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Single sample molecular chronology

Eric Lichtfouse

► To cite this version:

Eric Lichtfouse. Single sample molecular chronology. Accounts of Chemical Research, 2024, 57, pp.514-520. 10.1021/acs.accounts.3c00665. hal-04442996

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Submitted on 7 Feb 2024

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# Single Sample Molecular Chronology

Eric Lichtfouse*

**Accounts of Chemical Research**
https://doi.org/10.1021/acs.accounts.3c00665

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**CONSPECTUS:** This Account presents a new discipline, single sample molecular chronology (SSMC), which studies the relative age of an individual compound occurring in several temporal pools of a single sample in complex media. Geochemists have analytically observed for a long time that several pools of the same compound, e.g., a hydrocarbon or a pesticide, can be isolated from the same sample, e.g., a sediment or a soil, to yield a free compound pool obtained by solvent extraction and then a bound compound pool after treatment of the solid residue and further extraction. Yet the study on the significance of these pools has been limited due to the inherent lack of criteria to clearly distinguish the same compound present in various pools, and, as a consequence, the existence of these pools has been criticized as resulting from a default of extraction during analytical fractionation. Our breakthrough was to distinguish isotopically several temporal pools of a plant-derived C₃₁ *n*-alkane in a soil sample containing naturally ¹³C-labeled carbon and then to set up a method, ¹³C-relative dating, to calculate the relative age of these temporal pools. We observed wide differences in the relative age of the C₃₁ *n*-alkane in temporal pools of a single soil sample, ranging from –6.7 years for a soil humin-bound homologue to +25.1 years for the free homologue in the coarser soil particle-size fraction. Individual compounds can thus be used as molecular clocks to determine the relative age of temporal pools from the same sample. Moreover, our findings represented the first unambiguous proof that bound compounds are cycling slower and are somehow protected in a complex organo-mineral matrix, key information for the mechanism of carbon sequestration. SSMC could be developed in all disciplines of physical, biological, and environmental sciences manipulating complex media, to study the history of individual compounds. This chronochemistry should provide new information about the origin and transformation of individual compounds in biogeochemical systems. For example, historical information on drugs or pollutants encapsulated in temporal pools of a living organism would bring about critical new knowledge about the mechanisms of disease development. Investigations require isotope tracing using any isotope in natural or artificial abundance. Methods to separate temporal pools are suggested.

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## KEY REFERENCES

* Lichtfouse, E. ¹³C Labelling of soil *n*-hentriacontane (C₃₁) by maize cultivation. *Tetrahedron Lett.* **1995**, *36*, 529–530.¹ First determination of the ¹³C/¹²C ratio of a soil organic individual compound, allowing the measurement of the carbon turnover at the molecular level for the first time.

* Lichtfouse, E.; Wehrung, P.; Albrecht, P. Plant wax *n*-alkanes trapped in soil humin by non-covalent bonds. *Naturwissenschaft* **1998**, *85*, 449–452.² First observation that a compound bound in soil humin is isotopically different from the free homologue in the same soil sample.

* Lichtfouse, E. Temporal pools of individual organic substances in soil. *Analisis* **1999**, *27*, 442–444.³ Conceptualization of the existence of temporal pools of free and bound compound homologues in a single sample.

* Lichtfouse, E. ¹³C-dating, the first method to calculate the relative age of molecular substance homologues in soil. *Environ. Chem. Lett.* **2012**, *10*, 97–103.⁴ First method to calculate the relative age of homologues of an individual compound in isotope labeling experiments.

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## INTRODUCTION

Our understanding of current societal issues such as climate change, pollution, the food supply, and health security is limited by our limited knowledge of the molecular transformation of complex matter in environmental and living samples. For instance, soils represent a major carbon pool influencing atmospheric CO₂ levels and food production,⁵ yet soil organic carbon is a complex, partially macromolecular network, the molecular structure and dynamics of which are largely unknown and debated.⁶⁻⁸ Examples of unknown complex dead organic matter networks⁹ include humic substances in soils, sediments, aerosols, food and archeological remains, kerogen in sediments and coal, and dissolved organic matter¹⁰ in water and wastewater.
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Furthermore, abating pollution largely relies upon the precise knowledge of sources, transformation, and transport of contaminants. However, these processes are poorly known, and unexpected phenomena of health concern occur, such as the “resurrection” of contaminants in the environment.¹¹ This suggested the analogy of a “sponge” for soil organic matter trapping contaminants and then releasing them later, yet underlying mechanisms are poorly known. This lack of knowledge is not surprising because most environmental scientists prefer to study contaminants only as free compounds under pure conditions, which is understandable for limiting influencing factors yet unrepresentative because most contaminants likely occur in a bound form in environmental and living media. A bound compound is the trapped proportion of this compound encapsulated into or bound to a complex organo-mineral matrix. A bound compound is commonly isolated in three steps involving extraction of the sample to isolate the “free” homologue, treatment of the solid residue to break down organo-minerals, e.g., with acid, and then extraction to yield the “bound” homologue.

Similarly, while pollution is strongly linked to many diseases, there is actually little knowledge of the “history” of contaminants in living organisms, in particular regarding when the contaminant entered the organism, how much time it spent in it, and when it induced the disease. More generally, while there is indirect empirical evidence for the occurrence of bound compounds in complex media, the processes ruling their encapsulation, binding, transformation, storage, and eventual release are largely unknown (Figure 1).

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Empirical evidence has shown that individual organic substances, including contaminants, occur both as free, e.g., solvent-extractable, and bound homologues in complex organo-mineral media, yet mechanisms ruling their formation and fate are poorly known. Complex organic media include living organisms and environmental samples such as soils, sediments, aerosols, organic-rich waters, food, and meteorites. Bound compounds are also known as “bound residues”, which are compounds that are sequestrated by several poorly known processes such as physical encapsulation and chemical binding.</figcaption></figure>

Overall, these unknowns are mainly due to the lack of analytical methods to study the history and chronology of organic compounds in complex organic media. Here, I explain the discovery of temporal pools of plant wax *n*-alkanes in soils²,³,¹² based on the invention of ¹³C-relative dating to measure the relative age of molecular compounds.¹⁻⁴,¹² This discovery allows the definition of a new discipline, tentatively named single sample molecular chronology (SSMC), that studies temporal pools of structurally identical and isotopically different molecular compounds occurring in complex environmental and biological media. Methods to separate temporal pools and applications in various fields are suggested. The following sections present historical insights, dynamics of soil individual compounds, the discovery of temporal pools by ¹³C-

relative dating of molecular clocks, SSMC principles and conditions, and potential applications in earth, environmental, and health sciences.

## ■ HISTORICAL INSIGHTS

During my life, I got several hints that, once put together later, will lead us to discover temporal pools of individual compounds in complex media. First, as a child, I was attracted by the unknown, and I remember having spent hours dismantling a whole radio piece by piece, without any clear objective. Later, as a teenager, I was particularly puzzled by time-related concepts, such as the Einstein relativity theory and the time tombs of the Hyperion science-fiction novel by Dan Simmons. This certainly helped later to interpret data that led to the discovery of “ancient” contaminants in “modern” soils,¹³⁻¹⁵ plants,¹⁶ and sludges¹⁷ based on molecular and isotopic information.

During my Ph.D. thesis at Strasbourg University from 1986 to 1989, we were attempting to identify molecular fossils in sediments and petroleum,¹⁸,¹⁹ and several intriguing stories captivated my attention: first, my advisor, Dr. Pierre Albrecht, explained that molecular fossils can be used to measure the maturity of sedimentary organic matter.²⁰ To me, this suggested that some compounds could eventually be used as “molecular clocks”. Then, Dr. Jean-Michel Trendel recounted that the main reason that hopanes, as fossil molecules, have been discovered in sediments²¹ before their biological precursors,²² the bacterial hopanetetrols, was that hopanetetrols were amphiphilic and thus had been thrown into the sink with the water phase during the three-time wash of the organic phase during the classical lipid extraction procedure. This indicated to me that fractions that are usually trashed deserve more attention. And indeed, several years later, we successfully identified a temporal pool of bound soil *n*-alkane in a humin extract,² a fraction that was typically thrown away to purify humin because extracted compounds were believed to represent a contamination by free compounds.

I got a first hint of the occurrence of temporal molecular pools by analyzing kerogen, the insoluble fraction of sedimentary organic matter, during pyrolysis experiments at the French Petroleum Institute with Dr. Françoise Béhar in 1988. Specifically, we were comparing Jurassic fossil biomarkers that were present both as “free” compounds in the rock organic extracts and as bound compounds in the kerogen. In particular, we were interested in the S/R ratios of steranes isomers, which allowed us to measure the maturity of sedimentary organic matter.²³

To our surprise, we observed that all kerogen-bound biomarkers released by pyrolysis were less mature than free homologues from the rock extract of the same rock sample, in line with the observed phenomenon of “retarded maturation”.²⁴ This difference was unexpected because, first, the free and bound homologues came from the same rock sample and, second, the high temperature should have favored isomerization and production of the mature S isomer during kerogen pyrolysis. From that, I deduced that organic compounds that are trapped in a complex matrix could be somehow different and younger compared to free homologues. However, we did not have enough information to prove unambiguously an age difference because other processes such as sedimentary biodegradation and mineral catalysis could have also explained the S/R ratio differences.

I got a final hint of the importance of bound compounds in complex media at Lorraine University from Prof. Michel Schiavon, who was studying the persistence of pesticides in
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<figure><img src="image_7.png" /><figcaption>**Figure 2.** Field experiments of maize cultivation to trace the integration of plant carbon in soils. After plant death, plant wax *n*-alkanes from maize, a naturally ¹³C-enriched plant, are mixed with initially ¹³C-depleted soil *n*-alkanes, thus inducing an increase in the ¹³C/¹²C isotope ratios of soil *n*-alkanes. This allows for the measurement of the percentage of maize-derived carbon in individual soil *n*-alkanes and therefore an assessment of the dynamics of soil carbon at the molecular level.¹,²⁸</figcaption></figure>

soils using ¹⁴C-labeling. He told me that while three successive Soxhlet extractions of soils incubated with a ¹⁴C-pesticide (atrazine) should have extracted more than 99% of “extractable” compounds, if they would do a fourth Soxhlet extraction the next day, they would find an amount of ¹⁴C radioactivity higher than that of the previous extraction, which is contrary to the laws of successive extractions. To explain this phenomenon, he presumed that some spontaneous transformation occurred overnight in the soil sample and released the pesticide fraction that was previously bound to soil organic matter. However, there was no hard evidence for that because we had no information on the molecular structure of extracted radioactive compounds and we had no means to distinguish bound from free compounds.

Overall, I got several indications from various scientists that organic compounds bound in complex media may be temporally different from their free homologues, but this was hard to prove without a means to distinguish two molecules having the same structure. Therefore, I developed ¹³C-relative dating, as presented in the following sections.

## ■ DYNAMICS OF INDIVIDUAL SOIL COMPOUNDS

I did postdoctoral research on deep sedimentary rocks and oils in 1990 in the biogeochemical laboratories of Prof. John Hayes at Indiana University. He and his group pioneered the invention of compound-specific isotope analysis (CSIA)²⁵ by coupling a gas chromatograph to an isotope ratio mass spectrometer (GC-IRMS), which allowed the determination of isotope ratios, e.g., ¹³C/¹²C, of individual compounds occurring in complex mixtures, thus finding applications in many fields.²⁶ I started to apply this method to soil organic matter in 1992 at Pierre and Marie University, and we obtained the first ¹³C/¹²C ratios, reported as δ¹³C in ‰, of individual organic compounds in soils.¹,²⁷ This represented the first application of CSIA to soils, allowing for a more in-depth understanding of the underlying soil transformation processes and for the quantification of carbon transformation at the molecular level.

We were attempting to study the dynamics of soil organic carbon at the molecular level because most past investigations were solely focused on bulk soil organic matter, and thus, the possibly different kinetics of molecular substances were unknown. From the applied perspective, knowledge of

molecular dynamics would have been useful to understanding the mechanisms ruling soil properties such as carbon sequestration, water retention, fertility, and soil structure stability. Therefore, we analyzed ¹³C/¹²C ratios of soil samples from a 23-year field experiment of ¹³C-labeling at natural abundance by the cultivation of maize, a C₄ plant, on a soil that had been previously cultivated only by C₃ plants, e.g., wheat (Figure 2).

Since C₄ plants are ¹³C-enriched versus C₃ plants, after plant death, the ¹³C-enriched maize wax C₃₁ *n*-alkane, of δ¹³C of -20.6‰, is progressively integrated into the soil over time and mixed with the C₃₁ soil homologue, of δ¹³C of -35.7‰ before maize cultivation. As a consequence, the δ¹³C ratios of soil *n*-alkanes increase with time of cultivation, reaching -28.1‰ for the C₃₁ homologue after 23 years of cultivation (Figure 2). Using a simple mass balance (eq 1), this allowed for the first time to distinguish plant-derived carbon from the initial soil carbon in an individual soil compound and thus access the dynamics of soil carbon at the molecular level¹

$$M = 100 \times (\delta - \delta_0)/(\delta_m - \delta_0) \quad (1)$$

where *M* is the percentage of maize-derived carbon in the soil *n*-alkane, *δ* denotes the δ¹³C value of the soil *n*-alkane at a given time of maize cultivation, *δ*₀ depicts the δ¹³C value of the soil *n*-alkane before maize cultivation, and *δ*ₘ refers to the δ¹³C value of the plant *n*-alkane.

After 23 years of maize cultivation, the soil C₃₁ *n*-alkane contains 50.3% maize-derived carbon. By using a first-order kinetic law, the complete renewal of 99% of the initial soil C₃₁ *n*-alkane should take 148 years (eq 2).²⁸ This estimated time of molecular carbon renewal was much shorter than that of the total soil carbon of 192 years, which was rather unexpected because *n*-alkanes are lipids that are considered to be less prone to degradation compared to soil amino acids and carbohydrates. This difference in carbon turnover also confirmed the importance of studying molecular dynamics to decipher mechanisms ruling soil properties such as carbon sequestration. We have also used a similar mass balance approach to calculate, for the first time, the contributions of fossil fuel and modern carbon in individual soil *n*-alkanes¹³

$$M = 100 \times (1 - \exp(-0.031t_1)) \quad (2)$$
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where M is the percentage of maize-derived carbon in the soil n-alkane and t₁ denotes the time of maize cultivation in years.

Overall, we have measured for the first time the isotopic composition of an individual substance in soil, allowing for an unprecedented determination of the dynamics of soil carbon at the molecular level. Since we now had a tool to measure molecular dynamics and given previous insights suggesting that bound compounds may be temporally different from free compounds, I decided to analyze the isotopic ratios of the same compound in different pools of the same maize-labeled soil sample to see whether isotopic ratios were different. The results allowed us to define a new method, ¹³C-relative dating, presented in the next section.

## TEMPORAL POOLS OF INDIVIDUAL SUBSTANCES

We started to study soil humic substances because they almost always represent the major part of organic carbon in environmental samples, and their structure, origin, formation, and transformation were and still are largely unknown, despite their predominant role in biogeochemical cycles. Our results of isotopic, physical, and molecular characterizations of humic substances revealed several insights into the origin and dynamics of soil organic matter.⁸,²⁹⁻³¹

The major discovery occurred when we analyzed the ¹³C/¹²C isotope ratios of soil n-alkanes occurring in a free organic pool and two bound organic pools of the same soil sample that was partially labeled by enriched ¹³C from maize cultivation.² Free n-alkanes are homologues obtained by the CHCl₃-MeOH extraction of the soil. The first bound n-alkanes pool was obtained by treating the soil residue with HF/HCl to remove minerals and then by extracting the obtained humin with CHCl₃-MeOH. As mentioned previously, this extract was typically thrown away as a contaminant during the analysis of insoluble organic matter in sedimentary rocks, but I decided to analyze it because I suspected the presence of temporally different compounds. The second bound n-alkanes pool was obtained by pyrolysis of the pre-extracted humin.

We found that the C₂₇, C₂₉, C₃₁, and C₃₃ bound n-alkanes were ¹³C-depleted by 1.6–2.7‰ versus free homologues. This was the first unambiguous proof that free and bound compounds can be temporally different in a single environmental sample, leading to the discovery of temporal pools. Indeed, without a such molecular isotopic distinction, the presence of bound compounds could have been interpreted as a default of extraction, an argument that I had received from reviewers in previous investigations. The observed isotopic difference indicated that bound n-alkanes contain a lower proportion of maize-derived carbon and are therefore younger than their free counterparts. We thus obtained a method to calculate the relative age of individual compounds within a single sample.

We also identified temporal pools of soil n-alkanes in three particle-size fractions of the same soil sample.¹² The results showed much wider differences in δ¹³C ratios, of 4.1‰ for the C₃₁ n-alkane. The coarser 200–2000 µm particle size fraction contained the most ¹³C-enriched, and thus younger, homologue, and the finest 0–50 µm particle size fraction contained the least ¹³C-enriched, and thus older, homologue.

In 2012, by using the first-order law set up previously to calculate the turnover of individual soil compounds (eq 2),²⁸ I determined the relative age of individual n-alkanes occurring in six temporal pools of the same soil sample (eq 3): free homologues from the bulk extract, bound homologues from

humin extraction, bound homologues from humin pyrolysis, and free homologues in the three particle-size fractions.⁴ Results showed relative ages ranging from –6.7 years for the oldest homologues in humin to +25.1 years for the youngest homologues in the 200–2000 µm particle size fraction, compared to the homologues in the bulk soil extract (Figure 3). I named the corresponding method “¹³C-dating”,⁴ which

<figure><img src="image_8.png" /><figcaption>**Figure 3.** Calculation of the relative age of the C₃₁ n-alkane occurring in three temporal pools of the same soil sample from a field cultivated for 23 years with maize, a ¹³C-enriched plant.⁴,²⁸ The plot shows the percentage of maize-derived carbon in the C₃₁ n-alkane homologue. The C₃₁ n-alkane occurring in the bulk soil extract, of δ¹³C of –28.1‰, contains 50.33% maize-derived carbon; it is taken as the time reference and therefore has a relative age of 0. The humin-bound C₃₁ n-alkane, of δ¹³C of –29.7‰, contains less maize-derived carbon (39.74%) and thus has a relative age of –6.7 years, according to the turnover kinetic curve of the bulk extract homologue.²⁸ The C₃₁ n-alkane from the 200–2000 µm particle size-fraction, of δ¹³C of –24.0‰, contains more maize-derived carbon (77.48%) and thus has a relative age of +25.1 years. It is noteworthy that the calculated relative ages here are most probably overestimated because these calculations are based on the kinetic curve of the bulk soil extract C₃₁ n-alkane (eq 2), which is renewed faster than the total soil organic carbon.²⁸ Therefore, future investigations should determine the kinetic curves of each homologue in various pools. Reproduced with permission from ref 4. Copyright 2012 Springer Nature.</figcaption></figure>

enables the measurement of the relative age of individual compounds occurring in various temporal pools within a single sample

$$ \text{relative age} = t_1 - t_2 \qquad (3) $$

where t₁ refers to the time of maize cultivation calculated using eq 2 and t₂ denotes the time of maize cultivation of the soil sample, of 23 years in our example. Individual compounds can thus be used as “molecular clocks” to determine the relative age of temporal pools. It is noteworthy that compared to radiocarbon dating, ¹³C-relative dating does not give the absolute age and ratio of modern carbon but gives only relative information, that is, the time or age difference between molecular homologues in several temporal pools.

Overall, our isotope analysis revealed that an individual compound can exist in different temporal pools within a single environmental sample. Since most biological and environmental media are most probably made of complex mixtures of various compounds of various ages, such temporal pools should occur widely, yet this field is actually poorly explored. This possible
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occurrence of temporal pools is supported by the successful deconvolution of fossil versus modern carbon in individual soil n-alkanes by me and Dr. Timothy Eglinton in 1995.¹³ Similarly, in 2019, Timothy Eglinton's group distinguished young and old pools within individual fatty acids in sediments by deconvolution using molecular and ¹⁴C-dating analysis.³² It is noteworthy that since ¹³C-dating in soils using the C₃/C₄ plant isotopic difference might be restricted to short time scales, the use of ¹⁴C-dating at the molecular level in the future should nicely complement the application of our approach over longer time scales. Overall, our findings could represent the basis of a new discipline tentatively named single sample molecular chronology, as discussed in the next section.

## SINGLE SAMPLE MOLECULAR CHRONOLOGY

In this section, I attempt to imagine what could be the conditions to develop single sample molecular chronology (SSMC). I define SSMC as the study of temporal pools of structurally identical and isotopically different molecular compounds occurring in complex environmental and biological media. In other words, the SSMC investigates the history of an individual molecular compound occurring simultaneously in different fractions of the same sample. SSMC requires the following conditions:

* A situation where there is an isotopic difference within fractions of the same sample. For investigations at natural isotopic abundance, this is likely to occur in most geological, environmental, and biological samples, e.g., rocks, soils, sediments, aerosols, meteorites, archeological debris, food, and waste, because these media are made of matter often coming from isotopically distinct sources. Moreover, experiments at artificial isotopic abundance, where an isotopically enriched compound is introduced and evolved in a complex medium,³³ should allow the creation of temporal pools of individual compounds in single samples. In particular, the unintentional isotopic labeling of soils and other systems by the radiocarbon "bomb spike" of past nuclear tests³⁴ could be used to identify molecular temporal pools.

* A method to measure isotopic ratios at the molecular level, such as a compound-specific isotope analysis.²⁵,²⁶,³⁵,³⁶

* A method to separate distinct fractions from the same sample. Here, the procedures that have been developed in geochemistry to break down macromolecular organic networks³⁷ or in soil science to study bound compounds³⁸,³⁹ could be applied to any sample. Methods could comprise chemical, physical, and biological methods. For example, using various solvents with different properties should modify the complex organo-mineral network and thus extract compounds that are sequestrated by various mechanisms. Physical separation of organs from living organisms and further molecular isotopic analysis may lead to the occurrence of temporal molecular pools.

Chromatographic methods are also likely to separate temporal molecular pools. I suggest studying in particular all fractions that are usually thrown away in so-called "purification" pretreatments. Chemical cleavage is likely to release compounds that are strongly bound by covalent bonds. Acid treatment of samples should release compounds that are trapped in minerals.²,³⁰,³¹ Biodegradation as a pretreatment to remove

the bioavailable fraction of a compound, e.g., a toxic substance, followed by extraction or breakdown of the residue, should give access to bound homologues that may display toxicity in the long run.

## MOLECULAR CLOCKS

SSMC could potentially find applications in all fields, investigating complex samples that may contain temporally distinct pools of individual compounds. The potential of SSMC is illustrated in Figure 4 for the study of sedimentary records.

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Classical analysis of sedimentary cores is limited to data from bulk monotemporal samples, whereas the fractionation of a single sample into several temporal pools could potentially give access to data for each pool. The relative age of temporal pools is determined by isotopic analysis of "molecular clocks", which are isotopically distinct individual compounds.</figcaption></figure>

Here, classical research is monotemporal because it involves the analysis of bulk samples, yielding data, e.g., molecular information to reconstruct palaeotemperatures⁴⁰ and palaeoCO₂ levels,⁴¹ at a single age. For the new multitemporal approach, fractionation of the same sample into temporal pools could potentially give unique data for each pool. The number of temporal pools of a single sample is likely to be high due to the complex and diverse physical and chemical environments that organic matter encounters in natural systems. A temporal pool is relatively dated using an isotopically distinct individual compound, a "molecular clock"; therefore, all compounds occurring in the temporal pool can give information specific to the relative age of this pool. Similarly to genomic studies that use "molecular clocks" to constrain the timing of evolutionary processes over geologic time,⁴² SSMC is likely to give new temporal information on biogeochemical processes ruling the fate of organic compounds in ancient and modern media. In other words, a single sample could represent a sedimentary record in itself.

In archeology, where samples are commonly "single", SSMC could provide much more information from a single sample. In soil sciences, SSMC could provide information on the mechanisms ruling the sequestration of soil carbon at the molecular level; such knowledge would be useful to better understand the role of the global soil carbon pool as a source or a sink of atmospheric CO₂. For instance, our results have shown that organic compounds trapped in soil minerals are younger and thus better preserved.²,⁴ In geosciences, SSMC may reveal additional information on the history of past climate events such as forest/savana transitions⁴³ and extinction events.⁴⁴

In environmental sciences, SSMC may give information on the sources, transformation, and dynamics of contaminants that
are bound to the organo-mineral network of complex matter in waters, wastewaters, soils, aerosols, and sediments; this would be particularly useful for predicting the potential risks induced by contaminant release into ecosystems after years of sequestration. Similarly, in health sciences, SSMC could give insights on issues such as when a contaminant has entered a living organism, how long the contaminant has been stored in the living organism, whether there are different temporal pools of contaminants in living organisms, and the potential risks in terms of toxicity and diseases. A similar approach could be used to study the fate of drugs in living organisms because, so far, the unambiguous molecular characterization of temporal pools of a drug in complex matrixes has rarely been reported.⁴⁵,⁴⁶ SSMC could provide more information in any other field, manipulating complex media containing isotopically distinct individual compounds.

## ■ AUTHOR INFORMATION

### Corresponding Author

Eric Lichtfouse – State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, P. R. China; ● orcid.org/0000-0002-8535-8073; Email: eric.lichtfouse@icloud.com

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.accounts.3c00665

### Notes

The author declares no competing financial interest.

### Biography

Eric Lichtfouse graduated with a bachelor and a master of science in organic chemistry from Claude Bernard University, Lyon, and a Ph.D. thesis in organic geochemistry from Louis Pasteur University, Strasbourg, in 1989. After postdoctoral positions at Indiana University, USA, in 1990, and the KFA Juelich Research Center, in 1992, he did molecular and isotopic research on carbon and contaminants in soils, waters, plants, and the atmosphere at Pierre and Marie Curie University, Nancy University, Bourgogne University, and Aix-Marseille University. He is now a professor at Xi'an Jiaotong University.

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