# New frontiers for sulfur isotopic analyses

François Fourel, Christophe Lécuyer, Vincent Balter

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François Fourel, Christophe Lécuyer, Vincent Balter. New frontiers for sulfur isotopic analyses. Pro-cedia Earth and Planetary Science, 2015, New frontiers for sulfur isotopic analyses, 13, pp.232-239. 10.1016/j.proeps.2015.07.055. hal-01366836

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11th Applied Isotope Geochemistry Conference, AIG-11 BRGM

# New frontiers for sulfur isotopic analyses

Francois FOUREL<sup>a*</sup>, Christophe Lécuyer<sup>ab</sup>, Vincent Balter<sup>a</sup>

<sup>a</sup>Laboratoire de Géologie de Lyon "Terre, Planètes Environnement", CNRS-UMR 5276 Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon, Campus de la Doua, F-69622 Villeurbanne, France, <sup>b</sup> Institut Universitaire de France, 103 Boulevard Saint-Michel, 75005 Paris, France

## Abstract

Measurements of sulfur stable isotope ratios ($^{34}$S/$^{32}$S) have suffered in the past from technical difficulties in analyzing low-S materials reducing their use despite their undeniable scientific interest. The measurement of $^{34}$S/$^{32}$S ratios is a powerful tool for deciphering problems such as determining the sources of environmental pollutants, to detect adulteration, tracking the evolution of the redox state of the oceans, and quantifying the role of the bacterial activity in sulfide minerals genesis. We have used an innovative high-precision, easy and rapid method of simultaneous analysis of carbon ($^{13}$C/$^{12}$C), nitrogen ($^{15}$N/$^{14}$N) and sulfur ($^{34}$S/$^{32}$S) isotope ratios as well as elemental concentrations using a new combination of elemental analyzer and isotopic ratio mass spectrometer. This new technique provides multiple isotopic signatures on smaller sample aliquots with high precisions especially for sulfur determinations. The technique described here is Isotopic Ratio Mass Spectrometry (IRMS) on line in continuous flow mode with an Elemental Analyzer (EA) based on "purge and trap" technology rather than conventional packed-GC gas separation. Emphasis is put on the efficiency of the system to reliably combust sulfur-bearing compounds of both organic and inorganic origin with high conversion yields. This new technique demonstrates the high quality of $^{34}$S/$^{32}$S measurements for samples with S concentrations lower than 1 wt%. International calibrated references of diverse sulfur-bearing materials were used to calibrate two low (<1%) S-bearing phosphorites used as compositional reference material for future use as isotopic references: BCR32 and NBS120c. High quality of $^{34}$S/$^{32}$S determinations is illustrated using various international reference materials. Working calibrated material has been selected and characterized for all three isotopic signatures as well in order to fully use the capacities of the system in future work. The possibilities of such a system to reliably measure S isotopes as well as N and C within the same aliquot of sample opens up new fields of investigation in many domains where multi-isotopic approaches are required.

<footnote>* Corresponding author. Tel.: +33-0-472 44 62 42; fax: +33-0-472 43 35 88.
E-mail address: francois.fourelniv-lyon1.fr</footnote>
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the scientific committee of AIG-11

Keywords: carbon; nitrogen, sulfur; isotope; combustion; elemental-analyser

# 1. Introduction

Sulfur isotopic analyses have suffered for a long time from their high degree of technical difficulties compared to carbon or nitrogen isotopic analyses ¹. But beyond those technical difficulties, the interest of sulfur is that it occurs under different oxidation states (-2 to +6) and it is present under various organic and inorganic species. Sulfur at natural abundance shows four isotopic species ³²S (95.02%), ³³S (0.75%), ³⁴S (4.21%) and ³⁶S (0.02%). Due to those technical difficulties most studies concentrate on ³⁴S/³²S isotopic fractionation. To illustrate the complexity of the sulfur cycle we have reported in figure 1 a schematic outline of the natural sulfur cycle from Wynn et al 2008 ² and P. Wynn personal communication.

<figure><img src="image_2.png" /><figcaption>Figure 1: From Wynn et al. 2008 ²: Environmental sulfur cycle schematic illustrating the complexity of the different isotopic fractionations occurring naturally.</figcaption></figure>

This particular behavior of sulfur, once the technical aspect can be overcome, makes it a unique tool providing different or complementary information from conventional δ¹³C and δ¹⁵N measurements. δ¹⁵N, δ¹³C and δ³⁴S are accessible through continuous flow Elemental Analyser Isotopic Ratio Mass Spectrometry (EA-IRMS) techniques
since the early 90's usually in two steps: first the determination of δ¹⁵N, δ¹³C then the determination of δ³⁴S using different analytical setup and separating the analyte gases with a packed gas chromatography (GC) column.

The technique we present here is significantly different and uses “purge and trap” technology to separate the analyte gases which first improves significantly their separation and then allows the simultaneous determination of both δ¹⁵N, δ¹³C and δ³⁴S isotopic signatures as well as N, C and S elemental concentrations within 12 minutes.

## 2. Analytical techniques

The technical setup for NCS analyses using purge and trap technology is extensively described in Fourel et al 2014³ and is illustrated in figure 1.

<figure><img src="image_3.png" /><figcaption>Figure 2: Schematic principle of purge and trap EA technology configured in NCS mode. Compared to conventional EA techniques, the chromatographic column is replaced by 2 traps operating at room temperature which can then be heated to release the trapped gases.</figcaption></figure>

The system we used is an Elementar Pyrocube that consists of a combustion oven using tungsten oxide as an oxidant operating at 1150°C, then a reduction oven filled with reduced copper operating at 850°C connected to a chemical water trap filled with sicapent dryer. This is similar to traditional EA setup. After the water trap, the analyte gases are no longer separated on a packed GC column but enter the original purge and trap module. N₂ is not trapped and is then introduced first in the source of the IRMS. CO₂ and SO₂ are then trapped respectively in 2 separate traps. The CO₂ trap is then heated at 110°C to release CO₂ followed by the SO₂ trap which is then heated at 220°C. Analyte gases are then introduced in the source of an IsoPrime IRMS through an open split in parallel with pulses of N₂, CO₂ and SO₂ calibration gases. The resulting chromatogram is shown in figure 3.
<figure><img src="image_4.png" /><figcaption>Figure 3: Chromatograms for a typical NCS analysis using a Pyrocube purge and trap system comparing the SO₂ peak shape from a) conventional EA setup isotopic traces with b) purge and trap EA setup isotopic traces. SO₂ chromatographic peaks from masses 64 and 66 show significant improvement on the purge and trap chromatogram. C) shows the TCD traces measured from the Pyrocube software allowing the determinations of N, C and C compositions. The sample analysed here is our working calibrated material: rubeanic acid.</figcaption></figure>

What is noticeable from figure 3 is the quality of the SO₂ chromatographic peak resulting from the purge and trap technology which has a direct impact on the quality of the isotopic measurements. This chromatogram was generated from the combustion of one of our working calibrated material: rubeanic acid1 (RUB1: C₂H₄N₂S₂). Another advantage of this technique concerns the timings for heating the traps that are user defined. It means that the quality of the chromatography is not altered in the case of samples with high C/S or C/N ratios, thus opening up new possibilities for low sulfur content samples.

## 3. Results

### 3.1. $ ^{34}S / ^{32}S $ measurements with purge and trap EA-IRMS

Using the setup described above, we have first evaluated the quality of $ ^{34}S / ^{32}S $ measurements performed with this system. The capacity of such a system to reliably combust compounds for sulfur isotopic determinations has been evaluated and reported in Fourel et al 2014 ³. Both organic and inorganic sulfur samples were mixed in various proportions to test the conversion yields. We decided to use IAEA-S1 (Silver sulfide Ag₂S) and one of our working standard Ac. Rub.1 (rubeanic acid C₂H₄N₂S₂). We analysed sample amounts representing between 30µg and 350µg of S and calculated the yields based on the theoretical sulfur contents for Ag₂S and C₂H₄N₂S₂. The data calculated from the TCD recording gave us yields of respectively 99.1% (SD=1.7; n=11) and 101.2% (SD=2.0, n=15) for IAEA-S1 and Ac Rub 1. This demonstrates the capacity of the system to combust S-bearing samples into SO₂ as well as the linearity of such combustion yields, which is an important parameter when trying to tackle samples with low S concentrations.
Three batches of isotopic measurements were performed using IAEA-S1 (Silver sulfide Ag2S) δ³⁴SCDT = 0.30‰, IAEA-S2 (Silver sulfide Ag₂S) δ³⁴SCDT = 22.70‰, IAEA-S4 (native sulfur S) δ³⁴SCDT = 16.90‰, IAEA-S1 (Silver sulfide Ag₂S) δ³⁴SCDT = 0.3‰, IAEA-SO5 (Barium sulfate BaSO₄) δ³⁴SCDT = 0.5‰, IAEA-SO6 (Barium sulfate BaSO₄) δ³⁴SCDT = -34.10‰, and NBS127 (Barium sulfate BaSO₄) δ³⁴SCDT = 20.3‰ (SD=0.4) (IAEA TECDOC-825 1995) international reference standards to evaluate the technique. The reference material was selected in order to offer isotopic values bracketing the expected values for most studied samples.

<figure><img src="image_5.png" /><figcaption>Figure 4: δ³⁴SCDT Calibration run for NBS120c and BCR32. Working calibrated sample is Ac RUB-1 (rubeanic acid C₂H₄N₂S₂ admitted value δ³⁴SCDT = 4.43 (SD=0.28)).</figcaption></figure>

The quality of the results presented here for samples with less than 1 wt% of S allowed us to propose δ³⁴SCDT values of 18.16‰ (SD=0.31, N=23) and 18.33‰ (SD=0.39, n=20) for BCR32 and NBS120c, respectively.

### 3.2. $^{15}$N/$^{14}$N, $^{13}$C/$^{12}$C, $^{34}$S/$^{22}$S simultaneous measurements with purge and trap EA-IRMS

An important feature of the purge and trap system is the capacity of the system to measure simultaneously the δ¹³C, δ¹⁵N and δ³⁴S values of the same sample aliquot. Up to now there has been no reference calibrated material available for such multi-isotopic analyses. In order to evaluate the quality of these simultaneous isotopic measurements, samples of aspartic acid (an amino acid of chemical formula C₄H₇NO₄), which is used as an internal laboratory N and C calibration material, were mixed with samples of barium sulfate itself used as a calibration chemical compound for sulfur isotopic measurements (Figure 5)⁴. It is worthy to note that the aspartic acid had been previously calibrated against international reference material (IAEA-N1, IAEA-N2, IAEA-CH6, IAEA-CH-7, IAEA-C3, IAEA-C4). This test revealed that the isotopic reproducibility for the three isotopic ratios measured simultaneously compares well with the conventional individual determinations of δ¹³C, δ¹⁵N and δ³⁴S values (Table 1). Within the run, before and after the aliquots of our test mixture our two laboratory standards were also run individually with no significant differences between the values obtained as individual samples or mixed together.
<table><thead><tr><th></th><th>δ<sup>34</sup>S<sub>Mean</sub></th><th>SD<sub>δ34S</sub></th><th>N</th><th>δ<sup>15</sup>N<sub>Mean</sub></th><th>SD<sub>δ15N</sub></th><th>N</th><th>δ<sup>13</sup>C<sub>Mean</sub></th><th>SD<sub>δ13C</sub></th><th>N</th></tr></thead><tbody><tr><td>WS Mixture</td><td>20.22</td><td>0.24</td><td>12</td><td>-6.49</td><td>0.18</td><td>16</td><td>-24.80</td><td>0.12</td><td>16</td></tr><tr><td>Ref. Val.</td><td>20.30</td><td></td><td></td><td>-6.45</td><td></td><td></td><td>-24.85</td><td></td><td></td></tr></tbody></table>

Figure 5: Reproducibility for δ¹³C<sub>VPDB</sub>, δ¹⁵N<sub>AIR</sub> and δ³⁴S<sub>CDT</sub> values simultaneously determined from working laboratory calibration material (WS Mixture = aspartic acid C₄H₅NO₄ mixed with NBS127 barium sulfate BaSO₄ in proportion 2:1) used for N and C determinations and used for S determinations mixed together. SD = standard deviation. N = number of aliquots analysed. "Ref. Val." are the admitted values for aspartic acid and barium sulfate calibrated samples.

### 3.3. Multiple Isotopic Approach to investigate diets of ancient Egyptians

A significant application for the multi-isotopic approach was demonstrated when we investigated diets of ancient Egyptians⁵. Ancient Egypt emerged at the end of the Neolithic period (6000B.P.) and is particularly renowned for its exceptional longevity. This means that throughout their evolution, the Egyptian civilization underwent various major changes such as military conquests, climatic changes such as drought and flooding, revolutions, that might have influenced the day to day life and especially nutrition of the Egyptian people. Diets for those populations are difficult to estimate as the main sources of information are figurative depiction while food remains from graves are not necessarily fully representative of the whole Egyptian population. Stable isotope analyses of human tissues can generate significant information regarding diets of ancient Egyptians. For instance, it can indicate the various proportions of plant and animal proteins, the proportions of terrestrial or aquatic food or the proportions of C3 and C4 plants.

During this study⁵ we have determined δ¹³C values of hairs from ancient Egyptians. Those data are coherent with diets consisting of low amounts of C4 plant-derived food products as well as for inhabitants who lived in the Nile valley or oases despite different agricultural techniques. EA-IRMS δ¹³C values of hair are also coherent with δ¹³C determinations obtained from apatite carbonates analyzed by using acid digestion techniques. The lower δ¹³C values obtained for Coptic mummies might be explained by the consumption of olive oil introduced in Egypt during the Roman period.

Moreover, δ¹⁵N and δ³⁴S values of human and animal tissues from the studied area have been represented in figure 6. δ¹⁵N variations recorded in antelopes and gazelles are interpreted as resulting more from aridity than a specific diet. Thus, the use of δ¹⁵N alone may not be sufficient for reconstructing paleo-diets but it has to be coupled with another proxy such as δ³⁴S. Indeed, combinations of δ¹⁵N and δ³⁴S obtained from various human and animal samples tend to illustrate that the consumption of fish such as the Nile perch remained limited as indicated by the data obtained from hairs of Coptic mummies. This study appears as a good example of the importance of the multi-isotopic approach in the field of archeology.
<figure><img src="image_6.png" /><figcaption>Figure 6 : δ¹⁵N AIR and δ³⁴S CDT values of samples of human and animal tissues from Egypt (after Touzeau et al 2014 ⁵).</figcaption></figure>

3.4. *High precision δ<sup>34</sup>S measurements for medical applications*

Another field of application for high precision δ³⁴S determinations for samples with low (<1%) sulfur contents was investigated with natural stable isotope variations providing insights into copper and sulfur dysregulation in liver cancer ⁶. Copper isotope determinations were done with MC-ICPMS techniques and are beyond the purpose of this article. Sulfur stable isotope compositions (³⁴S/³²S) were performed using the purge and trap technology previously described. Blood and plasma samples analysed here represent another challenge for conventional isotopic NCS determinations as S concentrations are low (around 1 wt% of S) and the C concentrations are high (40 wt% of C) leading to high C/S ratios that are difficult to overcome with traditional chromatographic separations. The sulfur isotopic data are reported in figure 7.

The decrease of the sulfur isotope composition in blood of hepatocellular carcinomas HCC patients can be explained by the enhanced formation of sulfides, which δ³⁴S values can drop down to -20‰. Tumor-derived hydrogen sulfide and allosteric disulfide bonds in cancer-related proteins are the more likely candidates, but it is still unclear how prevalent is the formation of these compounds. Considering that the sulfur content in proteins is stoichiometric and therefore constant, the blood δ³⁴S value of HCC patients (δ³⁴S HCC = 4.0‰) can be considered to be a mixture of the normal sulfur isotope composition (δ³⁴S N = 5.4‰) and of total sulfides (δ³⁴STS = 20‰)

Our data indicate that about 6% of the sulfur in the blood of HCC patients originate from tumor-derived sulfides. The use of natural variations of stable isotope compositions, a technique originating from Earth sciences, provides new insights on tumor microenvironment and allows quantifying the imbalances that are produced in the body.
<figure><img src="image_7.png" /><figcaption>Figure 7 Sulfur isotope compositions in serum as a function of that of red blood cells. The black line stands for the least square correlation for all samples (y=1.05x-0.16, R²=0.86) from Balter et al 2014⁶.</figcaption></figure>

## 4. Conclusion

The results reported in this study demonstrate the capacities of the PyroCube Elemental Analyzer coupled online with an IRMS instrument to determine simultaneously δ¹³C, δ¹⁵N and δ³⁴S values from the same sample aliquot with high-levels of precision for the three isotopic systems (±0.2‰; ±0.2‰; ±0.4‰; respectively). This instrument opens up new possibilities for a multi-isotopic analysis of both organic and inorganic compounds without increasing either the analytical time or cost. Moreover, the results we are reporting in this study illustrate the capacities of the PyroCube system online with an IRMS instrument to reliably measure δ³⁴S_CDT values of phosphorite rocks (BCR32 and NBS120c) with low S concentrations, (0.4 to 0.7 wt%), although relative large aliquots (5 to 8 mg) are required to ensure enough precision (±0.3‰). The capacities of this technique are illustrated in two different fields of applications: paleo-diet reconstruction for archaeology and for medical applications to investigate liver cancer.

## References

1. B. Mayer & R.H. Krouse. (2004) Chapter 26, *Handbook of Isotope Analytical Techniques, Volume 1* 2004; 26: 538.

2. P.M. Wynn, I.J. Fairchild, A. Baker, S. Frisia, A. Borsato, J. Maldini & F. Mc Dermot (2008) *Geochim. Cosmochim. Acta*, **72**: 2465-2477.

3. F.Fourel, C. Lécuyer, F.Martineau, M. Seris (2014) Application of S isotopic analyses using purge and trap EA-IRMS technology to δ³⁴S analyses from natural phosphate material NBS120c and BCR32 *Geostandards and Geoanalytical Research*, **39**: 47-53.

4. F.Fourel, C. Lécuyer, F.Martineau, M. Seris (2014) Simultaneous N,C, S isotopic analyses using new purge and trap EA-IRMS technology. *Rapid Communications in Mass Spectrometry*, **28**: 2587-2594.

5. A. Touzeau, R. Amiot, J. Blichert-Toft, J.P. Flandrois, F. Fourel, V. Grossi, F. Martineau, P. Richardin & C. Lécuyer, (2014): Diet of ancient Egyptians inferred from stable isotope systematics. *Journal of Archaeological Science*, **46**: 114-124.

6. V. Balter, A. Nogueira da Costa, V. Paky Bondanese, K. Jaouen, N. Vincent, A. Lamboux, F. Fourel, P. Télouk, C. Lécuyer, A. Bréchot, F. Albarède & P. Hainaut, (2014): Natural stable isotope variations provide insights into copper and sulfur dysregulation in liver cancer *PNAS* **146**: 114-124.