# Development of a novel field micro-sampling technique using a tandem p-LIBS – particle recovery module: Application to calcium carbonate samples for 230Th/238U dating

Matthieu Boccas, J. Maréchal, Anne-Laure Ronzani, Fanny Claverie, M.

Maguregui, E. Pons-Branchu, C. Pécheyran

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Matthieu Boccas, J. Maréchal, Anne-Laure Ronzani, Fanny Claverie, M. Maguregui, et al., Develop- ment of a novel field micro-sampling technique using a tandem p-LIBS – particle recovery module: Application to calcium carbonate samples for 230Th/238U dating. Spectrochimica Acta Part B: Atomic Spectroscopy, 2025, 231, pp.107239. 10.1016/j.sab.2025.107239. hal-05136889

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# Development of a novel field micro-sampling technique using a tandem p-LIBS – particle recovery module: Application to calcium carbonate samples for $^{230}$Th/$^{238}$U dating

M. Boccas<sup>a,*</sup>, J. Maréchal<sup>a</sup>, A.-L. Ronzani<sup>a</sup>, F. Claverie<sup>a</sup>, E. Pons-Branchu<sup>b</sup>, M. Maguregui<sup>c</sup>, C. Pécheyran<sup>a</sup>

## ABSTRACT

One of the most challenging aspects of studying prehistoric cave walls is the sampling step which, despite extensive precautions, often falls short of meeting curators' requirements. Sampling is ideally avoided or accepted only under micro-invasive conditions. This challenge is particularly significant in dating prehistoric rock painting, as laboratory analyses require direct sampling of the painting. The uranium-thorium geo-chronometer offers an interesting alternative, as it allows to estimate the age of a painting without targeting it directly. Instead, it focuses on the precipitated carbonate, known as speleothems, which are abundant in their surroundings and may even overlay rock painting. The sampling step remains nevertheless a challenge, primarily because precise sampling is required, and because these analyses typically require at least mg-sized samples in optimal conditions. In this study, we modified a portable Laser Induced Breakdown Spectrometer (LIBS) to incorporate an induced particle collection module. Additionally, we developed an ablation strategy to remove approximately 1mg of material from calcium carbonate samples, including limestone slab, speleothems, and corals, producing square-based pyramidal ablation craters with maximal dimensions of about 900 x 900 x 700 µm (length x width x depth). Evaluation of the efficiency of the collection modules revealed a high recovery rate of nearly 80% of the total ablated mass. This sampling protocol was additionally applied to two known samples for which U/Th ages were determined after MC-ICP-MS analyses. Calculated U/Th ages were in good agreement with their expected values, suggesting that the proposed sampling methodology could offer a novel mean for micro-invasive investigations in carbonate caves and shelters.

## 1. Introduction

Due to their high patrimonial value, the study of archeological materials is often constrained by strict conservation requirements, especially concerning sampling. Prehistoric cave paintings are no exception and are likely among the most strictly controlled types of samples, given their considerable age, fragility, and the historical insights they provide. For these reasons, current studies increasingly favor the use of non-destructive analytical techniques, such as Raman spectroscopy, infrared spectroscopy, reflectance in the Vis-NIR-SWIR range or X-ray fluorescence (ED-XRF)[1], which are available in compact and portable formats suitable for in-situ analysis, even in narrow environments.

These instruments are typically used to provide qualitative information on the chemical and mineralogical composition of the pigments used in the paintings[2–5] and, in some cases, even quantitative data[6].

They can also help assess critical aspects of the paintings, such as their degree of degradation, and may even aid in distinguishing copies from authentic artworks[1].

However, they often have limitations. particularly in quantification [7] when dealing with complex matrices like cave walls[5], and are not suited for ultra-trace precise elemental and isotopic analyses essential for determining provenance or dating.

When possible, dating is preferentially done by the radiocarbon technique, which offers several advantages. This technique can date samples up to 55 kY[8] old and is well-suited for paintings ranging from a few hundred years to the Paleolithic era. It provides good precision (typically between 20 and 30 years at 1σ for samples around 10 kY[8]) and requires only small samples, in the tens of milligrams[9], minimizing the physical impact of sampling. However, it has two significant drawbacks: it can only date paintings made with organic materials,

<footnote><sup>a</sup> Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, Institute of analytical sciences and physico-chemistry for the environment and materials (IPREM), 2 Avenue du Président Pierre Angot, 64053 Pau, cedex 9, France</footnote>

<footnote><sup>b</sup> Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Bâtiment 12, avenue de la terrasse, 91198 Gif Sur Yvette Cedex, France</footnote>

<footnote><sup>c</sup> Department of Analytical Chemistry, Faculty of Pharmacy, University of the Basque Country UPV/EHU, Paseo de la Universidad 7, 01006 Vitoria-Gasteiz, Spain</footnote>

<footnote>* Corresponding author.
E-mail address: matthieu.boccas@univ-pau.fr (M. Boccas).</footnote>
which is not always the case (e.g. the ochres of Lascaux or direct engravings), and it directly targets the pigments which inevitably affect the integrity of the painting, even when only small samples are required.

U-series dating (or uranium-thorium dating), by contrast, presents an interesting alternative. This radiometric dating technique relies on the measurement of the isotopic activity ratios between $^{238}$U and its daughter isotopes $^{234}$U and $^{230}$Th. Typically, carbonated samples precipitated from natural waters tend to incorporate uranium but almost no thorium isotopes. This creates a disequilibrium situation in which the newly formed precipitate will accumulate $^{234}$U and $^{230}$Th over time. This accumulation occurs following a kinetic described by the following equation (1)[10], where kX represents the radioactive activity of the isotope k of element X, $\lambda_k$ the radioactive decay constant of the isotope k and t the time (ka) :

$$ (1) \left[ \frac{^{230}\text{Th}}{^{238}\text{U}} \right] (t) = 1 - e^{-\lambda_{230} t} + \left( \left[ \frac{^{234}\text{U}}{^{238}\text{U}} \right] - 1 \right) \left( \frac{\lambda_{230}}{\lambda_{230} - \lambda_{234}} \right) (1 - e^{-(\lambda_{230} - \lambda_{234})t}) $$

Measuring ($^{234}$U/$^{238}$U) and ($^{230}$Th/$^{238}$U) activity ratios becomes therefore equivalent to the measurement the time elapsed since the formation of the sample[11]. This geochronometer can determine ages up to 700 kY[12] and is particularly well-suited for dating carbonated precipitates (known as “speleothems”) that are abundant in caves. Among the various forms speleothems can take (e.g. stalagmites or stalactites), calcitic veils are particularly valuable for dating. Formed by water runoffs on cave walls, these veils can range from several hundred microns to a few millimeters in thickness[13–15] and sometimes overlay rock art[16]. Sampling and dating these veils allows for establishing a minimum age (“terminus ante quem”) or even constrain them in a time interval if the veils are organized in a sandwich-like structure surrounding the painting[13]. The main limitation of this technique is that it only provides relative dating, rather than direct, absolute dating. However, it can be used to date any artwork -paintings, drawings or engravings- regardless of the material used for their creation. Additionally, by targeting the surroundings rather than artwork itself, this approach avoids direct damage through sampling.

However, the implementation of these techniques still faces serious challenges, which are both analytical and practical. The first issue is that it relies on the measurement of $^{230}$Th, the last daughter isotope considered in the part of the radioactive decay chain of $^{238}$U relevant to U/Th dating. Given that uranium concentrations in speleothems are typically in the hundreds of ng/g range[17], $^{230}$Th can sometimes reach concentrations as low as a few tenths of pg/g[10]. For this reason, this technique was set aside for many years due to the significant amount of samples it required to be applied, typically in the range of several grams [18] in the late 1980s. Since then, significant advances in analytical instrumentation, especially with the advent of ICP-MS, have enabled U/Th dating with samples as small as few milligrams[10,13,19], making the technique more viable from a preservation perspective.

The second challenge is related to the sampling step. Traditionally sampling methodology for speleothems studies, such as mechanical drilling[20] or manual scrapping[21], offer limited spatial resolution. Spatially resolved sub-sampling technique such as micro-drilling or micro-milling can help overcome this limitation but cannot be applied directly in situ[22,23]. Samples must first be removed from the field before sub-sampling. This approach is not well-suited to veil sampling and requires the operator to personally select samples from the field, which, besides removing them from their context, introduces a certain level of subjectivity. Alternatively, laser ablation (LA), a well-established micro-sampling technique, offers a promising solution to this challenge. With spatial resolutions reaching as low as 10 µm in recent studies[10], this technique has been successfully coupled with ICP-MS in geochemical characterization studies since 1985[24] and was first used for isotopic ratio measurements as early as 1993[25]. Unfortunately, this instrumental coupling is confronted with the same limitation as the aforementioned manual sub-sampling techniques. Samples

must first be removed from the field to fit in ablation cells, which often restrict sample size to just a few centimeters or less in length, width, and height[26,27].

A portable open-cell LA device could bring a definitive solution to this problem by enabling direct, in-situ collection of any type of samples, without size limitations. By using a purpose-designed particle collection device, it is possible to take advantage of the benefits of LA (i.e. micro-scale in situ sampling) to transfer particles back to the laboratory for ICP-MS analysis after wet digestion. This approach allows for the detailed elemental and isotopic trace analysis capabilities of the instrument. A similar approach has already been reported in several studies[28–30] for trace element analysis in glass, gold and silver materials, wherein a custom built system was set up using a 532 nm diode-pumped solid state laser, an optical fiber, and a lens tube to collect particles on a membrane filter.

In this study, we leveraged the highly customizable LA parameters of a portable Laser Induced Breakdown Spectrometer (p-LIBS) device available in our laboratory and designed a reusable particle collection module attachable to the p-LIBS via a 3D-printed modified nose tip. In addition to developing this new sampling system, we established a LA sampling protocol specifically for mg-sized carbonate samples. Particle recovery rates were evaluated and induced particles and ablation craters were characterized on a limestone slab sample. This sampling protocol was also applied to two speleothems and one coral sample to evaluate variations in sampling across different types of carbonate materials. Finally, micro-samples collected from a known coral and a known speleothem sample were dated using the U/Th geochronometer and calculated ages were compared to their expected values to determine if the developed sampling technique was suited for carbonate cave investigations.

## 2. Experimental

### 2.1. Samples and chemicals

Different samples were used for this study. Three calcite speleothem samples (“YAAX-P”, “Salam 12”, and “Salam Coeur”) and one coral sample (Aqaba 13) were provided by the LSCE laboratory (Laboratoire des Sciences pour le Climat et l'Environnement, Paris) to evaluate the response of typical samples datable by the U/Th geochronometer to this sampling methodology. The “YAAX-P” speleothem is transparent with an apparent crystalline structure. “Salam 12” and “Salam Coeur” are two speleothems originating from different spots of the same concretion. They both are translucid but not to the extent of being completely transparent and present a whitish color. “Salam 12” has a smooth texture while “Salam Coeur” is powderier and leaves small deposits of particles when handled. “Aqaba 13” has a light ochre color and is opaque. This sample also presents two distinct zones, one quite homogeneous where laser ablations were performed, and another part with a delicate rosette-like structure called “corallite” that was avoided for this study. A detailed picture of the samples is available in Fig. 1 in SI.

As these samples were provided in limited quantities, a limestone slab sample was used as an in-lab reference material to evaluate the key characteristics of the sampling approach, including the evaluation of particle collection. This limestone slab is light ochre in color and opaque with an amorphous appearance. Its density was measured to be 2.5 after weighing a 16.0x16.0x3.7 mm parallelepiped sample.

Nitric acid solutions used for particles chemical digestion was prepared from a pure stock solution (JT Baker Instra-analysed 69.0-70%) previously purified by distillation in a Savillex DST-1000 acid sub-boiling distillation unit. Dilutions were prepared in ultra-pure water (18,2 MOhm, milli-Q, Millipore Corporation).

U/Th dating was tested on two sub-samples, one collected in the homogeneous zone of the Aqaba 13 coral sample and another one from an additional calcite speleothem, a flowstone collected on the Tinshemet archeological site (Israel) provided by the Archeoscience laboratory
<figure><img src="image_3.png" /><figcaption>Fig. 1. Presentation of the SciAps p-LIBS Z300 (A) and the particle collection module (B).</figcaption></figure>

from the University of Bordeaux. The coral sample presents a U concentration of $3.9 \pm 0.8 \mu g.g^{-1}$ and the flowstone a U concentration of $\sim 0.5 \mu g.g^{-1}$. Both samples had already been dated in previous studies to have reference ages. The “Aqaba 13” coral sample had thus been studied in a large scale study by Ribot et al. (2024) [31] comprising several sub-samples dated approximately between 200 and 225 ka. Similarly, the “Tinshemet” flowstone was dated in our laboratory by fsLA-ICP-MS to an age of $32.4 \pm 2.0$ ka (Zaidner et al. [32], in press).

## 2.2. Portable LA micro-sampling system

### 2.2.1. Portable laser ablation device

The portable laser ablation device used in this work is a p-LIBS, model Z300 from SciAps (see Fig. 1). The Z300 uses a PULSAR 1064-nm Nd:YAG pulsed laser which produces a 50 µm focused beam with a maximum power of 7.5 mJ within 1 ns long shots. This laser can operate at a maximum frequency of 50 Hz. The handle of the pistol possesses a small compartment that can host an argon cartridge to provide an argon flush prior to shooting. This helps with plasma stabilization and overall improves the quality of laser ablation, but this proves mostly useful for the LIBS function of the device, which are not critical in this work. For more information about the analytical performance of this instrument, see [33]. The device is also equipped with a touchscreen that grants access to different applications and settings for operating the device. A battery can be inserted at the base of the handle and provides an autonomy of about 2 hours when used extensively.

### 2.2.2. Software parameters

For better control over the different laser ablation parameters, the device can be connected to a computer with a µ-USB cable and the software “Profile Builder” (version used for this work 2.19.1a). For analytical purposes, the Z300 is initially designed to shoot at several locations organized on a raster to enhance sample ablation and consequently boost atomic emission lines intensity. The Z300 can thus ensure up to 600 shots over the course of a single acquisition. “Profile Builder” allows to customize this raster through a (x,y,z) shooting coordinates system, step-size between shots, number of shooting locations and shots per location.

The z coordinate is comprised in a [0:300] interval which allows to modify the focal plane of the laser. Whether for the shooting coordinates or the step-size value, 1 unit corresponds to 12,5 µm. With this system, it is possible to ensure focused shots at a maximum distance of 3.25 mm (at z=0) from the surface of the factory’s p-LIBS’s nose tip. The shooting parameters used for this work are available in Table 1. With these settings, 576 shots are distributed on a 12x12 grid that would cover a square area of 687,5 µm of side if the laser beam size was of 1 µm. With a beam size of 50 µm, and depending on the sample’s material and numbers of shots, this raster generally covers a square area of less than 1 mm x 1 mm. With a step-size value of 5 (62.5µm), shooting locations are

<table><caption>**Table 1**

Operating parameters used with p-LIBS Z300.</caption><thead><tr><td>Z300 parameters</td><td>Settings</td></tr></thead><tbody><tr><td>Number of locations</td><td>144</td></tr><tr><td>Data shots per location</td><td>4</td></tr><tr><td>Step size</td><td>5</td></tr><tr><td>Start location</td><td>160, 90, 0</td></tr><tr><td>End location</td><td>215, 145</td></tr><tr><td>Test rate</td><td>50 Hz</td></tr></tbody></table>

slightly overlapping due to the HAZ (Heat Affected Zone) being wider than the 50 µm size of the focalized laser beam. This choice was made to ensure maximum ablation efficiency, which would decrease if the shots were to be too far apart.

### 2.2.3. Modified 3D-printed nose-tip

To allow particle recovery, the p-LIBS’s factory nose tip was modified in order to install a particle collection module as close as possible to ablation sites. The Shapr3D CAD software was used to design a nose tip with a small 1/16” puncture to enable the connection to the particle collection modules. This fit for purpose nose-tip was then 3D resin printed with a pixel resolution of $50x50x20 \mu m^3$ (Length x Width x Height). The resin design is presented in Fig. 2. Here Anycubic’s Standard resin (white) was used. For this design, the initial “reverse pyramid” geometry of the internal face was replaced by a smoother conical geometry deprived from any sharp corners that could create areas for particle accumulation. To facilitate the positioning of the Z300 at the focal point, a set of 3 units of 1mm high merlons were used on the outer surface of the nose cone around the laser exit hole. Placed in contact with the sample, these 1 mm high merlons also serve to guide the flow of aspirated air.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Modified 3D model of the Z300’s nose-tip. A/ Inside view; B/ Outside view; C/ Section view; 1/ 1/16” puncture housing the PEEK pipe; 2/ Laser aperture; 3/ 1mm thick outer merlon units.</figcaption></figure>
### 2.2.4. Induced particles collection modules

Particles produced through laser ablation were sampled using re-
usable collection modules. These modules are made from a 13 mm
polycarbonate filter-holder (Sartorius) connected to a 12 cm long sec-
tion of 1/16" O.D // 1 mm I.D. PEEK tubing. The dimensions of this
PEEK tubing were chosen to minimize dead volume and therefore the
adsorption of particles on the internal walls while allowing the air pump
to maintain the required sampling flow rate. The connection between
the filter holder and the PEEK section was arranged by a 1.5 cm long
Tygon section of 3/16" O.D. // 1/16" I.D. For particle collection a 13
mm hydrophilic PTFE filter with a porosity of 0.45 µm (Omnipore PTFE
membrane, Millipore) was placed in the filter holder. A presentation of
the collection module is given Fig. 1 (right). To ensure particles
transportation to the filter holder, a portable aspirating pump (SKC
AirCheck TOUCH sample pump) was connected to the free extremity of
the filter holder. This field sampling pump offers an autonomy of several
hours and can reach aspirating flows of up to 5 L.min⁻¹. Since both the p-
LIBS and the sampling pump are battery-powered, the system allows
samples to be taken for more than 4 hours on its own (including a spare
battery for the p-LIBS). This is a significant advantage for use in the field
in isolated environments (caves).

The tubing-filter assembly generated a loss of pressure due to resistance to air flow. This resistance generally increased during sampling as the particles remain trapped in the filter. Although the motor of this pump can automatically compensate for this loss of pressure by accelerating to maintain a constant flow rate over time, it was not possible to maintain a flow rate above 1.7 L/min. To maintain a constant flow rate throughout the sampling time required to remove approximately 1 mg of material, the sampling flow rate was consequently set at 1.5 L/min.

To ensure maximum cleanliness and prevent contamination, each pump tubing was rinsed with 50 mL 10% HNO₃, followed by a 50 mL mQ water rinsing step. The polycarbonate filter-holders were also thoroughly acid-clean. For this, modules were disassembled and the filter-holders' parts were washed using an ETC EVO II unit (Analab). Filter-holders' parts were thus washed in 15% nitric acid (JT Baker Instra-analysed 69.0-70%) for 40 minutes at 90°C and rinsed with ultra-pure water (18,2 MOhm) for 1 hour. PEEK pipes and Tygon connections were first placed in a 2% nitric acid bath and ultrasonicated for 30 minutes then placed in a second bath of ultra-pure water (18,2 MOhm). The different parts of the modules were then put to dry in a laminar flow hood.

### 2.3. Sampling protocol for assessing particle collection efficiency

Considering a minimally invasive approach and taking into account the uranium and ²³⁰Th concentration generally found in speleothems, a sampling mass of 1 mg was targeted to provide enough signal for MC-ICP-MS analysis. The sample was positioned on X-Y-Z positioning stage (Techspec, Edmund Optics). Sample was then placed in contact with the merlon units of the 3D printed modified nose-tip and then lowered by 0.3 mm to ensure the laser focus on the sample's surface. A picture of the typical setup used for this experiment is shown in Fig. 3. Eight replicates of 15 consecutive shooting sequences with the conditions set as shown in Table 1 were then fired with the aspirating pump turned on. Ablated masses were determined simply by weighing the samples before and after the sampling step with a 10⁻⁵ g precision balance (Mettler Toledo, XP 205).

### 2.4. Sampling characterization

Ablation craters were characterized using a Keyence numerical mi-
croscope (VHX-7000 series), which allowed to measure craters' side
lengths, depths and volumes. Ablated particles size distribution was
evaluated on-line using a ELPI analyzer (Electrical Low-Pressure
Impactor, Dekati, Finland) equipped with 13 impactor stages, collect-
ing particles with diameters ranging from 7 nm to more than 10 µm. An

<figure><img src="image_5.png" /><figcaption>Fig. 3. Experimental setup used for the LA-assisted sampling protocol</figcaption></figure>

empty collection module thus was positioned on the modified 3D-
printed Z300 nose-tip and directly connected to the ELPI. A triplicate
of 18 consecutive shots fired on the limestone slab sample were thus
analyzed. An additional triplicate of 35 consecutive shots was also
performed on a speleothem sample to evaluate if the sampling induced
significant variations depending on the sample matrix. Filter particle
retention efficiency was additionally evaluated by repeating this pro-
tocol on the limestone sample but with a filter loaded in the collection
module.

## 2.5. Particle recovery evaluation

### 2.5.1. Particle solubilization

After sampling, collected particles were solubilized with 2% HNO₃. To avoid any losses, filters were kept inside their filter holder and the nitric acid solution was flowed throughout the entire collection module in the opposite direction as the aspiration flow, from the PEEK section down to the filter-holder compartment. A homemade system of peristaltic pumps (see Fig. 2 in **Supplementary Material**) was assembled for this purpose to ensure homogeneous solubilization conditions for up to 4 collection modules at a time. The nitric acid solution was passed through the modules with a 1.2 mL.min⁻¹ flow thanks to a 0.762 mm I.D. PEEK tubing connected to 1.02 mm I.D. peristaltic pump tubing. In order to assess the quantity of nitric acid solution required for total dissolution of the particles, 8 ml was passed through the collection modules by taking all the 0.4 ml fractions (i.e. 20 fractions) for analysis by ICP-MS. This procedure was repeated in triplicate.

The 3D nose tip was also immerged in 2% HNO₃ after each sample to
evaluate the amount of sample deposited on the cone-shaped surface.
The quartz laser window placed between the nose-tip and the LIBS's
main casing/enclosure followed the same procedure after noticing
particles deposition.

### 2.5.2. Solution analyses

Particle recovery rates were evaluated by analyzing calcium con-
centration. An ICP-MS (Agilent 8900 QQQ-ICP-MS) was used in single
quadrupole mode. A reference solution was prepared by dissolving 100
mg of the limestone slab in 50 mL 2% HNO₃ and further diluted by a
factor 200. Collection modules dissolution solutions were diluted by a
factor 20 whereas the quartz blast-shield and nose-tip solution were kept
as is. ⁴³Ca signals were then monitored for each dissolution solution and
compared to that of the limestone slab reference solution to quantify the
amount of collected particles in each part of the sampling system. Typical ICP-MS operating conditions are available in Table 2. Fractions collected to assess the elution kinetics were analyzed following a similar methodology. A calcium calibration curve up to 50 ng.mL⁻¹ was used to estimate the calcium content and the corresponding calcium carbonate mass, assuming a Ca ratio of 40% in CaCO₃. To correct signal fluctuations, signals were normalized using ¹⁰³Rh as internal standard spiked at 500 pg.mL⁻¹ in each solution.

## 2.6. ²³⁰Th/²³⁸U dating

Sampling for ²³⁰Th/²³⁸U dating was done subsequently to the characterization of the developed micro-sampling approach and adapted according to the different natures of the two tested samples. For the “Aqaba 13” coral sample, 30 consecutive shots were thus fired resulting in a total ablated mass of 0.55 mg. For the “Tinshemet” speleothem 15 consecutive shots were fired resulting in a total ablated mass of 0.49 mg. Two blank collection modules were also prepared for each sample using new, unused polycarbonate filter holders.

Sample preparation was then performed at the LSCE. Particle solubilization was done following the protocol detailed previously with the difference that the 2% nitric acid solution was manually flown through the modules using a 10 mL syringe. After addition of an isotopic double spike (²²⁹Th and ²³⁶U), uranium and thorium extraction and purification were performed using separation columns loaded with ion exchange resins (U-teva) following a protocol similar to the one presented by Pons-Branchu et al. (2014)[16]. Extractions were then dry-evaporated and redissolved in 200 µL of an acid mix (HNO₃/HF - 1N/0.05%) for the analysis.

Preparations were then analyzed with an MC-ICP-MS (Nu 1700, Nu Instruments) equipped with a desolvating nebulizer system (CETAC Aridus II) to increase signal intensities and limit hydride generation (especially on ²³⁸U)[34]. The typical operating conditions of the Nu 1700 are available in Table 3. Sample preparations were then injected as series of 5x10 µL micro-injections using a 0.5-10 µL electronic micropipette (BRAND Transferpette) in auto-aspiration mode (i.e. the peristaltic pump is only clamped on the drain tubing of the injection system). Auto-aspiration was chosen to ensure a smooth and regular sample aspiration. To enable micro-injections, the 0.5 mm I.D. UniFit PFA capillary of the nebulizer was cut to a length of approximately 20 cm and a 1 cm long section of a white-white peristaltic pump Tygon tubing (1.04 mm I.D.) was connected to the free extremity of the PFA section. This small Tygon section allows to easily insert micro-pipette's tips for injections. Each series of sample injections was preceded and followed by 3x10-µL micro-injections of an internal standard solution used to correct both for mass bias (exponential law as described by Maréchal et al. (1999)[35]) and U and Th extraction yields as the same isotopic double spike was used for standard and sample preparations.

<table><caption>**Table 2**

Operating parameters used for the Agilent 8900 QQQ-ICP-MS.</caption><thead><tr><th>8900 parameters</th><th>Settings</th></tr></thead><tbody><tr><td>Mode</td><td>Single Quad</td></tr><tr><td>RF Power</td><td>1500 W</td></tr><tr><td>Plasma gas flow rate</td><td>15.0 L.min<sup>-1</sup></td></tr><tr><td>Sampling Depth</td><td>7.2 mm</td></tr><tr><td>Data acquisition mode</td><td>Spectrum</td></tr><tr><td>Integration time</td><td>3 s</td></tr><tr><td>Number of replicates</td><td>10</td></tr><tr><td>Number of sweeps/replicates</td><td>30</td></tr><tr><td>Monitored isotope</td><td>⁴³Ca</td></tr><tr><td>Nebulizer gas flow rate</td><td>0.7 L.min<sup>-1</sup></td></tr><tr><td>Make-up gas flow rate</td><td>0.43. L.min<sup>-1</sup></td></tr><tr><td>Nebulizer liquid flow rate</td><td>350 µL.min<sup>-1</sup></td></tr><tr><td>Spray chamber</td><td>Scott-type</td></tr><tr><td>Scott chamber Temperature</td><td>2 °C</td></tr><tr><td>Reaction gas</td><td>No gas</td></tr></tbody></table>

<table><caption>**Table 3**

Operating parameters used for the Nu 1700 MC-ICP-MS (Top) and detector configurations used for the MC-ICP-MS analyses. L2 is a Faraday cup and ICs stand for Ion Counter (Bottom).</caption><thead><tr><th colspan="2">Nu 1700 parameters</th></tr></thead><tbody><tr><td>Coolant (Ar)</td><td>13 L.min<sup>-1</sup></td></tr><tr><td>Auxiliary gas</td><td>0.8 L.min<sup>-1</sup></td></tr><tr><td>Nebulizer gas</td><td>35-45 psi</td></tr><tr><td>Skimmer cone</td><td>Ni</td></tr><tr><td>Sampling cone</td><td>Ni</td></tr><tr><td>Nebulizer</td><td>Micromist 200 µL.min<sup>-1</sup></td></tr><tr><td>Sample introduction</td><td>Cetac Aridus II</td></tr><tr><td>Faraday cup amplifiers</td><td>10-11 Ohm</td></tr><tr><td>Acquisition Time</td><td>0.2 s</td></tr></tbody></table>

<table><thead><tr><th colspan="4">Uranium configuration</th><th colspan="3">Thorium configuration</th></tr><tr><th>²³⁸U</th><th>²³⁶U</th><th>²³⁵U</th><th>²³⁴U</th><th>²³²Th</th><th>²³⁰Th</th><th>²²⁹Th</th></tr></thead><tbody><tr><td>L2</td><td>IC0</td><td>IC1</td><td>IC2</td><td>L2</td><td>IC0</td><td>IC1</td></tr></tbody></table>

Collection modules' blank solutions were analyzed similarly at the beginning and end of the analysis.

Since the detectors configuration of the Nu 1700 does not allow the simultaneous detection of all isotopes of interests (²²⁹Th, ²³⁰Th, ²³²Th, ²³⁴U, ²³⁵U, ²³⁶U and ²³⁸U) two detector configurations were used, one for thorium and one for uranium (see Table 3).

After peak integration (peak area ratios), samples' average corrected ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U isotopic ratios were then expressed as isotopic activity ratios using the radioactive decay constants and half-lives values measured by Cheng et al. (2013)[12]. Associated uncertainties were calculated as 2xSD weighted by the square root of the number of micro-injections done for each sample. These ratios were then used for age calculation using a Monte Carlo algorithm with 10³ iterations. The JavaScript algorithm used is the same as used by Martin et al. (2022) [10].

# 3. Results and discussion

## 3.1. Characterization of the laser ablation process

### 3.1.1. Ablation craters characterization

Craters resulting from this ablation strategy showed a pseudo flat-top square based pyramidal geometry (see Fig. 4). Since the laser beam was not refocused over the course of the ablation, only the first shots were focused on the surface of the sample. Under these conditions, effective ablation could not be maintained over the entire square area defined in the trajectory parameters throughout the ablation. Therefore, as the laser beam defocused, a progressively smaller square area was ablated, resulting in an apparent pyramidal geometry. Side lengths, depths and volumes were measured for each crater for 5 replicates for the limestone slab reference sample. Average values of 940 ± 94 µm, 670 ± 160 µm and 0.30 ± 0.08 mm³ were measured respectively for side length, depth and volume (see Table 4). Surface overall flatness introduced some higher variability to depth and volume (of about 24 and 27 % relative uncertainty, relatively) compared to the 10% relative uncertainty obtained for side length. Indeed, depth and volume measurements strongly depend on the reference points taken around the crater. If the surface presents a slope, depth measurement's reference point will vary from one side of the crater to the other resulting in higher RSD in depth, and consequently volume. According to the average of these estimated volumes and considering a density of 2.5 for this carbonate slab, the ablated mass was roughly estimated at 0.75 ± 0.2 mg (±2xSD). Taking into account the uncertainty, this estimate is in agreement with the gravimetric measurement of the slab before and after ablation, measured at 1.08 ± 0.20 mg.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Typical ablation crater produced by the sampling protocol on a limestone slab sample. A/ Top view, side length measurements; B/ Profile view; C/ Depth measurements from the profile view. Pictures and data processing were obtained with a Keyence numerical microscope.</figcaption></figure>

Overall, similar dimensions were obtained for the speleothem and coral samples (see Table 4), but the number of shots had to be increased compared to the limestone slab as ablation efficiency seemed to be reduced. Indeed, after 15 consecutive shots on these samples, craters were visibly smaller compared to what was observed on the limestone slab. To obtain similar appearances, the number of shots was then gradually increased. For example, after increasing the number of shots

from 15 to 25 and 30, ablated masses of 1.16 ± 0.20 mg (n=6) and 1.10 ± 0.38 mg (n=6) were measured for Salam 12 and YAAX-P, respectively. Salam Coeur produced similar results, but the ablated mass was a little higher than targeted with 35 shots averaging at 1.40 ± 0.58 mg for 4 replicates. For the coral sample Aqaba 13, ablation efficiency was even more reduced since three 30 shot replicates gave an average ablated mass of 0.74 ± 0.23 mg. Considering that speleothems and corals are both of crystalline nature, which is not the case for the limestone slab, this suggests that crystalline structure seems to be responsible of lower ablation efficiencies. Additionally, crystallinity also seems to act in combination with transparency to influence the way laser ablation occurs, as crater's shapes and profiles were different between speleothems samples (both crystalline and transparent) on the one hand and the limestone slab and coral sample on the other hand (both opaque, but only the coral sample is crystalline). Indeed, for all speleothem samples, craters exhibited stepped profiles (see Fig. 5.C) and irregular shapes with sharp edges (see Fig. 5.B). Ablation craters with similar aspects have in fact already been reported by Hathorne et al.[36] on a biogenic calcite sample using a 213 nm ns-pulse laser. In this study, these irregularities were attributed to a preferential energy absorption by calcite lines of weakness and/or cleavage due to an incomplete coupling between the laser and calcite. By orienting energy diffusion, these lines favor the ejection of µm-sized particles during ablation, explaining the rough crater shapes. Corals are generally made of aragonite rather than calcite and thus present a different crystallography (orthorhombic while calcite is rhombohedral[37]) for the same composition (CaCO₃). Despite having a crystalline structure, the coral sample Aqaba 13 did not, however, exhibit a similar reaction to the ablation protocol as craters could show a much smoother profile, comparable to the ones obtained with the limestone slab (see Fig. 5.D and 5.C). This suggests that crystallinity is not the only factor responsible for the crater's shapes observed in the present work, at least with a 1064 nm – ns laser source. Transparency on the other hand seems to have a significant influence since the laser beam seemed to react markedly to it, as shown by the presence of a HAZ in the immediate surrounding of speleothem samples' craters (see Fig. 5.B), more and more visible the more transparent the speleothem is (YAAX-P > Salam 12 > Salam Coeur, see Fig. 3, 4 and 5 in Supplementary Material). This phenomenon is much likely due to laser energy diffusion owing to the transparency of the samples with respect to the laser wavelength, since infrared nanosecond laser ablation has already been reported to be responsible of surface and sub-surface stress-induced cracking for transparent mineral samples[38]. Taken together with the observations made by Hathorne et al. transparency could amplify irregular fracturing by being responsible of the appearance of weakness lines resulting from reaction of the sample to the ns-IR laser. These irregularities could indeed act in addition to the natural cleavage lines of crystalline samples by providing extra lines of preferential energy diffusion thus increasing irregular ablation patterns, which is in line with our observations. Additionally, in the same study by Hathorne et al., using a 193 nm ns-pulse laser rather than a 213 nm one resulted in the production of a much more defined circular ablation crater, associated with a significant decrease in the production of µm-sized calcite particles due to better coupling between the laser and the carbonate sample. This suggests that longer wavelengths tend to increase irregular fracturing which is coherent with our results.

Going back to the results presented in Table 4, when considering ablated masses, we can notice that ablation seems to be less reproducible as we increase the number of sampling shots, with RSD typically around 9 % for 25 shots or less, and in the 15-20 % range above. Considering that samples requiring a higher number of shots are of crystalline nature (calcite and aragonite), this might be linked to matrix effects and the ablation pattern described above. Indeed, if we consider that shooting more times increases the number of stress-induced cracks in the sample and by the same the occurrence of particles ejection along irregularities, then more element influencing the ablation are introduced which decreases overall reproducibility.
<table><caption>**Table 4**

Ablated mass and dimensions of the ablation craters obtained for the different samples tested.</caption><thead><tr><th>Sample ID</th><th>Replicate</th><th>Average side length (µm)</th><th>Average depth (µm)</th><th>Volume (mm3)</th><th>Ablated mass (mg)</th><th>Number of shots</th></tr></thead><tbody><tr><td rowspan="7">Limestone slab</td><td>1</td><td>911.1</td><td>707.3</td><td>0.32</td><td>1.10</td><td rowspan="7">15</td></tr><tr><td>2</td><td>1003.7</td><td>790.0</td><td>0.36</td><td>1.19</td></tr><tr><td>3</td><td>914.4</td><td>648.1</td><td>0.29</td><td>1.09</td></tr><tr><td>4</td><td>972.6</td><td>599.3</td><td>0.3</td><td>1.08</td></tr><tr><td>5</td><td>898.2</td><td>605.8</td><td>0.25</td><td>0.92</td></tr><tr><td><strong>Mean value</strong></td><td><strong>940.0</strong></td><td><strong>670.1</strong></td><td><strong>0.30</strong></td><td><strong>1.08</strong></td></tr><tr><td><strong>2 SD</strong></td><td><strong>91.4</strong></td><td><strong>159.3</strong></td><td><strong>0.08</strong></td><td><strong>0.20</strong></td></tr><tr><td><strong>2 RSD</strong></td><td><strong>10%</strong></td><td><strong>24%</strong></td><td><strong>27%</strong></td><td><strong>18%</strong></td></tr><tr><td rowspan="7">Salam 12</td><td>1</td><td>917.4</td><td>777.6</td><td>0.34</td><td>1.11</td><td rowspan="7">25</td></tr><tr><td>2</td><td>956.8</td><td>826.6</td><td>0.36</td><td>1.11</td></tr><tr><td>3</td><td>993.9</td><td>922.9</td><td>0.42</td><td>1.17</td></tr><tr><td>4</td><td>978.5</td><td>1051.1</td><td>0.42</td><td>1.30</td></tr><tr><td>5</td><td>902.9</td><td>752.8</td><td>0.34</td><td>1.02</td></tr><tr><td>6</td><td>918.0</td><td>734.7</td><td>0.34</td><td>1.22</td></tr><tr><td><strong>Mean value</strong></td><td><strong>944.6</strong></td><td><strong>844.3</strong></td><td><strong>0.37</strong></td><td><strong>1.16</strong></td></tr><tr><td><strong>2 SD</strong></td><td><strong>74.4</strong></td><td><strong>243.5</strong></td><td><strong>0.08</strong></td><td><strong>0.20</strong></td></tr><tr><td><strong>2 RSD</strong></td><td><strong>8%</strong></td><td><strong>29%</strong></td><td><strong>21%</strong></td><td><strong>17%</strong></td></tr><tr><td rowspan="7">YAAX-P</td><td>1</td><td>1008.2</td><td>511.2</td><td>0.30</td><td>0.93</td><td rowspan="7">30</td></tr><tr><td>2</td><td>975.9</td><td>700.8</td><td>0.38</td><td>1.06</td></tr><tr><td>3</td><td>902.2</td><td>553.2</td><td>0.29</td><td>0.94</td></tr><tr><td>4</td><td>856.3</td><td>759.6</td><td>0.29</td><td>1.32</td></tr><tr><td>5</td><td>873.1</td><td>672.7</td><td>0.30</td><td>1.36</td></tr><tr><td>6</td><td>951.7</td><td>755.8</td><td>0.36</td><td>1</td></tr><tr><td><strong>Mean value</strong></td><td><strong>927.9</strong></td><td><strong>658.9</strong></td><td><strong>0.32</strong></td><td><strong>1.10</strong></td></tr><tr><td><strong>2 SD</strong></td><td><strong>120.4</strong></td><td><strong>208.8</strong></td><td><strong>0.08</strong></td><td><strong>0.38</strong></td></tr><tr><td><strong>2 RSD</strong></td><td><strong>13%</strong></td><td><strong>32%</strong></td><td><strong>25%</strong></td><td><strong>35%</strong></td></tr><tr><td rowspan="7">Salam Cœur</td><td>1</td><td>986.7</td><td>965.4</td><td>0.42</td><td>1.24</td><td rowspan="7">35</td></tr><tr><td>2</td><td>949.7</td><td>1050.5</td><td>0.44</td><td>1.82</td></tr><tr><td>3</td><td>858.3</td><td>768.5</td><td>0.31</td><td>1.35</td></tr><tr><td>4</td><td>918.4</td><td>920.9</td><td>0.395</td><td>1.18</td></tr><tr><td><strong>Mean value</strong></td><td><strong>928.3</strong></td><td><strong>926.3</strong></td><td><strong>0.39</strong></td><td><strong>1.40</strong></td></tr><tr><td><strong>2 SD</strong></td><td><strong>108.8</strong></td><td><strong>236.3</strong></td><td><strong>0.11</strong></td><td><strong>0.58</strong></td></tr><tr><td><strong>2 RSD</strong></td><td><strong>12%</strong></td><td><strong>26%</strong></td><td><strong>29%</strong></td><td><strong>42%</strong></td></tr><tr><td rowspan="7">Aqaba 13</td><td>1</td><td>949.3</td><td>428.3</td><td>0.18</td><td>0.61</td><td rowspan="7">30</td></tr><tr><td>2</td><td>873.5</td><td>590.3</td><td>0.24</td><td>0.77</td></tr><tr><td>3</td><td>895.6</td><td>704.7</td><td>0.32</td><td>0.83</td></tr><tr><td><strong>Mean value</strong></td><td><strong>906.1</strong></td><td><strong>574.4</strong></td><td><strong>0.25</strong></td><td><strong>0.74</strong></td></tr><tr><td><strong>2 SD</strong></td><td><strong>78.0</strong></td><td><strong>277.8</strong></td><td><strong>0.14</strong></td><td><strong>0.23</strong></td></tr><tr><td><strong>2 RSD</strong></td><td><strong>9%</strong></td><td><strong>48%</strong></td><td><strong>57%</strong></td><td><strong>31%</strong></td></tr></tbody></table>

Concerning the temporal aspect of this sampling method, 1 shooting sequence takes about 1 minute to be performed when considering software loading times, as even if not used, the LIBS data still need to be processed. In addition to the handling of the system, collection modules must also be properly packaged after each sample. Ablating 1 mg of matter consequently takes between 20 and 40 minutes depending on the sample and the number of shooting sequences required.

### 3.1.2. Particles size distribution and filter retention efficiency

Particles size distribution was evaluated with a low-pressure cascade impactor (ELPI) on the reference limestone slab (18 shots in total) as well as on Salam Cœur (35 shots in total) to evaluate potential differences between an opaque and amorphous sample and a crystallized and transparent one. Particles mass repartition was also measured for each collector stage (corresponding to a specific aerodynamic diameter) but since the instrument could not be calibrated for absolute mass determination, these data were only considered in terms of proportions of total mass per stage. For Salam Cœur, data were considered for the 35 shots but also over the first 18 for comparison purposes with the limestone slab. These results are summarized in Fig. 6. For both types of samples, more than 99.8 % of the particles produced had an aerodynamic diameter less than 100 nm. The global particle repartition pattern was identical for Salam Cœur independently of the number of shots. However, these results confirm the decrease in ablation efficiency previously mentioned for crystalline samples as for the same number of shots, only 40.1 ± 8% of the total particles produced with the limestone slab were detected for Salam Cœur (see Table in Fig. 6).

Alternatively, while particles above 1.6 µm of diameter represented

only about 0.009% of the total number of particles detected for both samples, they accounted for about 95.5 % of the total ablated mass measured by the instrument. This could be problematic if this sampling technique were to be coupled with an ICP-MS for on-line LA-ICP-MS analyses, especially in the case of U/Th analyses. As reported in several articles (Guillong et al. (2003)[39], Hattendorf et al. (2003)[40], Hathorne et al. (2008, see Supplementary data[36]), increased particle size can be responsible of elemental fractionation that would be detrimental to precise measurements of U/Th ratio. However, the dissolution approach developed in this work (see dedicated section below) should overcome this issue if complete particle dissolution can be achieved. Additionally, adding a filter in the collection module resulted in an almost complete absence of signal on all collection stages. Moreover, the comparison of the two sets of ELPI analyses showed that 99.995 % of the induced particles that reached the collection module were efficiently trapped by the filters used in this work.

### 3.1.3. Particle size distribution as a function of the number of shots

Particle distribution was also plotted for each shot for both samples. Results are presented in Fig. 7 and in Figs 6, 7, 8 and 9 in SI. The number of particles produced by the sampling protocol first increases for each diameter as the number of shooting sequences fired on the sample increases, before reaching a threshold from which the number of particles produced by each shot for each diameter stabilizes. For both samples, most particle sizes show a similar overall evolution except between 55 nm and 156 nm in diameter for which significantly different patterns can be observed between the two samples. While the number of these particles increases then stabilizes after 5-10 shots for the
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Ablation crater produced by the sampling protocol on the YAAX-P speleothem sample. A/ Visible HAZ as a halo surrounding the crater; B/ Top view; C/ Depth measurements from the profile view; D/ Profile view of the ablation crater of the Aqaba 13 coral sample. Profiles measured with a Keyence numerical microscope.</figcaption></figure>

speleothem, the limestone slab shows an opposite tendency with particle
production first being close to its maximum before gradually decreasing
down to a threshold. This particular trend is specific to this range of
particle size only and becomes less marked as size increases
(55nm>93nm>156nm) before ultimately going back to the trend of
evolution first described in this section. Since the first few shots are the
most focused, this could be an effect of laser defocus specific to opaque
and/or transparent samples. The different evolution of each particle size
to a specific ablation threshold also impacts the proportion they repre-
sent as consecutive shots are fired. Between the first and last shooting
sequence of the ablation strategy used in this work the proportion of
particles below 7nm in diameter for the limestone slab thus increases
from about 40 % to about 70 % mainly at the expense of particles be-
tween 28 and 55 nm whose proportions decrease from about 45% to
about 30% and from 8% to 2%, respectively. For the speleothem samples
the trend is different with particles below 7 nm increasing from 45% of
the total particles to 60 %. While the proportions of 28nm and 55nm
particles also declines (from 45% to 37% and 10% to 4 %, respectively),
the decrease is less than observed for the limestone slab which indicates
that the interaction between the laser and crystalized and transparent
samples seems to favor the production of bigger particles compared to
opaque and amorphous ones. This could contribute to the rough crater
profiles described above. Even if a threshold is reached for each particle

diameter, ablation is nevertheless still effective for both sample types.
Indeed, when considering these data in a cumulative manner (see Fig. 8)
it appears clearly that ablation is maintained. More precisely, each
sample is characterized by a visible increase in ablation rate that con-
cerns essentially particles bellow 7 nm which significantly impact
overall particle production by 30 % and 60 % (slope ratio) for Salam
Coeur and the limestone slab, respectively. Inflection of these rates is
achieved after 14 and 8 shooting sequence for Salam Coeur and the
limestone slab respectively meaning that the ablation rate increase
occurred for 40 % less shots for the limestone slab, which is in line with
the results reported on the **table in Fig. 6**. Overall, it appears that the
laser defocus induced by the progressive removal of matter from the
sample surface is not sufficient to exceed the depth of field of the optical
system. This suggests that this sample protocol could be used to collect
larger samples up to a threshold that still needs to be determined.

3.2. Evaluation of particles dissolution and recovery rates

The amount of the 2% HNO₃ solution required to on-line dissolve the
particles collected on the filter module was evaluated by collecting 20
volumetric fractions of 0.4 ml. The elution profile plotted in **Fig. 10** in
**Supplementary Material** shows that more than about 92.5 % of the
total ⁴³Ca eluted during an 8mL 2% HNO₃ rinse of the modules was
collected within the first 2 mL of solution. 5 mL rinsing allowed to in-
crease particle solubilization to nearly 99%, showing that the applied
rinsing protocol should ensure quantitative dissolution of carbonate
particles collected through this sampling method. Under these condi-
tions, an elution volume of 5 mL of 2% HNO₃ was chosen.

Particle collection recovery was assessed for the eight collection
modules after ablating the limestone slab. It is important to point out
that our objective is to collect non-volatile elements like U and Th which
are of interest for U/Th dating. In our conditions, gravimetric approach
consisting in weighing collection modules before and after sampling
would not give reliable results due to matrix transformation during the
ablation process. Indeed, calcination of calcite into lime and carbon
dioxide (CaCO₃ → CaO + CO₂) occurs between 500 and 950 °C[41].
Since laser ablation with 1064 nm nanosecond pulses typically reaches
temperature as high as 10000 K[42], a significant amount of CO₂ would
likely be produced and not trapped on the filter or other parts of the
system. As a result, the mass measured by gravimetry would systemat-
ically underestimate the collection efficiency of these elements. As an
alternative, direct measurement of U and Th in the limestone slab and
filter would be much more reliable. However, the homogeneity of this
natural limestone slab in terms of U and Th is questionable and it was
preferred to refer to calcium, which as a non-volatile element and a
major constituent of the matrix was expected to be more homogeneous.

In order to obtain the response of the measured ⁴³Ca signal to the
mass of the slab, a reference solution was prepared by dissolving 100 mg
of slab in 50 ml of 2% HNO₃ solution as explained above. By comparing
the ⁴³Ca signal of this solution with that of a solution of particles trapped
on a filter for instance, it was possible to deduce the equivalent mass of
slab collected on this filter.

Results are presented in Table 5. The dissolved limestone slab
reference solution showed that for an average ablated mass of 1.08 ±
0.18 mg, 0.82 ± 0.09 mg of sample was collected in the modules, giving
a high recovery rate of 76.2 +/- 21.5% (as propagated uncertainty). This
indicates that it is necessary to target an ablated mass of 1.3 mg to ensure
1mg sample collection in the modules. On the other hand, the 3D nose-
tip and quartz blast shield rinsing solutions which are not supposed to
collect particles were found to contain 0.13 ± 0.02 mg and 0.007 ±
0.005 mg, which amounts to 12 % and 0.6% respectively. Taking into
account the propagated uncertainty the mass balance is respected (88.7
± 25.7 %). Indeed, even with a high velocity of the air stream (calcu-
lated at 3.2 m.s⁻¹ under our operating conditions) combined with very a
short distance between the collection pipe and the ablation site, it is
clear that some particles are unintentionally trapped next to the
<figure><img src="image_8.png" /></figure>

<table><caption>Fig. 6. ELPI collected particles' size distribution for the limestone slab and "Salam Cœur" samples. Error bars correspond to 2xSD (triplicate).</caption><caption>Proportion of particle counting for Salam Cœur compared to the limestone slab for the same number of shots</caption><thead><tr><th></th><th>&lt; 0.007</th><th>0.028</th><th>0.055</th><th>0.093</th><th>0.156</th><th>0.262</th><th>0.383</th><th>0.614</th><th>0.949</th><th>1.6</th><th>2.39</th><th>&gt; 4</th><th>Total particles</th></tr></thead><tbody><tr><th>Aerodynamic diameter (µm)</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Relative Counting (%)</th><td>33.8%</td><td>47.8%</td><td>67.7%</td><td>37.2%</td><td>38.5%</td><td>36.1%</td><td>37.4%</td><td>40.1%</td><td>39.5%</td><td>38.4%</td><td>37.6%</td><td>40.7%</td><td>40.1%</td></tr><tr><th>2 SD (%)</th><td>10.5%</td><td>11.3%</td><td>21.8%</td><td>13.4%</td><td>25.1%</td><td>17.5%</td><td>19.6%</td><td>16.0%</td><td>16.1%</td><td>16.5%</td><td>15.7%</td><td>18.1%</td><td>8.0%</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>Fig. 7. Average ablation profiles plotted as the number of particles detected per shot for each aerodynamic diameter (range detected by the ELPI analyzer for sampling triplicates of Salam Cœur and the limestone slab reference sample. Error bars correspond to 2xSD.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Average ablation profiles plotted as the sum of the number of particles detected after each shot for each aerodynamic diameter (range <7 nm - >4 µm) detected by the ELPI analyzer for sampling triplicates of Salam Cœur and the limestone slab reference sample. Error bars correspond to 2xSD (propagated).</figcaption></figure>

<caption>**Table 5**</caption>

Mass data (top) related to the determination of the sample recovery rate achieved by the proposed sampling methodology (bottom).

<table><thead><tr><th>Replicate</th><th>Ablated mass (mg)</th><th>Collected mass (mg)</th><th>Nose tip deposition (mg)</th><th>Laser window deposition (mg)</th></tr></thead><tbody><tr><td>1</td><td>1.03</td><td>0.76</td><td>0.12</td><td>0.010</td></tr><tr><td>2</td><td>1.10</td><td>0.84</td><td>0.13</td><td>0.009</td></tr><tr><td>3</td><td>0.92</td><td>0.78</td><td>0.12</td><td>0.007</td></tr><tr><td>4</td><td>1.21</td><td>0.88</td><td>0.14</td><td>0.002</td></tr><tr><td>5</td><td>1.09</td><td>0.88</td><td>0.13</td><td>0.006</td></tr><tr><td>6</td><td>1.19</td><td>0.82</td><td>0.14</td><td>0.009</td></tr><tr><td>7</td><td>1.08</td><td>0.79</td><td>0.13</td><td>0.006</td></tr><tr><td>8</td><td>1.04</td><td>0.87</td><td>0.12</td><td>0.006</td></tr><tr><td>Replicate average</td><td>1.08</td><td>0.82</td><td>0.13</td><td>0.007</td></tr><tr><td>2 SD</td><td>0.18</td><td>0.09</td><td>0.02</td><td>0.005</td></tr><tr><td>2 RSD</td><td>17%</td><td>11%</td><td>13%</td><td>79%</td></tr><tr><td></td><td>Collected mass</td><td>Nose tip deposition</td><td>Laser window deposition</td><td>Total</td></tr><tr><td>Proportion of ablated mass (average %)</td><td>76.2%</td><td>11.9%</td><td>0.6%</td><td>88.7%</td></tr><tr><td>Uncertainty (propagated)</td><td>21.5%</td><td>3.5%</td><td>0.6%</td><td>25.7%</td></tr></tbody></table>

collection pipe and therefore lost for the sampling. These results also suggest that the module should be replaced each time to avoid cross-contamination from one sample to the next. This is actually a very simple and quick operation as there are only two screws to arrange. In addition, pre-assembled collection modules in a clean room are certainly the best way to avoid the risk of contamination on site and are the preferred solution.

The design of the currently presented 3D printed nose-tip is the

<figure><img src="image_11.png" /></figure>

outcome of several trials using different materials and designs. The first model we developed copied the factory design of the nose-tip and was printed in PETG filament, which produces a rougher surface than the presented model. With this initial design, the amount of particles trapped on the nose-tip was slightly higher (by about 25%) and more dispersed with 0.16 mg ± 19.0 % for an average sampling of 1.05 mg ± 14.8 % (n=18). Changing the material and design of the 3D-printed nose-tip thus seems to provide a small overall improvement around the sampling interface.

### 3.3. $^{230}$Th/$^{238}$U dating

After tuning, optimization and detectors inter-calibration (gains between Faraday cups and ions counters) of the Nu 1700, a high sensitivity of $4.2 \times 10^7$ cps/ppb was achieved (measured on mass 238) owing to the Aridus II desolvating unit. Ensuring good sensitivity conditions was critical for this experiment given the reduced sample size (< 1mg) and low expected amounts of analytes, especially $^{230}$Th given the particularly long half-life of its parent isotope $^{238}$U ($T_{1/2} (^{238}\text{U}) = 4.468 \times 10^9$ y). Sampling blanks solutions indicated a significant contribution from the collection modules, especially on uranium. Indeed, collection modules contributed to 8% and 34% of the $^{238}$U signal for the Aqaba 13 coral and Tinshemet speleothem, respectively. For $^{230}$Th, this contribution amounted to only 0.7% for the “Aqaba 13” sample. However, for the “Tinshemet” speleothem, blanks represented about 10.5% of the sample’s signal. Since $^{230}$Th production is dependent both on the time elapsed since the precipitation of the carbonate and the amount of $^{238}$U initially incorporated, this higher contribution of the blanks to the signal of $^{230}$Th simply reflects the difference between both samples regarding age and U content (32.4 ka versus 200-225 ka and ~500 ng.g$^{-1}$ $^{238}$U versus ~4 µg.g$^{-1}$ $^{238}$U for Tinshemet and Aqaba 13, respectively).

The typical micro-injection peak shapes can be observed on Fig. 11 in **Supplementary Material**. With only 5 injections of 10 µL, inter-injection repeatability was excellent. For Aqaba 13, a $^{234}$U/$^{238}$U
isotopic ratio of $5.93 \times 10^{-5} \pm 5.27 \times 10^{-7}$ (2xSD) (or 0.44 % as 2xRSD) was measured which falls within the range of error values reported by Hoffman et al. (2007)[43] for the analyses of the CRM U010 ($^{234}$U/$^{238}$U = 5.466x$10^{-5} \pm 5.1 \times 10^{-7}$). For Tinshemet, peak ratio repeatability was slightly lower with $^{234}$U/$^{238}$U = $6.03 \times 10^{-5} \pm 1.26 \times 10^{-6}$ (2xSD) (or 1.04 % as 2xRSD). Concerning $^{230}$Th/$^{234}$U, the repeatability was better with $2.65 \times 10^{-1} \pm 6.54 \times 10^{-6}$ (2xSD) (or 0.002 % as 2xRSD) and $8.80 \times 10^{-2} \pm 2.67 \times 10^{-5}$ (2xSD) (or 0.03 % as 2xRSD), for Aqaba 13 and Tinshemet, respectively. This certainly results from blank variations and memory effects.

The calculated ages presented in Table 6 are in good agreement with the expected ages with the Aqaba 13 sample falling in the age range reported by Ribot et al. (2024)[31]. For Tinshemet, however, the age calculated is 13 % older than the expected value and 6% older if we consider the uncertainty interval. This is quite encouraging considering that these ages were obtained from much less sample mass than used in the reference studies. Indeed, when assuming a 76 % particle collection efficiency (as measured previously for the limestone slab sample), the actual mass of sample that was analyzed in the present works amounts to 0.42 mg and 0.37 mg for Aqaba 13 and Tinshemet respectively. For the former, this represents at least 20 times less sample than used by Ribot et al. (between 10 and 200 mg per sample) and about 3 times less sample than used to date the Tinshemet sample (between 1 and 2 mg) (Zaidner et al.[32], in press).

It should be noted that these ages are not corrected for detrital thorium. The potential contribution of detrital particles to the $^{230}$Th signal was indeed considered negligible as both samples present overall low levels of $^{232}$Th. This isotope is generally used as a proxy indicative of this type of contamination since it is not produced through the decay of $^{238}$U. However, the Tinshemet speleothem is much more heterogeneous than the Aqaba 13 coral and varying amounts of $^{232}$Th between a few ng. g$^{-1}$ and 1 to 2 µg.g$^{-1}$ were measured during the fsLA-ICP-MS analysis of the sample. In contrast, Ribot et al. reported $^{232}$Th concentrations for the Aqaba sample comprised between 0.1 and 35 ng.g$^{-1}$. It is therefore possible that the micro-sample collected on the Tinshemet speleothem may have contained detrital particles responsible for contamination with exogenous thorium, resulting in an artificial aging of the sample.

With only two samples collected in one replicate, these results must however be considered carefully and additional tests with more samples of various natures and ages should be done in the future to bring more insights regarding the potential of this approach as a field-sampling technique for accurate and precise U/Th dating. In particular, it would be interesting to confront this sampling approach with young samples (in the ka range) and/or samples with low U content.

## 4. Conclusion

In this work we developed a field micro-sampling technique capable of sampling about 1 mg of sample with a spatial resolution better than 1 mm$^3$, in about 30 minutes. This method was successfully adapted to different types of calcium carbonate samples, including limestone, speleothems and coral. The conjointly designed sample preparation and particle dissolution protocol allows to solubilize 99% of the particles collected in the sampling modules corresponding to a global particle recovery rate of 77%, under 5 minutes. Additionally, two carbonates of different natures and ages were accurately dated using the $^{230}$Th/$^{238}$U geochronometer from only ~0.5 mg of sample. In the context of uranium-thorium dating of speleothem samples, we believe that the designed system allows a targeted and respectful collection of archeological samples, particularly adapted to preservation imperatives associated with prehistoric wall painting preservation. Even if this work focuses on uranium thorium dating of carbonate samples, the simplicity of the design and the high control over the shooting parameters provided by the software of the Z300 enables easy modification and adaptation for varied applications.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.sab.2025.107239.

<table><caption>**Table 6**
Calculated $^{230}$Th/$^{238}$U ages and associated uncertainties.</caption><thead><tr><td>Sample</td><td>Expected age (ka)</td><td>Calculated age (ka)</td><td>2*SD/√5 (ka)</td></tr></thead><tbody><tr><td>Aqaba 13</td><td>200-225</td><td>203.7</td><td>0.5</td></tr><tr><td>Tinshemet</td><td>32.4 ± 2.0</td><td>36.6</td><td>0.1</td></tr></tbody></table>

## Credit authorship contribution statement

M. Boccas: Writing – original draft, Validation, Methodology, Formal analysis, Data curation, Conceptualization. J. Maréchal: Formal analysis. A.-L. Ronzani: Formal analysis. F. Claverie: Methodology. E. Pons-Branchu: Validation, Methodology, Conceptualization. M. Maguregui: Writing – review & editing, Supervision. C. Pécheyran: Writing – review & editing, Validation, Supervision, Software, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

## Acknowledgements

The authors wish to thank the “Laboratoire des Sciences du Climat et de l’Environnement” (LSCE) and the “Archeoscience” laboratory for providing the different coral and speleothems samples used in this study. This work was financially supported by the “Agence Nationale de la Recherche” (ANR), in the context of the “Apart” project (ref. 18-ce27-0004-01).

## Data availability

Data will be made available on request.

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