Glenn Woods
Agilent Technologies (UK) Ltd.

Keywords

lead, isotope, ratio, geochronology, dating, mercury, artifacts, precious metals, food, ammonia, on-mass 

Introduction

Lead isotope ratio analysis is important as it is used for Pb-Pb dating in geochronology, and to trace the origin of artifacts, precious metals and even foodstuffs. The natural isotopic pattern of lead varies more than any other element in the periodic table, because three of its isotopes are formed from the radioactive decay of uranium
(²³⁵U → ²⁰⁷Pb; ²³⁸U → ²⁰⁶Pb) and thorium (²³²Th → ²⁰⁸Pb). The Pb isotopic pattern can therefore vary depending upon the geology of the rocks and minerals from which the lead was extracted, and the age of the material. In geochronology, the constant rate of U/Th decay allows the Pb/Pb, U/Pb and Th/Pb ratios to be used to date the age of rocks using a so-called geological clock.

When Pb ratios are measured, it is often necessary to correct for the lead naturally present in the sample, and the only non-radiogenic isotope of Pb (²⁰⁴Pb; natural or common lead), is used for this purpose. For Pb-Pb dating, ²⁰⁴Pb is the reference isotope against which the radiogenic isotopes are compared (²⁰⁶Pb/²⁰⁴Pb; ²⁰⁷Pb/²⁰⁴Pb). Unfortunately ²⁰⁴Pb is directly overlapped by an isotope of Hg (²⁰⁴Hg), which makes accurate measurement of ²⁰⁴Pb impossible by ICP-MS. Mass resolution of 
²⁰⁴Pb from ²⁰⁴Hg is far beyond the capability of any commercial high-resolution (HR-) ICP-MS system, and until recently there has been no reliable chemical means to remove the Hg interference, so mathematical correction has been employed, which introduces error. Mercury does however undergo a gas-phase charge-transfer reaction with ammonia gas (NH₃), a reaction that can be utilized in the collision/reaction cell of a suitably equipped ICP-MS as follows:

Hg+ + NH₃ → Hg⁰ + “NH₃⁺”

This reaction offers the potential to remove the ²⁰⁴Hg interference from ²⁰⁴Pb, and could be applied to either solution or laser-based ICP-MS analysis.

Experimental

Instrumentation: Agilent 8800 #100.

Plasma conditions: Preset plasma/General purpose.

Ion lens tune: Soft extraction tune:
Extract 1 = 0 V, Extract 2 = -170 V. 

CRC conditions: NH₃ gas (10% in He) at 1.7 mL/min, Octopole bias = -8 V,
KED = -8 V.

Acquisition parameters: Three acquisition modes were compared:

• No gas: No reaction cell gas; Single Quad (SQ) mode with Q1 operating as an ion guide
• NH₃ bandpass: Ammonia reaction gas; SQ mode with Q1 operating as a bandpass filter
• NH₃ MS/MS: Ammonia reaction gas; MS/MS mode with Q1 operating as a mass filter at unit mass resolution

Results and discussion

Removal of ²⁰⁴Hg⁺ interference on ²⁰⁴Pb⁺

A preliminary study showed that Pb is almost unreactive with NH₃ cell gas
(<0.5% loss of Pb signal) indicating that
on-mass sensitivity for Pb should be maintained. On-mass measurement of Pb in NH₃ cell gas mode was therefore investigated in the presence of
Hg at 10 ppb. Figure 1 displays the spectra obtained in no gas (left) and NH₃ cell gas (right) modes. The ²⁰⁴Hg interference on ²⁰⁴Pb can be clearly seen in the no gas spectrum, while it has been completely removed under NH₃ reaction mode with MS/MS. A perfect isotopic pattern match was confirmed for Pb in NH₃ mode.

Effectiveness of MS/MS

The NH₃ reaction that removes the ²⁰⁴Hg interference would also work in the reaction cell of a quadrupole ICP‑MS (ICP-QMS), but ammonia is a highly reactive gas and can produce many adduct cluster ions, for example from Rare Earth Elements (REEs), see Table 1. The complex matrix composition of many natural samples means that the results obtained with NH₃ cell gas in ICP-QMS are often extremely unreliable. With the
8800 ICP-QQQ, MS/ MS mode allows all the co-existing matrix elements to be rejected by Q1, so only the target ions (²⁰⁴Pb and ²⁰⁴Hg) enter the CRC. The NH₃ reactions are therefore controlled and consistent, and no overlapping reaction product ions are formed from other elements in the sample.

To check the formation of cluster ions, the ICP-QQQ was operated with NH₃ cell gas; “Single Quad bandpass” and MS/MS modes were compared for the measurement of a 50 ppb REE mix. Figures 2a and 2b display the spectra obtained using bandpass and MS/MS conditions, respectively.

²⁰⁴Pb/²⁰⁸Pb isotope ratio analysis in presence of Hg 

To check the effectiveness of the ²⁰⁴Hg removal, the ²⁰⁴Pb/²⁰⁸Pb ratio was measured in a 1 ppb lead solution spiked with increasing Hg concentration. Table 2 displays the measured Pb ratio results (without any mass bias correction), showing that the Pb isotope ratio remained constant, regardless of the Hg content.

Conclusions

With the successful removal of the ²⁰⁴Hg interference on the natural ²⁰⁴Pb isotope, ICP-QQQ displays great promise for Pb/Pb and U/Pb dating and for other applications where accurate measurement of ²⁰⁴Pb is required.