High precision analysis of all four stable isotopes of sulfur (32S, 33S, 34S and 36S) at nanomole levels using a laser fluorination isotope-ratio-monitoring gas chromatography–mass spectrometry

doi:10.1016/j.chemgeo.2005.08.005

Chemical Geology

Volume 225, Issues 1–2, 5 January 2006, Pages 30-39

https://doi.org/10.1016/j.chemgeo.2005.08.005 Get rights and content

Abstract

The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric aerosols has had profound implications to our understanding of ancient and present atmospheric sulfur chemistry. We present a new technique that takes advantage of continuous He flow isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve precise analysis of all four stable sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) at nanomole level samples. The technique involves fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131 interference by a liquid-nitrogen ethanol slush at − 110 °C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Δ³³S and Δ³⁶S at 0.1 and 0.5‰ (2σ). Samples, as small as tens of nanomole, can be analyzed using this new method. One of the major sources of error in irm-GCMS is found to be tailing of the major ion beam (³²SF₅⁺) onto minor beams (³³SF₅⁺ and ³⁶SF₅⁺), which results in contraction of the measured δ³³S and δ³⁶S scales. This effect is corrected by measuring a series of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods.

Introduction

Sulfur has four stable isotopes, ³²S, ³³S, ³⁴S, and ³⁶S with fractional abundances of approximately 95.04%, 0.75%, 4.20%, and 0.015%, respectively (Ding et al., 2001). Measurements of three isotope ratios of sulfur, ³³S / ³²S, ³⁴S / ³²S, and ³⁶S / ³²S, have applications to studies of meteorites (Farquhar et al., 2000b, Gao and Thiemens, 1991), ancient rocks and minerals (Farquhar et al., 2000a, Mojzsis et al., 2003, Ono et al., 2003, Runnegar et al., 2002), polar ice (Savarino et al., 2003), and atmospheric samples (Romero and Thiemens, 2003). These materials are known to exhibit non-mass-dependent isotope effects that can only be seen by precise measurement of more than one sulfur isotope ratios.

Multiple isotope ratios of sulfur have been measured by a multi-collector secondary ion mass spectrometer (SIMS) (Farquhar et al., 2002, Greenwood et al., 2000, Mojzsis et al., 2003) and a conventional gas-source isotope-ratio mass spectrometer (IRMS) (Gao and Thiemens, 1991, Hoering and Prewitt, 1988, Hu et al., 2003, Hulston and Thode, 1965, Rumble et al., 1993). With the SIMS, polished samples are sputtered by a primary beam of Cs⁺ ions and the resulting secondary ions of ³²S⁻, ³³S⁻ and ³⁴S⁻ are collected simultaneously by multiple faraday cups. Precision for Δ³³S (see Notation section for definition) can be as good as 0.2‰ to 0.3‰ (2σ). SIMS has the best spatial resolution for in situ analysis (∼25 μm diameter) for measurements of two isotope ratios of sulfur (δ³³S and δ³⁴S).

Most laboratories measure δ³⁴S by gas source mass spectrometry in the form of SO₂ but analysis of Δ³³S is less precise compared to SF₆ method because of mass interferences due to three isotopes of oxygen (¹⁶O, ¹⁷O and ¹⁸O) (Hulston and Thode, 1965, Rees, 1978). The advantage of the SO₂ method in application of analysis of δ³³S and δ³⁴S, however, is high sample throughput when performed by combustion of sulfur by means of elemental analyzer and isotope analysis by isotope-ratio-monitoring gas chromatography–mass spectrometer (irm-GCMS) (Baublys et al., 2004).

Because fluorine has only one isotope (¹⁹F), SF₆ method has been the preferred method for high precision multiple sulfur isotope analysis. It involves fluorination of silver sulfide (or sulfide minerals) by either BrF₅ or F₂ and purification of SF₆ by gas chromatography (Gao and Thiemens, 1991, Hoering and Prewitt, 1988, Hu et al., 2003, Rumble et al., 1993) or a cryogenic trap (Beaudoin and Taylor, 1994). The accuracy and precision of the method is typically 0.2‰ for δ³⁴S and better than 0.1‰ and 0.2‰ for Δ³³S and Δ³⁶S respectively (Gao and Thiemens, 1991, Hoering and Prewitt, 1988, Hu et al., 2003). The SF₆ technique can be coupled with an IR laser (Beaudoin and Taylor, 1994) or a UV laser (Hu et al., 2003) for in situ analysis of sulfide minerals. However, a conventional dual inlet IRMS typically requires 1 to 5 μmol SF₆ for routine analysis. For pyrite, this corresponds to a ca. 300 to 500 μm diameter cylindrical pit of ca. 300 to 500 μm depth. Because modern laser sampling systems can achieve pit sizes more than ten times smaller than this, spatial resolution of the dual-inlet technique is limited not by the optics of the laser system but by the sample size requirement for dual inlet isotope analysis (Hu et al., 2003). This is also the case for the laser-sampling technique for oxygen isotopes analyses of minerals (Wiechert et al., 2002, Young et al., 1998).

Here, we describe the first application of an irm-GCMS to the laser ablation SF₆ technique to measure all four stable isotopes of sulfur. The advantage of irm-GCMS is analysis of small samples (nanomole to picomole level) by introducing sample gas in a stream of He. First, the precision and accuracy of the continuous flow SF₆ method is evaluated and discussed independent of assessment of errors associated with laser sampling. Overall precision and accuracy of the method for in situ analysis are tested by cross-comparison with conventional dual inlet methods and irm-GCMS. Significant improvement was made for measurements of ³⁶S that had been laborious and required careful analyses (Gao and Thiemens, 1991, Hoering and Prewitt, 1988).

Section snippets

Notation

Conventional delta notation is used to describe the isotope composition as: $δ^{x} S = ({}^{x}R_{sa} / {}^{x}R_{ref} - 1) \times 1000,$ where, x is 33, 34, or 36, and ^xR = ^xS / ³²S for the sample (R_sa) and the reference material (R_ref) such as Cañon Diablo Troilite. Capital delta notation is used to describe the deviation of the isotopic composition of a given sample from a reference mass fractionation line. We define (Hulston and Thode, 1965, Miller, 2002, Ono et al., 2003): $Δ^{33} S = [\ln (δ^{33} S / 1000 + 1) - {}^{33}λ \ln (δ^{34} S / 1000 + 1)] \times 1000 and$ $Δ^{36} S = [\ln$

Instrumentation

A laser fluorination SF₆ system was developed to measure four isotopes of sulfur (³²S, ³³S, ³⁴S, and ³⁶S) at the Geophysical Laboratory (Hu et al., 2003). We modified the earlier version of the system as described below. The system consists of three parts: a laser fluorination manifold, a gas chromatography purification system, and a gas-source isotope-ratio mass spectrometer (Finnigan MAT 253) (Fig. 1). The mass spectrometer has an acceleration voltage of 10 keV, and is equipped with faraday

Fluorination and sample injection

For in situ analysis, samples are prefluorinated at < 30 Torr F₂ without laser sputtering. This pre-fluorination is an important step for in situ laser analysis because compounds reactive with fluorine at room temperature (hydrocarbons, water, hydrous minerals, etc.) often interfere with fluorination reaction by producing S–O–F compounds (Beaudoin and Taylor, 1994, Hu et al., 2003). The prefluorination also minimizes a system blank that derives from spontaneous fluorination of fine-grained

Mass 131 interference

The relatively low abundance of ³⁶S (ca. 0.015%) and interferences on mass 131 (³⁶SF₅⁺) make measurement of δ³⁶S difficult. The mass interference on 131 is suggested to be C₃F₅⁺, which may be produced by fragmentation of fluorinated hydrocarbons (Rumble et al., 1993). Some protocols describe multiple GC purification or flushing GC column for hours between samples (Gao and Thiemens, 1991, Hoering and Prewitt, 1988, Hulston and Thode, 1965).

One of the advantages of irm-GCMS technique is real-time

Factors controlling precision and accuracy for Δ³³S and Δ³⁶S

For dual inlet SF₆ analysis, the major source of error is associated with fluorination reactions and incomplete transfer of the sample gas in the vacuum line. In the irm-GCMS system, shot noise error and ion scattering also introduce uncertainties. These can have a large effect on both δ³⁴S and Δ³³S, as evaluated in the following section.

Shot noise limited performance

We evaluate the irm-GCMS technique for shot noise error because this is the factor that ultimately limits the precision and accuracy of isotope ratio measurements. The shot noise error originates from the random distribution of SF₆ molecules in a capillary flow, and is estimated from counting statistics of the signal. The shot noise limited performance for sulfur isotope system can be written as (Merritt et al., 1994): $σ_{δ}^{2} = 2 \cdot 10^{6} \frac{(1 + {}^{x}R)}{{}^{x}R} \frac{1}{{}^{32}α E m N_{a}}$ where, x is 33, 34 or 36, ³²α is ³²S abundance

Peak tailing and abundance sensitivity correction

It was found that the major source of error in irm-GCMS analysis for SF₆ is from peak overlap of the major (127) beam and minor ion beams (128 and 131). All four SF₅⁺ peaks are well resolved for dual inlet analysis. However, upon introduction of He into the source (1.6 × 10^− 6 mbar source pressure) all peaks become broader and tailing occurs (Fig. 3).

Taking the 127 and the 128 beams as examples, when tailing of 127 beam overlaps onto 128, the measured isotope ratio ³³S / ³²S (³³R^m) is biased as, ${}^{33}R^{m} =$

Sample size dependence

The sample size dependence of the system was evaluated by injection of various sizes of SF₆. The SF₆ was produced by a single fluorination of a reference silver sulfide (IAEA S-3, Fig. 3) in order to avoid errors associated with fluorination reaction. Linear sample size dependence is found; 0.16, 0.04, and 0.38 ‰/100 nmol, for δ³³S, δ³⁴S, and δ³⁶S, respectively (Fig. 4). This linear sample size dependence is consistent with the expectation that the peak tailing contribution (¹²⁸i and ¹³¹i) is

Defining terrestrial fractionation line

Three IAEA reference materials (IAEA S-1, S-2 and S-3) and two pyrite separates from Permian ash beds (MD-99-33u and MZ-99-28B) were measured with the irm-GCMS in order to define reference fractionation line for the irm-GCMS system. Measurements were undertaken for fluorination of 0.4 mg silver sulfide or 0.1 mg of pyrite to produce 1.5 μmol of SF₆. The SF₆ was analyzed by sub-sampling SF₆ three times by expanding SF₆ to the injection loop, and the data for three analyses were averaged. First,

LA-irm-GCMS analysis of pyrite with mass-dependent sulfur isotope composition

The Geophysical Lab irm-GCMS system was tested for in situ analysis of a pyrite sample that was previously analyzed in our laboratory for δ³³S and δ³⁴S (Hu et al., 2003). For most analyses, laser pits of 150 μm diameter and 150 μm depth were produced with a UV laser (KrF), and yielded ca. 170 nmol SF₆. Smaller laser spots were made to test the analytical capability for small (< 100 nmol) samples. No systematic variations were observed between sample size, laser pit shape (depth/diameter), and

Conclusion

The LA-irm-GCMS described in this paper allows in situ analysis of sulfide minerals at laser spots of ∼150 μm diameter. Analysis of smaller spot size is currently possible with lower precision. Comparable precision is likely possible for smaller pit size upon optimization of the GCMS system (i.e. by increasing the split ratio). The technique offers an alternative to multi-collector SIMS for in situ analysis of sulfide minerals. Advantages of LA-irm-GCMS include relatively fast analysis and the

Acknowledgement

We thank R. Husted of Thermo Finnigan for technical support for mass spectrometer, G. Hu and P-L Wang for construction and various tests for initial SF₆ dual inlet system, M. Fogel, S. Shirey, and C. Henning for various inputs about irm-GCMS, and J. Eigenbrode and S. Bowring for providing samples. We also thank anonymous reviewers for their helpful comments. We acknowledge financial support from Carnegie Institution, NSF EAR-0125096 (Rumble) and NSF EAR-025953 (Rumble) and JPL Grand Challenge

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High precision analysis of all four stable isotopes of sulfur (32S, 33S, 34S and 36S) at nanomole levels using a laser fluorination isotope-ratio-monitoring gas chromatography–mass spectrometry

Abstract

Introduction

Section snippets

Notation

Instrumentation

Fluorination and sample injection

Mass 131 interference

Factors controlling precision and accuracy for Δ33S and Δ36S

Shot noise limited performance

Peak tailing and abundance sensitivity correction

Sample size dependence

Defining terrestrial fractionation line

LA-irm-GCMS analysis of pyrite with mass-dependent sulfur isotope composition

Conclusion

Acknowledgement

Geochimica et Cosmochimica Acta

Geochimica et Cosmochimica Acta

International Journal of Mass Spectrometry and Ion Physics

Geochimica et Cosmochimica Acta

Earth and Planetary Science Letters

Geochimica et Cosmochimica Acta

Earth and Planetary Science Letters

Geochimica Et Cosmochimica Acta

Organic Geochemistry

Geochimica et Cosmochimica Acta

Geochimica et Cosmochimica Acta

Earth and Planetary Science Letters

Geochimica et Cosmochimica Acta

Chemical Geology

High precision analysis of all four stable isotopes of sulfur (³²S, ³³S, ³⁴S and ³⁶S) at nanomole levels using a laser fluorination isotope-ratio-monitoring gas chromatography–mass spectrometry

Factors controlling precision and accuracy for Δ³³S and Δ³⁶S