer Cetac Aridus desolvating nebulizer

Table 1. Pb Isotope Ratiosa Pb/206Pb

Pb/206Pb

2.16606(46) 2.16607(30) 2.16601(16) 2.16627(55) 2.16559(66) 2.16751(58) 2.16752(33)

2.16657 2.16643

2.1679(18)

2.16689

2.16691(42)

2.16755(25)

2.16740(70)

2.1646(8)

2.16691(29)

2.16677(14)

0.91441(28) 0.91451(20) 0.91453(7) 0.91456(12) 0.91441(15) 0.91472(31) 0.91482(20)

0.91459 0.91459

0.91492(39)

0.91460

0.91470(10)

0.91486(6)

0.91461(18)

0.91404

0.91459(13)

0.91469(5)

0.914623(37) 0.91463(6)

0.91482(10)

2.16772(5) 2.16636(82) 2.1665(2)

0.914905(39)

0.91489(4)

0.91469(7) 0.91488(8)

0.91464 0.91459(13) 0.91475(35)

207

2.16781(12)

2.16768(12)

2.16770(21) 2.1677(2)

2.1681 2.16701(43) 2.16771(10)

208

Pb/204Pb

16.9315(118) 16.9308(54) 16.9287(28) 16.9319(47) 16.9264(56) 16.9429(112) 16.9423(53)

16.9369(39) 16.9373(11)

16.942(14)

16.9359(41)

16.9357(36)

16.9410(39)

16.941(6)

16.9467(76)

16.9366(29)

16.9364(55)

16.9311(90) 16.932(7)

16.9412(6)

16.9416(13)

16.9431

16.9409(22) 16.9417(29)

16.9377 16.9356(23) 16.9405(15)

206

Pb/204Pb

15.4824(91) 15.4835(24) 15.4818(25) 15.4853(62) 15.4777(76) 15.4980(90) 15.4992(23)

15.4904(34) 15.4907(12)

15.501(20)

15.4896(44)

15.49108

15.49864

15.4944

15.4899(39)

15.4900(17)

15.4912(51)

15.4856 15.487

15.4979(22)

15.5000(13)

15.5011

15.4956(26) 15.4996(31)

15.4919 15.4891(30) 15.4963(16)

207

Pb/204Pb

36.6746(220) 36.6733(79) 36.6678(70) 36.6791(193) 36.6557(231) 36.7239(200) 36.7229(76)

36.6949(87) 36.6935(39)

36.730(61)

36.6983(139)

36.69814

36.72046

36.7179

36.6825(78)

36.7000(23)

36.6969(128)

36.6800(21) 36.683

36.7237(19)

36.7262(31)

36.7273

36.7228(80) 36.724(9)

36.7226 36.7006(112) 36.7219(44)

208

16 13 20 12 15 16 13

42 29

114

47

89-92

16

114

33

48

30

8 15

5

119

14

41 36

11

N

8360


Cetac MCN 6000/H-cone Apex/X-cone Cetac MCN 6000/X-cone SIS/H-cone Apex/H-cone Cetac MCN 6000/H-cone Apex/X-cone Cetac MCN 6000/X-cone SIS/H-cone Apex/H-cone Cetac MCN 6000/H-cone Apex/X-cone Cetac MCN 6000/X-cone

Cetac MCN 6000/H-cone Apex/X-cone Cetac MCN 6000/X-cone SIS/H-cone Apex/H-cone

Cetac MCN 6000/H-cone Apex/X-cone Cetac MCN 6000/X-cone SIS/H-cone Apex/H-cone

introduction system

law law law law law

law law law law law

exponential law exponential law exponential law empirical correction empirical correction empirical correction empirical correction empirical correction empirical correction empirical correction empirical correction empirical correction empirical correction

exponential exponential exponential exponential exponential

exponential exponential exponential exponential exponential

normalization procedure Tl/203Tl

2.3888 2.3888 2.3888 2.3871 2.3871 2.3871 2.3871 2.3871 2.3871 2.3871 2.3871 2.3871 2.3871

2.3889 2.3885 2.3893 2.3888 2.3888

2.3888-2.3889 2.3883-2.3887 2.3890-2.3897 2.3887 2.3887

205

Pb/206Pb

2.16753(16) 2.16779(55) 2.16711(66) 2.208(32) 2.218(41) 2.219(53) 2.219(103) 2.206(19) 2.1975(59) 2.1996(47) 2.2046(23) 2.1867(18) 2.1918(43)

2.16762(16) 2.16752(55) 2.16756(66) 2.16758(46) 2.16759(30)

2.16760(17) 2.16755(44) 2.16756(40) 2.16749(46) 2.16750(30)

208

Pb/206Pb

0.91485(7) 0.91488(12) 0.91474(15) 0.930(8) 0.932(31) 0.925(16) 0.926(23) 0.922(5) 0.9203(13) 0.9220(10) 0.9221(5) 0.9188(4) 0.9200(9)

0.91487(7) 0.91483(12) 0.91483(15) 0.91474(28) 0.91483(20)

0.91487(7) 0.91483(10) 0.91483(8) 0.91472(28) 0.91482(20)

207

Pb/204Pb

16.9407(28) 16.9439(47) 16.9384(56) 17.56(145) 17.09(106) 17.51(85) 17.40(94) 17.25(9) 17.195(48) 17.169(37) 17.248(19) 17.101(14) 17.135(34)

16.9414(28) 16.9418(47) 16.9419(56) 16.9435(118) 16.9428(54)

16.9412(26) 16.9420(39) 16.9419(32) 16.9428(118) 16.9421(54)

206

Pb/204Pb

15.4983(25) 15.5017(62) 15.4942(76) 16.33(146) 15.93(56) 16.20(80) 16.12(128) 15.91(18) 15.824(64) 15.831(51) 15.905(25) 15.713(20) 15.765(47)

15.4992(25) 15.4988(62) 15.4990(76) 15.4988(91) 15.4999(24)

15.4990(24) 15.4991(51) 15.4990(42) 15.4979(91) 15.4989(24)

207

Pb/204Pb

36.7196(70) 36.7308(193) 36.7074(231) 38.77(363) 37.91(178) 38.85(225) 38.63(391) 38.04(53) 37.785(203) 37.765(163) 38.024(81) 37.396(63) 37.558(149)

36.7226(70) 36.7217(193) 36.7226(231) 36.7263(214) 36.7250(79)

36.7218(65) 36.7227(157) 36.7226(135) 36.7233(214) 36.7220(79)

208

20 12 15 16 13 20 12 15 16 13 20 12 15

20 12 15 16 13

20 12 15 16 13

N

a Obtained (a) using double-spike (DS) or triple-spike (TS) techniques (b) using Tl for mass bias correction with MC-ICPMS instruments combined with various introduction systems, (c) during this study for concentrations of Pb >10 ng g-1. Analytical errors (2 σ) given in parentheses correspond to the least significant digits; N is the number of replicates. Numbers in italics were calculated from the original literature data. b Average values from the DS and TS data of Galer and Abouchami,20 Thirwall,8,21 Thirwall and Anczkiewicz,18 and Baker et al.22

Empirical correction fPb adjusted for all days

Tl ratio adjusted - for all systems - for all days - fit to average DS values Empirical correction fPb adjusted each day

Tl ratio adjusted - for each system - for all days - fit to average DS values Modified exponential law

Tl ratio adjusted - for each system - for each day - fit to average DS values Modified exponential law

Table 1. Continued

different sessions was >400 ppm. Second, Collerson et al.4 argued that changing the 205Tl/203Tl cannot be justified as long as the thallium added to the measured solution originates from the same stock solution. However, we believe that this daily adjustment does not aim at determining a “true” 205Tl/203Tl, but rather is an artificial way of compensating for instrument parameters that can vary on a daily basis, since mass bias depends on the configuration of the system that varies with the session (e.g., type of introduction system, type and condition of the cones, position of the torch, extraction voltage and other focus potentials, gas flows, processes affecting thallium in solution in the nebulizer8,24). One can, still, argue that if the MC-ICPMS system is stable enough through extended periods of time, a daily adjustment of 205 Tl/203Tl may not be necessary. Therefore, we performed two supplementary sets of calculations to test whether our results were significantly modified by using (1) 205Tl/203Tl ratios (2.3885-2.3893) adjusted for all the sessions with a similar sample introduction configuration and (2) a single 205Tl/203Tl ratio (2.3888) optimized for all our analytical sessions. Results for lead concentrations of >10 ng g-1 are presented in Table 1 as well as in Figure 1, where they are compared to TIMS and MC-ICPMS literature data. In the first case, accuracies were in the order of 40-110 ppm for 208 Pb/206Pb, 3-120 ppm for 207Pb/206Pb, 15-100 ppm for 206Pb/ 204 Pb, 2-90 ppm for 207Pb/204Pb, 10-60 ppm for 208Pb/204Pb, and 10-100 ppm for 208Pb/207Pb. In the second case, the values were 60-280 ppm for 208Pb/206Pb, 10-95 ppm for 207Pb/206Pb, 30-170 ppm for 206Pb/204Pb, 25-240 ppm for 207Pb/204Pb, 40-450 ppm for 208Pb/204Pb, and 30-190 ppm for 208Pb/207Pb. Results obtained for lead concentrations of >10 ng g-1 using daily corrections seem more accurate than those achieved with a single 205Tl/203Tl ratio or with one ratio for all sessions using a similar configuration. However, when considering all data, multivariate statistical analysis including concentration, type of sample introduction system, and calculation method as independent variables showed no significant difference in accuracy or precision between the three 205Tl/203Tl adjustments (p ) 1; 95% confidence). Therefore, the use of a single 205Tl/203Tl ratio or of a value adjusted for each session did not produce statistically different results, suggesting that mass discrimination of lead and thallium was stable over the 15-month period covered by this study. Interestingly, the 205Tl/203Tl ratio optimized over all our data (2.3888, n ) 76) on a Neptune MC-ICPMS was remarkably close to the values found (1) using a similar method on a VG Plasma 546 (2.388 808), (2) from the analysis of silicate samples simultaneously spiked with Tl and Pb double spike on a Micromass IsoProbe8 (2.3889) and (3) from the log(rPb) versus log(rTl) plots of three independent lead isotope ratios on a Micromass IsoProbe4 (2.388 69), as shown in Table 1. These similarities suggest similar mass bias between the three instruments, validating the adoption of this new value as a reference for future measurements. Thus, the following discussion is based on lead ratios corrected for mass bias using the single 205Tl/203Tl ratio of 2.3888. Influence of Sample Introduction System and Concentration on Accuracy. For all the systems used, measured lead isotopic ratios were relatively stable and close to their accepted values for lead (24) Andre´n, H.; Rodushkin, I.; Stenberg, A.; Malinovsky, D.; Baxter, D. C. J. Anal. At. Spectrom. 2004, 19, 1217–1224.

Figure 2. Replicate measurements of 208Pb/204Pb and 207Pb/206Pb corrected with the modified exponential law (205Tl/203Tl ) 2.3888) versus lead concentrations. Trends for other ratios are similar. Inverted gray triangles refer to the SIS/H-cone setup, white triangles to the Apex/H-cone, gray circles to the MCN/H-cone, black triangles to the Apex/X-cone, and black circles to the MCN/X-cone combination. The dotted line represents the average of literature double- and triplespike values.8,18,20-22

concentrations of g10 ng g-1 (Figure 2). Below 10 ng g-1, all ratios departed from accepted values as concentrations decreased, with few differences in accuracies depending on the configuration (Figure 3). Accuracies for all systems and every isotopic ratio considered decreased by 1- 3 orders of magnitude over the range of concentration considered, from ∼5-450 ppm at 200 ng g-1 to 330-36 000 ppm at 0.2 ng g-1. Univariate statistical analysis showed that concentration influenced accuracy significantly (p e 0.04) for the ratios 208Pb/207Pb, 208Pb/206Pb, 207Pb/206Pb, and 206 Pb/204Pb and slightly (p e 0.06) for the ratios 208Pb/204Pb and 207 Pb/204Pb, probably as a result of the larger spread of data (i.e., poorer precision) observed with decreasing concentration. Accuracy was also significantly (p e 0.03; univariate analysis) influenced by the configuration of the sample introduction for ratios including 204Pb. This correlation was a reasonable observation, given that the introduction system impacts primarily sensitivity and that 204Pb is the least abundant of the lead isotopes. This conclusion is, however, only slightly supported by the examination of accuracy versus concentration plots (Figure 3). When using the H-cone configurations, no clear difference between the three types of introduction systems was observed, in spite of the 4.5fold difference in sensitivity between the SIS and the MCN configuration. Only a minor trend could be distinguished for the ratios including the low-abundance 204Pb, with accuracies improving with the sensitivity of the system, from the SIS to the Apex Analytical Chemistry, Vol. 80, No. 22, November 15, 2008

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Figure 3. Average accuracies ((SD, 1σ) and precision (2σ) obtained for the ratios 208Pb/204Pb and 207Pb/206Pb corrected with the modified exponential law (205Tl/203Tl ) 2.3888) versus lead concentrations. Trends for other ratios are similar. Inverted gray triangles refer to the SIS/ H-cone setup, white triangles to the Apex/H-cone, gray circles to the MCN/H-cone, black triangles to the Apex/X-cone, and black circles to the MCN/X-cone combination. Accuracy is defined as |r - R|/R, with r and R the measured and reference (average DS/TS; Table 1) isotope ratio, respectively. Precision defines the reproducibility of the data, and is calculated as SDr/r, with r the average measured isotope ratio and SDr the standard deviation (2σ) of this value.

and the MCN. This lack of clear trend was consistent with reports by others of similar or poorer accuracy for repeated runs of the NIST SRM 981 using a desolvating nebulizer compared to a spray chamber2,8 (Table 1). When the X-cone was combined with the Apex and the MCN, we observed similar or worse accuracies than with the H-cone at high concentrations. An exception was 208Pb/207Pb with the Apex/ X-cone setup that showed consistently better accuracies down to 2 ppm at 12 ng g-1 lead. But below 1 ng g-1, the trend reversed, and X-cone systems performed better than the H-cone ones. At 0.2 ng g-1 lead, the lowest concentration tested, the X-cone setups had accuracy values 1.4-12-fold lower that the H-cone systems. Accuracies reported in the literature for multiple analyses of pure standard solutions with MC-ICPMS for the ratios 208Pb/ 206 Pb, 207Pb/206Pb, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are in the order of 10-230 ppm.4,8,22 Since these data were obtained with solutions containing relatively high concentrations (50-200 ng g-1), they can be compared to our average data from repeated measurements on solutions containing more than 10 ng g-1 lead presented in Table 1. Our results, ranging from an accuracy of 10 to 450 ppm for the same ratios, are comparable. It is noteworthy that those published data do not show differences in accuracy related to the introduction system used, in agreement with our findings for this range of concentrations. Precision. As observed for accuracies, the external reproducibility (2σ) of the results was relatively stable for lead concentrations of >10 ng g-1 and increased as concentrations decreased, 8362


from 7-500 ppm at 200 ng g-1 to 200-88 000 ppm at 0.2 ng g-1 (Figure 3). Overall, there was no marked difference between the introduction systems when using the H-cone, although the SIS setup seemed to present slightly poorer reproducibility, especially for ratios containing 204Pb. Kamenov et al.2 and Thirwall8 both presented parallel data obtained with a spray chamber and a desolvation system (Table 1). While the former show an increase in external reproducibility by a factor of 2-4 with the desolvation unit, the latter do not show such a trend. Replacement of the H-cone with a high-efficiency X-cone had a different effect depending on the introduction system considered. With the Apex, the precision was not notably improved, while with the MCN, we observed better reproducibility for concentrations of lead of 10 ng g-1) were in those ranges or better for all ratios (∼5-250 pm for 208Pb/206Pb and 207Pb/206Pb,

∼10-550 for 206Pb/204Pb, ∼20-450 for 207Pb/204Pb, and ∼30-500 for 208Pb/204Pb). CONCLUSION Our analyses demonstrated that the choice of an appropriate sample introduction configuration is dependent on the concentration of the analyte considered for lead isotopic composition measurements. With samples containing >10 ng g-1 lead, none of the systems tested clearly stands out as being either significantly more precise or accurate than the others, although the performance of the SIS chamber was somewhat poorer. At those relatively high lead concentrations, the use of a highefficiency X-cone was unnecessary, as the improvement in sensitivity does not induce better data that would compensate for the potential damage caused by the intensive bombardment of ions on the defining slits of the mass spectrometer.17 As lead concentrations decrease, both accuracy and precision diminish, and for

Comparison of Mass Discrimination Correction Methods and Sample

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