Maïa Kuga
email: mkuga@crpg.cnrs-nancy.fr

Nathalie Carrasco

Bernard Marty

Yves Marrocchi

Sylvain Bernard

Thomas Rigaudier

Benjamin Fleury

Laurent Tissandier

Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles

Keywords: nitrogen isotopes, fractionation, organics, plasma, Titan's atmosphere, early Earth

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

suggest that the low δ 15 N values of Archean organics are partly the result of abiotic synthesis of organics that occurred at that time, and that the subsequent development of the biosphere resulted in shifts of δ 15 N towards higher values.

Introduction

Organic compounds are widely distributed in the Solar System, from very simple to more complex molecules. In contrast to what happen on Earth where organic matter has been mostly synthesized by life since the Archean, and possibly, the Hadean, eon(s), the formation of organic compounds is generally assumed to result from abiotic processes in the Solar System. Extraterrestrial organics are found mostly as amino-acids and kerogen-like material in primitive meteorites and as organic hazein planetary atmospheres such as Titan's. Among such organic molecules, the nitrogen-bearing ones are of particular prebiotic interest, as nitrogen is a key element of proteins and nucleic acids. Furthermore, nitrogen seems to play a pivotal role during the production of organic aerosols in the N 2 -rich atmosphere of Titan [START_REF] Israël | Complex organic matter in Titan's atmospheric aerosols from in situ pyrolysis and analysis[END_REF], Carrasco et al., 2013).

Nitrogen has two stable isotopes, 14 N and 15 N. The 15 N/ 14 N ratio (3.676 x 10 -3 for the terrestrial atmospheric N 2 ) is often expressed in permil deviation to terrestrial atmospheric N 2 isotope composition (AIR)as δ 15 N = [( 15 N/ 14 N) sample / ( 15 N/ 14 N) AIR -1] x 1000 (in ‰). Relative abundances of 14 N and 15 N fractionate upon physical, chemical and biological transformations of N-bearing compounds. Remarkably, the isotopic composition of nitrogen presents dramatic variations among solar system objects and reservoirs, which are not fully understood. N 2 was probably the main N-bearing species in the protosolar nebula (PSN, [START_REF] Grossman | Condensation in the primitive solar nebula[END_REF] and was 15 N-poor ( 15 N/ 14 N = 2.27 ± 0.03 x 10 -3 , that is, δ 15 N = -383 ± 8 ‰; [START_REF] Marty | A 15N-poor isotopic composition for the solar system as shown by Genesis solar wind samples[END_REF] whereas all other objects and reservoirs of the Solar System (with the exception of a few like Jupiter's atmosphere) are richer in 15 N by several hundreds to thousands of permil. Most meteorite families and the inner planetary bodies including the Earth have comparable δ 15 N values within a few tens of ‰ whereas cometary CN and HCN are enriched in 15 N by a factor of 3 relative to the PSN value [START_REF] Bockelée-Morvan | Large Excess of Heavy Nitrogen in Both Hydrogen Cyanide and Cyanogen from Comet 17P/Holmes[END_REF]. 15 N enrichments can be dramatic at the micron scale in meteoritic organics, with δ 15 N values up to 5,000 ‰ [START_REF] Briani | Pristine extraterrestrial material with unprecedented nitrogen isotopic variation[END_REF]. Some of these enrichments may be related to atmospheric processing, as proposed for the atmosphere of Mars (δ 15 N N2 = +660 ‰, [START_REF] Owen | The composition of the atmosphere at the surface of Mars[END_REF], but in other cases these 15 N enrichments relative to the PSN nitrogen require other types of extensive isotope fractionation that are poorly understood.

Exothermic ion-molecule reactions at low temperature might have led to extensive N isotope fractionation under specific cold yet dense environments (e.g., dense cores, outer solar system, [START_REF] Terzieva | The possibility of nitrogen isotopic fractionation in interstellar clouds[END_REF][START_REF] Rodgers | Nitrogen Isotopic Fractionation of Interstellar Nitriles[END_REF][START_REF] Aleon | Multiple Origins of Nitrogen Isotopic Anomalies in Meteorites and Comets[END_REF][START_REF] Hily-Blant | The 15N-enrichment in dark clouds and Solar System objects[END_REF]. Alternatively, photodissociation of N 2 associated or not to self-shielding [START_REF] Clayton | Solar System: Self-shielding in the solar nebula[END_REF][START_REF] Lyons | Timescales for the evolution of oxygen isotope compositions in the solar nebula[END_REF][START_REF] Chakraborty | Huge Isotope Effect in VUV Photodissociation of N2: Implications for Meteorite Data[END_REF], might have led to 15 N-rich radicals prone to incorporation into forming organics. Such a photochemical induced fractionation has been invoked to address the 15 N-rich HCN relative to N 2 in Titan's atmosphere [START_REF] Vinatier | The Titan 14N/15N and 12C/13C isotopic ratios in HCN from Cassini/CIRS[END_REF][START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF][START_REF] Croteau | Measurements of Isotope Effects in the Photoionization of N2 and Implications for Titan's Atmosphere[END_REF]. However, those theoretical studies have focused on the isotopic composition of very simple gaseous N-bearing compounds, and the propagation and conservation of such a large N isotopic fractionation upon polymerization of organic solids has not yet been fully investigated.

Several experimental works have simulated the synthesis of gaseous and solid organic compounds in gas mixtures similar to Titan's atmosphere (see [START_REF] Coll | Can laboratory tholins mimic the chemistry producing Titan's aerosols? A review in light of ACP experimental results[END_REF], for a review) early Earth's atmosphere [START_REF] Miller | A Production of Amino Acids Under Possible Primitive Earth Conditions[END_REF][START_REF] Marais | Prebiotic Organic Syntheses and the Origin of Life[END_REF] or the PSN [START_REF] Dzizckniec | Characterization of an N2 flowing microwave postdischarge by OES spectroscopy and determination of absolute ground-state nitrogen atom densities by TALIF[END_REF]Lumkin, 1981, Kerridge et al., 1989), either by UV photons or by electron energy deposition.

However, only a few of them have focused on the nitrogen incorporation into refractory organics from N 2 dissociation [START_REF] Trainer | Nitrogen Incorporation in CH4-N2 Photochemical Aerosol Produced by Far Ultraviolet Irradiation[END_REF][START_REF] Gautier | Experimental simulation of Titan's aerosols formation[END_REF], and the extent of related N isotope fractionation is essentially undocumented.

Whatever the original mechanism of isotope selection, N 2 dissociation and nitrogen compound ionization are believed to play an important yet not fully understood role. Plasmas were used in the present study because of the strong covalent bond of N 2 , which needs energies above 9.8 eV (< 120 nm) to break. Recent experimental simulations have used VUV photons as incident energy but production of aerosols was not reported [START_REF] Imanaka | Formation of nitrogenated organic aerosols in the Titan upper atmosphere[END_REF]Smith, 2010, Peng et al., 2013). So far, electron energy deposition stays the easiest energy source to simulate aerosol productions from irradiated gas mixtures. Here, we investigate the isotopic fractionation of nitrogen during synthesis of solid organics by plasma discharge using N 2 -CH 4 and N 2 -CO gaseous mixtures as proxies of Titan's atmosphere and the PSN, respectively.

Experimental methods

Aerosols production setups

Two experimental plasma setups were used for this study in order to produce nitrogenrich aerosols: (i) the PAMPRE experiment (LATMOS, Guyancourt, France), designed to investigate Titan's ionosphere processes; and (ii) the Nebulotron experiment (CRPG, Nancy, France), dedicated to simulate young solar nebula processes. Main experimental conditions and plasma characteristics are described below and in the Table 1.

The PAMPRE experiment

The PAMPRE (for Production d'Aérosols en Microgravité par Plasma REactif) experiment consists of a stainless steel reaction chamber, where a radiofrequency discharge (RF,13.56 MHz) is generated between two electrodes in a metallic cage confining the plasma [START_REF] Sciamma-O'brien | Titan's atmosphere: An optimal gas mixture for aerosol production?[END_REF]Szopa et al., 2006, Fig. 1a).

A gas mixture of high purity N 2 and CH 4 is flowed continuously through the plasma discharge in which electrons dissociate and ionize N 2 and CH 4 . This initiates chemical reactions and molecular growth that results in the production of hydrocarbons and N-bearing molecules that eventually end up forming solid particles. These solid particles grow up in suspension in the plasma and fall in a glass vessel surrounding the metallic cage. After typical runs of 8 h, the produced solid particles are collected for ex-situ analysis. These aerosols have the appearance of a very fine powder, orange to brown in color [START_REF] Szopa | PAMPRE: A dusty plasma experiment for Titan's tholins production and study[END_REF].

In this work, experiments were performed with a continuous 55 sccm (standard cubic centimeter per minute) N 2 -CH 4 gas flow, containing 1%, 2%, 5% and 10% CH 4 in N 2 . These gas proportions are representative of the composition of Titan's atmosphere [START_REF] Niemann | The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe[END_REF][START_REF] Waite | Ion neutral mass spectrometer results from the first flyby of Titan[END_REF]. For all experimental conditions, the injected RF power was fixed to 30 W, the pressure in the reactor was 0.9 mbar and the neutral gas temperature, measured by [START_REF] Alcouffe | Capacitively coupled plasma used to simulate Titan's atmospheric chemistry[END_REF], ranged from 310 K to 340 K depending on experimental conditions (Tables 1 and2).

The Nebulotron experiment

The second experimental setup used in this study, the Nebulotron (CRPG-CNRS), consists of a vacuum glass line in which adjustable gas mixtures can be flown through a microwave (2.45 GHz) plasma discharge [START_REF] Robert | Deuterium exchange rate between and organic CH bonds: Implication for D enrichment in meteoritic IOM[END_REF]. The aim of this setup is to simulate processes occurring in a CO-N 2 atmosphere. CO and N 2 are believed to have been the main gaseous species hosting C and N in the protosolar nebula [START_REF] Grossman | Condensation in the primitive solar nebula[END_REF]. The experimental setup consists of a quartz reactor (Fig. 1b) where a gas mixture of CO (purity 99.5%) and N 2 (purity 99.995%) is flowed continuously through the plasma discharge at a pressure of 1 mbar. As for the PAMPRE experiment, electrons dissociate and ionize N 2 and CO. In this setup however, organic aerosols grow up on the quartz tube surfaces that are cooled down by compressed air. After a typical 6 hours-long experiment, the plasma is turned off and the glass line is pumped out for 12 hours before opening the reactor to atmospheric pressure. The orange to dark solids are then recovered by gently scratching the quartz tube and stored in microvials for ex-situ analysis. As for the PAMPRE setup, not all the solids produced can be collected and the mass production rates presented here (Table 2) must be considered as minimum values. No leak has been detected on the Nebulotron reactor when isolated from the pumping group and from the gas injection system. Nonetheless, contrary to the PAMPRE setup, this one is not fitted with a secondary turbo pump, allowing a 10 -2 mbar limit vacuum only. This does not allow for sufficient desorption of water adsorbed on the reactor walls. This expected water contamination is discussed in the 3.1 Section. So far, the electronic features of the Nebulotron plasma setup have not been characterized yet but, as the volume of the plasma is much smaller than for the PAMPRE experiment, with a similar input power (30 W, Table 1), the temperature of neutral gases and the electron density in the Nebulotron setup are expected to be higher than in the PAMPRE plasma [START_REF] Fridman | Plasma chemistry[END_REF], Es-Sebbar et al., 2009[START_REF] Gries | Diagnostics and modeling of CH4-CO2 plasmas for nanosmooth diamond deposition: Comparison to experimental data[END_REF].

Here, a 6 hour-long experiment was performed with a 30 W injected power and a 6 sccm CO-N 2 gas mixture containing 80% CO in N 2 (Table 2). This experimental C/N ratio is comparable to what is estimated for the protosolar nebula [START_REF] Lodders | Solar System Abundances and Condensation Temperatures of the Elements[END_REF].

Elemental composition analysis of the aerosols

The elemental composition (C, H, N and O) of the Nebulotron aerosols has been analyzed at the Service d'Analyses Elementaires (Université de Lorraine, France) by combustion and pyrolysis techniques. The PAMPRE aerosol elemental composition was measured and reported previously by Sciamma-O'Brien et al., 2010.

Carbon-and Nitrogen-edge XANES analysis

Measurements were done using the STXM (Scanning Transmission X-Ray Microscopy) located on beamline 5.3.2.2 (Polymer STXM beamline - [START_REF] Kilcoyne | Interferometer-controlled scanning transmission X-ray microscopes at the Advanced Light Source[END_REF] at the Advanced Light Source (ALS, Berkeley, USA). This beamline uses soft X-rays (250 -600 eV) generated via a bending magnet while the electron current in the storage ring is held constant in top-off mode at 500 mA at a storage ring energy of 1.9 GeV. The microscope chamber was evacuated to 0.1 mbar after sample insertion and back-filled with helium. The energy calibration was carried out using the well-resolved 3p Rydberg peak at 294.96 eV of gaseous CO 2 for the C K-edge. In order to obtain partly X-ray transparent samples which do not completely absorb the incident light at the C and N K-edges, the PAMPRE and Nebulotron aerosols have been finely ground and deposited on SiN windows. The C-and N-XANES data shown here have been collected following the procedures for X-ray microscopy studies of radiation sensitive samples recommended by [START_REF] Wang | Radiation damage in soft X-ray microscopy[END_REF]. Alignment of images of stacks and extraction of XANES spectra were done using the aXis2000 software (ver2.1n). Normalization and determination of spectral peak positions were determined using the Athena software package [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]. Extensive databases of reference Cand N-XANES spectra are available for organic compounds (e.g., [START_REF] Leinweber | Nitrogen Kedge XANES-an overview of reference compounds used to identifyunknown'organic nitrogen in environmental samples[END_REF][START_REF] Solomon | Carbon (1s) NEXAFS spectroscopy of biogeochemically relevant reference organic compounds[END_REF].

Nitrogen isotope analysis techniques

Nitrogen isotopic ratio in the N 2 initial gases

Two N 2 tanks were used in the PAMPRE experiment: pure N 2 and a N 2 -CH 4 mixture (90-10%), which were mixed up in the flow in order to get 1, 2, 5 and 10% CH 4 in the initial gas mixture. One tank of pure N 2 was used in the Nebulotron setup, which was mixed up with pure CO. The isotopic compositions of the two pure N 2 tanks (PAMPRE and Nebulotron) were obtained by filling a glass vessel with N 2 gases from the respective tanks and by analyzing it by dual-inlet technique on a MAT253 mass-spectrometer at CRPG (Nancy, France).

The N 2 -CH 4 tank used in the PAMPRE experiment could not be directly measured because of its high CH 4 content. In order to remove CH 4 from the N 2 , an aliquot of this mixture was oxidized during 45 min in a CuO furnace at 900°C to oxidize CH 4 into CO 2 , which was then removed by adsorption on a cold trap held at -172°C. The CuO furnace was then cooled down to 450°C and the purified N 2 gas was transferred to a Cu furnace at 600°C in order to trap residual oxygen that could still be present in the line. The yield of the purification was close to 100% thus implying no isotope fractionation effect during purification. The purified N 2 was then measured by dual-inlet mass spectrometry for its isotopic composition.

Data were normalized against the NSVEC-air standard (International Atomic Energy Agency, Vienna, Austria). Blanks were negligible and errors (±0.04 ‰ on δ 15 N, 1ζ) include external reproducibilities on the standards and on the samples obtained following 10 analyses of each gas.

Bulk nitrogen content and isotopic ratio of aerosols by EA-IRMS

Bulk measurements of nitrogen content and isotopic ratio of aerosols were performed with an elemental analyzer (EuroVector) coupled to an isotope ratio mass spectrometer (IsoPrime, GV Instruments) at CRPG (Nancy, France). N 2 (purity 99.999%) was used as the reference gas. The elemental analyzer was calibrated with urea (46.65% N), glutamic acid (9.52% N) and ammonium sulphate (21.2% N) for nitrogen in the range of 0.04-0.18 mg.

Certified nitrogen isotopic standards (IAEA-N-1, δ 15 N = 0.4 ± 0.2‰ and USGS-25, δ 15 N = -30.4 ± 0.4‰, International Atomic Energy Agency, Vienna, Austria) were used to calibrate the mass spectrometer and correct sample isotopic ratios. Two aliquots of each aerosol sample was analyzed and bracketed by several standards (elemental and isotopic standards) analysis.

The external reproducibility of isotopic standards was 0.2 ‰ (1ζ) on the δ 15 N values.

Stepwise pyrolysis extraction and static mass spectrometry

We used a static stepwise pyrolysis extraction and a static mass spectrometry method [START_REF] Marty | Nitrogen and argon isotopes in oceanic basalts[END_REF] in order to check the evolution of the nitrogen isotopic composition of aerosols during their thermal degradation. Aerosols were wrapped in Pt foil and pre-heated at 100°C for 24h under high vacuum (< 10 -8 mbar) before extraction. Aerosols were thermally degraded by stepwise pyrolysis up to 950°C (3 to 5 steps, each of 20 min long) in a double-walled quartz tubing furnace [START_REF] Yokochi | Nitrogen in peridotite xenoliths: lithophile behavior and magmatic isotope fractionation[END_REF]. Extracted gases were purified by oxidation through a CuO furnace at 800°C. Most oxidized gases were removed using a cold trap held at -172°C, and only NO x compounds and minor amounts of CO and organic volatiles remained in the purification line. The CuO furnace was then cooled to 450°C to reduce NO x to N 2 . Purified N 2 was then expanded and analyzed in a mass spectrometer working in static mode [START_REF] Marty | Nitrogen and argon isotopes in oceanic basalts[END_REF][START_REF] Marty | Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assesment of shallowlevel fractionation and characterization of source composition[END_REF]. δ 15 N external reproducibility on air standards was 1.8‰ (1ζ). Blanks were less than 1% of the samples. Two samples from the PAMPRE experiment (1% and 5% CH 4 ) and an aliquot of the Nebulotron aerosols were analyzed with this technique (Fig. 5). Interestingly, the H/C values of the PAMPRE aerosols increase while their N/C values decrease with increasing CH 4 concentration in the initial gas mixture. The oxygen content of these aerosols is interpreted as resulting from water adsorption and/or from partial oxidation in air after recovery [START_REF] Sciamma-O'brien | Titan's atmosphere: An optimal gas mixture for aerosol production?[END_REF]. In contrast to the PAMPRE aerosols, the Nebulotron aerosols are characterized by a high O/C ratio, consistently with the high initial concentration of CO. As no hydrogen was present in the CO-N 2 gas mixture of the Nebulotron experiment, the high H/C value of the Nebulotron aerosols may only come from water contamination. Indeed, as suggested before, the complete removing of the water adsorbed on the reactor walls would have required baking the reactor and secondary pumping.

Results

Elemental composition of aerosols: H/C, N/C and O/C ratios

Moreover, solids that are produced in plasmas are known to be highly reactive: water adsorption on the Nebulotron solids when put out to the atmosphere cannot be excluded. The Nebulotron and the PAMPRE aerosols exhibit similar N/C ratios despite an initial N 2 concentration in the Nebulotron experiment ~5 times lower than in the PAMPRE one. This highlights a nitrogen incorporation from gas to solid much more efficient in the Nebulotron setup than in the PAMPRE one.

STXM based XANES spectroscopy: carbon and nitrogen speciation

Synchrotron-based XANES spectroscopy at the C and N K-edges provides key insights on carbon and nitrogen speciation in the PAMPRE and Nebulotron aerosols. These two aerosol mixtures are spectroscopically very homogeneous: over 20 particles of each aerosol, 19 exhibit very similar C-and N-XANES spectra. Representative C-and N-XANES spectra are shown in Fig. 3.

At the C K-edge, the low intensity of the absorption feature that can be seen in both samples at 285.1 eV (C1) is commonly attributed to electronic transitions of aromatic or olefinic carbon groups (C=C) (Bernard et al., 2010a, Bernard et al., 2010b). This is consistent with previous chemical studies which have pointed out an organic structure dominated by methyl groups and sp 2 carbons; and a lack or low abundance of any protonated aromatic or heteroaromatic rings in the PAMPRE aerosols [START_REF] Quirico | New experimental constraints on the composition and structure of tholins[END_REF][START_REF] Derenne | New insights into the structure and chemistry of Titan's tholins via 13C and 15N solid state nuclear magnetic resonance spectroscopy[END_REF].

The peak at 286.8 eV (C2), identified in C-XANES spectra of both samples, is generally assigned to electronic transitions of ketonic or phenolic groups (C=O) and/or nitrile groups (C ≡ N) [START_REF] Cody | Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES[END_REF][START_REF] Solomon | Carbon (1s) NEXAFS spectroscopy of biogeochemically relevant reference organic compounds[END_REF]. Both functions may be present within Nebulotron areosols, while only nitrile groups contribute to this peak in spectra of PAMPRE aerosols as attested by Infra-Red spectroscopy data [START_REF] Quirico | New experimental constraints on the composition and structure of tholins[END_REF], Gautier et al., 2012).

The peak at 288.1 eV (C3), also seen in spectra of both types of aerosols, is usually assigned to electronic transitions of amidyl groups (CO-NH x ) [START_REF] Cody | Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES[END_REF][START_REF] Nuevo | XANES analysis of organic residues produced from the UV irradiation of astrophysical ice analogs[END_REF] and/or aliphatics (CH 1-3 ) (Bernard et al., 2012a, Bernard et al., 2012b;[START_REF] Buijnsters | Hydrogen stability in hydrogenated amorphous carbon films with polymer-like and diamond-like structure[END_REF].

In contrast to the sharp and intense peakthat can be seen in the spectra of the Nebulotron aerosols, for which both of these functionalities may contribute, the broader absorption feature located at 288.1 eV in spectra of the PAMPRE aerosols can only be related to the absorption of various types of aliphatics, mostly connected to nitrogen atoms as indicated by FTIR and NMR data [START_REF] Quirico | New experimental constraints on the composition and structure of tholins[END_REF], Gautier et al., 2012[START_REF] Derenne | New insights into the structure and chemistry of Titan's tholins via 13C and 15N solid state nuclear magnetic resonance spectroscopy[END_REF]. Finally, the peak located at 289.3 eV (C4), only observed in spectra of Nebulotron aerosols, is attributed to electronic transitions of hydroxylated-or ether-linked C species [START_REF] Cody | Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES[END_REF][START_REF] Solomon | Carbon (1s) NEXAFS spectroscopy of biogeochemically relevant reference organic compounds[END_REF], confirming the oxidized nature of these aerosols.

These results notably show that a part of the hydrogen incorporated into the Nebulotron aerosols is linked to carbon atoms in aliphatic and aromatic functions; thus confirming the dissociation of a hydrogenated molecule, most probably H 2 O, and further active chemistry to form C-H bonds into the plasma. Even if H 2 O partial pressure in the plasma is small, hydrogen is probably very efficiently incorporated into the organic structure, for which hydrogen is necessary to build organic bonds and for polymerization. The presence of terminal -OH functions (C4 peak) in the Nebulotron aerosols' spectra likely traces oxidation and water adsorption on the aerosols' surface when they are extracted from the reactor.

At the N K-edge, three main peaks have been identified in the N-XANES spectra of the PAMPRE and Nebulotron aerosols at 398.1 eV (N1), 399.7 eV (N2) and ~ 401 eV (N3) [START_REF] Mitra-Kirtley | Determination of the nitrogen chemical structures in petroleum asphaltenes using XANES spectroscopy[END_REF], Mullins et al., 1993[START_REF] Leinweber | Nitrogen Kedge XANES-an overview of reference compounds used to identifyunknown'organic nitrogen in environmental samples[END_REF][START_REF] Cody | Quantitative organic and light-element analysis of comet 81P/Wild 2 particles using C-, N-, and O-μ-XANES[END_REF][START_REF] Nuevo | XANES analysis of organic residues produced from the UV irradiation of astrophysical ice analogs[END_REF]. These peaks can be attributed to electron transitions of imine (C=N), nitrile (C≡N) and amidyl groups (CO-NH x ), respectively. Nitrogen-bearing heterocycles may also contribute to those peaks (Mitra-Kirtley et al., 1993, and references therein).

Consistently, saturated amines have been identified in the PAMPRE aerosols in FTIR data [START_REF] Quirico | New experimental constraints on the composition and structure of tholins[END_REF], Gautier et al., 2012). In XANES spectra, saturated amines generally appear at ~ 406 eV [START_REF] Mitra-Kirtley | Determination of the nitrogen chemical structures in petroleum asphaltenes using XANES spectroscopy[END_REF], Mullins et al., 1993) and their contribution to the Nebulotron and the PAMPRE aerosols spectra is certainly masked by the diffuse absorptionobserved at the same energy likely corresponding to highly delocalized excited states or to the overlapping contribution of electronic and atomic resonances (Feshbach resonances, [START_REF] Stöhr | NEXAFS spectroscopy[END_REF].

Interestingly, although nitrogen in the Nebulotron aerosols appears to be mainly within amidyl groups whereas the PAMPRE aerosols are richer in imine and nitrile groups, nitrogen seems entirely linked to C or H atoms, as no N-O functionalities have been identified. This suggests that -NH and -CN bonds are favored despite the high oxygen content in the starting gas mixture of the Nebulotron. Thus, the volatile precursors of the nitrogen incorporation into the aerosols might be comparable in the Nebulotron and the PAMPRE experiments.

Nitrogen isotopic composition of aerosols

Because the nitrogen isotopic ratio of initial N 2 in the experimental gas mixtures is slightly different from that of air (δ 15 N=0‰ by definition), we use the Δ notation, which represents the difference between the isotopic composition of the products (aerosols) and that of the reactant (initial gas mixture):

Δ 15 N = δ 15 N-aerosols -δ 15 N-initial gas (in ‰)

The δ15 N values measured by EA-IRMS for the experimental aerosols from the PAMPRE and the Nebulotron setups and the corresponding Δ 15 N values are reported in the Table 3.

All aerosols show a systematic depletion in the heavier nitrogen isotope 15 N compared to the initial gas N 2 , with Δ 15 N ranging from -15 to -25‰ whatever the setup used (Fig. 4).

Previous Δ 15 N values of -3 to -17‰ for nitrogen trapped in plasma-synthetized organics were reported by [START_REF] Kerridge | Isotopic characteristics of simulated meteoritic organic matter: 1-Kerogen-like material[END_REF]. However, neither these data nor the experimental setup were discussed.

Step-heating data (Table 4) confirm such negative values and show overall a good homogeneity for N isotope fractionation, except for the lowest temperature step of the Nebulotron experiment (Fig. 5, top panel), which value is closer to air and may result from atmospheric nitrogen contamination.

Overall, isotopic data from step-heating and bulk measurements show a remarkable consistency, implying that these organic aerosols are globally isotopically homogeneous, whatever the setup production and the oxidation degree of the initial gas mixture.

Discussion and implications

Origin of the nitrogen isotopic fractionation in plasma experiments

The PAMPRE and the Nebulotron aerosols share comparable 15 N depletions relative to the initial N 2 gas around -20‰, although experimental setups have different geometry, carbon source and electric discharge characteristics. This heavy isotope depletion is relatively large compared to the natural nitrogen isotopic variations observed in terrestrial organic molecules or geological objects, for which δ 15 N lower than -10‰ are rare [START_REF] Coplen | Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrial materials and reagents[END_REF]. No such However, the experimental nitrogen isotopic fractionation measured in this study is small compared to the large isotopic variations that are observed in planetary atmospheres, e.g., Titan, or among primitive reservoirs and objects of the solar system, To explain such variations, several studies have focused on the dissociation of N 2 by photons in the far UV range [START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF][START_REF] Croteau | Measurements of Isotope Effects in the Photoionization of N2 and Implications for Titan's Atmosphere[END_REF][START_REF] Lyons | Timescales for the evolution of oxygen isotope compositions in the solar nebula[END_REF][START_REF] Chakraborty | Nitrogen Isotopic Fractionation in VUV Photodissociation of N2: Implications for the Early Solar System[END_REF][START_REF] Chakraborty | Huge Isotope Effect in VUV Photodissociation of N2: Implications for Meteorite Data[END_REF]. Photodissociation of N 2 could lead in principle to large 15 N enrichments (several 100 to several 1000 ‰) either by (i) isotopic selective photoabsorption and further self-shielding of N 2 in a dense atmosphere [START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF][START_REF] Lyons | Timescales for the evolution of oxygen isotope compositions in the solar nebula[END_REF] and/or by (ii) indirect (also called accidental) predissociation of N 2 excited states [START_REF] Lorquet | Isotopic effects in accidental predissociation. The case of the C2Σ+u state of N+2[END_REF][START_REF] Lefebvre-Brion | Chapter 7 -Photodissociation Dynamics[END_REF][START_REF] Van De Runstraat | Excitation and decay of the C2Σ+u state of N+u in the case of electron impact on N2[END_REF][START_REF] Muskatel | On the strong and selective isotope effect in the UV excitation of N2 with implications toward the nebula and Martian atmosphere[END_REF]. In contrast, in the present study, (i) the N isotope fractionation leads to compounds depleted, and not enriched, in 15 N, and (ii) the isotopic effect is much more modest than expected to account for N isotope variations in the Solar System.

In plasmas, N 2 dissociation mostly occurs via (i) electron impact at electron energies above the dissociation threshold of N 2 (9.8 eV) and (ii) heavy particles collisions [START_REF] Fridman | Plasma chemistry[END_REF][START_REF] Alves | Capacitively coupled radio-frequency discharges in nitrogen at low pressures[END_REF]. As electron energy of impacting electrons is continuous and not quantified as for photons, several excited states of atoms and molecules can be reached by electron impact in plasmas. As a result, electron impact cross-sections of N 2 dissociation are not as structured as photoabsorption cross-sections [START_REF] Zipf | On the dissociation of nitrogen by electron impact and by E.U.V. photo-absorption[END_REF]. Because of the continuous distribution of electron energies, 14 N 2 and 14 N 15 N dissociation rates should not be different. Moreover, atoms and molecules in plasma are not subjected to self-shielding, as electrons are isotropically produced by the electric discharge, in contrast to what occur in photon-induced reactions.

Although indirect predissociation of N 2 might be invoked, this scenario has never been documented in plasma electron energy ranges (0-20 eV) relevant to the present study. In any case, this dissociation mechanism is quite rare in plasma, and isolated events may certainly not affect the final nitrogen isotopic composition of aerosols, even with an isotopic fractionation of a few thousands of ‰ [START_REF] Muskatel | On the strong and selective isotope effect in the UV excitation of N2 with implications toward the nebula and Martian atmosphere[END_REF].

As a result, other processes are responsible for the 15 N-depletion observed in the Nebulotron and PAMPRE aerosols. These 15 N-depleted values may result from one or several fractionation processes all along the reaction chain that leads to aerosol formation that favor the light 14 N isotope to be produced in the plasma by N 2 dissociation and/or isotopically light molecules to be incorporated into the aerosols via polymerization of gaseous precursors.

Because the two setups and both type of aerosols are very different (e.g., elemental abundances and XANES characteristics) the fractionation process(es) must be independent of the chemistry and the oxidation degree of the environment. Kinetic Isotopic Fractionation (KIF, O'Neil, 1986, Li et al., 2009) is likely to occur during such experiments, as the plasmas used in the present study are out of equilibrium: chemical reactions occurring in the gas phase and during aerosol polymerization are largely controlled by kinetic competitions [START_REF] Fridman | Plasma chemistry[END_REF]. KIF during aerosol polymerization was already invoked to account for the 13 C isotopic fractionation observed in plasma-synthetized carbonaceous solids (Des [START_REF] Marais | Molecular carbon isotopic evidence for the origin of geothermal hydrocarbons[END_REF][START_REF] Marais | Prebiotic Organic Syntheses and the Origin of Life[END_REF].

KIF is a mass-dependent fractionation process. Considering two molecules, A-14 N and B-15 N, isotopically substituted, and assuming, to a first approximation, a Maxwell-Boltzmann distribution of velocities in the gas phase in our plasma setups, the KIF α factor can be written as: 𝛼=; [START_REF] Carrasco | Volatile products controlling Titan's tholins production[END_REF]. On the other hand, the 10% CH 4 -PAMPRE aerosol at -14.6‰ requires a heavier molecule than HCN (or CH 2 NH) that could imprint a less 15 N-depleted signature. In the PAMPRE setup context, one could think of acetonitrile (CH 3 CN, m = 41, ε = -12‰) or even larger N-bearing molecules with 4, 5 or more carbon atoms that were identified in the gas phase of the PAMPRE experiment [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF], Carrasco et al., 2012).

We thus suggest that the 15 N-depleted isotopic composition of the experimental aerosols relative to the initial N 2 is due to a KIF during the aerosol growth by polymerization of Nbearing molecules, mainly HCN and/or CH 2 NH. The strong relationship that is observed between the PAMPRE aerosols nitrogen isotopic composition and the initial CH 4 concentration at which they were produced (Fig. 4) requires the involvement of other Nbearing molecules that will enhance (at low CH 4 %) or reduce (at high CH 4 %) the 15 Ndepleted signature of HCN and CH 2 CN; namely NH 3 (or N) and CH 3 CN (or even larger Nbearing molecules), respectively. Such an hypothesis is in good agreement with FTIR and High Resolution Mass Spectrometry (HRMS) data obtained on the PAMPRE aerosols [START_REF] Gautier | Mid-and far-infrared absorption spectroscopy of Titan's aerosols analogues[END_REF][START_REF] Gautier | Experimental simulation of Titan's aerosols formation[END_REF], which clearly show an increase of the saturated amine (C-NH 2 bonds) content and an increase of light N-rich compounds incorporated into the aerosols as the initial CH 4 % decreases.

In summary, isotopic fractionation during synthesis of organic aerosols in plasma is consistent with kinetic isotopic fractionation (KIF). KIF is not specific to plasma and is likely to occur in every abiotic system, whether it is an electron, photon or temperature dominated environment. However, larger isotope effects in specific conditions such as photodissociation in the far UV light range may hide its 15 N-depleted typical signature.

Cosmochemical and planetary implications  Nitrogen in meteoritic organics

Most of the organic compounds encountered in primitive meteorites are insoluble polyaromatic macromolecules, referred as the so-called -Insoluble Organic Matter‖ (IOM, [START_REF] Derenne | Model of molecular structure of the insoluble organic matter isolated from Murchison meteorite[END_REF]. Bulk nitrogen isotopic compositions of these organics show little variations and are very close to the terrestrial value of δ 15 N = 0‰, although some rare compounds exhibit extreme isotopic heterogeneities at very fine scales (-hot-spots‖, [START_REF] Busemann | Interstellar Chemistry Recorded in Organic Matter from Primitive Meteorites[END_REF][START_REF] Briani | Pristine extraterrestrial material with unprecedented nitrogen isotopic variation[END_REF][START_REF] Bonal | Highly 15N-enriched chondritic clasts in the CB/CH-like meteorite Isheyevo[END_REF]. The process, timing and location of the formation of these large isotopic fractionations are still not resolved, but are certainly closely related to the processes having led to the formation of these meteoritic organic compounds, processes that are still poorly understood [START_REF] Alexander | The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter[END_REF][START_REF] Bonal | Highly 15N-enriched chondritic clasts in the CB/CH-like meteorite Isheyevo[END_REF][START_REF] Hily-Blant | The 15N-enrichment in dark clouds and Solar System objects[END_REF].

The Nebulotron organic aerosols were experimentally produced from a gas mixture relatively close to the protosolar gases (N 2 -CO withH 2 O contamination). As discussed above, the use of an electric discharge to dissociate N 2 does not allow the production of very large isotopic fractionations as expected for UV photons. Thus, the reactions occurring in the Nebulotron setup clearly do not explain the extreme 15 N-enrichments observed at the micron scale in the IOM. Nonetheless, the bulk nitrogen isotopic composition of these organics is close to 0‰, suggesting that the IOM formed essentially in non-photonic environments, in which KIF was likely to be the main isotopic fractionation process.

 Titan's aerosols

On Titan, The dissociation in the ionosphere of Titan of the two main gases N 2 and CH 4 results in an efficient integration of nitrogen in the gas products, as shown by the detection by Cassini of numerous N-containing neutrals atoms [START_REF] Nixon | Nitrogen in the Stratosphere of Titan from Cassini CIRS Infrared Spectroscopy[END_REF] and ions by instruments on boardCassini (Ref Nixon ici), both on the dayside and the nightside [START_REF] Cui | Analysis of Titan's neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements[END_REF].

Aerosols are initiated in the upper atmosphere, where a dusty plasma chemistry occurs [START_REF] Lavvas | Aerosol growth in Titan's ionosphere[END_REF] possibly supporting that probably controlled the significant nitrogen amount detected in Titan's aerosols close to the surface by the Huygens probe [START_REF] Israël | Complex organic matter in Titan's atmospheric aerosols from in situ pyrolysis and analysis[END_REF]. The PAMPRE experiment seems well representative of the chemistry occurring in Titan's atmosphere, as both gas and solid products are in good agreement with Cassini data [START_REF] Gautier | Nitrile gas chemistry in Titan's atmosphere[END_REF], Carrasco et al., 2012, Carrasco et al., 2013).

Although the dominant energy source available for N 2 dissociation in the upper Titan's atmosphere is solar radiation, suprathermal electrons may play a significant role in the neutral and ion chemistry occurring in Titan's atmosphere. Indeed, electron impact may account for ~ 10% of the total N 2 dissociation rate during dayside, at an altitude of ~ 1000 km, where initiation of aerosol growth is expected [START_REF] Lavvas | Energy deposition and primary chemical products in Titan's upper atmosphere[END_REF]. On the nightside, solar photons cannot operate and electrons are the main source for ionization and dissociation of atmospheric molecules, despite a lower density than for the dayside [START_REF] Cravens | Model-data comparisons for Titan's nightside ionosphere[END_REF](Cravens et al., , Ågren et al., 2009)).

Based on the present experimental KIF data, isotopic effects in the non-photonic dissociation of N 2 and further aerosol synthesis are not expected to play a major role in determining the large 15 N-enrichment of HCN relative to N 2 in Titan's upper atmosphere [START_REF] Vinatier | The Titan 14N/15N and 12C/13C isotopic ratios in HCN from Cassini/CIRS[END_REF][START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF][START_REF] Niemann | Composition of Titan's lower atmosphere and simple surface volatiles as measured by the Cassini-Huygens probe gas chromatograph mass spectrometer experiment[END_REF]. However, the PAMPRE nitrogen isotopic data may give new constraints for nitrogen isotopic modeling in Titan's atmosphere, knowing that nitrogen isotopic composition of Titan's aerosols has not been measured yet. N 2 photodissociation and self-shielding isotopic effects calculated by [START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF] resulted in a factor of ~ 2.4 too large 15 N/ 14 N ratio for HCN ( 15 N/ 14 N = 4.35 10 -2 , [START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF]) compared to the observations ( 15 N/ 14 N = 1.79 ± 0.26 10 -2 , [START_REF] Vinatier | The Titan 14N/15N and 12C/13C isotopic ratios in HCN from Cassini/CIRS[END_REF]). These authors thus suggested that an additional source of unfractionated atomic nitrogen, produced by ion/electron-impact induced N 2 dissociation was required to counterbalance the too large effect of self-shielding. Isotopic effects described in the present study may contribute to counterbalance the calculated self-shielding 15 N-enrichment. We can roughly quantify by a simple mass balance calculation the importance of KIF for nitrogen isotopes in Titan's atmosphere. Assuming a -20‰ isotopic kinetic effect with 10 % of electron-impact dissociated N 2 on the dayside [START_REF] Lavvas | Energy deposition and primary chemical products in Titan's upper atmosphere[END_REF]) and 100% of electronimpact dissociated N 2 on the nightside, such a calculation provides a final 15 N/ 14 N ratio of 2.25 10 -2 . Hence, the discrepancy between the HCN isotopic ratio calculated by the selfshielding model and the measured one is reduced by factor 2. This demonstrates that KIF may play a role in the nitrogen isotopes balance in Titan's atmosphere; yet, such a calculation stays a rough quantification and should be taken carefully. Besides, as first pointed out by Croteau et al., 2011, photoionization, photodissociation and non-photonic dissociation of N 2 provide distinct nitrogen isotopic fractionation signatures. The results of the present work may thus allow discriminating between putative aerosol formation pathways when nitrogen isotopic measurements in Titan's aerosols and organic molecules will become feasible.

 Organics on early Earth

The Earth's Archean atmosphere (3.8 to 2.5 Ga ago) was anoxic and mainly consisted of N 2 and CO 2 , with possibly minor amounts of H 2 and CH 4 [START_REF] Zahnle | Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere[END_REF][START_REF] Pavlov | UV shielding of NH3 and O2 by organic hazes in the Archean atmosphere[END_REF][START_REF] Tian | A hydrogen-rich early Earth atmosphere[END_REF][START_REF] Wordsworth | Hydrogen-nitrogen greenhouse warming in Earth's early atmosphere[END_REF][START_REF] Charnay | Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM[END_REF][START_REF] Marty | Nitrogen isotopic composition and density of the Archean atmosphere[END_REF]. In the presence of methane, a Titan-like hydrocarbon haze might have existed before widespread oxygenation around 2.5 Ga ago [START_REF] Zahnle | Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere[END_REF][START_REF] Sagan | The Early Faint Sun Paradox: Organic Shielding of Ultraviolet-Labile Greenhouse Gases[END_REF][START_REF] Domagal-Goldman | Organic haze, glaciations and multiple sulfur isotopes in the Mid-Archean Era[END_REF][START_REF] Tian | Revisiting HCN formation in Earth's early atmosphere[END_REF][START_REF] Zerkle | A bistable organicrich atmosphere on the Neoarchaean Earth[END_REF]. Organic aerosol formation in such mildly reduced atmospheres has been demonstrated experimentally [START_REF] Trainer | Organic haze on Titan and the early Earth[END_REF][START_REF] Dewitt | Reduction in haze formation rate on prebiotic Earth in the presence of hydrogen[END_REF]. The formation of organic aerosols may also account for the isotopic fractionation of xenon in the ancient atmosphere evidenced from the analysis of ancient sedimentary rocks [START_REF] Hébrard | Coupled noble gas-hydrocarbon evolution of the early Earth atmosphere upon solar UV irradiation[END_REF].

Without an ozone layer, the EUV photons from the Sun may have triggered atmospheric photochemical reactions such N 2 photodissociation and organic production. Even though the present experimental atmospheres (N 2 -CH 4 and N 2 -CO-H 2 O) do not represent early Earth's atmosphere composition, it seems reasonable to assume that, in the expected mildly oxidized early Earth's atmosphere, active incorporation of nitrogen into organic aerosols have occurred as the result of photochemistry or lightning. The isotopic composition of Earth's atmospheric N 2 has not changed since the Paleoarchean era, 3.5 to 3 Ga ago [START_REF] Marty | Nitrogen isotopic composition and density of the Archean atmosphere[END_REF], confirming that the thin Earth's atmosphere has not been prone to nitrogen isotopic selfshielding as it is expected for Titan [START_REF] Liang | Source of Nitrogen Isotope Anomaly in HCN in the Atmosphere of Titan[END_REF][START_REF] Croteau | Measurements of Isotope Effects in the Photoionization of N2 and Implications for Titan's Atmosphere[END_REF]. Thus, the present experimental results suggest strongly that, in the absence of such dramatic isotopic enrichments, nitrogen fixation into abiotic Archean haze from atmospheric N 2 would have acquired a 15 N-depletion down to -25‰.

In the Archean, the atmospheric production of abiotic organic matter and its delivery to oceans and continental areas might have been low, but comparable to that produced by early biological activity [START_REF] Kasting | Evolution of a habitable planet[END_REF]. Later in the Archean, abiotic organic haze might have co-existed with life and biogenic organic production [START_REF] Zerkle | A bistable organicrich atmosphere on the Neoarchaean Earth[END_REF]. 15 Ndepleted abiotically formed aerosols could directly account for a significant fraction of sedimentary organics and thus leave their isotopic imprint in Archean geological records. In line with this possibility, negative δ 15 N values in the range of -11 to -6.2 ‰ measured in Paleoarchean kerogens and metasedimentary rocks have been attributed to nitrogen cycle operated by microorganisms in a reducing environment [START_REF] Beaumont | Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmosphere chemistry?[END_REF]Robert, 1999, Pinti et al., 2001, Fig. 6). Alternatively, the present experimental results suggest that these low δ 15 N might be in part of abiotic origin. The progressive oxygenation of the Earth's atmosphere, leading to a haze-free atmosphere, resulted in a oceanic nitrogen cycle dominated by the biomediated denitrification process, and δ 15 N of organic matter evolved toward positive values (Boyd, 2001, Fig. 6). Considering the very negative δ 15 N of the abiotic organics synthetized in the present study, the gradual enrichment in 15 N of organic matter from the Archean to Present might also be the result of a transition from abiotic production to biotic one, and may therefore trace the temporal development of the biosphere.

Conclusions

We have analyzed the nitrogen content, the N speciation and the N isotopic composition in organic aerosols synthetized by plasma from two starting gas mixtures containing N 2 : N 2 -CH 4 (PAMPRE setup) and N 2 -CO (Nebulotron setup). This study demonstrates the efficient incorporation of nitrogen into the synthesized solids, independently of the oxidation degree and the N 2 content of the starting gas mixture. Both the PAMPRE and the Nebulotron aerosols show a comparable 15 N depletion of about -20‰ relative to the initial N 2 . The experimental organic solids were found to be isotopically homogeneous, with negligible air contamination. This isotopic signature is attributed to mass-dependent kinetic isotopic fractionation, which could occur at any step in the synthesis of aerosols but most likely during polymerization. Such kinetic isotopic fractionation is too small and goes in the wrong direction to account for the large 15 N enrichments observed in some primitive Solar System materials compared to the protosolar 15 N/ 14 N value. Processes such as specific isotopic effects associated to N 2 photodissociation, self-shielding, or isotopic exchange at very low temperature remain the best candidates. Nonetheless, in environments where these processes do not occur, as in Titan's atmosphere on the nightside or in early Earth's atmosphere, kinetic isotopic effects may imprint a 15 N-depleted signature. Such negative δ 15 N values may characterize abiotic production of organics during the Archean eon, rather than specific metabolic pathways.

Table captions:

Table 1: Qualitative comparison of the two plasma setups used in this study. For comparison, δ 15 N measured in old kerogen and modern marine sediments are given (bottom box). Sources of the data: NH 4 + assimilation [START_REF] Hoch | Isotope fractionation associated with ammonium uptake by a marine bacterium[END_REF], Macko et al., 1987[START_REF] Waser | Nitrogen isotope fractionation during the uptake and assimilation of nitrate, nitrite, ammonium, and urea by a marine diatom[END_REF], chemosynthesis [START_REF] Brooks | Deep-Sea Hydrocarbon Seep Communities: Evidence for Energy and Nutritional Carbon Sources[END_REF][START_REF] Conway | Stable isotopes in the study of marine chemosyntheticbased ecosystems[END_REF], N 2 uptake [START_REF] Macko | Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms[END_REF][START_REF] Zerkle | Metal limitation of cyanobacterial N2 fixation and implications for the Precambrian nitrogen cycle[END_REF], NO 3 -cycle [START_REF] Macko | Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms[END_REF][START_REF] Mariotti | Experimental determination of nitrogen kinetic isotope fractionation: some principles; illustration for the denitrification and nitrification processes[END_REF][START_REF] Boyd | Ammonium as a biomarker in Precambrian metasediments[END_REF], Past kerogen data (compilation from [START_REF] Thomazo | Biogeochemical Cycling of Nitrogen on the Early Earth[END_REF] and Modern marine sediments and dissolved NO 3 - [START_REF] Peters | Correlation of carbon and nitrogen stable isotope ratios in sedimentary organic matter[END_REF], Sweeney et al., 1978).

Fig. 2

 2 Fig. 2 shows the H/C, N/C and O/C ratios of the Nebulotron aerosols, as well as the

Fig. 1 :

 1 Figure captions:

Fig. 2 :

 2 Fig. 2: Elemental composition of the PAMPRE (open symbols) and Nebulotron (filled symbols) experiments. Elemental ratios of the PAMPRE aerosols are presented as a function of the initial CH 4 concentration in the gas mixture (data from Sciamma-O'Brien et al., 2010). Error bars are smaller than the symbol sizes.

Fig. 3 :

 3 Fig. 3: XANES spectra and band assignment in the C-and N-K edges ((a) and (c) respectively) of the PAMPRE (5% initial CH 4 ) and Nebulotron aerosols. (b) and (d) are a zoom of the shaded area in the (a) and (c) plots respectively.

Fig. 4 :

 4 Fig. 4: Bulk nitrogen isotopic composition of the PAMPRE (open diamonds) and Nebulotron (filled diamonds) aerosols, expressed as a Δ 15 N (in ‰) relative to the isotopic composition of the initial N 2 gas (represented by the shaded area). Δ 15 N of the PAMPRE aerosols is presented as a function of the initial CH 4 concentration in the gas mixture. Error bars (1ζ) are smaller than the symbol sizes.

Fig. 5 :

 5 Fig. 5: Nitrogen isotopic composition (δ 15 N in ‰, left axis, dark circles) and quantity of released nitrogen (in % of total nitrogen extracted, right axis, grey squares) as a function of pyrolysis temperature of the Nebulotron and the PAMPRE aerosols. Solid lines: weighted average and standard deviation of the nitrogen isotopic ratios over all extraction steps. Red diamonds:EA-IRMSbulk δ 15 N measurements (error bars (1ζ) are smaller than the symbol size).

Fig. 6 :

 6 Fig.6: Variations in the nitrogen isotopic composition of organic matter and biomass resulting from abiotic (this study, top box) and principal biomediated (middle box) processes. For comparison, δ 15 N measured in old kerogen and modern marine sediments are given (bottom box). Sources of the data: NH 4 + assimilation[START_REF] Hoch | Isotope fractionation associated with ammonium uptake by a marine bacterium[END_REF], Macko et al., 1987[START_REF] Waser | Nitrogen isotope fractionation during the uptake and assimilation of nitrate, nitrite, ammonium, and urea by a marine diatom[END_REF], chemosynthesis[START_REF] Brooks | Deep-Sea Hydrocarbon Seep Communities: Evidence for Energy and Nutritional Carbon Sources[END_REF][START_REF] Conway | Stable isotopes in the study of marine chemosyntheticbased ecosystems[END_REF], N 2 uptake[START_REF] Macko | Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms[END_REF][START_REF] Zerkle | Metal limitation of cyanobacterial N2 fixation and implications for the Precambrian nitrogen cycle[END_REF], NO 3 -cycle[START_REF] Macko | Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms[END_REF][START_REF] Mariotti | Experimental determination of nitrogen kinetic isotope fractionation: some principles; illustration for the denitrification and nitrification processes[END_REF][START_REF] Boyd | Ammonium as a biomarker in Precambrian metasediments[END_REF], Past kerogen data (compilation from[START_REF] Thomazo | Biogeochemical Cycling of Nitrogen on the Early Earth[END_REF] and Modern marine sediments and dissolved NO 3 -[START_REF] Peters | Correlation of carbon and nitrogen stable isotope ratios in sedimentary organic matter[END_REF], Sweeney et al., 1978).

Table 2 :

 2 Experimental conditions of aerosol production in the PAMPRE and Nebulotron setups.

Table 3 :

 3 Nitrogen isotopic composition in initial gas mixtures and aerosols from the PAMPRE and Nebulotron experiments from EA-IRMS measurements.

Table 4 :

 4 Nitrogen isotopic composition in aerosols from the PAMPRE and Nebulotron experiments from step-heating and static mass-spectrometry measurements.

N-depleted values were observed for abiotic and open-system experiments comparable to the PAMPRE and Nebulotron simulations performed in the present study.

Acknowledgments:

We thank L. Zimmermann (CRPG, Nancy) and V. Busigny (IPGP) for their help with the nitrogen isotopes measurements by stepwise pyrolysis and the N 2 -CH 4 purification, respectively. C. France-Lanord kindly put at disposition the CRPG stable isotope facility. We are grateful to P. Cartigny for helpful discussions and to G. Cernogora for the -plasma lessons‖ and for his wise scientific comments. M.K. gratefully acknowledges support from the French Ministry of Higher Education through a PhD grant. This study was partly funded by the French Programme National de Planétologie (PNP) and by the European Research Council under the European Community's Seventh Framework Program (FP/7 2007-2013, grant agreement number 267255 to B.M.).


Jennifer Mabry

Tefang Lan

Pete Burnard

Bernard Marty

High-precision helium isotope measurements in air

Helium has two natural isotopes which have contrasted, and variable sources and sinks in the atmosphere ( 3 He/ 4 He air = 1.382 ± 0.005 x 10 -6 ). Variations in atmospheric helium isotopic composition may exist below typical measurement precision thresholds (0.2 to 0.5%, 2σ). In order to investigate this possibility, it is necessary to be able to consistently measure helium isotopes in air with high precision (below 0.2% 2σ). We have created an air purification and measurement system that improves the helium isotope measurement precision. By purifying a large quantity of air at the start of a measurement cycle we can make rapid standard-bracketed measurements. Controlling the amount of helium in each measured aliquot minimizes pressure effects. With this method we improve the standard errors by 2x over measuring the same amount of gas in a single step. Individual measurements have standard errors of 0.2 to 0.3% (2σ), with three repeat samples needed to reach 0.1% or better errors. The long-term reproducibility of our calibration sample is 0.033% (2σ).

Introduction

The atmospheric content of helium (5.24 ppm vol. 1 ) is much lower than would be predicted from degassing of the mantle and crust because helium escapes from the atmosphere through thermal and non-thermal processes 2 . Therefore, the atmospheric abundance of its two natural isotopes, 3 He and [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He, results from a balance of outgassing of the solid earth, input from precipitation of solar wind at the poles, and loss into space, with a residence time on the order of 10 6 years 3 . The isotopic composition of the atmosphere ( 3 He/ 4 He = 1.382 ± 0.005 x 10 -6 ) [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] is distinctly different from crustal helium ( 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He ~10 -8 ) which is dominated by radiogenic [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He produced during the alpha decays of crustal U and Th. Several authors [START_REF] Oliver | [END_REF][6][7] have proposed that the amount of excess crustal helium entering the atmosphere due to modern fossil fuel extraction may be enough to upset the balance of the helium composition in the atmosphere on a timescale short enough to detect with modern measurement techniques.

Several groups have searched for evidence of temporal [6][7][8][9][10][11][12] variations in the helium isotopic composition of the atmosphere, however results thus far have not been definitive. Groups 6,7,11 measuring pre-industrial samples report elevated 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He relative to modern air by as much as 4%, but the errors associated with these measurements are typically quite large, up to 50%. Groups searching for temporal variations on a shorter, decadal scale have found a range of results from no change at the 0.2% (2σ) level 10 , to a decrease of the atmospheric 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He of (0.023 ± 0.008)%/year 12 up to (0.094± 0.156)%/year 9 . In order to resolve these discrepancies will require further improvement in the precision of 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He measurements from air samples.

Obtaining such a high level of precision (below 0.2%) for 45 helium measurements presents several analytical challenges. The low abundance of helium in the atmosphere means that large quantities of air must be well-purified to obtain a reasonable helium signal and that static mass spectrometry must be used. High-sensitivity noble gas ion sources are notorious for being 50 pressure sensitive, which means it is imperative to closely match the amounts of gas measured in each sample or standard aliquot.

There are six orders of magnitude difference in abundance of the two isotopes of helium, making simultaneous detection challenging. Finally, 3 He has several isobars, which must either 55 be resolved or corrected for.

In this paper, we detail the system we have developed in order to mitigate these challenges. It features a unique standard-sample bracketing system that will mimic as closely as possible the rapid switching between standard and sample which is possible with 60 dynamic mass spectrometry. Our system allows the purification and preparation of enough sample gas for multiple analyses of the same sample, thus gaining some of the benefits of the dynamic system. Additionally, the amount of helium in each sample aliquot is matched to the amount in an aliquot of the standard to 65 minimize pressure effects. Data acquired by this method are compared to the more standard measurement technique of measuring all of the purified sample in a single aliquot for statistical comparison.

Method Sample Gas Purification

We purify a relatively large amount of air sample (~15-20 cm 3 ) all at once before any measurements are made. The air is first exposed to activated charcoal held at 77K (liquid nitrogen temperature) to trap the majority of the unwanted gases including the heavier non-reactive gases such as argon. Then the remaining gas is sequentially exposed to two hot (~400C) titanium getters which chemically trap reactive gases and to an additional activated charcoal finger (Fig. 1A). Each step lasts ten minutes.

In order to monitor the consistency of the purification from sample to sample, we separate a small amount of the purified gas and analyze it with a quadrupole mass spectrometer (MKS Microvision 2™, Fig. 1C). After purification, each sample consists of > 98% neon and helium isotopes and < 2% impurities (such as H 2 , N 2 and CO 2 ). Regular repetitions show no significant variations.

The purified sample is then held in an adjustable volume (Fig. 1D) with a pipette (Fig. 1E) which is used to take multiple aliquots and perform standard-sample bracketing. Repeated measurements draw on the same reservoir of purified sample, which is important, as typically using a "standard bracketing" method with static mass spectrometry actually means using a newly purified sample for each measurement 13 .

Running Standard and Sample Air

The standard we used for our standard bracketing measurements is a bottle of purified air obtained from the Bretagne region of 30 France, just on the shore of the Atlantic. A 2.3L volume of air was collected in a steel tank and passed through silica gel to remove some of the water. This gas was later purified in the lab.

The air was sequentially exposed to two titanium getters (~800C) for approximately 125 hours and then to cold (77K) activated 35 charcoal for 2.5 hours. Finally the gas was expanded into a 10L bottle with a 0.1 cm 3 pipette for use. Twice, we expanded this bottle to a portion of our extraction line for approximately 15 hours in order to expose it to activated charcoal and remove further traces of argon and other impurities.

40

For the sample measurements described in this paper we use air that was collected in a 2.3L tank from a park on a bluff near the CRPG Institute: Parc de Brabois, Villers-lès-Nancy, France (48.661 N , 6.149 E). This location is 85 km from the closest nuclear power plant; the closest industrial plant is a steel factory 45 about 7 km away. This bottle is connected to an ~7 cm 3 pipette volume. From this tank we draw approximately 20 cm 3 (3 pipettes to start, increasing as the tank becomes depleted) of air for each measurement. This volume is sufficient to take multiple measurement aliquots from each sample after purification.

50

We used an air standard, rather than an artificially mixed standard which could have a higher 3 He content (and thus lower error associated with our calibrations), because we wanted the standard and sample to be as close as possible in composition to eliminate biases in the mass spectrometer. However, despite 55 precautions, the standard gas is fractionated relative to Brabois air, being depleted in 3 He by 3.6%. This fractionation was discovered early on in the life of the standard and has remained stable over time. Thus we believe that it occurred during the preparation of the standard.

We have considered several possible sources of this fractionation and found them all to be improbable. The long times during purification make it unlikely that a light gas such as helium would be fractionated by thermal diffusion. We also considered thermophoresis, which can cause steady state concentration gradients under some conditions, but this effect should be negligibly small for helium atoms at atmospheric or lower pressure 14 . Finally, the possibility that large enough quantities of helium were adsorbed on the silica gel at room temperature to cause such a level of fractionation is not plausible 15 . Thus further experiments would be needed to determine the cause of the fractionation, which is beyond the scope of this work.

Pressure Effect

It is essential to control the amount of gas let into the mass spectrometer with each measurement, because the ion source (Nier-type) is extremely sensitive to pressure 16 . For example, in measuring multiples of our standard, we found the measured 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He ratio changes nonlinearly by about 13% for a 20-fold increase in the [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He amount (Fig. 2). Mitigating this effect was one of the major considerations when designing this new method, and we approach the problem in two steps.

First, the volume where the purified sample gas is held is adjustable, consisting of a bellows connected to a stepper motor which is computer controlled. This serves two purposes: (1) to allow us to match the amount of gas in a sample aliquot with the size of an aliquot of our standard (since different samples can have different amounts) and ( 2) to compensate for the depletion of the reservoir as aliquots are pulled out and the pressure decreases. Carefully matching the size of each sample aliquot to the standard and previous sample aliquots bypasses the need for a pressure correction in the data. In practice we found that keeping the [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He amount of the sample within 2% of the standard amount was close enough to eliminate the pressure effect. This level was chosen because there are no correlations with amount seen at this level (Fig. 3). At the 10% variation, there is a clear correlation between the amount and the ratio, but this disappears at the 2% level. Second, we expose both the sample gas and standard gas to a 45 cryogenic trap (Oxford ICE, Fig. 1G) to remove the neon before admittance to the mass spectrometer. The trap is set to 35K (uncalibrated temperature, set point determined experimentally where most neon is trapped without trapping helium) and is stable to ± 0.01K. This keeps the pressure in the mass spectrometer as 50 low and as consistent as possible.

Mass Spectrometer

Measurements are made on a new Thermo Helix Split Flight Tube (SFT) multi-collector noble gas mass spectrometer, which was installed in February 2012. This machine is designed for helium isotope measurements with a split flight tube which physically separates the 3 He and [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He beams. The two paths terminate in an electron multiplier and Faraday cup to allow for simultaneous detection of 3 He and [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He. The mass resolution of the multiplier is ~700 which is sufficient to resolve 3 He isobars (HD and 3 H), and the resolution of the Faraday cup is 425. An electrostatic filter before the multiplier helps to further improve isotope separation and block out stray ions. The helium sensitivity is 2.35×10 -4 A/torr at 400 µA trap current. We typically operate at 350 µA trap current and 4500V acceleration voltage.

An SAES getter installed on the mass spectrometer volume keeps hydrogen levels low which reduces the amount of HD interference at mass 3. Previously, a charcoal finger was used to keep the background of argon and other heavier isotopes low during a static measurement. However, we found that for these measurements, it was not stable enough and the small pressure changes resulting from modest changes to the liquid nitrogen level were enough to disrupt the measurement (Fig. 4). For example, the addition of approximately 2 cm of liquid N 2 could change the sensitivity of the [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He measured by 2% (Fig. 4a).

Filling the liquid N 2 only before an analysis (and not during) sometimes worked, but could have unpredictable effects depending on the length of the analysis and the temperature in the room which would change how much evaporation occurred over the course of the measurement. Abandoning liquid N 2 altogether (Fig. 4b) made for very unstable measurements. These problems disappeared with the replacement of the charcoal finger by the cryogenic trap (Fig. 4c). This trap is set to 25K (uncalibrated temperature, set point determined experimentally at lowest temperature not trapping helium) and is stable to ± 0.05K.

Measurements and Results

In order to evaluate whether the changes to our extraction line and measurement method led to a real improvement in the statistics of the measured data, we performed two different sets of 45 measurements over the course of three months. All of these measurements were made using the Brabois (local air) sample gas tank as described above. We made a set of measurements employing the multiple standard bracketed technique described above, making multiple measurements of each sample. And to compare with this, we also amount of sample gas but then letting all of the gas into the mass spectrometer for a single measurement of each sample, also bracketed by measurements of the standard. These will be referred to as the multi-aliquot and single-aliquot methods respectively. The initial procedure such as gas purification is identical for both methods, only the measurement procedure differs (Fig. 5). The total amount of gas measured by each method is approximately the same, however the total time of the measurement is much less for the single-aliquot method: ~15 minutes for the single-aliquot vs ~150 minutes over all measurements for the multi-aliquot method.

Multi-Aliquot Measurements

To perform the multi-aliquot measurements, approximately 20 cm 3 of air is purified and passed through the extraction line to the bellows volume as described above. A small aliquot of this gas is quickly measured in the mass spectrometer to determine the pressure of He in the bellows volume and then the bellows position is adjusted such that the amount of [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He in one aliquot is within 2% of the amount in one pipette of the standard. Once this is reached, the measurement proceeds with multiple sample aliquots being drawn out from the bellows volume for separate measurements, each one bracketed by standard measurements (Fig. 5). The total time for sample preparation and measurement is around 9 hours.

Table 1 Multi-aliquot measurement data. Shown are the total number of measurements (N), the average measured [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He relative to the standard (A m ), the average 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He relative to the standard (R m ), and the standard error (2σ). 1 summarizes the measurements made by the multialiquot method. The quantity A m in Table 1 represents how closely the amount of helium in the sample measurements is matched to the standard. First, the aliquot amount, A a , is calculated, as the measured [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He of the sample aliquot, divided by the average of the amounts in the standards measured immediately before and after the sample aliquot:

N

.

Then for each sample, the average of all of the individual A a are made to give the final for the sample: ∑ ⁄ , where N is the number of standard-bracketed aliquots measured from 40 that sample. Typically, it is possible to measure 11 aliquots from one sample, however if the value of A a was greater than 1.02 or less than 0.98 (i.e. if the amount of [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He in a sample aliquot differed than the standard by more than ±2%), then that measurement was thrown out.

45

The ratios, R m (Table 1) are calculated the same way as the amounts. First, the aliquot ratio, R a , is calculated, as the measured 3 He/ [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He of the sample aliquot, divided by the average of the measured ratios of the standards measured immediately before and after the sample aliquot: Then for each sample, the average of all of the individual R a are made to give the final for the sample: ∑ ⁄ . The standard errors (2σ) are between 0.2% and 0.3% for one sample. This error is dominated by the scatter in the 3 He measurement. For the 19 measurements shown, the average value is 1.0363 with 55 a standard error (2σ) well below 0.1% at 0.0005 (0.051%).

Single-aliquot measurements

To perform single-aliquot measurements, the procedure is identical to that of the multi-aliquot measurements for gas purification (Fig. 5). However, for the actual measurement, all of 60 the sample gas is let into the mass spectrometer and measured at once. Two standards are measured before the sample gas, and two more after. The total time for sample preparation, standard and sample measurement is around 3 hours. As with the multi-aliquot measurements, we show (Table 2) the amount of gas in the single sample aliquot relative to the standard (A s ), which is the amount of [START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He in the sample aliquot divided by the average of the two standards measured before and the two standards measured after:

∑ 4 ⁄ .
In contrast to the multi-aliquot method, we are not trying to match the sample size to the standard, but simply to match the amount of helium between one sample and the next. We rejected any data that varied by more than ±2% from the mean of all the A s values. The ratio R s is also relative to the average ratios measured from the standards measured with the sample:

∑ 4 ⁄ .
The errors given for each ratio are the calculated errors based on the measurement error of the five (four standards, one sample) individual measurements and are typically 0.3% to 0.4% for each single measurement. The mean for these 17 measurements is 0.9268 with standard error is 0.001 (0.1%, 2σ). Note that this ratio differs from the multi-aliquot ratio (by ~11%). This is a consequence of the pressure effect discussed above; although both are normalized to the same internal standard, the singlealiquot measurements have about 13.5 times more gas than the multi-aliquot measurements.

Discussion

For purposes of comparing these two measurement methods, we have normalized these two data sets to their respective mean values and show them together in Fig. 6. Each point represents approximately the same total amount of sample gas measured.

The multi-aliquot method has noticeably less amount of scatter and the standard deviation is half that of the single-aliquot data. To determine whether or not the difference between these two methods is statistically significant, we make a one-tailed F-test. First, for the test to be valid the data sets should be normally distributed, so we make a normal probability plot for each data set (Fig. 7) and check that the result is linear. We use a one-sided 45 test because we developed the multi-aliquot method specifically to improve upon the single-aliquot method. The details of this calculation are shown in Table 3 and indicate that the lower variance of the multi-aliquot method compared to the singlealiquot method is statistically significant at a 95% confidence In a direct comparison, for the same total amount of gas measured, but different total measurement times, the multialiquot method has errors on a single sample of about 60% of the single-aliquot method and a long term reproducibility that is about twice as good. Although it seems obvious that increasing the measurement time will improve the statistics, this is not always the case. A significant increase in measurement time, such as in this case, increases the probability that external factors (drift in electronics, gain of detectors, temperature, day/night variations) may affect the measurement and thus hinder an improvement in statistics.

Next, there is the question of whether the improvement is worth the extra measuring time. This depends on several factors such as the time available for measurements, the precision needed, and the number of samples available. Based on these data, it takes only on average 3 separate sample measurements by the multi-aliquot method to reach a standard error of 0.1% (2σ) for a total laboratory time around 30 hours. In contrast, the single-aliquot method approached a plateau at an average standard error of 0.11% (2σ) after 11 separate samples measured (total laboratory time ~33 hours). Thus, if reaching 0.1% precision is important, the multi-aliquot method is the better choice, especially if one is limited in the number of samples one can acquire.

Finally, we have made measurements using the multi-aliquot method over an extended time period beyond this comparison study in order to assess the long-term reproducibility. The measurements made over a period of about 270 days were very stable (Fig. 8), with no trend detected; a linear fit gives an R 2 = 0.0012. This time period includes the replacement of the multiplier around day 200 (necessitating opening the mass 35 spectrometer to atmosphere).

The 2σ standard error over this entire period is 0.033%. The earliest measurements (first 30 days) have a higher amount of scatter (SD = 0.41% 2σ) compared to later measurements (SD = 0.27% 2σ). We attribute this to the fact that the first 40 measurements were made soon after the installation of the new mass spectrometer and some details were still being finalized such as climate control of the laboratory.

Conclusions

We have constructed an extraction line to purify relatively large 45 quantities of atmospheric helium and monitor the consistency of the purification process with a quadrupole mass spectrometer. An adjustable volume is used to control the quantity of this helium to be let into the mass spectrometer and thus mitigate pressure effects in the source. A cryogenic trap on the line removes the 50 neon from the sample gas for further control of the pressure in the mass spectrometer. The addition of a second cryogenic trap on the mass spectrometer volume maintains low and stable background levels which are essential for high precision helium isotope measurements.

55

We compared our modified multi-aliquot measurement method to the more 'traditional' single-aliquot method. In both cases we monitor and control the amount of gas let in for a measurement. We found a 2x improvement in the error when using the multialiquot measurement technique over the single-aliquot method. 60 With the multi-aliquot measurement technique, we can measure with a long-term reproducibility over several months of 0.033% (2σ). Single measurements have standard errors of 0.2 to 0.3% (2σ), but only three replicate samples are needed to reach 0.1% or better errors. This is a level sufficient to probe further into the 65 question of temporal helium isotopic variations.

Fig. 1

 1 Fig. 1 Schematic of the sample purification and inlet system.

40Fig. 2

 2 Fig. 2 Effect of the pressure in the mass spectrometer on the measured 3 He/ 4 He ratio. The ratio decreases nonlinearly by about 13% for a 20-fold increase in 4 He amount.

Fig. 3

 3 Fig.3 Measured 3 He/

Fig. 4

 4 Fig. 4 Single measurements of 4 He with an activated charcoal finger on the mass spectrometer volume: (a) adding roughly 2 cm of liquid N 2 near end of the measurement, (b) without any liquid N 2 , and (c) after replacing the charcoal finger with a cryogenic trap.

  50

Fig. 5

 5 Fig. 5 Flow diagram of the procedure for the single-aliquot and multialiquot methods.

  55 made measurements in the 'classic' way by purifying the same This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00-00 | 5

Fig. 6 Fig. 7

 67 Fig.6 Normalized measured ratios for the single-aliquot (purple circles) and multi-aliquot (black diamonds) method, shown in the order in which

  40

  This journal is © The Royal Society of Chemistry [year]Journal Name, [year], [vol], 00-00 | 7

Fig. 8

 8 Fig. 8 All (83) multi-aliquot measurements made over 270 days. The red line is a linear fit of the data and shows no significant trend over time. The grey shaded area shows 2 standard errors (SE) and the thick black lines show 2 standard deviations (SD).

Table 2

 2 Single-aliquot measurement data. The average measured[START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He 65 relative to the standard (A s ), the average 3 He/[START_REF] Sano | The Noble Gases as Geochemical Tracers[END_REF] He relative to the standard (R s ), and the standard error (2σ) are shown.

	A s 13.51	R s 0.9238	Standard error (2σ) 0.0034
	13.38	0.9281	0.0044
	13.54	0.9273	0.0033
	13.59	0.9272	0.0023
	13.64	0.9264	0.0029
	13.45	0.9281	0.0036
	13.51	0.9288	0.0067
	13.56	0.9272	0.0037
	13.56	0.9269	0.0023
	13.53	0.9260	0.0035
	13.61	0.9252	0.0050
	13.62	0.9246	0.0025
	13.64	0.9281	0.0026
	13.53	0.9307	0.0030
	13.56	0.9243	0.0030
	13.52	0.9293	0.0034
	13.43	0.9236	0.0039
		0.9268 ± 0.0010 (0.105%) -standard error
		± 0.0040 (0.42%)-standard deviation

Table 3

 3 One-tailed F-test calculation comparing the standard deviations of the multi-aliquot (S m ) and single-aliquot (S s ) measurements. The calculation indicates that the difference between them is significant.

	Formulate null and alternative hypotheses Calculate F	H 0  S m 2 = S s 2 H 1  S m 2 2 < S s F = S s 2 = (0.00200) 2 /(0.00111) 2 2 / S m = 3.2465
	Critical F value for α = 0.05 Is F > F crit ?	F crit = 2.2496 3.2465 > 2.2496 

He ratio for different varying amounts of 4 He relative to the internal standard: a) all measurements, some which have 40% different 4 He amount from the standard; b) filtering out points more than

5% away from the standard amount; and c) filtering out points more than 2% away from the standard amount. At the 10% level, there is a clear correlation between the amount and the ratio, but this disappears at the 2% level.

Acknowledgments

This experiment is funded by grant VIHA2 from the Agence Nationale de la Recherche (ANR), and by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013 grant agreement no. 267255).

ARTICLE TYPE

This journal is © The Royal Society of Chemistry [year]

[journal], [year], [vol], 00-00 | 8

Notes and references

a CRPG-CNRS, 15 rue Notre Dame des Pauvres, Vandoeuvre-les-Nancy, France. E-mail:jmabry@crpg.cnrs-nancy.fr