Isotope ratio measurement by hexapole ICP-MS: mass bias effect, precision and accuracy

Abstract

Isotope ratios were measured from the low mass (²⁵Mg/²⁶Mg) to the high mass (²⁰³Tl/²⁰⁵Tl) region, using an ELAN 5000 and a Platform Hex-ICP-MS, to investigate mass bias effects on the two instruments. For the ELAN 5000, the percentage mass discrimination per atomic mass unit (MD%) varies from 5% for ²⁵Mg/²⁶Mg to 0.7% for ²⁰³Tl/²⁰⁵Tl, whereas for the Platform it ranges from 10% to 0.11%, respectively. The Platform has higher mass bias at low mass and lower mass bias at the high mass region compared to the ELAN 5000. The difference in mass bias between the two instruments is attributed to the different ion transferring lens system, i.e., electrostatic lens system in the ELAN 5000 and hexapole collision cell in the Platform. For the Platform Hex-ICP-MS, the addition and variation of collision/reaction gases (He and H₂) does not introduce any additional mass bias. The precision and accuracy of isotope ratio measurement by the Platform Hex-ICP-MS were assessed using the NBS982 Pb isotope reference material and an elemental Sn standard spiked with ¹¹⁷Sn-enriched spike. The overall precision was found to be 0.1–0.6% relative standard deviation.

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Introduction

Precise and accurate isotope ratio measurements have traditionally been carried out by thermal ionization mass spectrometry (TIMS). In the past few years inductively coupled plasma source mass spectrometry (ICP-MS), with both a quadrupole mass analyzer-single collector (Q-ICP-MS) and a magnetic sector mass analyzer, single or multiple collector configuration (MC-ICP-MS), have increasingly been used in high precision and accuracy isotope ratio measurement.^1–3 Although Q-ICP-MS cannot achieve the precision and accuracy in isotope ratio measurement attainable by TIMS and MC-ICP-MS, the technique can still play an important role in isotope ratio measurement for such applications as isotope dilution analysis, isotope labelled metal bioavailability and toxicity studies, metal speciation studies and natural isotope variations.^2,4–6

Several aspects of instrument performance negatively affect the precision and accuracy of isotope ratio measurement by ICP-MS: specifically poor sensitivity, signal stability, spectral interferences, and mass discrimination. One way to reduce or eliminate spectral interference is to employ a high resolution magnetic sector mass analyzer. However, a drawback of the high resolution mass analyzer is the inherent decrease in instrument sensitivity.² Alternatively, a “chemical resolution” technique has been successfully applied to quadrupole ICP-MS. The technique utilizes a “collision” or “reaction” cell filled with various gases (e.g., He, H₂, NH₃ or Xe) to reduce or eliminate spectral interferences.^7–12 In most current models of ICP-MS that utilize the “chemical resolution” technique, the collision/reaction cell also acts as ion focusing optics that homogenize ion energy and hence increase ion transmission efficiency.^9,10,13

Mass bias is the deviation of measured isotope ratios by a mass spectrometer from the “true value”, due to mass discrimination that occurs in all mass spectrometers. In particular, several factors have been found to affect bias in ICP-MS, such as detector “dead time”, the electrostatic ion focusing lens and the “space charge” effect occurring at the interface region.^1,14 In general, mass bias in TIMS is time dependent, whereas mass bias change with time in ICP-MS has been demonstrated to be much slower under given instrumental parameters such as nebulization gas flow rate, torch position, and rf power. Hence, mass bias in ICP-MS can be predicted and more accurate corrections can be applied.¹⁵ However, with the addition of a “collision cell”, some concerns have arisen as to whether this introduces unpredictable mass bias effecs.^16,17

In this study, isotope ratios were measured at low and high mass regions, using a conventional ICP-MS and a hexapole “collision cell” ICP-MS, to compare the mass bias of the two. The effect of varying collision gases on the mass bias of the hexapole ICP-MS (Hex-ICP-MS) was also investigated. The precision and accuracy of isotope ratio measurement using a Hex-ICP-MS was assessed by the measurement of Pb isotope ratios in NBS-982 standard, and the Sn isotope composition of an elemental Sn standard with natural isotope abundance, but spiked with ¹¹⁷Sn-enriched isotope in various proportions.

Experimental

Samples and reagents

Pure elemental standards (Inorganic Ventures, Inc., Lakewood, NJ, USA) were used to prepare the diluted multi-element solutions. For Sn isotope ratios, in addition to diluted pure elemental standard, three solutions of pure elemental standard spiked with ¹¹⁷Sn-enriched spike (Oak Ridge Laboratory, USA) in different ratios were also prepared. Pb isotope ratios were measured using NBS 982 standard solution (NIST, Gaithersburg, MD, USA). Double distilled HNO₃ and deionized water (>18.2 MΩ, Barnstead, Dubuque, IA, USA) were used for sample dilution. All sample preparations were carried out in a clean room environment.

Instrumentation

The ICP-MS instruments used in this study were a PerkinElmer Elan 5000 (PE-Sciex, Concord, ON, Canada) and a Micromass Platform (Manchester, UK). The former is a conventional ICP-MS with four ion focusing lenses and a photon stop, whereas the latter is equipped with a hexapole collision/reaction cell. The collision cell filled with gases at 10⁻³ mbar (e.g., He and H₂) serves two functions: (1) reduction and elimination of Ar-based polyatomic interferences through collisional dissociation and reaction; and (2) that of an ion focusing lens through thermalization of ions that reduce the dispersion of ion energy, thus enhancing ion transmission.^9,10,12–14 Schematics of instrumental configuration are shown in Fig. 1. A standard sample introduction system consisting of a Meinhard-type nebulizer, a Scott type double path spray chamber, and a Fassel quartz torch were used on both instruments. Operation parameters for both instruments are listed in Table 1.

Fig. 1 Schematics of instrument configuration of the Elan 5000 (a) and Platform ICP-MS (b).

Table 1 Instrument operation parameters

a Instrument read-back.
PE Elan 5000—
Rf forward power	1100 W
Cool gas flow rate	10 L min^−1
Intermediate gas flow rate	0.8 L min^−1
Nebulizer gas flow rate	0.76 L min^−1
Sample uptake rate	1 ml min^−1
Sampler/skimmer cones	Ni, 1.1 and 0.89 mm diameter, respectively
Lens settings/V^a	P: −69, B: +9.1, S2: −8.3, E1: +8.2
Acquisition	Peak hopping
Dwell time	30 ms peak
Point peak	1
Micromass platform—
Rf forward power	1350 W
Cool gas flow rate	14 L min^−1
Intermediate gas flow rate	0.85 L min^−1
Nebulizer gas flow rate	0.76 L min^−1
Sample uptake rate	0.70 ml min^−1
Sampler/skimmer cones	Ni, 1.1 and 0.7 mm diameter, respectively
Extraction lens	−400 V
Exit lens	400 V
Hexapole bias	−1.6 V
Ion energy	2 V
Multiplier	450 V
He gas flow rate	Variable, optimum 5.0 ml min^−1
H2 gas flow rate	Variable, optimum 2.8 ml min^−1
Acquisition mode	Single ion measuring (peak hopping)
Dwell time	200 ms
Point/peak	1

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Results

Table 2 presents the results of isotope ratio measurements from low mass (Mg) to high mass (Tl) by the Elan 5000 and Platform. Also listed in the table are mass bias factors (f_MD) and percentage mass discrimination per atomic mass unit (MD%). The f_MD and MD% are defined as follows:

f_MD = R_true/R_measured (1)

MD(%) = (f_MD−1) × 100/Δm (2)

where R_true and R_measured are the true ratio and measured ratios, respectively. Most isotope ratios presented in Table 2 are considered invariable in nature, except Cu (⁶³Cu/⁶⁵Cu = 2.2321−2.2425) and Sr (⁸⁶Sr/⁸⁸Sr = 0.1207−0.1185).¹⁸ Accordingly, the representative isotope composition recommended by IUPAC¹⁸ were used as true ratios. The concentration for each of the elements in the multi-element solutions used in isotope ratio measurement was 10 ng ml⁻¹ for the Platform and 100 ng ml⁻¹ for the Elan 5000.

Table 2 Mass bias comparison between Elan 5000 and Platform ICP^a

Platform^b	^25Mg/^26Mg	^63Cu/^65Cu	^86Sr/^88Sr	^95Mo/^96Mo	^107Ag/^109Ag	^151Eu/^153Eu	^184W/^186W	^203Tl/^205Tl
1	0.8218	2.0278	0.1147	0.9427	1.0476	0.9056	1.0790	0.4186
2	0.8210	2.0054	0.1148	0.9438	1.0424	0.9118	1.0689	0.4148
3	0.8234	1.9939	0.1143	0.9422	1.0385	0.9060	1.0721	0.4170
4	0.8318	2.0053	0.1162	0.9480	1.0450	0.9065	1.0671	0.4182
5	0.8228	2.0107	0.1149	0.9344	1.0472	0.9042	1.0776	0.4228
6	0.8132	2.0178	0.1142	0.9358	1.0359	0.9052	1.0734	0.4139
7	0.8213	2.0285	0.1154	0.9267	1.0384	0.9032	1.0816	0.4202
8	0.8209	2.0147	0.1144	0.9386	1.0452	0.9069	1.0734	0.4169
9	0.8168	2.0243	0.1140	0.9323	1.0457	0.9001	1.0682	0.4192
10	0.8232	2.0111	0.1145	0.9433	1.0488	0.9096	1.0696	0.4184
Average	0.8216	2.0139	0.1147	0.9388	1.0435	0.9059	1.0731	0.4180
Standard	0.0048	0.0110	0.0007	0.0065	0.0044	0.0032	0.0049	0.0026
RSD (%)	0.58	0.55	0.58	0.69	0.43	0.36	0.46	0.61
f MD	1.1055	1.1140	1.0406	1.0167	1.0315	1.0112	1.0043	1.0022
MD(%)	10.58	5.71	2.03	1.67	1.58	0.56	0.22	0.11

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Elan 5000	^25Mg/^26Mg	^63Cu/^65Cu	^86Sr/^88Sr	^95Mo/^96Mo	^107Ag/^109Ag	^151Eu/^153Eu	^184W/^186W	^203Tl/^205Tl
a 10 ppb solution for Platform and 100 ppb for Elan 5000. b Collision gas settings: He = 5.0 ml min^−1, H2 = 2.8 ml min^−1. c From IUPAC (1997).
1	0.8669	2.0332	0.1139	0.9466	1.0139	0.8772	1.0640	0.4265
2	0.8412	2.0388	0.1211	0.9179	1.0449	0.8896	1.0256	0.3972
3	0.9357	1.9527	0.1155	0.9049	1.0361	0.8720	1.0696	0.4042
4	0.9342	2.1382	0.1137	0.9556	1.0524	0.8567	1.0466	0.3992
5	0.8406	1.9774	0.1161	0.9431	1.0174	0.8959	1.0640	0.4224
6	0.8519	2.0064	0.1127	0.9366	1.0497	0.8797	1.0576	0.4103
7	0.8621	2.0363	0.1100	0.9210	0.9914	0.8733	1.0986	0.4105
8	0.8349	2.1430	0.1131	0.9266	1.0418	0.8875	1.0777	0.4048
9	0.8462	1.9467	0.1041	0.8725	1.0149	0.8961	1.0609	0.4226
10	0.8216	2.0532	0.1139	0.9430	1.0362	0.8789	1.0538	0.4303
Average	0.8635	2.0326	0.1134	0.9268	1.0299	0.8807	1.0618	0.4128
Standard	0.0398	0.0677	0.0043	0.0245	0.0196	0.0121	0.0192	0.0118
RSD	4.61	3.33	3.83	2.64	1.90	1.38	1.80	2.87
f MD	1.0518	1.1038	1.0528	1.0298	1.0452	1.0402	1.0150	1.0148
MD(%)	5.19	5.20	2.65	2.99	2.26	2.01	0.75	0.74
Natural ratio^c	0.9083	2.2436	0.1194	0.9544	1.0764	0.9161	1.0777	0.4189

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For the Platform ICP-MS, the relative standard deviation (RSD) of measured isotope ratios varies between 0.4 and 0.7%. In all isotope ratios f_MD is >1, consistent with light isotopes being transmitted less efficiently in ICP-MS.¹⁹ The mass discrimination per atomic mass unit (MD%) varies from 10% for ²⁵Mg/²⁶Mg to 0.11% for ²⁰³Tl/²⁰⁵Tl, similar to previously determined MD% values by the same type of instrument.²

For the Elan 5000 the RSD of isotope ratio measurement is between 2 and 5%, significantly higher than that of the Platform, despite the elemental concentration in the analyte being higher for the Elan 5000 than for the Platform. The mass bias factor (f_MD) is also >1, and the MD% varies from 5% for ²⁵Mg/²⁶Mg to 0.7% for ²⁰³Tl/²⁰⁵Tl, similar to those reported by Heumann et al.¹ on an Elan 5000.

There is also a difference in MD(%) between the Elan 5000 and Platform (Table 2 and Fig. 2). Specifically, the Platform appears to have larger MD% at low mass, but smaller MD% at high mass compared with the Elan 5000. Heumann et al.¹ also found differences in mass bias among three quadrupole ICP-MS instruments (Spectromass 2000, Elan 5000, and HP4500), and attributed the difference to the different instrument design.¹

Fig. 2 Comparison of mass bias per amu versus mass between the Elan 5000 and Platform ICP-MS. See text for details of measurement and instrument parameters.

Discussion

Detector dead time

Elan 5000 uses a channel electron multiplier and a pulse counting system for ion signal measurement. At a high count rate, a continuous dynode electron multiplier may register fewer counts than actually occur, a phenomenon termed “detector dead time”.¹⁹ A consequence of detector dead time in isotope ratio measurement is the deviation of measured ratio from “true ratio” due to the dead time effect in measuring the more abundant isotope, and a correction may be necessary. For the Elan 5000 used in this study, Xie and Kerrich²⁰ found that measured ⁹¹Zr/⁹⁰Zr ratios were within error at analyte concentrations between 80 and 1000 ng g⁻¹ (equivalent to 10⁵–10⁶ counts s⁻¹), suggesting that at this count rate dead time correction is not necessary. The analyte concentration used for the Elan 5000 in this study was 100 ng ml⁻¹ (equivalent to 1 × 10⁵ counts s⁻¹).

The Platform uses a “Daly-type photomultiplier” with a linearity up to 1 × 10⁸ counts s⁻¹. At count rates >1 × 10⁸, a strong “after glow” will occur, and subsequent counts will not be registered, a phenomenon similar to the dead time in a continuous dynode electron multiplier. Fig. 3 shows the instrument response with varying Ba concentration measured on the Platform ICP-MS. The data indicates that the instrument response remains linear up to 2 × 10⁶ counts s⁻¹, corresponding to a Ba concentration of 10 ng g⁻¹. The analyte signal on the Platform in this study was between 1 × 10⁵ and 1 × 10⁶. Thus, a detector “dead time” correction was considered unnecessary.¹⁰ Accordingly, detector dead time is unlikely to be the reason that the two instruments have different mass bias.

Fig. 3 Count rate versus Ba concentration measured by the Platform, showing linear response of the detector up to 2 × 10⁶ counts s⁻¹.

Mass bias in ICP-MS with and without collision cell

Two major factors have been found to affect mass discrimination in ICP-MS: (1) electrostatic ion focusing lens; and (2) “space charge” and “nozzle separation” effect at the interface region.^1,14 Mass discrimination occurs when ion transmission efficiency in the ion transfer lens system is different with different ion energies. A plasma source produces ions with a large energy spread. In a particular ion transfer lens system, there will usually be an optimum lens setting that will transfer ions with a certain energy more efficiently than ions with different energy, introducing mass discrimination against either light ions or heavy ions, depending on the optimum focal energy.^14,21 For example, Xie and Kerrich²⁰ found that changing the B lens setting of the Elan 5000 from +6 to +14 V (40–80 digipots), ⁹⁰Zr/⁹¹Zr and ¹⁷⁸Hf/¹⁷⁹Hf ratios first increase, and then decrease, after an optimum setting of 60. They attributed such change in mass discrimination to the different energy focal point under different lens settings.²⁰ Other studies have also shown the effects of ion lens settings on mass bias in ICP-MS.^1,22 Such mass discrimination induced by the ion transfer lens system may be against light or heavy isotopes depending on ion energy. This mass discrimination effect is different from the mass discrimination caused by “space charge” and “nozzle separation” effects that occurs in the interface region.

In all ICP-MS designs, there is a substantial column of positively charged particles behind sampler and skimmer cones; positively charged particles are mutually repelled. In this region, the light ions tend to disperse more than heavy ions, resulting in mass discrimination against light isotopes. This is termed “space charge” effect.²³ More recently, Heumann et al.¹ suggested that when a supersonic jet of charged particles passes through the ICP-MS interface region, light isotopes should preferably be pumped down and therefore more light isotopes leave the central beam compared with the heavier isotopes. They termed this the “nozzle separation” effect. “Nozzle separation” effect also introduces mass discrimination against light ions. However, because the “space charge” and “nozzle separation” produce mass bias in the same direction, it is difficult to distinguish the two. The “space charge” effect has been considered to have the strongest mass bias effect in ICP-MS, inducing a mass discrimination consistently against the light isotope.²³

For an ICP-MS with an ion focusing lens system, such as the Elan 5000, the total bias is a combination of ion lens system and “space charge/nozzle separation” effects, and is dependent on ion mass and energy. In contrast, for the Platform ICP-MS equipped with a collision cell, the total bias is dominated by “space charge” and “nozzle separation” effect. Furthermore, the collision cell also homogenizes ion energy, reducing energy spread to <1 eV.²⁴ Hence, the total bias in a Platform is dependent on mass, but independent of ion energy. This is, perhaps, the reason that the Elan 5000 has lower apparent mass bias at low mass, but higher apparent bias at high mass (Fig. 2), whereas the Platform appears to have mass bias that is correlated exponentially with mass (Fig. 2). Palacz recently also showed the different mass bias between a hexapole single focusing MC-ICP-MS and a double focusing MC-ICP-MS, and suggested that the ESA employed in a double focusing MC-ICP-MS may offset the bias–mass correlation.¹⁵ The exponential law has been considered a good approximation for mass bias occurring in an ICP-MS interface.^25–29

Effect of varying collision gas on mass bias

As described above, the Platform uses a collision cell filled with gases as an ion transfer system, instead of an electrostatic ion focusing lens system, to guide ions to the quadrupole mass filter. The addition of He and H₂ serves two functions: (1) thermalization of ions to reduce ion energy spread; (2) reduction of Ar-based polyatomic interference through collisional dissociation and reaction. A question arises as to whether additional mass bias is induced by hexapole gases. To investigate the effect of collision gases on instrumental mass bias, isotope ratios ranging from ²⁵Mg/²⁶Mg to ²⁰³Tl/²⁰⁵Tl were measured with variable gas settings.

Table 3 and Fig. 4 present the f_MD and MD(%) of various isotope ratios measured under different gas settings. All other parameters remain the same (see Table 1). The f_MD varies from 1.1 at low mass (²⁵Mg/²⁶Mg) to 1.002 at high mass (²⁰³Tl/²⁰⁴Tl). There is no significant difference in bias per amu under different gas settings, except for ²⁵Mg/²⁶Mg, where MD% changes from 10.5 under He = 5.0 ml min⁻¹ and H₂ = 2.8 ml min⁻¹ to 7.7 under He = 5.0 ml min⁻¹ and no H₂ (Table 3, Fig. 4). These results suggest that change of hexapole gas settings does not significantly affect mass bias in the Platform. In contrast, changing lens setting in a conventional ICP-MS with an electrostatic ion focus lens system can induce mass bias against light or heavy isotopes depending on ion energy.^{14,19,20,22,23} It appears that in hexapole ICP-MS, mass bias occurs predominantly in the interface region induced by “space charge” and “nozzle separation” effects.

Fig. 4 Mass bias per amu (MD%) under three different hexapole gas settings for the Platform. The three settings are: (□) He = 5.0 ml min⁻¹ and H₂ = 2.8 ml min⁻¹; (△) He = 5.0 ml min⁻¹ and H₂ = 0 ml min⁻¹; and (○) He = 2.5 ml min⁻¹ and H₂ = 2.8 ml min⁻¹. Other parameters remain the same (see Table 1 and text for details).

Table 3 Mass bias for Platform ICP-MS with variable collision gas settings^a

	^25Mg/^26Mg	^63Cu/^65Cu	^86Sr/^88Sr	^95Mo/^96Mo	^107Ag/^109Ag	^151Eu/^153Eu	^184W/^186W	^203Tl/^205Tl
a Measured with 10 ppb multi-element solution.
He = 5.0 ml min^−1; H2 = 2.8 ml min^−1
f MD	1.1055	1.1140	1.0406	1.0167	1.0315	1.0112	1.0043	1.0022
MD(%)	10.55	5.70	2.03	1.67	1.58	0.56	0.22	0.11
He = 5.0 ml min^−1; H2 = 0 ml min^−1
f MD	1.0779	1.0748	1.0203	1.0156	1.0203	1.0111	1.0054	1.0017
MD(%)	7.79	3.74	1.02	1.56	1.01	0.55	0.27	0.09
He = 5.0 ml min^−1; H2 = 2.8 ml min^−1
f MD	1.0748	1.1104	1.0204	1.0140	1.0168	1.0047	1.0039	1.0020
MD(%)	7.48	5.52	1.02	1.40	0.84	0.24	0.19	0.10

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Precision and accuracy

Two measurements were made to assess the precision and accuracy of isotope ratio determination attainable with a hexapole ICP-MS. The ¹¹⁷Sn/¹²⁰Sn ratio was measured on three solutions (S1, S2 and S3). The three solutions were prepared from pure elemental Sn solution spiked with ¹¹⁷Sn-enriched isotope spike, with final Sn concentrations of about 6, 11 and 16 ppb, respectively. The spiked ratios based on gravimetric sample preparation are listed in Table 4. Also listed in Table 4 are the signal intensity and bias corrected measured ¹¹⁷Sn/¹²⁰Sn ratios. A separate 10 ppb Sn solution with natural isotopic composition was also measured to determine mass bias. Natural Sn isotope composition from IUPAC¹⁸ was used to calculate mass bias factor of ¹¹⁷Sn/¹²⁰Sn, and the calculated mass bias factor was used for bias correction of samples.

Table 4 Measured ¹¹⁷Sn/¹²⁰Sn ratio of natural Sn standard solutions spiked with ¹¹⁷Sn-enriched isotope spike^a

Measurement #	S1 (6 ppb)			S2 (11 ppb)			S3 (16 ppb)
	117 cps^b	120 cps	^117Sn/^120Sn^c	117 cps	120 cps	^117Sn/^120Sn^c	117 cps^b	120 cps	^117Sn/^120Sn^c
a Measured with: dwell time 0.2 s; inter-channel delay 0.02 s; 1 acquisition/measurement, total time/measurement 1 min. Other parameters as in Table 1. b Counts per second. c Bias corrected ratios. See text for details.
1	287997	295552	1.0014	363128	602349	0.6195	411103	863655	0.4892
2	282433	291080	0.9971	352519	587534	0.6166	403128	849517	0.4877
3	277396	287252	0.9924	351191	584561	0.6174	405763	851065	0.4900
4	279189	286293	1.0022	350850	583329	0.6181	410988	861023	0.4889
5	273643	280157	1.0038	353887	588990	0.6175	407849	856734	0.4889
Average	280132	288067	0.9994	354315	589353	0.6178	407766	856399	0.4889
Standard deviation	5423	5733	0.0046	5071	7609	0.0011	3430	6123	0.0008
Relative standard deviation (%)	1.94	1.99	0.46	1.43	1.29	0.18	0.84	0.71	0.17
Spiked ratio			1.0103			0.6236			0.4931
Deviation from true ratio			−0.0109			−0.0058			−0.0041

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The precision of ¹¹⁷Sn/¹²⁰Sn ratio based on 5 measurements ranges from 0.46% in S1 to 0.17% in S3. The accuracy, measured by the difference between the bias-corrected observed ratio and the spike-adjusted, true ratio, ranges from −0.0109 in S1 to −0.0041 in S3 (Table 4). The difference between bias corrected and spiked ratios suggests that there is still a residual error in bias correction. Such residual error most likely stems from the external mass bias correction, i.e., mass bias factors were measured on a different solution and applied to samples assuming they have similar mass bias. As mass bias in ICP-MS occurs predominantly in the interface region and is dependent on sample matrix, such external correction may not fully correct the bias effect. The experiments indicated that the instrument mass bias was stable over 4 h.

Internal correction is also commonly used in isotope ratio measurement by mass spectrometry. For example, in Nd isotope ratio measurement, mass bias is determined using ¹⁴⁶Nd/¹⁴⁴Nd and applied to other isotope ratios (e.g., ¹⁴³Nd/¹⁴⁴Nd) in the same sample. In cases where there is no pair of stable isotope (e.g., Pb) or all isotopes are to be measured (Ca, Fe, Cu, etc.), a different element close in mass can be used. For example, Tl has been used for Pb isotopes,²² and Zn for Cu isotopes.²⁹Table 5 presents Pb isotope measurement of the standard NBS982 by hexapole ICP-MS, using Tl for internal mass bias correction. An NBS982 solution with 20 ppb Pb was spiked with 10 ppb high purity Tl solution (Inorganic Ventures, Inc.). Mass bias factor was determined using measured ²⁰³Tl/²⁰⁵Tl and the IUPAC value (0.4189), and applied to Pb isotope ratios using an exponential correction law.²⁵

Table 5 Pb isotope ratio measurement of NBS982

Measurement #	^206Pb/^204Pb	^207Pb/^204Pb	^208Pb/^204Pb	^207Pb/^206Pb	^208Pb/^206Pb	^208Pb/^207Pb
a Ratios were corrected for mass bias using the ^203Tl/^205Tl ratio. See text for details.
1	36.8380	17.0503	35.9616	0.4628	0.9910	2.1411
2	36.3725	16.8636	35.6922	0.4636	0.9953	2.1468
3	36.4566	17.0327	35.9056	0.4672	1.0013	2.1432
4	36.5009	17.0850	35.7566	0.4681	0.9978	2.1317
5	36.6340	17.0631	35.9459	0.4658	0.9953	2.1369
6	36.7084	17.0923	36.1047	0.4656	0.9995	2.1465
7	36.5952	17.2067	36.1416	0.4702	1.0018	2.1306
8	36.9915	17.2687	36.4923	0.4668	1.0027	2.1479
9	36.6806	17.1249	36.2489	0.4669	1.0023	2.1470
10	36.4334	17.0139	35.7641	0.4670	0.9956	2.1319
11	35.8323	16.8769	36.0798	0.4710	1.0069	2.1378
12	36.2122	16.9513	36.1913	0.4681	0.9994	2.1350
13	36.1443	16.9717	35.8753	0.4696	0.9926	2.1138
14	36.1170	16.9091	36.1563	0.4682	1.0011	2.1383
15	36.2262	16.9605	36.5097	0.4682	1.0078	2.1526
16	35.9458	16.7921	36.1034	0.4672	1.0044	2.1500
17	36.3160	17.0106	36.4107	0.4684	1.0026	2.1405
18	36.3110	16.9584	36.4819	0.4670	1.0047	2.1513
19	35.9790	16.9331	36.5892	0.4706	1.0170	2.1608
20	36.2698	16.9670	36.1680	0.4678	0.9972	2.1317
21	36.0520	16.8923	36.0346	0.4686	0.9977	2.1332
Average	36.3627	17.0011	36.1245	0.4676	1.0007	2.1404
Standard deviation	0.3053	0.1147	0.2611	0.0020	0.0058	0.0102
Relative standard deviation (%)	0.84	0.67	0.72	0.43	0.58	0.48
Certified values	36.7385	17.1595	36.7443	0.4671	1.0002	2.1413
Deviation from true ratios	−0.3757	−0.1583	−0.6199	0.0005	0.0005	−0.0009

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The precision of bias corrected Pb isotope ratios ranges from 0.43% to 0.84% (Table 5). The RSD% for ratios involving ²⁰⁴Pb is higher than other isotope ratios. This is due to: (1) lower abundance in ²⁰⁴Pb; and (2) background correction of Hg contribution on mass 204. The deviation from the certified value ranges from 0.0005 (0.1%) for ²⁰⁷Pb/²⁰⁶Pb to −0.6199 (1%) for ²⁰⁸Pb/²⁰⁴Pb. The accuracy is also poorer for ratios involving ²⁰⁴Pb. A precision of 0.1% RSD or less has been achieved on Pb isotope ratio measurement of NBS981 with 100 ng g⁻¹ Pb on a Platform.¹⁰ Mason et al. obtained a precision of 0.03–0.05% in ³⁴S/³²S measurement of 10–50 mg L⁻¹ solutions using a Platform with He, H₂ and Xe as collision gases.¹¹

Conclusions

This study suggests that mass bias in hexapole ICP-MS appears to occur predominantly in the interface region, and has a bias–mass relation in agreement with previously established exponential form (Fig. 2). Compared with conventional ICP-MS, the hexapole ICP-MS has high mass bias at low mass (Mg) region, but lower mass bias at higher mass. Addition of hexapole gases (e.g., He and H₂) does not introduce a significant mass bias effect, in contrast to conventional ICP-MS equipped with an electrostatic ion focusing lens system, which may induce mass bias against light or heavy isotopes. The precision and accuracy were assessed by the measurement of ¹¹⁷Sn/¹²⁰Sn ratios on natural Sn solutions spiked with ¹¹⁷Sn-enriched spike, and the measurement of Pb isotopes on NBS982 using Tl for internal mass bias correction. The measured isotope ratios agree well with the expected values. For example, the measured ²⁰⁷Pb/²⁰⁶Pb on NBS982 is 0.46755, compared with a certified value of 0.46707. The overall precision of isotope ratio measurement by hexapole ICP-MS in this study is 0.2–0.6%.

Acknowledgements

The Platform Hex-ICP-MS was purchased with a Natural Science and Engineering Research Council of Canada (NSERC) equipment grant. This work was supported by an NSERC Major Facility Access grant, University of Saskatchewan, and the George McLeod endowment to the Department of Geological Sciences. We thank Fadi R. Abou-Shakra and Micromass Ltd. staffs for their help and continuous support. Incisive critiques and suggestions from the two journal reviewers have greatly improved the manuscript.

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