# Unravelling the molecular significance of Rock-Eval® S2 pyrograms from soil and geological samples

Jérémy Jacob, Frédéric Delarue, Yoann Copard, Claude Le Milbeau, Laurent Grasset, Patrick Brockmann

► To cite this version:

Jérémy Jacob, Frédéric Delarue, Yoann Copard, Claude Le Milbeau, Laurent Grasset, et al. Unravelling the molecular significance of Rock-Eval® S2 pyrograms from soil and geological samples. Journal of Analytical and Applied Pyrolysis, 2025, 191, pp.107211. 10.1016/j.jaap.2025.107211. insu-05101532

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1 Unravelling the molecular significance of Rock-Eval® S2 pyrograms from soil and geological samples

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3 Jérémy Jacob¹, Frédéric Delarue², Yoann Copard³, Claude Le Milbeau⁴, Laurent Grasset⁵, Patrick

4 Brockmann¹

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6 ¹ Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, Université

7 Paris-Saclay, 91198, Gif-sur-Yvette, France

8 ² Sorbonne Université, UPMC, CNRS, EPHE, PSL, UMR 7619 METIS, 4 place Jussieu, F-75005 Paris,

9 France

10 ³ Normandie Univ, UNIROUEN, UNICAEN, CNRS, M2C, France, 76000 Rouen, France

11 ⁴ Université d'Orléans, CNRS/INSU, BRGM, Institut des Sciences de la Terre d'Orléans (ISTO), UMR

12 7327, 45071 Orléans, France

13 ⁵ Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285 CNRS, Université de Poitiers,

14 Poitiers, France

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16 Abstract

17 Rock-Eval® pyrolysis is dedicated to rapidly determine the quality and quantity of organic matter

18 (OM) in environmental and geological samples. It has been proposed that detailed examination of S2

19 pyrograms acquired from a Flame Ionization Detector during programmed pyrolysis of samples can

20 provide complementary information to classical Rock-Eval® parameters. Previous mathematical

21 deconvolution of the S2 pyrogram has been suggested to semi-quantify distinct thermal fractions
that are supposed distinct in terms of chemical and/or biological lability. To date, there is no direct
molecular support for such an assumption. This work proposes a methodological approach based on
coupling a temperature-programmed pyrolyser to a standard mass spectrometer (Py-MS). A set of
reference soil samples was analysed. The set was completed by dissolved OM, source rock and coal
samples in order to test the insights of this methodological approach to various OM types. Data
analysis was based on Tpeak values defined at temperatures of maximum m/z fragment production
during the temperature ramp. These Tpeak values were grouped into clusters of distinct m/z values
for which a chemical attribution is proposed owing to an extensive literature survey. For the soil
samples, all the pyrolytic fractions described in the literature from mathematical deconvolution of
the S2 pyrogram were identified except for the most thermolabile fraction (280-320 °C). Fragments
with Tpeak values between 330 and 390 °C were mainly attributed to proteins, lignin and
carbohydrates. Tpeak values found in the 391-499 °C range corresponded to m/z fragments
attributed to aliphatics, lignin and aromatics while fragments with Tpeak values higher than 480 °C
were only assigned to aromatics. In all samples, these fragments displayed a singular pattern of
decreasing m/z with increasing temperature, the significance of which remains to be fully elucidated.
This preliminary study provides key methodological guidelines for re-exploring Py-MS applications to
disentangle the chemical nature of OM.

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40 **Keywords:** Rock-Eval® pyrolysis, S2 deconvolution, organic matter, soil, environmental matrices, Py-MS

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# 1. Introduction

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Rock-Eval® pyrolysis is a common technique providing information on organic matter (OM) quality and quantity in various environmental and geological samples [1-5]. During the last decades there has been continuous efforts to generate more information than the classically used parameters (Tmax, Total Organic Carbon, Hydrogen and Oxygen Index...). These efforts were mainly focused on the multi-lobed S2 pyrolysis curves observed during soil, sediment and rock samples analyses, on the argument that Tmax values of natural samples can be divided into several main temperature fractions. Morphometric analyses and mathematical deconvolution of S2 curves into simple Gaussians (considering the shape of the S2 curve of purified products; Sebag et al., 2006) were proposed to define OM types within which a coherent thermal stability and biogeochemistry is assumed [6-9]. Sebag et al. [7] proposed four elementary Gaussian components or pyrolytic families (named F1 to F4) to resolve multi-lobed S2 curves obtained from soil samples. F1 (mean Tmax = 307 °C) corresponds to labile bio-macromolecules found abundantly in fresh plants and soil litter. F2 (mean Tmax = 371 °C) corresponds to resistant biopolymers such as lignin and cellulose. F3 (mean Tmax = 436 °C) corresponds to immature geo-macromolecules also (somewhat debatably) called "humic substances". Finally, F4 (mean Tmax = 481 °C) corresponds to "mature geo-macromolecules". This approach has been applied to a large set of samples and scientific questions, with some distinctions on the range and significance of the pyrolytic families [10-16]. An alternative to mathematical deconvolution of the S2 peak consists in integrating the S2 peak area over fixed temperature ranges [17-20].

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These methods, and the numerous indexes built upon the pyrolytic families (R330 [21]; R400 [22]; S2I<sub>350</sub> and S2I<sub>380</sub> [17]; I-index [7, 18, 20]; and R-index [20]) were then applied by several authors to assess the proportions of thermolabile and thermo-recalcitrant OM in soils [8, 11, 13, 17, 23, 24], peats [25], riverine sediments [8], compost associated with pure minerals [26] and OM in aquifer sediments [27]. Comparison of the results and indexes obtained by mathematical deconvolution (or
68 integration between temperature ranges) of S2 peaks with other techniques (Fourier Transform
69 Infrared Spectroscopy [21], ¹³C NMR [19, 26]) provided indirect molecular evidence by improving our
70 knowledge on the functional groups associated with the defined pyrolytic families. Similarly,
71 statistical analyses on large sets of samples were used to better discriminate the factors controlling
72 the distribution of the pyrolytic families [20].

73 Despite these laudable efforts, the molecular rationale for mathematically deconvoluting S2
74 pyrolytic curves obtained on soil samples into four to five Gaussian curves of homogeneous thermal
75 and chemical stability remains to be proven. As stated by Derenne and Quenéa [28], there is
76 currently no molecular information directly comparable to the Rock-Eval® pyrolysis signals from soils.
77 This prevents us from filling the gap between bulk and molecular analyses and thus achieving the
78 complete molecular elucidation of natural soil OM composition [29].

79 For instance, there is a large consensus that the first, thermolabile fraction (F1) comprises
80 carbohydrates whereas the late, thermo-recalcitrant fraction (F4) is mostly aromatic [25].
81 Conversely, the definition of the F2 and F3 fractions appears more elusive, with mentions of cellulose
82 and lignin, but also the so-called "humic substances". In addition, the number of Gaussian curves
83 required to satisfactorily explain the S2 signal can vary significantly between studies (from 4 to 11) in
84 agreement with the high variability of Tmax values recorded for soil samples, and the boundaries of
85 each defined fraction can vary amongst authors. Nevertheless, some common features are found
86 throughout these studies. There is a clear distinction between the OM that is responsible for the
87 signal below 330 °C ("thermally labile biological compounds" [6] or "little resistant bio-
88 macromolecules" [7]), between 330 and 400 °C ("cellulose, lignin and polypeptides" [6] or "resistant
89 biological constituents" [7]), and OM that is responsible for the signal above 400 °C ("immature
90 humic substances" [6] or "immature non-biotic constituents"/"immature geo-macromolecules" [7]).
91 Higher temperature fractions are defined either as "mature/reworked OM and charcoal" [6], or as
92 "mature refractory fraction" [7]. This distinction into several fractions of distinct thermal stability is
the foundation for the definition and use of different ratios that reflect the proportion of each
fraction to the S2 signal, as discussed above.

As pointed out by Sebag et al. [7], four or five Gaussian components cannot account for the
large diversity of biological constituents and their derivatives in soils and geological samples, and
there is currently no consensus on the adequate number of Gaussian curves that are necessary to
explain the S2 signal. In petroleum studies, the molecular composition of the S1 and S2 peaks has
long been addressed by means of offline, preparative, or online pyrolysis-GC-MS with cryo-trapping
[30, 31]. Cadeau et al. [32] recently proposed a new system that recovers hydrocarbons released
during Rock-Eval® analysis of standards, oils and source rocks to determine the molecular
composition of the Sh0, Sh1 and Sh2 peaks, those corresponding to free and absorbed hydrocarbons
in liquid-rich rock samples [33, 34, 35]. These approaches provided molecular clues about the
chemical composition of rather simple S1 and S2 peaks for oil shales and other petroleum-related
samples. They are not appropriate, however, to reveal the more complex chemical nature of soil
samples.

In the 90s, in-source pyrolysis mass spectrometry (Py-MS) and direct temperature-resolved
Field Ionization Mass spectrometry (FIMS) or Thermogravimetry/mass spectrometry were developed
for the molecular analysis of soil, dissolved and particulate marine and estuarine OM, dissolved
aquatic humic substances, plants [36-49] and sewage sludge and biowaste [50, 51]. These devices
make it possible to determine the chemical nature of materials by monitoring the evolution of
fragment emitted by pyrolysis under increasing temperatures directly in the source of the mass
spectrometer. An alternative to pyrolysis upstream the mass spectrometer is thermogravimetry. For
example, Lopez Capel et al. [52, 53] determined by this means three pools of soil OM: a thermally
labile fraction between 300 and 350 °C; a recalcitrant fraction between 400 and 500 °C and a
refractory fraction between 430 and 530 °C.
The molecular significance of Rock-Eval® S2 curves is here explored by applying a rather simple, rapid, and cost-effective method inspired by the above-mentioned work to a set of standard soil and geological samples. For this purpose, a temperature programmable pyrolyser was connected to a standard single quadrupole mass spectrometer. Although this approach involves stronger fragmentation and precludes the screening for molecular ions - unlike field ionization mass spectrometry (FIMS) or in-source Py-MS - this allow to obtain molecular information in conditions comparable to that of RE6 pyrolysis. In a previous work [54], we compared Rock-Eval® and Py-MS results acquired on this set of samples by focusing on Total Ion Current (TIC) traces of Py-MS. This allowed us evidencing time lags and temperature corrections, quantitative relationships and differences in response between the two methods. Here, we discuss the evolution of the abundance of *m/z* fragments during temperature-programmed pyrolysis to S2 curves obtained by Rock-Eval® pyrolysis, which is discussed in terms molecular significance of the mathematical deconvolution of S2 peaks.

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## 2. Materials and methods

### 2.1. Samples

The set of samples was presented in Jacob et al. [54]. It is constituted by a series of international standards of soils (“Elliott”, “Vertisol” and “Chernozem”), peat soil (“Pahokee”), source rocks (“160000” and “Green”), coal (“Pocahontas”) and dissolved OM (“Suwannee”) that display significant differences in RE6 parameters (TOC, HI, OI, Tmax) and in the shape of the S2 signals due to distinct sources, diagenesis and maturity (see references in [54]).

### 2.2. Rock-Eval® pyrolysis and pyrolysis-mass spectrometry

Both methods were extensively described in Jacob et al. [54]. Briefly, for Rock-Eval® pyrolysis, samples were analysed using "RecentSM" program: isothermal at 200 °C during 5 min, then an
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increase to 650 °C at 30 °C/min and a final cessation of heating during 3 min down to ~540°C.

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Samples were analysed at least twice. In the following, we will use TpS2 values as the temperature effectively determined at the maximum of the S2 peak. Tpeak values stand for the temperature determined either at the maximum of the Gaussian curves resulting from mathematical deconvolution or at the maximum of the *m/z* curves acquired during programmed pyrolysis. TpS2 and Tpeak values are hence directly comparable.

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Py-MS was realised with a Pyroprobe 5150 (CDS Analytical Inc.) pyrolyser directly coupled to a single quadrupole ISQ7000 mass spectrometer (ThermoScientific), the Trace1310 GC gas chromatograph (ThermoScientific) being only used as a bypass (deactivated capillary column, id 0.1, 60 cm length, oven maintained at 300°C mm). The temperature program set for the coil was similar to that of Rock-Eval® pyrolysis. Samples were analysed at least three times.

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## 2.3. Data management

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In order to evaluate the extent to which there were clusters of *m/z* fragments issued from organic compounds released at specific temperatures, we determined the Tpeak values of each *m/z* Py-MS curve for several samples and replicates, at each nominal mass between *m/z* 50 and 150

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Py-MS curves were extracted from Xcalibur .raw files after having converted .raw files into .cdf files using the "Thermo File Converter" software. An in-house Python notebook was developed for data management and data visualization. The script accomplishes two operations: first, extracting and reorganizing 3d (*m/z*, scan/time and intensity) data from the converted .cdf file; then, smoothing the curve (over 100 scans) and visualizing data as the evolution of intensity for selected *m/z* along scans. Scan numbers were converted into time of analysis and then to temperature of pyrolysis by using the temperature program equation over the temperature ramp (200 to 650 °C at 30 °C/min).

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The package "peakutils" was then used to detect the temperature of maximum abundance of each *m/z* (Tpeak), for each sample. The list of Tpeak values together with their corresponding *m/z* was then exported to a .csv file and compiled for all samples and replicates.

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The Python notebook can be tested with Binder on selected result files acquired on standard samples at the following address: https://github.com/PBrockmann/Pyrograms.

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The result files can also be downloaded here:

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https://thredds-su.ipsl.fr/thredds/catalog/ipsl_thredds/brocksce/Pyrograms/catalog.html

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For data management reasons, we only considered Tpeak values of fragments in the *m/z* 50-150 range. These fragments constitute by far the largest proportion of the TIC signal (estimated from absolute abundances in the total spectrum, integrated over the whole TIC peak) in Green (91.4 ± 1%), Elliott (93.7 ± 0.7%), Vertisol (85.2 ± 4.8%), Chernozem (91.8 ± 1.5%), Suwannee (90.1 ± 1.2%), Pahokee (92.9 ± 1.9%) and Pocahontas (78.9 ± 7.2%) samples. These values are higher than those reported, for example, for Time-Resolved Pyrolysis-Field Ionization Mass Spectrometry [1, 2] due to stronger ionization and fragmentation with the device used. Tpeaks were individually verified for all samples and exported to an Excel sheet. This operation led to at least 200 Tpeak values (observable; obs) per sample over at least two analytical replicates (n) for every standard sample (Elliott: n=4, obs=443; Vertisol: n=3, obs=278; Chernozem: n=3, obs=251; Pahokee: n=3, obs=365; Suwannee: n=3, obs=309; Pocahontas: n=3, obs=318; Green: n=2, obs=200; 160000: n=2, obs=202). Finally, Tpeak values were converted into temperatures equivalent to TpS2 values as measured by Rock-Eval® (Tpeak<sub>conv</sub> = (Tpeak Py-MS - 114.69)/0.8134; see Jacob et al. [54]). The inventory of Tpeak values in the *m/z* 50-150 range was not exhaustive since some Tpeaks of double peaks or shoulders could not be automatically determined. Although this work refers to the mass spectrometric literature for molecular attributions of *m/z* values (cf. section 4.7.), *m/z* attribution to a specific class of compounds is not unequivocal. In addition to multiple peaks and shoulders in some *m/z* curves that
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indicate a distinct thermal behaviour of the compounds they derive from, the same *m/z* fragments
can be produced from compound families that are chemically distinct.

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Clustering of Tpeak<sub>conv</sub> values was done with the k-means classification function in the XLSTAT® software. Although 3 to 5 clusters, depending on the sample, were defined with hierarchical clustering (realized with the HCA function in XLSTAT®), it was decided to set the k-means classification with 4 clusters for homogeneity, and in order to be in line with the number of Gaussian curves into which S2 pyrolysis curves are deconvoluted in most cases.

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### 3. Results

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S2 and TIC signals have been compared previously [54]. Here we focus on the analysis of *m/z* curves and compare the distribution of their Tpeak<sub>conv</sub> values to literature data on mathematical deconvolution of S2 signals.

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#### 3.1. Total mass spectra

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Total mass spectra illustrate the relative abundance of *m/z* fragments within the time range of the TIC peak (Fig. 1 for Elliott and Green and Fig. S1 for all samples). Some common features were observed in the total mass spectra of all standard samples: a decreasing trend of abundance towards higher *m/z* and strong abundance of the *m/z* 91 fragment in nearly all samples, although with different relative abundances.

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In Green and 160000, *m/z* 91 emerges only slightly from a decreasing trend of *m/z* relative abundance with increasing *m/z*. In Pahokee, Pocahontas and Suwannee, *m/z* 91 strongly emerges or dominates. In addition to *m/z* 91, *m/z* 94 is also abundant in Pahokee and Suwannee samples (Fig. S1). In soil samples Chernozem, Elliott and Vertisol, the relative abundance of *m/z* 91 compared to other fragments is similar. In addition, clusters of *m/z* can be identified in all mass spectra at *m/z* 51-

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211 58, m/z 63-71, m/z 76-85, m/z 89-97 and 102-113. Clusters of heavier fragments (m/z 115-123, 127-135, 141-147) are less evident. Fragments larger than m/z 200 are barely discernible.

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## 3.2. Evolution of selected *m/z* with time/temperature

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A convenient manner of depicting the distinct evolution of *m/z* fragment intensity with time/temperature consists in plotting the intensity of selected fragments (here *m/z* 57, 78, 91, 92, 94 and 107) with temperature. In Fig. 2 (for Elliott and Green samples, in Fig. S2 for all samples), the intensity of *m/z* fragments is plotted against temperature by converting Py-MS acquisition time to temperature, using the temperature program, and then converting to Tpeak<sub>conv</sub> for comparison with TpS2 (see Methods). Only data for the 5-20 min time range, corresponding to the linear increase in temperature with time, are shown, excluding data acquired during the isothermal steps. From the selected *m/z* fragments of which the intensity is plotted over temperature, it becomes evident that fragments are recorded over distinct time ranges, with differing maximum release temperatures. This illustrates the sequential release and cracking of compounds during programmed pyrolysis.

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The curve of Py-MS TIC evolution with temperature of Green is a simple Gaussian with few variations in the Tpeak<sub>conv</sub> values. This is in agreement with the observation by Sebag et al. [7] that the S2 pyrolysis curves of source rocks, in contrast to soils and recent sediments, only require a single Gaussian curve to be mathematically deconvoluted. This is indeed true for those with limited organic sources such as Green that is Type I OM mainly composed of algal OM. The Py-MS TIC curve is more complex for the Elliott sample. It is built upon *m/z* curves whose Tpeaks are distinct. In addition, some of the *m/z* Py-MS curves display shoulders or are bimodal, for example, the curves of *m/z* 57 and 78 in Elliott (Fig. 2). This suggests that (1) distinct compounds affording the same fragments are released at distinct times or that (2) the same compounds (thus displaying the same fragments) are released at two distinct times and/or have distinct thermal behaviours. From a quantitative point of view, the main features identified from total mass spectra (Fig. 1) are also revealed from *m/z*
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evolution with temperature. Upon the selected fragments displayed in Fig. 2, *m/z* 57 is dominant in

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Green whereas *m/z* 91 is the most abundant fragment in Elliott, associated with abundant *m/z* 94

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and 107.

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### 3.3. Clusters of Tpeak<sub>conv</sub> values

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Tpeak<sub>conv</sub> values determined in the *m/z* 50 to 150 range for replicate analyses of standard

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samples were grouped into four clusters whose characteristics are shown in Table 1. In soil samples,

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C1 was centered around 365 °C, C2 around 430 °C, C3 around 460 °C and C4 around 490 °C, with

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significant intra-cluster variance. In geological samples of higher thermal maturity, the clusters were

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shifted towards higher temperature values, specifically for Pocahontas. Note that the clusters

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defined for the Green and 160000 samples were defined on a narrower range of temperatures and

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displayed lower variance.

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248 4. Discussion

249 4.1. Tpeak<sub>conv</sub> distribution in soil samples

250 "Bulk" quantitative differences in the eight standard samples were in Jacob et al. [54]. Major
251 distinctions in *m/z* abundance are also noted in the total mass spectra of samples (Fig. 1 and Fig. S1).

252 This denotes disparities in the molecular composition of the studied OM samples in accordance with
253 (1) their origin and (2) the degree of degradation or thermal alteration they were subjected to. This is
254 also highlighted by the Py-MS curves of selected *m/z* fragments (Fig. 2) illustrating various Tpeaks for
255 distinct *m/z*. Although Fig. 2 does not provide a full picture of Tpeak<sub>conv</sub> variability within or across
256 samples, it suggests that the chemical composition of OM that emits those *m/z* fragments is distinct
257 at distinct temperatures. This is in agreement with the mathematical deconvolution of RE6 S2 curves
258 that makes the assumption of the sequential cracking of chemically and thermally distinct OM during
259 pyrolysis. These types of OM are classically grouped into four to five families, each characterized by
260 rather homogeneous temperatures of maximum release and thought to be chemically or physically
261 homogeneous. In order to test the hypothesis that there are such pyrolytic families of homogeneous
262 temperatures of maximum release, the Tpeak<sub>conv</sub> values determined for each *m/z* fragment in the 50-
263 150 range were used as a key parameter. The distribution of Tpeak<sub>conv</sub> values against *m/z* fragments
264 is displayed for four replicates the Elliott sample in Fig. 3.

265 For the Elliott sample, Tpeak<sub>conv</sub> values of individual *m/z* curves range from 332 to 581 °C. There
266 is a rather good reproducibility of Tpeak<sub>conv</sub> values between the four replicates of the Elliott sample,
267 with a standard deviation from 0 to 13 °C (average 2.7 °C on 443 Tpeak<sub>conv</sub> values over the four
268 replicates). On examining the distribution of Tpeak<sub>conv</sub> values by increasing temperatures (Fig. 3a),
269 several slope breaks can be distinguished. This suggests that Tpeak<sub>conv</sub> values are not randomly
270 distributed and that several groups can be distinguished. Distribution of the Tpeak<sub>conv</sub> values against
271 *m/z* and individualised by replicate analyses (Fig. 3b) confirms that the Tpeak<sub>conv</sub> values are not
272 randomly distributed. Four clusters can be distinguished (Fig. 3c; Table 1 and Table 2). Within the
273 third and fourth groups, a singular pattern can be observed with several series of decreasing
274 Tpeak<sub>conv</sub> values with increasing *m/z*, a pattern that is more evident between *m/z* 114 and 124, *m/z*
275 128 and 134 and *m/z* 142 and 148 (Fig. 3b).

276 The distribution of Tpeak<sub>conv</sub> values (averaged over replicates) against *m/z* in Chernozem,
277 Vertisol and Pahokee is similar to that observed for Elliott (Fig. 4). As observed for Elliott, and with
278 few exceptions, the Tpeak<sub>conv</sub> values determined for each *m/z* are consistent within replicates (mean
279 standard deviation=2.4 °C for Elliott, 120 observables, n=4; 4.6 °C for Vertisol, 98 observables, n=3;
280 3.6 °C for Chernozem, 96 observables, n=3; 3.5 °C for Pahokee, 124 observables, n=3). In addition,
281 the Tpeak<sub>conv</sub> values determined for each *m/z* fragment are relatively consistent across samples,
282 except for Pahokee, which displays, on average, higher Tpeak<sub>conv</sub> values for the same *m/z* fragments
283 in the 440-520 °C range (clusters 3 and 4). Despite these rather good agreements of Tpeak<sub>conv</sub> values,
284 some discrepancies can be evidenced between samples. In cluster 1 (340-400 °C), as an example,
285 more *m/z* fragments were found in Elliott and Pahokee than in Vertisol and Chernozem. This denotes
286 chemical differences between soil samples that are additional to differences in relative abundance of
287 *m/z* fragments, not discussed here. Some minor differences were observed in Tpeak<sub>conv</sub> values in the
288 440-520 °C temperature range (clusters 3 and 4) between soil samples. In most cases, and for a given
289 *m/z* fragment, Tpeak<sub>conv</sub> values are higher for Pahokee, then Chernozem, then Vertisol and Elliott.
290 Although this must be confirmed by further studies, these differences may illustrate mineral matrix
291 effects [55] on the cracking temperature of these polymers. For instance, Dembicki [56] showed that,
292 due to catalytic effects, clay minerals decrease activation energies to lower values. This is likely
293 explaining why Tpeak<sub>conv</sub> values are lower in Elliott (silt loamy soil) than in Vertisol and Chernozem
294 (sandy clay), and in Pahokee (peaty soil). Finally, the pattern of decreasing Tpeak<sub>conv</sub> values with
295 increasing *m/z* in the 450-520 °C range for several series of *m/z*, already observed for Elliott, is also
296 noted for Chernozem, Vertisol and Pahokee.
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## 4.2. Comparison of S2 and of Py-MS TIC deconvolution and Tpeak<sub>conv</sub> clustering for soils

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Left panel of Figure 5 depicts the clusters of Tpeak<sub>conv</sub> values and Tpeak and Tpeak<sub>conv</sub> values of Gaussian curves obtained after mathematical deconvolution of the S2 and TIC peaks determined on standard soil samples (from [54]). These results are compared to the temperature ranges of fractions defined in the literature (central panel, Fig. 5), and to the same parameters as in the left panel but for non-soil samples (right panel, data from [54]). The parameters of each cluster are given in Table 1. Although the deconvolution of the S2 pyrolysis curve resulted in Gaussian curves centred around 316 °C in Pahokee and 321 °C in Elliott (black hyphens, Fig. 5), no fragment was found for which a Tpeak<sub>conv</sub> value was determined in the 290-330 °C range that corresponds to the F1 [7], F2 [8] or A1 [19, 20] pyrolytic fractions in the samples. This range corresponds to the pyrolytic family defined as "thermally labile biological compounds" [6-8, 19, 20]. Because these compounds are essentially found in soil litters, they are not expected to represent a large fraction of the signal in the standard soil samples since these were taken from deeper horizons and can thus be devoid of fresh material. Nonetheless, this is in contradiction with the appearance of a Gaussian curve at low temperatures from the deconvolution of the S2 pyrolysis curves [54]. An alternative explanation would imply that compounds released at this temperature are light/small compounds with minimal production of fragments higher than *m/z* 50, as discussed in [54].

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The first (by order of increasing temperatures) series of clusters of Tpeak<sub>conv</sub> values that could be defined falls within the 330-390 °C range (332-385 °C for Elliott, 329-395 °C for Chernozem, 343-393 °C for Vertisol and 332-387 °C for Pahokee; Table 1 and Fig. 5). These ranges are fairly homogeneous and in agreement with ranges defined for F2 [7], F3 [8], A2 [19, 20], and the higher temperature range of the S2a [18] pyrolytic fractions that are attributed to cellulose, lignin and, tentatively, polypeptides by Disnar et al. [6]. Tpeak<sub>conv</sub> values of the second Gaussian obtained by deconvolution of the TIC peak fall within the temperature ranges of this cluster (grey hyphens in Fig. 5), but not
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Tpeak values defined on the second Gaussian curve from deconvolution of the S2 peak, which are
always higher.

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The second series of clusters of Tpeak<sub>conv</sub> values ranges between 391 and 455 °C (391-433 °C for Elliott, 416-455 °C for Chernozem, 398-449 °C for Vertisol, and 396-448 °C for Pahokee (Table 1 and Fig. 5). These ranges fall within the range of temperatures proposed in the literature for the third pyrolytic fraction (defined as F3, F4, A3, and the lower temperature range of S2b, depending on the authors; Fig. 5) considered to consist, for soils, of "immature humic substances" [6].

325

326

327

328

329

The third series of clusters range from 434 to 499 °C (434-464 °C for Elliott, 456-486 °C for Chernozem, 450-486 °C for Vertisol and 451-499 °C for Pahokee, Table 1 and Fig. 5). They thus overlap by ~20 °C with the range of the second series of clusters. These ranges are consistent with the F5 pyrolytic fraction of Copard et al. [8], with the lower values of the F4 and A4 fractions, and with the upper range of the S2b fraction (Fig. 5). These pyrolytic fractions are defined in the literature as characteristic of "mature" or "reworked" OM. Within this series of clusters, there is a better agreement with Tpeak<sub>conv</sub> values defined on the third Gaussian resulting from deconvolution of the S2 peak than for the other series of clusters.

330

331

332

333

334

335

336

337

The fourth series of Tpeak<sub>conv</sub> clusters ranges between 465 and 588 °C (465-581 °C for Elliott, 487-588 °C for Chernozem, 487-588 °C for Vertisol and 500-581 °C for Pahokee, Table 1 and Fig. 5). Again, this series of clusters overlaps with the previous one by ~30 °C. It could correspond to the F6 fraction of Copard et al. [8] considered as reflective of recalcitrant, fossil OM, but also to the S2c of Gillespie et al. [18] and to the upper temperature range of the A4 fraction of Albrecht et al. [19] and Sebag et al. [20].

338

339

340

341

342

343

Although both S2 mathematical deconvolution and Tpeak clustering resulted in four fractions, these are thus not systematically comparable. The first fraction defined from S2 deconvolution (F1, F2, A1 and lower range of S2a) could not be identified. Only the first and last Tpeak clusters are in general good agreement with F2, [7], F3 [8], the upper part of S2a [18] and A2 [19, 20] on the one

344

345

346
hand and F6 [8], S2c [18] and the upper part of A4 [19, 20] on the other hand. The second and third
Tpeak clusters more or less correspond to F3 and F4 [7], F4 and F5 [8], A3 and the lower part of A4
[19, 10], and S2b [18], with some overlaps between Tpeak clusters. Thus, clusters 2 and 3 (and to a
lesser extent cluster 4) could be considered as a continuous series, calling into question a possible
artificial clustering of a homogeneous set of Tpeak<sub>conv</sub> values.

Overall, the examination of Tpeak<sub>conv</sub> values for soil samples appears to validate a cluster of
temperatures in the 330-390 °C range that could correspond to the second pyrolytic fraction
commonly reported in the literature. The same applies to the high temperature cluster. The picture is
more complex for intermediate Tpeak<sub>conv</sub> values with no clear clustering and variable correspondence
with pyrolytic families defined from Rock-Eval® S2 deconvolution.

### 4.3. Comparison of S2 and of Py-MS TIC deconvolution and Tpeak<sub>conv</sub> clustering for other samples

The series of clusters in Suwannee is not very distinct from those of the soils, except that the four clusters appear to be systematically set at lower temperatures than their equivalents in soils. Conversely, the series of clusters in Pocahontas is distinct, with no Tpeak<sub>conv</sub> value below 399 °C (except for m/z 64 at 352-360 °C not presented, cf. below) and maximum temperatures recorded at 631 °C, due obviously to the higher thermal maturity recorded by this sample. Except for a cluster that was defined between 349 and 394 °C in Green (but with only 4 Tpeak<sub>conv</sub> values), Green and 160000 samples are characterized by concentrated clusters in a narrower range of temperatures (444-495 °C for Green and 416-503 °C for 160000) than those observed for soils. For non-soil samples such as Pocahontas, Green and 160000 there is hence no or little correspondence with the temperature ranges defined in the literature for soil samples.

### 4.4. Chemical attribution of m/z Tpeak<sub>conv</sub> values

Graphical and statistical treatments only provided a partially satisfactory distinction of classes of
fragments depending on the temperature at which their maximum was released. As a matter of fact,
some Tpeak<sub>conv</sub> values of replicate analyses of one standard sample were found in two distinct clusters, which shows the limit of clustering. In order to better understand the molecular significance of pyrolytic fractions, fragments were organised according to both their chemical attribution and their Tpeak<sub>conv</sub> values. m/z fragments were tentatively attributed to specific chemical families following an extensive literature survey (see Supplementary Table S1 and related references).

Although the attribution of m/z fragments to specific chemical families is non-unique and thus debatable, it provided a first step to assess the nature of the compounds that are progressively produced during the course of pyrolysis. m/z fragments were then tentatively attributed to distinct chemical families for the Elliott sample. The result of this attribution is displayed in the m/z 50-150 range in Table 3. When two Tpeak<sub>conv</sub> values were defined for a single m/z fragment, the m/z fragment was attributed to the most probable chemical family, depending on the Tpeak<sub>conv</sub> value and chemical attribution of neighboring (in terms of temperature) m/z fragments. Sulfur was identified in all samples through the m/z 64 fragment.

Five chemical families were distinguished. "Aromatics" are composed of m/z fragments identified in the literature as originating from benzene, "alkylaromatics", "alkylbenzenes", or "naphthalenes". m/z fragments interpreted in the literature as representative of "aliphatic polymers", "hydrocarbons", "alkane fragments", "low-mass olefins" or "lipids" were grouped into "Aliphatics". The "Carbohydrates" family comprises fragments attributed to "carbohydrates with pentose and hexose subunits", "mono- and polysaccharides" or "cellulose". The "Lignin" family is composed of fragments attributed to "phenols and lignin monomers", "dihydroxybenzenes", "alkyl-substituted dihydroxybenzenes" and "alkylphenols". Finally, fragments relative to "proteins", "N-compounds or N-containing compounds", "peptides", "amino-acid species and amides", "alkyl pyrroles and indoles" were grouped in the "Proteins" chemical family.

Despite being produced with different temperature programs, the result of m/z attribution is in general agreement with results acquired on soils by Py-FIMS [44, 57, 58] and with results from the
396

Rock-Eval® analysis of pure organic materials [10, 22]. In other words, the Tpeak<sub>conv</sub> values found for
chemical families are in line with the temperatures of maximum release defined in the literature.

397

398

Fig. 6 reports the ranges of Tpeak<sub>conv</sub> values for the 5 chemical families of m/z fragments in the 50-150 range, in the Elliott sample. As already discussed, and considering the nature of the samples, the most labile fraction described in the literature between 280 and 320 °C could not be identified by significant Tpeak<sub>conv</sub> values. The cluster of Tpeak<sub>conv</sub> values that occurs between 340 and 400 °C is constituted by m/z fragments attributed to Sulphur, Carbohydrates, Phenols and Proteins (Fig. 6). This result is in line with the chemical attribution of the fraction of soil OM that cracks between 330 and 400 °C to "cellulose, lignin and polypeptides" [6] or "resistant biological constituents" [7]. A meticulous examination of differences in Tpeak occurrences between soil samples in this 340-400 °C cluster could give valuable information on the chemical differences in thermolabile OM between soil samples, although quantitative data could be more efficient. For example, and contrary to Elliott, a Tpeak<sub>conv</sub> value could not be defined in Chernozem for m/z 98 (Carbohydrates), m/z 61 and 124 (Phenols) and m/z 74 (Protein).

399

400

401

402

403

404

405

406

407

408

409

410

Most fragments of the 415 to 480 °C Tpeak<sub>conv</sub> cluster relate to Aliphatics, Lignin and Aromatics, and to a lesser extent to Carbohydrates (m/z 50, 82, 114, 144, 146), although some of these fragments appear isolated in the 415-440 °C range. Due to the difficulties in defining distinct Tpeak clusters in this temperature range (cf. 4.2.), it is considered that all fragments in the 415-480 °C range represented a homogeneous, although chemically complex, polymeric OM.

411

412

413

414

415

416

4.5. A singular pattern in the 415-480°C range

417

The distribution of m/z fragments against Tpeak<sub>conv</sub> values in the 415-480°C range is characterized by the already mentioned singular pattern with Tpeak<sub>conv</sub> values decreasing with increasing m/z (Fig. 3, 4 and 7). This pattern is more evident in the m/z 110-150 range. For lower m/z,

418

419
420 abundant fragments attributed to Lignin might mask this pattern. Three series of *m/z* (~115-123,
421 ~128-135 and ~140-149), separated by ~14 units, are each characterized by a ~50 °C drop in
422 temperature (Fig. 7) in most samples, except for Green and 160000, for which the difference is lower.
423 This pattern was identified in all soil samples but also in Suwannee, Green and 160000. In Pocahontas
424 this pattern was also observed but occurs at higher temperatures (500-560 °C). This singular and
425 similar pattern regardless of the samples, but recorded at distinct Tpeak<sub>conv</sub> values, suggests that the
426 considered *m/z* fragments are representative of a coherent pool of OM mostly composed, for soils,
427 of Aliphatics and Lignin, with some Aromatics and Carbohydrates.

428 The considered pattern suggests a continuum of cracking in this cluster. This could reflect the
429 progressive cracking of components of the polymers based on thermochemical considerations
430 (degree of unsaturation, aromaticity, double bond location on the alkyl chains, branching...). For
431 example, as the unsaturation increases (leading to a series of *m/z* with 2 mass unit differences),
432 lower activation energies may be involved [60]. The lower temperature range recorded for Green
433 and 160000 samples for this cluster could be related to their more aliphatic nature compared to
434 other samples, as already illustrated by the low relative abundance of *m/z* 91 in the total mass
435 spectra of Green and 160000 (Fig. 1). For soil samples, these results thus provide new insight into this
436 yet poorly resolved or understood RE pyrolytic fraction considered in the literature as "immature
437 humic substances" [6] or "immature non-biotic constituents"/"immature geo-macromolecules" [7].

438 Surprisingly, the pattern is also valid for the Pocahontas sample, but with Tpeak<sub>conv</sub> values of the
439 corresponding *m/z* fragments are about ~70 °C higher than for soils (Fig. 7). Mineral matrix effects
440 cannot be invoked because Pocahontas is devoid of a significant mineral fraction (as Suwannee). The
441 higher Tpeak<sub>conv</sub> values recorded for Pocahontas is likely due to the higher degree of
442 reticulation/condensation of the polymers and/or higher aromaticity since Pocahontas is mainly
443 composed of a network of alkylbenzenes and alkylnaphthalenes [60]. This probably explains the
444 scarcity of Tpeak<sub>conv</sub> values lower than 500 °C for this sample: (1) *m/z* 64 Tpeak<sub>conv</sub> value was found at
356 °C and attributed to Sulphur; (2) m/z 83, 97, 98, 99, 111, 112, 113 and 125 Tpeak<sub>conv</sub> values were found in the 417-470 °C range. These were attributed to Aliphatics in soil samples, as well as in Pocahontas. All other Tpeak<sub>conv</sub> values defined for Pocahontas (500-600 °C) encompass the range defined for the fourth cluster of Tpeak<sub>conv</sub> values (470-590 °C) defined in soil samples. Very consistent Tpeak<sub>conv</sub> values were found for m/z 78, 128 and 138 in both Pocahontas (Fig. 7) and soil samples (Figs. 3, 4 and 7). Thus, these fragments and their associated Tpeak<sub>conv</sub> values could attest to a contribution of fossil organic carbon in soil samples. Results are thus consistent with interpretations of the high temperature fraction (~480 °C) defined from the mathematical deconvolution of the S2 pyrogram as "mature/reworked OM and charcoal" [6] or "mature refractory fraction" [7]. It appears peculiarly relevant for Chernozem in which Schmidt et al. [61] noted a significant contribution of charred organic carbon. The monitoring of such fragments in environmental samples should be examined in order to determine the contribution of charcoal/fossil carbon/black carbon.

457

## Conclusion

Temperature-resolved pyrolysis directly coupled to a mass spectrometer applied to various environmental samples provides direct evidence for the molecular composition of pyrolytic fractions defined from the mathematical deconvolution of Rock-Eval® S2 pyrograms. Only the most thermolabile fraction (280-320 °C) described in the literature for soil samples was not detected. Then, and according to their thermal lability, four main clusters of Tpeak values were described. The first cluster of Tpeak values (340-400 °C) is formed by fragments assigned to proteins, lignin and carbohydrates. The second and third clusters (415-480 °C) correspond to fragments attributed to aliphatics, lignin and aromatics. In all samples, this cluster displays a singular pattern of decreasing m/z with increasing temperature, the significance of which remains to be elucidated. The fourth cluster (Tpeak values > 480 °C) is constituted by fragments exclusively attributed to aromatic moieties.
These results show that the mathematical deconvolution of S2 curves of soil samples obtained from Rock-Eval® pyrolysis can be partly validated by direct molecular information, although the complex 415-480 °C cluster should probably not be considered as a simple Gaussian and requires further attention. For other types of samples such as mature geological samples, the strategy must be adapted, and the attribution of *m/z* fragments could differ. This approach opens new windows for the elucidation of the molecular nature of complex organic matrices. Further work will require a closer scrutiny of the significance of *m/z* achieved by using higher mass resolution and lower energy. Extensive data mining of the results could afford more relevant variables than the Tpeak values alone.

479

480 Acknowledgments

The authors wish to thank David Curry, Thomas Gentzis and Joseph Curiale for their constructive comments on an earlier version of the manuscript. Rachel Boscardin at ISTO laboratory in Orléans is acknowledged for Rock-Eval® analyses. This work received financial support from the OTARIE project (Région Centre Val de Loire, European Council through the FEDER fund) and from the Labex VOLTAIRE (ANR-10-LABX-100-01). The authors appreciated interactions with Benoit Auguin (Quad Service, France) from the initiation to the finalization of the project. J. Jacob, F. Delarue and Y. Copard are indebted to J.R. Disnar for raising their awareness of the potential of Rock-Eval® pyrolysis and for long-standing inspiring discussions on this project. M.F. Dignac and V. Grossi are thanked for their advice during data interpretation. E. Rowley-Jolivet is acknowledged for English language editing of the manuscript.
491 Tables

492 Table 1: Barycentre, intra-cluster variance, minimum (Min) and maximum (Max) values and standard
deviation (Stdev) for the 4 clusters of Tpeakconv values defined in the *m/z* 50-150 range of the eight
493
494 standard samples.

<table><thead><tr><th>Sample</th><th>Cluster</th><th>Barycentres (°C)</th><th>Intra-cluster variance</th><th>Min (°C)</th><th>Max (°C)</th><th>Stdev (°C)</th><th>Observables</th></tr></thead><tbody><tr><td rowspan="4">Elliott</td><td>C1</td><td>354</td><td>101</td><td>332</td><td>385</td><td>8,3</td><td>71</td></tr><tr><td>C2</td><td>418</td><td>51</td><td>391</td><td>433</td><td>5,5</td><td>78</td></tr><tr><td>C3</td><td>449</td><td>66</td><td>434</td><td>464</td><td>7,6</td><td>194</td></tr><tr><td>C4</td><td>480</td><td>630</td><td>465</td><td>581</td><td>16,7</td><td>100</td></tr><tr><td rowspan="4">Chernozem</td><td>C1</td><td>352</td><td>308</td><td>329</td><td>395</td><td>14,4</td><td>15</td></tr><tr><td>C2</td><td>443</td><td>79</td><td>416</td><td>455</td><td>7,2</td><td>95</td></tr><tr><td>C3</td><td>469</td><td>46</td><td>456</td><td>486</td><td>5,5</td><td>100</td></tr><tr><td>C4</td><td>504</td><td>551</td><td>487</td><td>588</td><td>15,2</td><td>41</td></tr><tr><td rowspan="4">Vertisol</td><td>C1</td><td>361</td><td>128</td><td>343</td><td>393</td><td>8,7</td><td>24</td></tr><tr><td>C2</td><td>433</td><td>139</td><td>398</td><td>449</td><td>10,3</td><td>94</td></tr><tr><td>C3</td><td>466</td><td>74</td><td>450</td><td>486</td><td>7,3</td><td>131</td></tr><tr><td>C4</td><td>508</td><td>767</td><td>487</td><td>588</td><td>21,2</td><td>29</td></tr><tr><td rowspan="4">Pahokee</td><td>C1</td><td>355</td><td>185</td><td>332</td><td>387</td><td>12,3</td><td>70</td></tr><tr><td>C2</td><td>424</td><td>267</td><td>396</td><td>448</td><td>14,5</td><td>19</td></tr><tr><td>C3</td><td>475</td><td>92</td><td>451</td><td>499</td><td>7,9</td><td>254</td></tr><tr><td>C4</td><td>524</td><td>769</td><td>500</td><td>581</td><td>23,9</td><td>22</td></tr><tr><td rowspan="4">Suwanee</td><td>C1</td><td>360</td><td>133</td><td>334</td><td>379</td><td>9,7</td><td>47</td></tr><tr><td>C2</td><td>400</td><td>80</td><td>381</td><td>419</td><td>7,5</td><td>74</td></tr><tr><td>C3</td><td>439</td><td>91</td><td>422</td><td>455</td><td>8,6</td><td>74</td></tr><tr><td>C4</td><td>471</td><td>181</td><td>455</td><td>514</td><td>10,6</td><td>114</td></tr><tr><td rowspan="4">Pocahontas</td><td>C1</td><td>430</td><td>224</td><td>399</td><td>458</td><td>13,1</td><td>23</td></tr><tr><td>C2</td><td>514</td><td>92</td><td>492</td><td>535</td><td>7,9</td><td>121</td></tr><tr><td>C3</td><td>555</td><td>92</td><td>535</td><td>572</td><td>8,4</td><td>113</td></tr><tr><td>C4</td><td>589</td><td>159</td><td>572</td><td>631</td><td>10,7</td><td>61</td></tr><tr><td rowspan="4">Green</td><td>C1</td><td>372</td><td>484</td><td>349</td><td>394</td><td>19,8</td><td>4</td></tr><tr><td>C2</td><td>454</td><td>26</td><td>444</td><td>462</td><td>4,3</td><td>26</td></tr><tr><td>C3</td><td>469</td><td>8</td><td>462</td><td>475</td><td>2,3</td><td>118</td></tr><tr><td>C4</td><td>482</td><td>23</td><td>476</td><td>495</td><td>4,2</td><td>52</td></tr><tr><td rowspan="4">160000</td><td>C1</td><td>429</td><td>43</td><td>416</td><td>438</td><td>5,9</td><td>36</td></tr><tr><td>C2</td><td>448</td><td>20</td><td>440</td><td>457</td><td>4,0</td><td>102</td></tr><tr><td>C3</td><td>467</td><td>33</td><td>459</td><td>476</td><td>5,5</td><td>54</td></tr><tr><td>C4</td><td>492</td><td>67</td><td>482</td><td>503</td><td>7,4</td><td>10</td></tr></tbody></table>

495

496

497
498

499 Table 2: Affectation of *m/z* fragments to distinct Tpeak<sub>conv</sub> clusters in the Elliott sample. When two

500 Tpeak<sub>conv</sub> values were found for the same fragment, the Tpeak<sub>conv</sub> values are indicated.

<table><thead><tr><th>Tpeak<sub>conv</sub> cluster</th><th>m/z fragment</th></tr></thead><tbody><tr><td>340 - 400 °C</td><td>58, 59, 60, 61, 64, 68, 73, 74, 86, 87, 96, 98, 109, 110, 111, 112, 119, 121, 124, 125, 137, 138</td></tr><tr><td>400 - 440 °C</td><td>53, 54, 58, 61, 67, 72, 80, 81, 82, 87, 95, 96, 109, 110, 111, 114, 123, 124, 125, 133, 135, 136, 137, 138, 148, 149, 150</td></tr><tr><td>440 - 470 °C</td><td>50, 51, 55, 56, 57, 61, 62, 64, 65, 66, 67, 69, 70, 71, 77, 79, 83, 84, 85, 86, 91, 92, 93, 94, 97, 98, 99, 100, 104, 105, 106, 108, 111, 112, 113, 117, 118, 119, 120, 121, 122, 125, 126, 132, 133, 134, 137, 140, 145, 146, 147, 148</td></tr><tr><td>470 - 591 °C</td><td>63, 64, 74, 75, 76, 78 (470 °C), 78 (570 °C), 89, 90, 101, 102, 103, 107, 114, 115, 116, 127, 128 (475 °C), 128 (565 °C), 129, 130, 131, 139, 141, 142, 143, 144, 145, 146</td></tr></tbody></table>

501

502 Table 3: Attribution of *m/z* fragments to chemical families in the Elliott sample according to literature

503 data (Table S1).

<table><thead><tr><th>Chemical family</th><th>m/z</th></tr></thead><tbody><tr><td>Aromatics</td><td>78, 92, 106, 120, 128, 134, 138, 142, 148</td></tr><tr><td></td><td>55, 56, 57, 58, 65, 67, 70, 74, 76, 81, 83, 84, 87, 89, 91, 92, 93, 96, 97, 98, 99, 100, 101, 102, 103, 105, 106, 107, 108, 109, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 148</td></tr><tr><td>Aliphatics</td><td></td></tr><tr><td>Carbohydrates</td><td>50, 58, 60, 68, 82, 86, 96, 98, 110, 112, 114, 144, 146</td></tr><tr><td>Lignin</td><td>51, 54, 61, 69, 71, 72, 75, 77, 79, 80, 85, 86, 90, 94, 95, 110, 124, 136, 140, 150</td></tr><tr><td>Proteins</td><td>59, 73, 74, 87, 109, 111, 125, 137</td></tr><tr><td>Sulfur</td><td>64</td></tr><tr><td>Non-attributed</td><td>145, 53, 62, 63, 64, 66, 78, 104, 119, 121, 137, 138, 139, 141, 143, 147, 149</td></tr></tbody></table>

504

505
506 **Figure captions**

507 Figure 1: Total mass spectra of Elliott and Green standard samples analysed by Py-MS in the *m/z* 50-508 300 range. Total mass spectra illustrate the relative abundance of *m/z* fragments recorded during the 509 time interval of the TIC peak. Background subtraction is performed before and after the TIC peak.

510 Figure 2: Evolution of TIC (red curve) and relative abundance of selected *m/z* (57, 78, 91, 92, 94 and 511 107) with temperature (converted to be comparable to TPs2) for Elliott and Green samples. The 512 same curves for the eight standard samples are displayed in Fig. S2. Tpeak<sub>conv</sub> values are indicated for 513 both TIC and *m/z* curves. The amount of material used for each analysis (in mg) is indicated after the 514 sample name.

515 Figure 3: Tpeak<sub>conv</sub> values determined for individual *m/z* values (obs=443) in the 50-150 range for four 516 replicate analyses of the Elliott standard sample (11.1, 14, 14.4 and 16.7 mg). (a) Ordered by 517 increasing temperature; (b) Ordered against *m/z* values and organized by replicate analysis. Arrows 518 represent a singular pattern of decreasing *m/z* with increasing Tpeak<sub>conv</sub> values discussed later; (c) 519 Grouped after cluster analysis. Horizontal dotted line corresponds to the limits of defined clusters.

520 Figure 4: Comparison of Tpeak<sub>conv</sub> values (averaged per sample) organized versus *m/z* values in the 521 522 523 524 525 526 527 528 529

Figure 5: Comparison of cluster analysis of Tpeak<sub>conv</sub> values for soil standard sample with ranges of temperatures defined in the literature for Tpeak values of Gaussian curves resulting from mathematical deconvolution of the S2 pyrolytic curve (boxes and whisker plots; see Jacob et al. [54]). In order to allow direct comparison of soil Tpeak<sub>conv</sub> clusters with literature data, these are presented next to each other. Clusters of Tpeak<sub>conv</sub> values for non-soil samples (Suwannee, Pocahontas, Green and 160000) are presented in the four panels on the right. Parameters of the Tpeak<sub>conv</sub> clusters are
given for each standard sample in Table 1. Tpeak values defined from the deconvolution of the S2 pyrolysis curve are indicated, for each sample, as black hyphens (horizontal bars), and Tpeak values defined on each Gaussian curve obtained from the mathematical deconvolution of the TIC peak are indicated as grey and thick hyphens (from Jacob et al. [54]). Note that *m/z* 64 was isolated before clustering in the Pocahontas sample, and appears as a single point.

Figure 6: Tpeak<sub>conv</sub> values for each *m/z* in the 50-150 range, organized according to their chemical attribution. Tpeak<sub>conv</sub> values represent average values for 4 replicate analyses of the Elliott sample.

Figure 7: Distribution on Tpeak<sub>conv</sub> values of *m/z* fragments in the 100-150 range for replicate analyses of Elliott, Vertisol, Chernozem, Pahokee, Green, Pocahontas and Suwannee standard samples.

540

541
542 References

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544 Méthode rapide de caractérisation des roches mètres, de leur potentiel pétrolier et de leur degré d'évolution. Revue de l'Institut Français du Pétrole 32, 23-42.

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548 [3] Espitalié, J., Deroo, G., Marquis, F., 1985b. La pyrolyse Rock-Eval® et ses applications; deuxième partie. Revue de l'Institut Français du Pétrole 40, 755-784.

550 [4] Lafargue, E., Marquis, F., Pillot, D., 1998. Rock-Eval® 6 applications in hydrocarbon exploration, production, and soil contamination studies. Revue de l'Institut Français du Pétrole 53, 421-437.

552 [5] Behar, F., Beaumont, V., Penteado, H.D.B., 2001. Rock-Eval® 6 technology: performances and developments. Oil Gas Science and Technology – Revue de l'Institut Français du Pétrole 56, 111-134.

554 [6] Disnar, J.R., Guillet, B., Keravis, D., Di-Giovanni, C., Sebag, D., 2003. Soil organic matter (SOM) characterization by Rock-Eval® pyrolysis: scope and limitations. Organic Geochemistry 34, 327-343.

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704 German chernozemic soils. European Journal of Soil Science 50, 351-365.

705
Supplementary data
<table><caption>707 Table S1: List of *m/z* fragments together with the chemical families they were attributed to, from the literature.</caption><thead><tr><th>m/z</th><th>Chemical family</th><th>Ref.</th><th>m/z</th><th>Chemical family</th><th>Ref.</th><th>m/z</th><th>Chemical family</th><th>Ref.</th><th>m/z</th><th>Chemical family</th><th>Ref.</th></tr></thead><tbody><tr><td>50</td><td>Polysaccharides</td><td>[1]</td><td>81</td><td>Aliphatic polymers</td><td>[1]</td><td>103</td><td>N-containing compounds</td><td>[11]</td><td>124</td><td>Alk-dihydroxybenz</td><td>[7]</td></tr><tr><td>51</td><td>Lignin</td><td>[1]</td><td>81</td><td>Proteins</td><td>[1]</td><td>103</td><td>N-compounds</td><td>[10]</td><td>124</td><td>Dihydroxybenzenes</td><td>[9]</td></tr><tr><td>51</td><td>Polyphenols</td><td>[1]</td><td>81</td><td>N-containing compounds</td><td>[11]</td><td>105</td><td>Lipids</td><td>[1]</td><td>124</td><td>Phenols and lignin monomers</td><td>[11]</td></tr><tr><td>52</td><td>Lignin</td><td>[1]</td><td>81</td><td>N-compounds</td><td>[10]</td><td>106</td><td>Alkylaromatics (alkylbenzenes)</td><td>[2]</td><td>124</td><td>Lignin monomers and phenols</td><td>[10]</td></tr><tr><td>54</td><td>Lignin</td><td>[1]</td><td>82</td><td>Mono- and polysaccharides</td><td>[2]</td><td>106</td><td>M,D,T,P-benzene</td><td>[6]</td><td>125</td><td>Lipids</td><td>[1]</td></tr><tr><td>54</td><td>Proteins</td><td>[1]</td><td>82</td><td>Carbo-pent/hexose</td><td>[6]</td><td>106</td><td>Ethyl- to docosyl-benzene</td><td>[6]</td><td>125</td><td>N-containing compounds</td><td>[11]</td></tr><tr><td>54</td><td>Polyphenols</td><td>[1]</td><td>82</td><td>Cellulose</td><td>[4]</td><td>106</td><td>Benzene</td><td>[4]</td><td>125</td><td>N-compounds</td><td>[10]</td></tr><tr><td>55</td><td>Aliphatic polymers</td><td>[1]</td><td>82</td><td>Methyl furan</td><td>[4]</td><td>106</td><td>Lipids</td><td>[1]</td><td>126</td><td>Mono- and polysaccharides</td><td>[2]</td></tr><tr><td>55</td><td>Hydrocarbons</td><td>[3]</td><td>82</td><td>Aliphatic polymers</td><td>[1]</td><td>106</td><td>Benzenes</td><td>[7]</td><td>126</td><td>Carbohydrates</td><td>[12]</td></tr><tr><td>56</td><td>Low-mass olefins</td><td>[6]</td><td>82</td><td>Polysaccharides</td><td>[1]</td><td>106</td><td>Alkylbenzenes</td><td>[9]</td><td>126</td><td>Carbo-pent/hexose</td><td>[6]</td></tr><tr><td>56</td><td>Proteins</td><td>[1]</td><td>82</td><td>Polysaccharides</td><td>[5]</td><td>106</td><td>Alkylaromatics</td><td>[11]</td><td>126</td><td>Phenols</td><td>[6]</td></tr><tr><td>56</td><td>Alkanes</td><td>[7]</td><td>82</td><td>Carbohydrates</td><td>[11]</td><td>106</td><td>Alkylaromatics</td><td>[10]</td><td>126</td><td>Cellulose</td><td>[4]</td></tr><tr><td>57</td><td>Aliphatic polymers</td><td>[1]</td><td>82</td><td>Pent/hexose-carbo</td><td>[10]</td><td>107</td><td>Nitrogen compounds</td><td>[2]</td><td>126</td><td>Lipids</td><td>[1]</td></tr><tr><td>57</td><td>Hydrocarbons</td><td>[3]</td><td>84</td><td>Mono- and polysaccharides</td><td>[2]</td><td>107</td><td>Aliphatic polymers</td><td>[1]</td><td>126</td><td>Polysaccharides</td><td>[1]</td></tr><tr><td>57</td><td>Peptides</td><td>[11]</td><td>84</td><td>Carbo-pent/hexose</td><td>[6]</td><td>108</td><td>Phenols and lignin monomers</td><td>[2]</td><td>126</td><td>Polysaccharides</td><td>[5]</td></tr><tr><td>57</td><td>Peptides</td><td>[10]</td><td>84</td><td>Low-mass olefins</td><td>[6]</td><td>108</td><td>Lignin monomers</td><td>[12]</td><td>126</td><td>Polysaccharides</td><td>[8]</td></tr><tr><td>58</td><td>Mono- and polysaccharides</td><td>[2]</td><td>84</td><td>Cellulose</td><td>[4]</td><td>108</td><td>Phenols</td><td>[6]</td><td>126</td><td>Levoglucosenone / Carbohydrates</td><td>[9]</td></tr><tr><td>58</td><td>Acetone</td><td>[6]</td><td>84</td><td>Hydroxyfuran</td><td>[4]</td><td>108</td><td>Phenol</td><td>[4]</td><td>126</td><td>Carbohydrates</td><td>[11]</td></tr><tr><td>58</td><td>Polysaccharides</td><td>[1]</td><td>84</td><td>Polysaccharides</td><td>[1]</td><td>108</td><td>Aliphatic polymers</td><td>[1]</td><td>126</td><td>Pent/hexose-carbo</td><td>[10]</td></tr><tr><td>58</td><td>Alkane fragments</td><td>[7]</td><td>84</td><td>Alkanes</td><td>[7]</td><td>108</td><td>Phenols</td><td>[7]</td><td>126</td><td>Lignin monomers and phenols</td><td>[10]</td></tr><tr><td>59</td><td>N-containing compounds</td><td>[12]</td><td>84</td><td>Carbohydrates</td><td>[11]</td><td>108</td><td>Alkylphenols</td><td>[9]</td><td>127</td><td>Lipids</td><td>[1]</td></tr><tr><td>59</td><td>Acetamide</td><td>[6]</td><td>84</td><td>Peptides</td><td>[11]</td><td>108</td><td>Phenols and lignin monomers</td><td>[11]</td><td>128</td><td>Mono- and polysaccharides</td><td>[2]</td></tr><tr><td>59</td><td>N-compounds</td><td>[6]</td><td>84</td><td>Pent/hexose-carbo</td><td>[10]</td><td>108</td><td>Lignin monomers and phenols</td><td>[10]</td><td>128</td><td>Polysaccharide</td><td>[4]</td></tr><tr><td>59</td><td>Acetamide</td><td>[9]</td><td>85</td><td>N-containing compounds</td><td>[12]</td><td>109</td><td>Trimethyl pyrrole</td><td>[6]</td><td>128</td><td>Lipids</td><td>[1]</td></tr><tr><td>59</td><td>N-containing compounds</td><td>[11]</td><td>85</td><td>Amino-acid species and amides</td><td>[12]</td><td>109</td><td>Aliphatic polymers</td><td>[1]</td><td>128</td><td>Polysaccharides</td><td>[1]</td></tr><tr><td>59</td><td>N-compounds</td><td>[10]</td><td>85</td><td>Lignin</td><td>[1]</td><td>109</td><td>N-containing compounds</td><td>[11]</td><td>128</td><td>Naphtalenes</td><td>[7]</td></tr><tr><td>60</td><td>Mono- and polysaccharides</td><td>[2]</td><td>86</td><td>Mono- and polysaccharides</td><td>[2]</td><td>109</td><td>N-compounds</td><td>[10]</td><td>129</td><td>Lipids</td><td>[1]</td></tr></tbody></table>
60 Amino-acid species and amides [12]

60 Carbohydrates [12]

60 Carbo-pent/hexose [6]

60 Acetic acid [6]

60 Cellulose [4]

60 Acetic acid [4]

60 Lignin [1]

60 Polyphenols [1]

60 Polysaccharides [5]

60 Carbohydrates [11]

60 Pent/hexose-carbo [10]

61 Lignin [1]

61 Polyphenols [1]

63 Lignin [1]

64 Lignin [1]

65 Aliphatic polymers [1]

67 Nitrogen compounds [2]

67 Pyrrole [6]

67 Prot-alk-pyrroles/indoles [4]

67 Aliphatic polymers [1]

67 Proteins [1]

67 N-containing compounds [11]

67 N-compounds [10]

68 Cellulose [4]

68 Furan [4]

68 Lignin [1]

69 Lignin [1]

69 Hydrocarbons [3]

70 Amino-acid species and amides [12]

70 Low-mass olefins [6]

70 Polysaccharides [1]

86 Lignin [1]

87 Amino-acid species and amides [12]

87 Aliphatic polymers [1]

87 Peptides [11]

87 Peptides [10]

88 Lignin [1]

89 Aliphatic polymers [1]

90 Lignin [1]

91 Lipids [1]

91 Peptides [11]

91 Peptides [10]

92 Alkylaromatics (alkylbenzenes) [2]

92 M,D,T,T,P-benzene [6]

92 Benzene [4]

92 Benzenes [7]

92 Alkylbenzenes [9]

92 Alkylaromatics [11]

92 Alkylaromatics [10]

93 N-compounds [6]

93 Aliphatic polymers [1]

93 Proteins [1]

93 N-containing ions [5]

94 Phenols and lignin monomers [2]

94 Lignin monomers [12]

94 Phenols [6]

94 Phenol [4]

94 Lignin [1]

94 Phenols [7]

94 Alkylphenols [9]

94 Phenols and lignin monomers [11]

94 Lignin monomers and phenols [10]

110 Phenols and lignin monomers [2]

110 Mono- and polysaccharides [2]

110 Carbohydrates [12]

110 Carbo-pent/hexose [6]

110 Phenols [6]

110 Cellulose [4]

110 Aliphatic polymers [1]

110 Polysaccharides [1]

110 Polysaccharides [5]

110 Alk-dihydroxybenz [7]

110 Dihydroxybenzenes [9]

110 Carbohydrates [11]

110 Phenols and lignin monomers [11]

110 Pent/hexose-carbo [10]

110 Lignin monomers and phenols [10]

111 Aliphatic polymers [1]

111 N-containing compounds [11]

111 N-compounds [10]

112 Mono- and polysaccharides [2]

112 Carbohydrates [12]

112 Carbo-pent/hexose [6]

112 Cellulose [4]

112 Aliphatic polymers [1]

112 Polysaccharides [1]

112 Carbohydrates [11]

112 Pent/hexose-carbo [10]

113 Lipids [1]

114 Mono- and polysaccharides [2]

114 Carbohydrates [12]

114 Carbo-pent/hexose [6]

114 Polysaccharide [4]

129 Peptides [11]

129 Peptides [10]

130 Lipids [1]

131 Nitrogen compounds [2]

131 N-compounds [6]

131 Prot-alk-pyrroles/indoles [4]

131 Lipids [1]

131 Proteins [1]

132 Mono- and polysaccharides [2]

132 Carbohydrates [12]

132 Carbo-pent/hexose [6]

132 Lipids [1]

132 Polysaccharides [1]

132 Alk-tetralins/benzofurans [7]

132 Carbohydrates [11]

132 Pent/hexose-carbo [10]

133 Lipids [1]

134 Alkylaromatics (alkylbenzenes) [2]

134 M,D,T,T,P-benzene [6]

134 Ethyl- to docosyl-benzene [6]

134 Lipids [1]

134 Alkylbenzenes [9]

134 Alkylaromatics [11]

134 Alkylaromatics [10]

135 Amino-acid species and amides [12]

135 Lipids [1]

135 Peptides [11]

135 Peptides [10]

136 Lipids [1]

136 Phenols [7]

137 N-containing compounds [11]
<table>
  <tbody>
    <tr>
      <td>70</td>
      <td>Lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>70</td>
      <td>Polyphenols</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>70</td>
      <td>Alkanes</td>
      <td>[7]</td>
    </tr>
    <tr>
      <td>70</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>70</td>
      <td>Peptides</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>71</td>
      <td>N-containing compounds</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>71</td>
      <td>Lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>71</td>
      <td>Polyphenols</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Carbohydrates</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Carbo-pent/hexose</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Cellulose</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Polysaccharides</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Alkane fragments</td>
      <td>[7]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Carbohydrates</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>72</td>
      <td>Pent/hexose-carbo</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>73</td>
      <td>Amino-acid species and amides</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>73</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>73</td>
      <td>Peptides</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>74</td>
      <td>Amino-acid species and amides</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>74</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>74</td>
      <td>Polysaccharides</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>74</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>74</td>
      <td>Peptides</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>75</td>
      <td>Lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>75</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>75</td>
      <td>Peptides</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>76</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>77</td>
      <td>Lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>78</td>
      <td>Benzene</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>78</td>
      <td>Proteins</td>
      <td>[1]</td>
    </tr>
  </tbody>
</table>

<table>
  <tbody>
    <tr>
      <td>95</td>
      <td>Nitrogen compounds</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>95</td>
      <td>Hydroxypyr</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>95</td>
      <td>Prot-alk-pyrroles/indoles</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>95</td>
      <td>Lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>95</td>
      <td>Polyphenols</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>95</td>
      <td>N-containing compounds</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>95</td>
      <td>N-compounds</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Mono- and polysaccharides</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Carbohydrates</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Carbo-pent/hexose</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Cellulose</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Furfural and/or pyrone</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Polysaccharides</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Polysaccharides</td>
      <td>[5]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Polysaccharides</td>
      <td>[8]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Carbohydrates</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>96</td>
      <td>Pent/hexose-carbo</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>97</td>
      <td>Amino-acid species and amides</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>97</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>97</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>97</td>
      <td>Peptides</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Mono- and polysaccharides</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Carbohydrates</td>
      <td>[12]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Carbo-pent/hexose</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Low-mass olefins</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Cellulose</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Furfuryl alcohol</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Lipids</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Polysaccharides</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>98</td>
      <td>Polysaccharides</td>
      <td>[8]</td>
    </tr>
  </tbody>
</table>

<table>
  <tbody>
    <tr>
      <td>114</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>114</td>
      <td>Polysaccharides</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>114</td>
      <td>Polysaccharides</td>
      <td>[8]</td>
    </tr>
    <tr>
      <td>114</td>
      <td>Carbohydrates</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>114</td>
      <td>Pent/hexose-carbo</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>115</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>115</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>115</td>
      <td>Peptides</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>116</td>
      <td>Lipids</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>117</td>
      <td>Nitrogen compounds</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>117</td>
      <td>N-compounds</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>117</td>
      <td>Prot-alk-pyrroles/indoles</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>117</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>117</td>
      <td>Proteins</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>117</td>
      <td>N-containing ions</td>
      <td>[5]</td>
    </tr>
    <tr>
      <td>118</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>119</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Phenols and lignin monomers</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Alkylaromatics (alkylbenzenes)</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>M,D,T,T,P-benzene</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Ethyl- to docosyl-benzene</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Phenol</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Alkylbenzenes</td>
      <td>[9]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Alkylaromatics</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Peptides</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>120</td>
      <td>Alkylaromatics</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>122</td>
      <td>Phenols and lignin monomers</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>122</td>
      <td>Phenols</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>122</td>
      <td>Aliphatic polymers</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>122</td>
      <td>Phenols</td>
      <td>[7]</td>
    </tr>
  </tbody>
</table>

<table>
  <tbody>
    <tr>
      <td>137</td>
      <td>N-compounds</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Phenols and lignin monomers</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Alkylaromatics (alkylbenzenes)</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Phenols</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Monomeric lignins</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Guaiacyl units from lignin residues</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Phenol</td>
      <td>[4]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Monomeric products from lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Alk-dihydroxybenz</td>
      <td>[7]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Dihydroxybenzenes</td>
      <td>[9]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Phenols and lignin monomers</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>138</td>
      <td>Lignin monomers and phenols</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>139</td>
      <td>N-containing compounds</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>139</td>
      <td>N-compounds</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>140</td>
      <td>Phenols</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>140</td>
      <td>Monomeric lignins</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>140</td>
      <td>Monomeric products from lignin</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>140</td>
      <td>Phenols and lignin monomers</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>142</td>
      <td>Alk-naph</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>142</td>
      <td>M,D,T,T,P-benzene</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>142</td>
      <td>Naphtalenes</td>
      <td>[7]</td>
    </tr>
    <tr>
      <td>142</td>
      <td>Alkylaromatics</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>142</td>
      <td>Alkylaromatics</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>144</td>
      <td>Mono- and polysaccharides</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>144</td>
      <td>Carbo-pent/hexose</td>
      <td>[6]</td>
    </tr>
    <tr>
      <td>144</td>
      <td>Polysaccharides</td>
      <td>[1]</td>
    </tr>
    <tr>
      <td>144</td>
      <td>Polysaccharides</td>
      <td>[8]</td>
    </tr>
    <tr>
      <td>144</td>
      <td>Carbohydrates</td>
      <td>[11]</td>
    </tr>
    <tr>
      <td>144</td>
      <td>Pent/hexose-carbo</td>
      <td>[10]</td>
    </tr>
    <tr>
      <td>145</td>
      <td>Nitrogen compounds</td>
      <td>[2]</td>
    </tr>
    <tr>
      <td>146</td>
      <td>Tetralins / benzofurans</td>
      <td>[7]</td>
    </tr>
  </tbody>
</table>
<table><tr><td>78</td><td>Benzenes</td><td>[7]</td><td>98</td><td>Alkanes</td><td>[7]</td><td>122</td><td>Alkylphenols</td><td>[9]</td><td>148</td><td>Alkylaromatics (alkylbenzenes)</td><td>[2]</td></tr><tr><td>79</td><td>Nitrogen compounds</td><td>[2]</td><td>98</td><td>Carbohydrates</td><td>[11]</td><td>122</td><td>Phenols and lignin monomers</td><td>[11]</td><td>148</td><td>M,D,T,P-benzene</td><td>[6]</td></tr><tr><td>79</td><td>Pyridine</td><td>[6]</td><td>98</td><td>Pent/hexose-carbo</td><td>[10]</td><td>122</td><td>Lignin monomers and phenols</td><td>[10]</td><td>148</td><td>Ethyl- to docosyl-benzene</td><td>[6]</td></tr><tr><td>79</td><td>N-compounds</td><td>[6]</td><td>99</td><td>N-containing compounds</td><td>[12]</td><td>123</td><td>Tetramethyl pyrrole</td><td>[6]</td><td>148</td><td>Alkylbenzenes</td><td>[9]</td></tr><tr><td>79</td><td>Lignin</td><td>[1]</td><td>99</td><td>Amino-acid species and amides</td><td>[12]</td><td>123</td><td>Aliphatic polymers</td><td>[1]</td><td>148</td><td>Alkylaromatics</td><td>[11]</td></tr><tr><td>79</td><td>Proteins</td><td>[1]</td><td>99</td><td>Lipids</td><td>[1]</td><td>123</td><td>N-containing compounds</td><td>[11]</td><td>148</td><td>Alkylaromatics</td><td>[10]</td></tr><tr><td>79</td><td>N-containing ions</td><td>[5]</td><td>99</td><td>Peptides</td><td>[11]</td><td>123</td><td>N-compounds</td><td>[10]</td><td>150</td><td>Phenols and lignin monomers</td><td>[2]</td></tr><tr><td>79</td><td>N-containing compounds</td><td>[11]</td><td>99</td><td>Peptides</td><td>[10]</td><td>124</td><td>Phenols and lignin monomers</td><td>[2]</td><td>150</td><td>Monomeric lignins</td><td>[6]</td></tr><tr><td>79</td><td>N-compounds</td><td>[10]</td><td>100</td><td>Aliphatic polymers</td><td>[1]</td><td>124</td><td>Phenols</td><td>[6]</td><td>150</td><td>Guaiacyl units from lignin residues</td><td>[4]</td></tr><tr><td>80</td><td>Lignin</td><td>[1]</td><td>101</td><td>Aliphatic polymers</td><td>[1]</td><td>124</td><td>Monomeric lignins</td><td>[6]</td><td>150</td><td>Phenol</td><td>[4]</td></tr><tr><td>81</td><td>Nitrogen compounds</td><td>[2]</td><td>102</td><td>Cellulose</td><td>[4]</td><td>124</td><td>Guaiacyl units from lignin residues</td><td>[4]</td><td>150</td><td>Monomeric products from lignin</td><td>[1]</td></tr><tr><td>81</td><td>Methyl pyrrole</td><td>[6]</td><td>102</td><td>Aliphatic polymers</td><td>[1]</td><td>124</td><td>Phenol</td><td>[4]</td><td>150</td><td>Phenols</td><td>[7]</td></tr><tr><td>81</td><td>N-compounds</td><td>[6]</td><td>103</td><td>Benzonitrile</td><td>[6]</td><td>124</td><td>Lipids</td><td>[1]</td><td>150</td><td>Phenols and lignin monomers</td><td>[11]</td></tr><tr><td>81</td><td>Prot-alk-pyrroles/indoles</td><td>[4]</td><td>103</td><td>Aliphatic polymers</td><td>[1]</td><td>124</td><td>Monomeric products from lignin</td><td>[1]</td><td></td><td></td><td></td></tr></table>

708

709 References

710 [1] Hempfling, R., Schulten, H.-R., 1990. Chemical characterization of the organic matter in forest soils by Curie point pyrolysis-GC/MS and pyrolysis-field ionization mass spectrometry. Organic Geochemistry 15, 131-145.

712 [2] Leinweber, P., Schulten, H.-R., Horte, C., 1992. Differential thermal analysis, thermogravimetry and in-source pyrolysis-mass spectrometry studies on the formation of soil organic matter. Thermochimica Acta 200, 151-167.

714 [3] Gibson, E.K., 1973. Thermal analysis-mass spectrometer computer system and its application to the evolved gas analysis of Green River Shale and Lunar soil samples. Thermochimica Acta 5, 243-255.

716 [4] Saiz-Jimenez, C., Haider, K., Meuzelaar, H.L.C., 1979. Comparisons of soil organic matter and its fractions by pyrolysis mass-spectrometry. Geoderma 22, 25-37.

717 [5] Schulten, H.-R., 1987. Pyrolysis and soft ionization mass spectrometry of aquatic/terrestrial humic substances and soils. Journal of Analytical and Applied Pyrolysis 12, 149-186.

718 [6] Schnitzer, M., Schulten, H.-R., 1992. The analysis of soil organic matter by pyrolysis-field ionization mass spectrometry. Soil Science Society of America 56, 1811-1817.

719 [7] Simmleit, N., Schulten, H.-R., Yun, Y., Meuzelaar, H.L.C., 1992. Thermochemical Analysis of U.S. Argonne Premium Coal Samples by Time-Resolved Pyrolysis-Field ionization Mass Spectrometry. In: Meuzelaar H.L.C. (Ed.), Advances in Coal Spectroscopy. Modern Analytical Chemistry. Springer, Boston, MA, USA, pp. 295-337.

720 [8] Boon, J.J., 1992. Analytical pyrolysis mass spectrometry: new vistas opened by temperature-resolved in-source PYMS. International Journal of Mass Spectrometry and Ion Processes 118-119, 755-787.

723 [9] Schulten, H.-R., Schnitzer, M., 1993. Temperature-resolved in-source pyrolysis-soft ionization mass spectrometry of soil humic acids. Organic Geochemistry 20, 17-25.

724 [10] Monreal, C.M., Schulten, H.R., Kodama, H., 1997. Age, turnover and molecular diversity of soil organic matter in aggregates of a Gleysol. Canadian Journal of Soil Science 77, 379-388.

725 [11] Schulten, H.-R., Leinweber, P., 1999. Thermal stability and composition of mineral-bound organic matter in density fractions of soil. European Journal of Soil Science 50, 237-248.

726 [12] Leinweber, P., Schulten, H.-R., 1995. Composition, stability and turnover of soil organic matter: investigations by off-line pyrolysis and direct pyrolysis-mass spectrometry. Journal of Analytical and Applied Pyrolysis 32, 91-110.
728

Table S2: Tpeak values determined for each m/z fragment in replicate analyses of standard samples

729

and organized by clusters.

730

<figure><img src="image_2.png" /></figure>

See attached file

731

Figure S1: Total Mass spectra illustrate the relative abundance of *m/z* fragments recorded during the

732

time interval of the TIC peak. Background subtraction is performed before and after the TIC peak.

733

The amount of sample used is indicated after the sample name.

734

Figure S2: Evolution of TIC (red curve) and relative abundance of selected *m/z* (57, 78, 91, 92, 94 and

735

107) with temperature (converted from time of Py-MS analysis) for the eight standard samples.

736

Tpeak values are indicated for both TIC and *m/z* curves. The amount of material used for each

737

analysis (in mg) is indicated after the sample name.