Geerke H Floor
email: geerke.floor@aquatrain.eu

Romain Millot

Mónica Iglesias

Philippe Négrel

Influence of methane addition on selenium sensitivity and spectral interferences

Keywords: selenium, isotopes, methane, multi collector ICPMS, interferences, precision, hydride generation

The measurements of stable Se isotopic signatures by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) are very challenging, due to the presence of spectral interferences and the low abundance of Se in environmental samples. We systematically investigated the effect of methane addition on the signal of selenium and its interferences. It is the first time that the effect of methane addition has been assessed for all Se isotopes and its potential interferences using hydride generator multi collector inductively coupled plasma mass spectrometry (HG-MC-ICP-MS). Our results show that a small methane addition increases the sensitivity. However, the response differs between a hydride generator and a standard introduction system, which might be related to differences in the ionization processes. Both argon and hydrogen based interferences, the most common spectral interferences on selenium isotopes in HG-MC-ICP-MS, decrease with increasing methane addition. Therefore analyte-interference ratios and precision are improved. Methane addition has thus a high potential for the application to stable Se isotopes ratios by HG-MC-ICP-MS.

Introduction

Selenium (Se) is an element with both environmental and toxicological interest, due to its narrow range between essential and poisonous concentrations. As for other elements, its speciation plays a key role in its toxicity and mobility [START_REF] Johnson | Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions[END_REF] . Therefore, an understanding of the biogeochemical transformations of selenium is of primary importance. Selenium has six stable isotopes (74, 76, 77, 78, 80 and 82) that may fractionate during biological and geochemical processes. [START_REF] Johnson | Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions[END_REF][START_REF] Johnson | Selenium stable isotope ratios as indicators of sources and cycling of selenium: Results from the northern reach of San Francisco Bay[END_REF][START_REF] Johnson | A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes[END_REF][START_REF] Johnson | Selenium isotope fractionation during reduction by Fe(II)-Fe(III) hydroxide-sulfate (green rust)[END_REF][START_REF] Johnson | Mass-Dependent Fractionation of Selenium and Chromium Isotopes in Low-Temperature Environments[END_REF][START_REF] Herbel | Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions[END_REF][START_REF] Herbel | Selenium stable isotope ratios in California agricultural drainage water management systems[END_REF][START_REF] Rouxel | Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes[END_REF][START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Ellis | Stable isotope fractionation of selenium by natural microbial consortia[END_REF][START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Zhu | High Precision Measurement of Selenium Isotopic Composition by Hydride Generation Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a 74Se-77Se Double Spike[END_REF] Consequently, stable Se isotope measurements could potentially lead to a better understanding of Se mobility in the environment.

Selenium determinations are challenging for several reasons. Firstly, Se has a low abundance in geological matrixes. e.g. [START_REF] Johnson | Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions[END_REF][START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF] For example the average concentration in upper continental crust is as low as 0.083 mg kg -1 . [START_REF] Wedepohl | The composition of the continental crust[END_REF] Secondly, the sensitivity of Se is lower compared to most other elements in ICP techniques due to its high ionization energy that results in a poor ionization yield (≈33%) in an argon plasma. [START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF][START_REF] Moellmer | ICP-MS Analysis of Trace Selenium in the Great Salt Lake[END_REF] Thirdly, four out of six Se isotopes ( 74 Se, 76 Se, 77 Se, 82 Se) have a low natural abundance (<10%). [START_REF] Elwaer | Comparative performance study of different sample introduction techniques for rapid and precise selenium isotope ratio determination using multi-collector inductively coupled plasma mass spectrometry (MC-ICP/MS)[END_REF] Lastly, spectral interferences overlap with Se isotopes of which the plasma derived Ar polyatomic ions (on m/z + 76, 78 and 80) are a major concern. [START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF][START_REF] Moellmer | ICP-MS Analysis of Trace Selenium in the Great Salt Lake[END_REF][START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Evans | Interferences in inductively coupled plasma mass spectrometry. A review[END_REF] Isotopic measurements differ from concentration measurements because more interference free isotopes are needed. For element determination only the intensity of one single isotope (or two for quality control) is used. In contrast, for isotopic measurements a minimum of two (using standard bracketing for mass bias correction [START_REF] Rouxel | Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes[END_REF][START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Carignan | Scaling NIST SRM 3149 for Se isotope analysis and isotopic variations of natural samples[END_REF] ) or four (using a double-spike for mass bias correction [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Zhu | High Precision Measurement of Selenium Isotopic Composition by Hydride Generation Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a 74Se-77Se Double Spike[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF] ) are needed. To plot the results on the mass fractionation line as control that interferences are well resolved, even one more interference isotope is required. Moreover, in MC-ICP-MS a total resolution of interferences is more important than in single collector ICP-MS, since a small deviation in signal intensity has a significant impact on the Se isotope ratio. Quadropole ICP-MS can be equipped with a reaction-collision cell to resolve interferences [START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF][START_REF] Moellmer | ICP-MS Analysis of Trace Selenium in the Great Salt Lake[END_REF][START_REF] Warburton | Methane mixed plasma-improved sensitivity of inductively coupled plasma mass spectrometry detection for selenium speciation analysis of wheat-based food[END_REF][START_REF] Goenaga Infante | Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP-MS detection by ultrasonic nebulisation or carbon-loaded plasma[END_REF][START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF] . However, most multi-collector ICP-MS apparatuses lack the presence of a reaction-collision cell. Although high resolution has been used in Se determinations in sector field ICP-MS, high resolution for Se isotopes in a multi-collector ICP-MS has never been reported to date, due to the small mass differences and the lower resolving power of multi collector compared to traditional high sector field ICP-MS [START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF][START_REF] Larsen | Carbon-enhanced Inductively Coupled Plasma Mass Spectrometric Detection of Arsenic and Selenium and Its Application to Arsenic Speciation[END_REF][START_REF] Vanhaecke | Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis[END_REF] . Therefore, other approaches to improve the analyte-interference ratio are essential in isotopic measurements.

For selenium isotope ratio applications, high measurement precision is of extreme importance, because the variations in isotopic composition are very subtle (few promille). [START_REF] Johnson | Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions[END_REF][START_REF] Johnson | Selenium stable isotope ratios as indicators of sources and cycling of selenium: Results from the northern reach of San Francisco Bay[END_REF][START_REF] Johnson | A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes[END_REF][START_REF] Johnson | Selenium isotope fractionation during reduction by Fe(II)-Fe(III) hydroxide-sulfate (green rust)[END_REF][START_REF] Johnson | Mass-Dependent Fractionation of Selenium and Chromium Isotopes in Low-Temperature Environments[END_REF][START_REF] Herbel | Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions[END_REF][START_REF] Herbel | Selenium stable isotope ratios in California agricultural drainage water management systems[END_REF][START_REF] Rouxel | Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes[END_REF][START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Ellis | Stable isotope fractionation of selenium by natural microbial consortia[END_REF][START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Zhu | High Precision Measurement of Selenium Isotopic Composition by Hydride Generation Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a 74Se-77Se Double Spike[END_REF] Precision is affected by the analyte signal (count statistics). Since Se levels are low in natural samples, improvement of Se sensitivity is therefore a major issue in isotopic Se measurements.

For above mentioned reasons, Se isotope measurements can be improved by increasing the Se sensitivity and increasing the analyte-interference ratio. An online hydride generator system coupled to the MC-ICP-MS increases the sensitivity, because it significantly enhances the efficiency of the analyte transfer to the plasma and the ionization compared to standard wet aerosol introduction. [START_REF] Rouxel | Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes[END_REF][START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Zhu | High Precision Measurement of Selenium Isotopic Composition by Hydride Generation Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a 74Se-77Se Double Spike[END_REF][START_REF] Hwang | Diomiguardi A hydride generator system for a 1-kW inductively coupled plasma[END_REF] Additional, the hydride generation only transfers volatile hydride forming elements to the plasma, thereby excluding some sample derived spectral interferences such as 66 Zn [START_REF] Elwaer | Comparative performance study of different sample introduction techniques for rapid and precise selenium isotope ratio determination using multi-collector inductively coupled plasma mass spectrometry (MC-ICP/MS)[END_REF] O + , 48 Ti 14 N 14 N and 39 K 39 K. [START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Warburton | Methane mixed plasma-improved sensitivity of inductively coupled plasma mass spectrometry detection for selenium speciation analysis of wheat-based food[END_REF][START_REF] Goenaga Infante | Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP-MS detection by ultrasonic nebulisation or carbon-loaded plasma[END_REF][START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF][START_REF] Larsen | Carbon-enhanced Inductively Coupled Plasma Mass Spectrometric Detection of Arsenic and Selenium and Its Application to Arsenic Speciation[END_REF][START_REF] Vanhaecke | Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis[END_REF][START_REF] Hwang | Diomiguardi A hydride generator system for a 1-kW inductively coupled plasma[END_REF][START_REF] Floor | Selenium determination in volcanic soils by ICP-QMS: influence of reaction cell pressurization and methanol addition on the occurrence of spectral interferences[END_REF] On the other hand, carbon addition to the plasma shows beneficial effects on Se determination in ICP techniques. Both organic solvents added to the aqueous solutions or volatile carbon compounds introduced into the spray chamber can be used as carbon source. [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Evans | Interferences in inductively coupled plasma mass spectrometry. A review[END_REF][START_REF] Warburton | Methane mixed plasma-improved sensitivity of inductively coupled plasma mass spectrometry detection for selenium speciation analysis of wheat-based food[END_REF][START_REF] Goenaga Infante | Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP-MS detection by ultrasonic nebulisation or carbon-loaded plasma[END_REF][START_REF] Larsen | Carbon-enhanced Inductively Coupled Plasma Mass Spectrometric Detection of Arsenic and Selenium and Its Application to Arsenic Speciation[END_REF][START_REF] Allain | Signal enhancement of elements due to the presence of carbon-containing compounds in inductively coupled plasma mass spectrometry[END_REF][START_REF] Rodushkin | Improved multi-elemental analyses by inductively coupled plasma-sector field mass spectrometry through methane addition to the plasma[END_REF][START_REF] Hill | Investigations into the application of methane addition to the nebulizer gas in inductively coupled plasma mass spectrometry for the removal of polyatomic interferences[END_REF][START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF][START_REF] Evans | Effect of Organic Solvents and Molecular Gases on Polyatomic Ion Interferences in Inductively Coupled Plasma Mass Spectrometry[END_REF][START_REF] Park | Application of methane mixed plasma for the determination of Ge, As, and Se in serum and urine by ICP/MS[END_REF][START_REF] Ebdon | Evaluation of Ethene Addition to the Nebulizer Gas in Inductively Coupled Plasma-Mass Spectrometry for the Removal of Matrix-, Solvent-, and Support-Gas-Derived Polyatomic Ion Interferences[END_REF] Carbon addition has two effects on Se determination. Firstly, a carbon loaded plasma increases the Se sensitivity. This enhancement is explained by several mechanisms, including a charge transfer reaction from C + -species to analyte atoms. [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Allain | Signal enhancement of elements due to the presence of carbon-containing compounds in inductively coupled plasma mass spectrometry[END_REF][START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF] Secondly, carbon loaded plasmas show lower levels of interferences, related to the competitive formation of carbides. [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Evans | Interferences in inductively coupled plasma mass spectrometry. A review[END_REF][START_REF] Rodushkin | Improved multi-elemental analyses by inductively coupled plasma-sector field mass spectrometry through methane addition to the plasma[END_REF][START_REF] Hill | Investigations into the application of methane addition to the nebulizer gas in inductively coupled plasma mass spectrometry for the removal of polyatomic interferences[END_REF][START_REF] Evans | Effect of Organic Solvents and Molecular Gases on Polyatomic Ion Interferences in Inductively Coupled Plasma Mass Spectrometry[END_REF] Both effects increase the analyte-interference ratio. However, until now there is no systematic evaluation of the Se sensitivity and interferences for all Se isotopes.

In this study we asses the potential of a methane mixed plasma for the determination of Se isotopes using the MC-ICP-MS Neptune. For both a hydride generator and a standard introduction system, we studied the response of the Se signal on methane addition. To our knowledge no previous work exists on the combination of a methane-mixed plasma with a hydride generator introduction system. However, this is of great importance for the potential application to Se isotope measurements, since this is the common introduction system in Se isotopic analyses. Although the use of a hydride generator injector system eliminates some of the interferences, both argon based interferences (e.g. ArAr, ArCl, ArArH) and hydride forming elements (e.g. Ge, GeH, SeH and AsH) still form interferences on Se isotopes. Therefore, we assessed the influence of methane addition on these interferences. Moreover, we compared the precision and mass bias between a methane mixed plasma and the traditional approach. As far as we are aware, it is the first time a carbon loaded plasma has been used in multi collector ICP-MS.

Methods

Instrumentation and experimental conditions

We performed all measurements with a NEPTUNE MC-ICP-MS (Finnigan, ThermoElectron). Two different introduction systems were used: 1) the ThermoFinnigan Stable Introduction System (SIS), which consists of a quartz glass dual spray chamber arrangement (cyclonic and standard Scott double pass) combined with a PFA® nebulizer [START_REF] Weyer | High precision Fe isotope measurements with high mass resolution MC-ICPMS[END_REF] and 2) a Hydride Generator Introduction System (HGIS). The sample gas was mixed with methane (CH 4 = 2.013 ± 0.040 mol% in Ar) before the torch with a Yconnection part. For the SIS the sample was transferred to the plasma by self-aspiration (Figure 1a). For the hydride generator, the sample was pumped at a flow rate of about 0.25 ml/min. The sample and reducing agent were merged at least 40 cm before they entered a gas-liquid separator to ensure good mixing. A low-flow of argon (0.03 L/min) gas entered the gas-liquid separator in order to get efficient transport towards the plasma (indicated as Extra (Ar) in Figure 1b). The volatile selenium hydride was mixed with a sample gas flow, sometimes containing a small percentage of CH 4 (< 1 %). Typical instrumental conditions for both set-ups are shown in Table 1. The ion optics and gas flows were daily tuned for optimum Se sensitivity without methane addition. After methane addition, only the sample gas flow and the radio frequency (RF) power were retuned for maximum sensitivity. For the comparison of final isotopic ratios we measured a 5 ppb Se solution with and without methane for 10 minutes (using 150 integrations of 4 seconds each).

Reagents and solutions

For the SIS we used a Se solution (out of 1000 ppm Se, CPI international) with a concentration one order of magnitude higher compared to the HGIS. However, the concentration used does not influence the results, as all data are normalized to the response without methane. Furthermore, the solution matrix was changed. For the SIS we used a 3 % v/v nitric acid matrix (Merck Suprapure). HCl was avoided to maintain a low background level on m/z + 77, otherwise influenced by 40 Ar 37 Cl + . [START_REF] Moellmer | ICP-MS Analysis of Trace Selenium in the Great Salt Lake[END_REF][START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF] For the HGIS, we used 2N HCl (double distilled) in order to obtain efficient selenium-hydride formation. HCl has a strong reducing nature favouring the selenium hydride formation. [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Hwang | Diomiguardi A hydride generator system for a 1-kW inductively coupled plasma[END_REF][START_REF] Verlinden | Optimization of the determination of selenium by atomic-absorption spectrometry: Comparison of two hydride generation systems[END_REF] With the use of a HGIS only part of the Cl will enter the plasma and cause a relatively small interference on m/z + 77. [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF] For the HGIS the reducing agent consisted of 0.5% w/v NaBH 4 (Merck reagent grade) stabilized by addition of 0.5% NaOH (Merck reagent grade).

Results and discussion

Se sensitivity: comparison between the introduction systems

Changes in Se sensitivity due to the addition of methane are shown in Figure 2 for the SIS (circle) and the HGIS (square). The results are based on the measurements on m/z + 82 in a Se solution, but other Se isotopes not (strongly) interfered (e.g. m/z + 77 and 78) show a similar pattern. All results are normalized to the Se sensitivity without any methane addition. The maximum signal enhancement is around 2.5 at a methane flow rate of 4 mL min -1 for the SIS. For the HGIS the maximum signal enhancement is lower (~1.5) and at a lower flow rate (~1 mL min -1 ). In this case methane addition above 2 mL min -1 gives lower sensitivity compared to no methane addition, while for the SIS the sensitivity increased for all used methane flows (up to 8 mL min -1 ). The sensitivity decrease for higher carbon loading with the use of organic solvents is thought to be related to the cooling of the plasma. [START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF] However, this cannot explain the sensitivity decrease using volatile carbon compounds. Roduskhin et al [29] explained the signal decrease by higher diffusion losses of ions from the analytical zone in the methane-mixed plasma. Although slightly low, the enhancement factors fit within the range (between 1.5 and 9) previously reported. [START_REF] Warburton | Methane mixed plasma-improved sensitivity of inductively coupled plasma mass spectrometry detection for selenium speciation analysis of wheat-based food[END_REF][START_REF] Goenaga Infante | Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP-MS detection by ultrasonic nebulisation or carbon-loaded plasma[END_REF][START_REF] Rodushkin | Improved multi-elemental analyses by inductively coupled plasma-sector field mass spectrometry through methane addition to the plasma[END_REF][START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF] The different responses of the two injector systems might be related to the different forms of Se which enters to the plasma. For the SIS, the solution arrives as an aerosol. Consequently, some plasma energy is consumed for volatilization of the matrix and to convert the aerosols to gas. As a result, Se is not efficiently ionized. For the HGIS the Se enters the plasma as a gas. In this case the plasma energy is only needed for the atomization and ionization. Therefore, the ionization processes can be more efficient. The different response on the RF power for each introduction system confirms this hypothesis. The RF power is a parameter which is related to the temperature in the plasma. The optimum RF power without methane addition is higher for the SIS than for the HGIS (Table 1). Another difference is that for the SIS the RF optimum power depends on the amount of methane present in the plasma, while for the HGIS the optimum RF does not change with methane addition (Figure 3). The lower RF power for a carbon loaded plasma in the SIS, suggests that the methane facilitates the ionization [START_REF] Hwang | Diomiguardi A hydride generator system for a 1-kW inductively coupled plasma[END_REF] .

Another difference between the two systems concerns the sensitivity. Taking into account the different transport efficiencies and the flow rates, the Se loading towards the plasma vary about one order of magnitude between the two systems. However, the observed sensitivity difference is around 20-30 times.

Thus, the discrepancy in sensitivity cannot be only explained by variations in the sample transfer and related Se loading within the plasma. This is an additional argument that differences in ionization efficiency occur between both systems. It also explains the dissimilarity in the enhancement factors between the two systems. Since the ionization of the SIS is less efficient, the ionization can be more improved than for the HG. Therefore, a higher enhancement factor is observed for SIS compared to HG.

Interferences on Se isotopes by HG-MC-ICP-MS

Despite that the use of a hydride generator eliminates potential interferences on Se isotopes, several interferences can still occur (Table 2). In this section we discuss the effect of a methane mixed plasma on the occurrence of argon based interferences (e.g. Ar dimers, ArCl), elements transferred to the plasma by the HGIS causing spectral interferences at Se isotopes (so called hydride forming elements, such as Ge, As) and other interferences (e.g. plasma derived Kr, sample cone derived NiO). It is essential to understand how methane influences these interferences, since even if interferences can be corrected, it is important to obtain the highest analyte-interference ratios possible. Although transition metals (e.g. Fe 2+ , Cu 2+ and Co 2+ ) can interfere with the hydride formation, these elements can easily be removed using a thiol-based sample preparation. [START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Vanhaecke | Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis[END_REF] . Moreover, they do form matrix effects rather than spectral interferences on Se isotopes. Therefore, we have not further investigated the effects of these elements.

Ar-based polyatomic ions

Major ArAr + interferences occur at m/z + 76, 78 and 80. [START_REF] Elwaer | Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry[END_REF][START_REF] Moellmer | ICP-MS Analysis of Trace Selenium in the Great Salt Lake[END_REF][START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Evans | Interferences in inductively coupled plasma mass spectrometry. A review[END_REF] High resolution has never been performed for Se isotopes with MC-ICP-MS. Instruments equipped with a hexapole cell can eliminate the Ar interferences due to collisions with a reactive gas. [START_REF] Rouxel | Subsurface processes at the lucky strike hydrothermal field, Mid-Atlantic ridge: evidence from sulfur, selenium, and iron isotopes[END_REF][START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Carignan | Scaling NIST SRM 3149 for Se isotope analysis and isotopic variations of natural samples[END_REF] However, most MC-ICP-MS do not contain a collision cell and therefore mathematical corrections for the Ar based interferences are required. The 40 Ar 40 Ar + has such a high intensity that precise 80 Se + data are excluded. On peak zero corrections (subtraction of signal intensities in blank solutions) almost completely removes the smaller ArAr + beams at m/z + 76 and 78. However, a residual ArAr + intensity forms due to drift in the Ar dimers. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF] These can be corrected using the measured intensity at m/z + 80 and subtraction of the estimated Se intensity to obtain a calculated ArAr + intensity. This can be applied to calculate the Ar-dimer contribution at m/z + 76 and 78. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF] Although corrections exist, it is better to have the smallest Ar dimer beam as possible. Changes in Ar dimer signal due to the addition of methane are shown in Figure 4 (squares). The results are based on the measurements on m/z + 80 in the blank solution (2N HCl), whilst other Ar dimer masses (e.g. m/z + 76) show a similar pattern. All results are normalized to the Ar dimer signal without any methane addition.

The intensity of the Ar dimer decreases with increasing methane levels.

The ArCl + variation is minimized using matrix-matched blanks and solutions (within 5%). Since the intensity of the ArCl + is small, an on peak zero correction is enough. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Vanhaecke | Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis[END_REF] We monitored m/z + 77 in a blank solution to investigate the effect of methane on the ArCl + . Although ArArH + also contributes to the 77 intensity, this beam is 2 orders of magnitude smaller compared to the ArCl + . Due to methane addition, the ArCl + decreases (circles in Figure 4).

The Ar 2 H + and Ar 2 H 2 + beams form interferences with respectively 77 Se + and 82 Se + . ArArH/ArAr ratios are normally below 1%, depending on the tuning and daily conditions (data not shown). Ar 2 H 2 + is a small beam which can be corrected using on-peak zero corrections. Methane addition significantly decreases the intensity of Ar 2 H + /ArAr (monitored at m/z + 81/80 corrected for 81 Br + using 79 Br + , rhombus in Figure 4) and therefore probably also for Ar 2 H 2 + .

The decrease of polyatomic interferences in carbon loaded plasmas have been observed before. [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Evans | Interferences in inductively coupled plasma mass spectrometry. A review[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Rodushkin | Improved multi-elemental analyses by inductively coupled plasma-sector field mass spectrometry through methane addition to the plasma[END_REF][START_REF] Hill | Investigations into the application of methane addition to the nebulizer gas in inductively coupled plasma mass spectrometry for the removal of polyatomic interferences[END_REF][START_REF] Ebdon | Evaluation of Ethene Addition to the Nebulizer Gas in Inductively Coupled Plasma-Mass Spectrometry for the Removal of Matrix-, Solvent-, and Support-Gas-Derived Polyatomic Ion Interferences[END_REF] This has been related to the competitive formation of carbides. The ArArH + beam decreases faster than for the ArAr + ion. The difference in signal decrease might be because three instead of two atoms are involved. Another reason can be that more signal decrease is observed for smaller signals. In both cases, the signal decrease on Ar 2 H 2 + is expected to be more than for ArArH + .

Hydride forming elements

The hydride-forming ions such as arsenic (As) and germanium (Ge) form spectral interferences which hinder the measurements of Se isotopes. 9.12,20 Therefore, it is important to know the effect of methane addition on the signal of As, Ge, Br and their hydride ions. Germanium can be quantitatively removed from the sample matrix using sample preparation. However, reagents can contain Ge. [START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Evans | Effect of Organic Solvents and Molecular Gases on Polyatomic Ion Interferences in Inductively Coupled Plasma Mass Spectrometry[END_REF] Mathematical corrections can be performed using the intensity on m/z + 72 or 73 to calculate the Ge + contribution of m/z + 74 and m/z + 76. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF] Arsenic typically has a one or two order of magnitude higher concentration than Se in geological materials [START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF] . Although most As can be removed during the sample preparation step using thiol-cellulose/cotton, several percent of the initial As could remain into the solution. [START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Carignan | Scaling NIST SRM 3149 for Se isotope analysis and isotopic variations of natural samples[END_REF][START_REF] Vanhaecke | Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis[END_REF] AsH corrections are difficult [START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF] , because AsH/As cannot be directly determined due to overlap with Ar dimers.

However, in most samples after sample preparation As concentrations are so low that a correction is not required. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF] However, for samples much more enriched in As than Se, such as shales and hydrothermal waters, interference corrections are needed. SeH + interferences at m/z + 77 and 78 can be approximated using the intensity on m/z + 82 and 83 to calculate the SeH/Se ratio. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Zhu | High Precision Measurement of Selenium Isotopic Composition by Hydride Generation Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a 74Se-77Se Double Spike[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF] HBr + overlaps with Se isotopes at m/z + 80 and 82. However, only volatile species of bromide are transferred to the plasma using a hydride generator. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF] Volatile Br species in samples can be removed by N 2 bubbling [START_REF] Zhu | High Precision Measurement of Selenium Isotopic Composition by Hydride Generation Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a 74Se-77Se Double Spike[END_REF] , eliminating the potential HBr + interference on Se isotopes. Although m/z + 79 is monitored as a control, HBr + corrections are normally not needed. Therefore, we did not investigate the effect of methane on the HBr beam. Other hydride-forming elements such as Te, Sb and Sn do not form spectral interferences with Se and are found not to have any adverse affect on the Se signal. [START_REF] Rouxel | Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Elwaer | Comparative performance study of different sample introduction techniques for rapid and precise selenium isotope ratio determination using multi-collector inductively coupled plasma mass spectrometry (MC-ICP/MS)[END_REF] The effect of methane addition on the hydride forming elements and their polyatomic ions are reported in Figure 5. It shows that the Ge signal is not enhanced, and therefore the relative influence of the Ge interference decreases. The optimum methane percentage for Se and As is the same. However, in agreement with previous work [START_REF] Muñoz-Olivas | Sensitive determination of selenium by inductively coupled plasma mass spectrometry with flow injection and hydride generation in the presence of organic solvents[END_REF][START_REF] Allain | Signal enhancement of elements due to the presence of carbon-containing compounds in inductively coupled plasma mass spectrometry[END_REF][START_REF] Rodushkin | Improved multi-elemental analyses by inductively coupled plasma-sector field mass spectrometry through methane addition to the plasma[END_REF][START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF] , the As signal is more enhanced than Se. Although the As signal is more enhanced than the Se signal, we are only interested in AsH formation which interferes with the 76 Se isotope. As explained before, it is difficult to assess the AsH formation. However, we can estimate the SeH formation by monitoring m/z + 83. The SeH intensity on m/z + 83 decreases with methane addition (Figure 5). The SeH/Se ratio change depends on the relative change on Se intensity and SeH intensity.

The hydride formation for Se decreased by a factor 0.7 at the optimum methane concentration whilst at this value As has around 1.2 times higher signal than Se. Therefore, when assuming a similar response for AsH as for SeH, the estimated relative importance of AsH on m/z + 76 is expected to decrease.

Other interferences

Kr interferences on Se isotopes occur at m/z + 78, 80 and 82. Trace Kr is present in the Ar gas used to generate the plasma. Moreover, Kr can dissolve from the atmosphere into sample solutions. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF][START_REF] Carignan | Scaling NIST SRM 3149 for Se isotope analysis and isotopic variations of natural samples[END_REF] Kr interferences can be corrected measuring the intensity at m/z + 83 or 84. [START_REF] Elwear | Selective separation of selenium (IV) by thiol cellulose powder and subsequent selenium isotope ratio determination using multicollector inductively coupled plasma mass spectrometry[END_REF][START_REF] Layton-Matthews | Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry[END_REF] However, when samples and blanks are equilibrated with the atmosphere, an on peak-zero correction (subtraction of signal intensities in blank solutions) is normally sufficient. Nevertheless, it is useful to monitor the Kr intensity. [START_REF] Clark | Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study[END_REF] The 84 Kr + beam decreases with methane addition (Figure 6). Kr has an ionization energy of 14.00 eV, which is above the ionization energy of C (11.20 eV). Previous work showed that no enhancement is observed for elements with an ionization energy above this threshold value because charge transfer reactions cannot occur. [START_REF] Allain | Signal enhancement of elements due to the presence of carbon-containing compounds in inductively coupled plasma mass spectrometry[END_REF][START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF] The sample and skimmer cones are made of Ni. NiO can overlap on different m/z + 74, 76 (and with low abundance also at m/z + 77, 78 and 80). However, these signals are very small and probably constant. It is difficult to assess the effect of methane on NiO, because the beams are so small. Nevertheless, previous work shows that oxide levels decrease using a carbon loaded plasma. [START_REF] Rodushkin | Improved multi-elemental analyses by inductively coupled plasma-sector field mass spectrometry through methane addition to the plasma[END_REF][START_REF] Hu | Volatile organic solvent-induced signal enhancements in inductively coupled plasma-mass spectrometry: a case study of methanol and acetone[END_REF][START_REF] Ebdon | Evaluation of Ethene Addition to the Nebulizer Gas in Inductively Coupled Plasma-Mass Spectrometry for the Removal of Matrix-, Solvent-, and Support-Gas-Derived Polyatomic Ion Interferences[END_REF]

Implications for Se isotope analyses

Isotopic ratios corrected for the above discussed interferences are shown in Figure 7 for both with and without methane addition together with the theoretical mass fractionation line (exponential law). It shows that the same isotopic ratios ( 76 Se/ 82 Se, 77 Se/ 82 Se and 78 Se/ 82 Se) are obtained in both conditions. The fractionation from the natural isotopic abundance is a combination of natural fractionation (i.e. the isotopic signature of the sample) and the instrumental mass bias. Since in both cases we observe a similar fractionation and we used the same Se solution, we can conclude that methane addition does not significantly affect the instrumental mass bias. The precision of measurements was determined using signal monitoring during ten minutes (150 integrations of 4 seconds). The stability of the signal intensity is not better by the optimum methane addition (1 mL/min). Although the count statistics are improved in a methane-mixed plasma due to higher sensitivity, the variations in the low gas flow of the methane-argon mixture induces an extra error. However, variations in signal intensity are compensated since all masses are collected simultaneously (static mode). Indeed, the isotopic ratios after interference correction show a better precision for a methane mixed plasma. Using the traditional approach a standard error of ~0.2‰ has been obtained. However, in the case of a methane mixed plasma the standard error is better (~0.07‰).

This means that the standard error is decreased three times. The enhanced precision due to methane addition is related to the higher analyte-interference ratios.

Conclusions

We investigated the effect of methane addition on the sensitivity and interferences on all Se isotopes using a multi-collector ICP-MS. Firstly, the effect on Se sensitivity for both a standard and a hydride generator introduction system has been evaluated. The Se sensitivity is significantly enhanced for both introduction systems. However, for the standard introduction system (SIS), the maximum enhancement factor is higher and observed at a higher carbon loading compared to hydride generator introduction system (HGIS). This result can be explained by different ionization processes in both systems related to the Se form that enters into the plasma. This is also confirmed by differences in the optimum RF power (associated with the plasma temperature) and different sensitivities that are not related to changes in Se loading.

Secondly, the effect of methane addition on common interferences in HG-MC-ICP-MS has been assessed.

Intensities of polyatomic interferences (such as ArAr, ArCl and several hydride containing species) decrease with methane addition, due to the competitive formation of carbides. Germanium also decreases its intensity. Arsenic intensity is more enhanced than Se, however, the AsH beam is probably less enhanced than the Se signal.

Thirdly, interference corrected Se isotopic ratios, mass bias and precision have been compared in both measurement conditions. Corrected isotopic ratios and mass bias are similar with and without methane addition. However, the precision is improved three times due to a better signal-blank ratio.

Therefore, we can conclude from this study that methane addition has a high potential for the determination of Se isotope ratios. The Se sensitivity is increased, while the intensities of the interferences are considerably decreased. These improvements are very important for the measurement of stable Se isotopes in environmental samples, which can significantly enhance our knowledge about Se cycling in the environment.

Table captions

Table 1: Instrumental conditions Table 2: Se isotopes and their potential interferences using a hydride generator injection system

Figure 1 :

 1 Figure 1: experimental set-up a) for a stable introduction system and b) for a hydride generator introduction system. S= sample; R= reducing agent; W= waste; F=filter.

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2: The effect of methane addition on Se sensitivity Figure 3: The effect of RF power for Stable Introduction System (SIS) and hydride generator Introduction System (HGIS) for different carbon loading. The intensity is normalized to optimum settings without methane addition Figure 4: The response of Ar-based polyatomic interferences on methane addition. Grey bar indicated optimum methane flow (with methane in Figure 7).

Figure 5 :

 5 Figure 5: The responses of hydride forming elements on methane addition. Grey bar indicated optimum methane flow (with methane in Figure 7).

Figure 6 :

 6 Figure 6: The response of Kr-intensity on methane addition. Grey bar indicated optimum methane flow (with methane in Figure7).

Figure 7 :

 7 Figure 7: Interference corrected Se isotopic ratios 76 Se/ 82 Se and 77 Se/ 82 Se vs. 78 Se/ 82 Se with and without methane addition. Open symbols: 76 Se/ 82 Se. Closed symbols: 77 Se/ 82 Se.

Acknowledgements

We would like to acknowledge M. Robert for her help in the Neptune laboratory. Thanks to T.M. Johnson for providing the hydride generator. T.D. Bullen and T.M. Johnson are acknowledged for discussions. Thanks to M. Kuijpers for her help with Figure 1 and to A. Hegan for critical reading of the manuscript. This is a contribution of the AquaTRAIN Marie Curie Research Training Network (Contract No. MRTN-CT-2006-035420) funded under the European Commission Sixth Framework Programme (2002-2006) Marie Curie Actions, Human Resources & Mobility Activity Area -Research Training Networks. It reflects the views of the authors but not necessarily those of the European Community, which is not liable for any use that may be made of the information contained therein. Additional funding was obtained from the Research Division of BRGM. This is BRGM contribution n° 6437.


Op. cit Benjamin Schwartz

Li For

Hongzhang

Stanley Spector

Hung-Chang Li

Huai

The Relationship between Chinese Learning and Western

Learning according to Yan Fu (1854-1921) Frédéric Wang, Inalco, fwang@inalco.fr

During the nineteenth century, translations in Chinese from Western languages were done essentially by missionaries with the help of Chinese collaborators or by some Chinese intellectuals in contact with Japan. For instance, the translation of Henry Wheaton's Elements of International Law (1836) as Wanguo gongfa 萬國公法 in 1864 by W. A. P. Martin is often regarded as an epoch-making event. But, the main fields of translation remained science and technology. Yan Fu 嚴復 (1854-1921) was the first Chinese translator of the works of Adam Smith, John Stuart Mill, Thomas Henry Huxley, Herbert Spencer, and Montesquieu, among others. He wrote many commentaries on these authors and their works in his translation. His three criteria of translation: faithfulness (xin 信), comprehensibility (da 達), elegance (ya 雅), published in the preface of Tianyan lun 天演論 (Translation of Evolution and Ethics of Huxley, 1898), have become classical ones in the Chinese theory of translation. 1 Before him, Chinese elite had begun to discuss the relationship between hanxue 漢學 and xixue 西學 commonly rendered as Chinese Learning and Western Learning: the first was considered as "Constitution", "Foundation" or "Substance" (ti 體, "body"), the second one as "Function" or "Practical Application" (yong 用). Yan Fu first examined this relationship between ti and yong in the perspective of political thought taking Freedom as ti and Democracy as yong. The objective of this paper is to understand how Yan Fu conceives this thesis and to discuss some recent evaluations of Benjamin Schwartz's pioneering work on Yan Fu. 2 I begin with a presentation of Yan Fu's life and his role in the introduction of European human and social sciences.

Yan Fu was born in 1854 in Fujian province. During his early years, he was immersed in the traditional cultural. As Benjamin Schwartz points out, "in his own individual existence, as a matter of fact, he remains a traditional gentleman no matter how far his ideas on general political and social issues may stray." 3 He encountered Western science, when he was 14 years old, in the English-language section of the Fuzhou Shipyard School, where he studied arithmetic, geometry, algebra, analytical geometry, trigonometry, physics, mechanics, chemistry, geology, astronomy and navigation. In 1877, he travelled to Britain, where he studied seamanship, first at Portsmouth, and then at the Royal Naval College in Greenwich. After returning to China in 1879, he taught at the Fuzhou Shipyard School before moving to the Beiyang Naval Academy in Tianjin created by Li Hongzhang 李鴻章 (1823-1901). 4 Meanwhile, Yan Fu did not play an important role in the entourage of Li. This is why he also tried since 1885 to pass several times the imperial examinations, but he failed every time. After the Sino-Japanese War (1895), he finally found his voice and became one of the leading Chinese intellectuals. He published in Zhibao 直報, a Chinese newspaper founded in Tianjin by the German Constantin von Hannecken (1854-1925), some of his most famous essays: "On the Speed of World Change" (Lun shi bian zhi ji 論世變之亟), "On the Origin of Strength" (Yuan qiang 原強), "On our Salvation" (Jiuwang jue lun 救亡決論), and "In Refutation of Han Yu" (Pi Han 辟韓). Yan Fu's political ideas were clearly elaborated in these essays. Since China has been defeated by Britain in the Opium War (1840), Chinese elite realised that in order to defeat the West, and make the country strong, the only solution was to learn from the West. The Qing court thus decided to launch a reform, and began to develop industry and technology. During this period of "self-strengthening", called also "Yangwu Movement" (Yangwu yundong 洋務運動), the makers of modern China, as Zeng Guofan 曾囯藩 (1811-1872), Li Hongzhang 李鴻章, understood that changes needed in China would have to go beyond mere technological innovation. But they believed that this should be done within the framework of Chinese cultural tradition. The topic of the "roots" (ben 本) and the "branches" (mo 末), of the substance (ti 體) and the function (yong 用) was vehemently discussed by this generation and later by Zhang Zhidong 張之洞 (1837-1909), future governor of Canton, who strongly believed that China could flourish only if the new "branches" of Western methods and ideas were grafted on the "roots" of Chinese culture and tradition. For all of them, the Western Learning was a function or application (yong) and the Chinese Learning was a "Constitution" or "Foundation" (ti) (zhongxue weiti, xixue weiyong 中學為體,西學為用, slogan of Zhang Zhidong). Even Tan Sitong 譚嗣同 (1865-1898), a radical theorist and philosopher of Wuxu Reform (Hundred Days' Reform, 1898), based the explanation of Western Learning on Chinese Learning. Yan Fu's view is quite different. According to him, Substance (ti) and Function (yong) are inseparable both for Chinese Learning and Western Learning. He said:

"Substance (body) and function are two aspects of the same thing. The body of an ox has the function to carry a load; the body of a horse has the function of long-distance racing. I never heard that one took ox's body and horse's function. Chinese Learning and Westerns Learning are not homogeneous, just like the difference of the appearance between Chinese and Westerners. We can not force their resemblance. Therefore, Chinese Learning as well as Western Learning owns its proper Substance and Function. Their division allows their coexistence and a union of two leads to their mutual disappearing."

體用者,即一物而言之也。有牛之體,則有負重之用;有馬之體,則有致遠之用。未聞 以牛為體,以馬為用者也。中西學之為异也,如其种人之面目然,不可強謂似也。故中學 有中學之體用,西學有西學之體用,分之则並立,合之则兩亡。 5
For Yan Fu, Western Learning and Chinese Learning are two parallel fields. One should master both the substance and function of them. Thus, he reproaches explicitly Zhang Zhidong's view which sets up a hierarchy between these two types of learning. Among his essays, we can find a key word: liberty or freedom (ziyou 自由) in his comparison between China and the West. In "On the Speed of World Change" (Lun shi bian zhi ji 論世變之亟), Yan Fu shows two aspects constituting the vital pulse of wealth and power of the West: in the area of knowledge, the Westerners reject the falseness and respect the truth; in the area of justice, they privilege the public good instead of the private interest. The final reason is that they are free: "In fact, all sages in ancient Chinese history were afraid of the theory of liberty and therefore never established it as a doctrine. But the Westerners say that because heaven alone produces man and gives birth to all equally, all people are granted liberty in the same way. For this reason, all people and all states enjoy liberty, the only restriction being that they do not harm each other. Those who violate the liberty of others are considered as violating the principles of heaven and betraying the proper conduct of man. To kill or hurt somebody or to steal someone else's property are extreme cases of violating the liberty of others. Therefore, not even the ruler of a country may violate the liberty of a single person; and laws and punishments were established in order to ensure this."

夫自由一言,真中國歷古聖賢之所深畏,而從未嘗立以為教者也。彼西人之言曰:唯天 生民,各具賦畀,得自由者乃為全受。故人人各得自由,國國各得自由,第務令毋相侵損 而已。侵人自由者,斯為逆天理,賊人道。其殺人傷人及盜蝕人財物,皆侵人自由之極致 也。故侵人自由,雖國君不能,而其刑禁章條,要皆為此設耳。 6
This is to say that freedom was not included in Chinese tradition or Chinese Learning. The principles of equality and democracy, which are also extolled, are, as it were, corollaries of the principle of freedom. In using the old Chinese substance / function (ti-yong) dichotomy to describe the relation of freedom and democracy, Yan Fu says in "On the Origin of Strength" (Yuan qiang 原強) that in the modern West, "Freedom is the substance, democracy the function". Obviously, he wishes a transplantation of this occidental value that he idealizes to Chinese case. In this essay, he refers to Darwin and especially Spencer's social Darwinism. China, for his survival, should adopt the principle of evolution based on the social struggle. China can not only venerate his past, but should face the present and the future as the modern West does. Yan Fu alluded also to Spencer's sociology translated by him as "study of groups" (qunxue 群學). At this point, he was influenced by Xunzi 荀子 (~298-~235) who states that man's superiority over the beast lies in his ability to form a social group. For Benjamin Schwartz, "Yen Fu's rapturous embrace of Spencer by no means marks a total rupture with 5 YAN Fu, "Yu Waijiao bao zhuren shu" 與《外交報》主人書 (Letter to the Editor of the Waijiao bao on Education) (1902), in Yan Fu ji 嚴復集, Vol. 3, 1986, pp. 558-559. 6 YAN Fu, "Lun shi bian zhi ji" 論世變之亟 (1885), in Yan Fu ji, Vol. 1, p. 5. XIONG Yuezhi, "'Liberty', 'Democracy', 'President': The Translation and Usage of Some Political Terms in Late Qing China", in Michael Lackner, Iwo Amelung and Joachin Kurtz (eds.), New terms for New Ideas. Western Knowledge and Lexical Change in Late Imperial China, Leiden: Brill, 2001, p. 72. Chinese traditional thought on all levels." 7 The most radical statement of Yan Fu's ultimate commitment to democracy is to be found in his essay entitled "In Refutation of Han Yu" (Pi Han 辟 韓 ). In this essay, Yan Fu criticized the "Transmission of the Confucian Way" (daotong 道統) established by the famous writer of Tang period Han Yu (768-824), who advocated the personal assimilation of Confucian values through the Classics, making them part of one's life. Yan Fu's critique of the authoritarianism in imperial China is more vehement than what Huang Zongxi 黃宗羲 (1610-1695) did in his Mingyi dai fang lu 明夷待 訪錄 (Waiting for the Dawn). 8 B. Schwartz notes that Yan Fu's persistent preoccupation with the wealth and power of the state marks a change of paradigm form those who are presumably committed to the "preservation of the faith" (bao jiao 保教), or to what Zhang Zhidong referred to as the "essential substance" (ti) of the Chinese "Way": For Yan Fu, the "preservation of the state" (bao guo 保國) is more important than the "preservation of the faith". According to Schwartz, this ideology guided Yan Fu's enterprise of translation. Thus, Yan Fu makes his personal adoption of Western liberal thought. For example, Yan suggests that "the value of individual liberty lies in its power to advance the wealth and power of the nation-state". For Schwartz, Yan Fu discovers also some theoretical problems not clarified by the liberal thinkers that he translated. Therefore, the depth of his translation surpasses sometimes this of original writers.

Before Schwartz, the philosopher He Lin 賀麟 (1902-1992), one of the best Chinese specialists of Hegel at his time, said that Yan Fu "was not interested in any truth in the doctrines he translated. He was just using these doctrines as medicine for China's illness": "When he translated the texts on the theory of evolution, he emphasized the concepts of 'the weak are the prey of the strong' and 'survival of the fittest' to warn the Chinese people. He introduced British utilitarianism with the purpose of encouraging the Chinese people to search for wealth and power. As for the former theory, he ignored its biological and embryological aspects. As for the latter, he neglected democratic ideas stressing laissez-faire, tolerance, liberty, and equality, along with ideas about social reform, social welfare, and the practical improvement of living conditions." [START_REF] Huang | The Reception of Yan Fu in Twentieth-Century China[END_REF] He Lin emphasized the utilitarian aspect of Yan Fu's translation while Benjamin Schwartz analysed with finesse some distortions made by Yan Fu of essential concepts of Western liberal thought in translating Spencer and Mill. "If liberty of the individual is often treated in Mill as an end in itself, in Yan Fu it becomes a means to the advancement of 'the people's virtue and intellect', and beyond this to the purpose of the state." 10 Thus Yan Fu was responsible of Chinese misunderstanding of Western liberalism: the sacrifice of individual rights for the benefit of state-nation. This explains way Yan Fu rendered On Liberty by John Start Mill as Qunji quanjie lun (On the boundary between self and group). This point of view, supported by Chang Hao 張 灝 11 , has been recently challenged by many scholars, both Chinese and Western. For scholars like Huang Ko-wu 黃克武, Li Qiang 李強, Wang Hui 汪暉 12 etc., there exists an evident continuity between Chinese tradition and Yan Fu's work. Some of them are influenced by the post-colonial studies, and do not share the Max Weber's view in the interpretation of Yan Fu's work by Schwartz. They do not agree with his thesis that the introduction of social Darwinism by Yan Fu is decisive in changing Chinese traditional "culturalism" into nationalism. [START_REF] Wang Jian | Yan Fu xianxiang jiedu: jian yu Shi Huaci duihua" 嚴復現象解讀--兼與史華慈對話 (The lecture of the phenomena of Yan Fu: a dialogue with Schwartz)[END_REF] For Wang Jian 王健, the return of Yan Fu to Confucius and Mencius "is not a back to the opposite side between Western Learning and Chinese Learning." It reflects Yan Fu's universal concern and his search for a real resource of thinking which could make China more powerful and the world more rational. 14 Li Qiang follows Thomas Metzger's view that the Chinese modern thinkers found the way to realise Confucian ideals via the West. According to Li Qiang, Schwartz not only misunderstands Yan Fu's work, but also misreads Spencer, Mill and Smith. For example, Li Qiang notes that Spencer used only the term of physical energy but not the "famous triad of physical, intellectual, and moral energies", as pretended by Schwartz. [START_REF] Benjamin | [END_REF] For Li Qiang, Yan Fu's "triad" is based essentially on his lecture of the Dauxe 大學 (Great Learning), one of Confucian canonical Four Books (Sishu 四 書 ). Only the term of "physical energy" is probably borrowed from Spencer's social Darwinism. [START_REF] Li Qiang | Zhongguo jindai sixiang de zhuanxing: jianping Shi Huaci Xunqiu fuqiang: Yan Fu yu xifang" 嚴復與中國近代思想的轉型--兼評史華慈《尋求富強:嚴復與西方》 (Yan Fu and the transition of modern Chinese thought: a commentary of Schwartz's In Search of Wealth and Power[END_REF] For Li Qiang, Yan Fu and his contemporaries have realised two transitions: 1. They successfully overcame the leading role of the traditional Confucian thought which, even though they belied on it in the latter years of their life. 2. They saw the problems of the model of Western civilisation after embracing it with enthusiasm. Their disillusion leaves us an unresolved question: the search for another outstanding cultural model transcending the dichotomy of China and the West. In rejecting the discontinuity of the thought of Yan Fu with the Confucian tradition, these scholars assume explicitly that the main concern of Yan Fu and the intellectuals of his generation is not only the salvation of China but also the enlightenment of Chinese people, which constitute two aspects of one thing. More implicitly, their point of view coincides with this of new Confucian's: Chinese could work out (are capable of development) science and democracy from Chinese traditional morality and culture.

For Wang Hui, the case of Yan Fu illustrates that in the early stage of modernity, Chinese intellectuals did not completely turn their back on tradition and hence their affirmation of modernity simultaneously entailed a critique. According to Wang Hui, "History, experience and knowledge are the source from which we can constantly surpass ourselves, but they are also the limits we find difficult to overcome." 17 We can assert without hesitation that Yan Fu tried to adapt his system of knowledge to the historical context of his time focusing his attention to awakening of the people:

"What are China's principal troubles? Are they not ignorance, poverty, and weakness? In a nutshell, any method which can overcome this ignorance, cure this poverty, lift us out of this weakness, is desirable. The most urgent of all is the overcoming of ignorance, for our failure to cure poverty and weakness stems from our ignorance. In overcoming our ignorance we must exert our utmost efforts to seek out knowledge. We have no time to ask whether this knowledge is Chinese or Western, whether it is new or old. If one course leads to ignorance and thus to poverty and weakness, even if it originates with our ancestors or it is based on the authority of our rulers and teachers, not to speak of persons of lower order, we must cast it aside. If another course is effective in overcoming ignorance and thus leads us to the cure of poverty and weakness, we must imitate it, even if it proceeds from barbarians and wild beasts, not to speak of persons of a higher order."

今吾國之所最患者,非愚乎?非貧乎?非弱乎?則徑而言之,凡事之可以瘉此愚、 療 此 貧 、 起 此 弱 者 皆 可 為 。 而 三 者 之 中 , 尤 以 瘉 愚 為 最 急 。 何 則 ? 所 以 使 吾 日 由 貧 弱 之 道 而 不 自 知 者 , 徒 以 愚 耳 。 繼 自 今 , 凡 可 以 瘉 愚 者 , 將 竭 力 盡 氣 皸 手 繭 足 以 求 之 。 惟 求 之 能 得 , 不 暇 問 其 中 若 西 也 , 不 必 計 其 新 若 故 也 。 有 一 道 於 此 , 致 吾 於 愚 矣 , 且 由 愚 而 得 貧 弱 , 雖 出 於 父 祖 之 親 , 君 師 之 嚴 , 猶 將 棄 之 , 等 而 下 焉 者 無 論 已 。 有 一 道 於 此 , 足 以 瘉 愚 矣 , 且 由 是 而 療 貧 起 弱 焉 , 雖 出 於 夷 狄 禽 獸 , 猶 將 師 之,等而上焉者無論已。 18
This stresses again the importance of "intellectual energy" in Yan Fu's thought. In this passage, the distinction between Chinese Learning and Western Learning, between the present and the past disappears. In some way, here Yan Fu seems to prefigure Wang Guowei 王國維 (1877-1927) who will reject in 1911 all qualifiers of learning (xue 學): new or ancient, Chinese or Western, useful or useless (學無新舊也,無中西也,無有用無用也). But Yan Fu remained a theorist. He was seldom involved into political movements in his time: hundred days' Reform, the Xinhai Revolution, the movement of May 4 th 1919… He was a man of contradiction. He attempted to awake the people, but in his translation he used an elegant classical Chinese language of Tongcheng School 19 in the time where other intellectuals promoted the vernacular one. His works were intended to the elite but not to the people. He criticized the imperial examinations, but he passed it three times. All his life was connected to the Western liberalism, but his search of freedom led him finally in the last years of his life to support Yuan Shikai's restoration. Perhaps we can not use the dichotomy continuity / discontinuity of Chinese tradition and this of Western Learning / Chinese Learning to him, because he was constantly facing these two aspects of one concern: the knowledge. Yan Fu's writings and translations exercised a considerable influence on his younger contemporaries and on the generation of Chinese intelligentsia and political elite. Liang Qichao 梁啟超 (1873-1929), Hu Shi 胡适 (1891-1964), Lu Xun 魯迅 (1881-1936), and Mao Zedong 毛澤東 (1893-1976) recognized all their debt by the works of Yan Fu. The latter continues to arouse a vast interest of contemporary Chinese intellectuals. The debate launched by Chinese and Taiwanese scholars around Benjamin Schwartz's pioneering work on Yan Fu is keeping with the general pattern of the theory of modernisation.

  From 1898 to 1909, he translated theses following major works of Western liberal thought: Evolution and Ethics by Thomas H. Huxley as Tianyan lun 天演論 (On evolution), The Study of Sociology by Herbert Spencer as Quanxue yiyan 群學肄言 (A Study of sociology), The Wealth of Nations by Adam Smith as Yuanfu 原富 (On wealth), On Liberty by John Start Mill as Qunji quanjie lun 群己權界論 (On the boundary between self and group), A System of Logic by John Start Mill as Mule mingxue 穆勒名學 (Mill's logic), The Spirit of the Laws by Montesquieu as Fayi 法意 (The meaning of the laws), Primer of Logic by William Stanley Jevons as Mingxue qianshuo 名學淺説 (An outline of logic), 1909. In 1902, Yan Fu became Director of the Bureau of Compilation and Translation of Books. He was nominated for the president of the University of Fudan in 1906 then of the University of Peking after the Xinhai Revolution (1911). The First World War plunged Yan Fu into a profound doubt of Western civilisation and he made a radical return to the Confucian tradition. Since 1913, Yan Fu published some essays where he promoted the lecture of Chinese Classics, on filial piety and loyalty. He took part in the Association of Confucianism (Kongjiao hui 孔 教會) and supported Yuan Shikai 袁世凱 (1859-1916)'s restoration of imperial system. Yan Fu died in 1921 of an asthma attack.

WANG Hui, "Yan Fu de sange shijie" (Yan Fu's Three Worlds), 1997. Translation by Viren MURTHY, "Retrieving the Past from the Present: Wang Hui, Yan Fu and the intellectual Discourse of the 1990s", http://cosa.uchicago.edu/virenmurpthy2.htm

"Letter to the Editor of the Waijiao bao on Education", op. cit., translation byBenjamin Schwartz, op. cit., p. 49.

The representative of this school was Yan Fu's mentor: Wu Rulun 吳汝倫 (1840-1903) who prefaced his translation of Huxley's Evolution and ethics.