Evaluation of different types of paper as solid substrates and their effects on LIBS emission signals

Abstract

In the present work, different types of paper were evaluated as a solid substrate for LIBS measurements, aimed at verifying their effects on the emission signals, taking into account their grammage, thickness and metal ions naturally present (minor components/impurities). Cu(II) and Mn(II) ions were used in the present study. The emission signals for Cu I and Mn I significantly varied depending on the paper type, whose intensities decreased after rinsing with nitric acid solution. No correlation was found between signal intensity and paper grammage or paper thickness, whose effect could be hidden by the metal ions present in the paper, as signals significantly decreased after their removal. In addition, after washing with nitric acid solution, the wettability of the paper (measured as the contact angle with water) increased, allowing a more effective absorption of the metal ion solution by the paper, a fact that can also contribute to decreasing the signal intensity. The addition of Na(I) and Ca(II) ions in the paper sheet previously rinsed with nitric acid solution caused an increase in the signal emission of Cu I and Mn I, as these ions can contribute to the electron density of the plasma, increasing its temperature. However, the addition of Fe(II) ions and other transition metal ions in the paper also provided an increase in the emission of Cu I and Mn I and signal intensities can be recovered by adding a mix of these metal ions on the paper. The sensitivity of the measurements (taken as the slope of the calibration curve) provided by using office paper as a substrate was around 14 and 2.7 times higher than those obtained with quantitative filter paper for Cu and Mn, respectively. Therefore, depending on the analyte to be determined, ordinary paper can be used to improve the sensitivity of the desired LIBS method.

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1 Introduction

Laser-Induced Breakdown Spectroscopy (LIBS) is a technique for multielemental analysis, with the capability of performing in situ determinations in a real-time regime. The LIBS technique can be used in different areas, such as the food industry, environmental chemistry, deep-sea geochemistry, space exploration, biology, and mineralogy, among others.^1,2 The advantages of the LIBS technique include rapid analysis, minimal or no sample preparation, non-destructive character, simple operation, and applicability to the analysis of solid, liquid or gaseous samples.³ Despite several advantages of LIBS, the lack of reproducibility and even accuracy can be observed, which is mainly associated with the matrix effects caused by the physical and chemical properties of the sample that can affect the sample ablation process, as well as the analytical signal intensity.^2,3 One of the main limitations of the technique is the direct analysis of liquid samples, in which part of the laser energy is attenuated due to solvent vaporisation, causing rapid cooling of the plasma and signal attenuation. Furthermore, the splash effect generated by ablation can damage optical components and microbubbles or particulate materials present in the liquid sample can affect the laser pulse focus and angle of incidence of the beam, generating plasma instability.^4–6

To overcome the limitations of analysing liquid samples by LIBS, the strategy of changing the liquid phase to a solid phase is used, with sample freezing being one of these approaches.⁷ Another strategy reported in the literature is the use of different solid substrates, to which the analytes present in a liquid sample are transferred, with subsequent analysis of the material after drying. The transfer of analytes aims to obtain better signal-to-noise ratios and emission signals of high intensities, eliminating the effect of liquid sample splashes. Various substrates have been used for transfer and pre-concentration of analytes, such as graphite, wood, polymers, exchange membranes, filter paper, coal, oxides of calcium and zinc, porous fibres and metallic substrates. The use of these solid substrates can improve the detection limit, reproducibility and sensitivity in LIBS measurements.^8–10

Among the different substrates used, a special focus has been placed on cellulosic substrates such as papers due to their low cost, ease of handling and availability. Paper has been used as a substrate for analytical purposes in different applications, such as pH testing,¹¹ chromatography,¹² electrophoresis,¹³ manufacturing microfluidic devices¹⁴ and even in rapid detection of infectious diseases.¹⁵ Some types of paper offer an economical alternative to numerous applications in analytical chemistry, including deposition of liquid samples and retention of analytes.¹⁶ A recent contribution has demonstrated that the application of extremely porous materials, such as paper, behaves similarly to a solid surface, showing no significant difference from a glass substrate.¹⁷

The contributions reported in the literature describe an efficient use of papers as solid substrates, to which the analytes present in the liquid sample are transferred, with subsequent analysis by LIBS. For instance, Maji et al.¹⁸ used a filter paper combined with a colloid formation process to determine Cu and Cr in liquid samples, obtaining LOD ranging from 0.020 to 0.025 μg mL⁻¹, with satisfactory recovery values in tests with tap and rain water samples. Allegra et al.¹⁹ also used paper as a solid substrate for the determination of Ca, Na, K, Mg, C, H, O, N, and CN in serum samples, aiming at developing a fast, less invasive, cost-effective method combined with machine learning for diagnosis and staging of human malignancies. The combination of the ring-oven technique with filter paper was successfully applied by Pasquini and Filho²⁰ for pre-concentration and determination of Cu in an alcoholic beverage. The authors achieved a LOD of 0.3 mg L⁻¹, with results similar to those obtained by the flame atomic absorption reference method. A commercially available ordinary printing paper was chosen by Youli et al.²¹ for the determination and enrichment of toxic metals, based on a simple immersion method, achieving LOD of 0.033 mg L⁻¹ and 0.026 mg L⁻¹ for Pb and Cr, respectively. The results prove the efficiency of LIBS in determining toxic metals in liquid samples, and the methods developed have sensitivity similar to the classical techniques, such as X-ray fluorescence spectroscopy (XRFS), flame atomic absorption spectroscopy (FAAS) and inductively coupled plasma optical emission spectroscopy (ICP OES).^22–24 Recently, Cicconi and Lazic²⁵ performed surface and in-depth characterization of commercial papers by LIBS for their classification and demonstrated that plasma emission was higher for the first few laser pulses because the ablation of fillers, such as calcium carbonate, is more intense as they are soft materials.

LIBS has been used for environmental applications aiming at monitoring toxic metals, such as Cu(II) and Mn(II) in water, which is a relevant topic due to the various environmental degradation processes that have caused serious pollution issues. These metals are transferred to the food chain, where they accumulate in biological tissues, strongly impacting proteins and various enzymes in organisms, causing chronic toxicity and various diseases.^26–28 Based on this fact, monitoring these metal ions in liquid samples using analytical methods such as LIBS combined with liquid–solid conversion using a solid substrate is essential in order to control contamination.

Although paper has frequently been used as a substrate for LIBS analysis of liquids, there is no systematic study regarding the effect of the paper type, as well as the physical characteristics and chemical composition of these papers on the emission intensity of metals in LIBS analysis. Therefore, the present work is aimed at evaluating the effect of different types of papers, their grammage and minor components on LIBS emission signals. Cu(II) and Mn(II) ions were taken as a model and measurements were performed by depositing sample solutions onto the paper substrate, which was dried in an oven before LIBS analysis.

2 Experimental

2.1 Reagents and solutions

All solutions used in the present work were prepared with analytical grade reagents. The working solutions of Cu(II) and Mn(II) ions were prepared from 1000 mg per L Cu(II) and 1000 mg per L Mn(II) standard solutions (Dynamics, Rio de Janeiro, Brazil). HNO₃ (Dynamics, Rio de Janeiro, Brazil), NaCl (Synth, Diadema, Brazil), CaCl₂ (Synth, Diadema, Brazil) and FeCl₂ (Merck, New Jersey, United States) solutions were prepared in deionized water (resistivity of 18.2 MΩ cm) obtained from a water purification system (Gehaka, model GS200 UV, São Paulo, Brazil).

2.2 Paper substrates and sample preparation

Table 1 lists the 17 types of paper used in the present study, whose characteristics are described in Table S1.

Table 1 Different types of paper tested as solid substrates

Substrate	Type
A	Thermal
B	Recycled
C	Photographic
D	Tracing paper I
E	Colour set
F	Craft
G	Wood
H	Tracing paper II
I	Laid I
J	Office
K	Eggshell
L	Linen
M	Coffee filter
N	Laid II
O	Quantitative
P	Chromatographic
Q	Qualitative

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Paper sheets were cut with a circular paper punch to obtain discs with a diameter of 25 mm, which were placed in acrylic lids with a diameter of 27 mm for sample preparation. The sample deposition and sample drying were carried out by adding a volume of 600 μL of the working solution containing Cu(II) and Mn(II) ions onto the paper and dried for 120 minutes in an oven at 60 °C for subsequent analysis by LIBS. The solution volume used as well as the time and temperature for drying the paper were based on a previous work.²⁹

2.3 Instrumentation

The LIBS experimental setup used (Fig. S1) was similar to that described by Silva & Raimundo.²⁹ A Nd:YAG laser source (Quantel, Brilliant, Newbury, United Kingdom) operating at 1064 nm, with a pulse duration of 5 ns and a repetition rate of 20 Hz, was used. The laser beam was reflected by a dichroic mirror (Ealing Electron-Optics, 456 196 000, Tokyo, Japan) positioned at 45° and then focused on the sample surface through a plano-convex lens, with a focal length of 100 mm (Thorlabs, BK 7, New Jersey, United States). The distance between the focusing lens and the sample was fixed at 95 mm to avoid plasma breakdown in air. A 5 mm diameter lens with a 40 mm focal length (Ocean Optics, 74 UF, Dunedin, United States) was used to collect the radiation emitted by the plasma, focusing it on the tip of a 105 μm diameter optical fiber (Thorlabs, FG105UCA, New Jersey, United States), responsible for guiding the radiation to the echelle polychromator (Andor Technology, Mechelle 5000, Belfast, Northern Ireland) and an ICCD detector (Andor Technology, iStar DH 734, Belfast, Northern Ireland) that perform the acquisition of the analytical signal.

For LIBS measurements, the solid substrate containing the analytes was placed on a sample holder, composed of a polytetrafluoroethylene (PTFE) rotating platform connected to a direct current motor, which was maintained at a rotation of 10 rpm. As the laser pulses were delivered at a repetition rate of 20 Hz, it was possible to accumulate the emission signals provided by 120 pulses, which were fired side by side, fulfilling a complete circumference. To acquire the spectra, a laser pulse energy of 90 mJ (5.5 ns pulse, spot diameter of 150 μm) was employed, setting a delay time of 2.0 μs and an integration time of 3.0 μs. The emission lines of Cu I 324.76 nm and Mn I 403.23 nm were monitored. All results presented in this study are the average of three measurements. Use of laser safety eyeglasses is obligatory to perform LIBS measurements. Spectral data processing was performed using Origin® 9.0 and MATLAB® software. The baseline correction of the spectra was performed with the PLS Toolbox using the Whittaker filter.^30,31 Signal intensities were obtained by subtracting the peak intensity from the baseline.

2.4 Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and contact angle analysis

The morphology of the samples was examined with a field emission scanning electron microscope (Thermo Fisher Scientific, Quanta FEG 250, Waltham, United States) equipped with an energy dispersive spectrometer (Oxford, X MAX50, Abingdon, United Kingdom). The equipment was operated in the environmental mode (ESEM), with an acceleration voltage of 20 kV, using the gaseous secondary electron detector (GSED) for evaluating the morphology and distribution of cellulose fibres of the solid substrate. The contact angle was estimated on the surface of the solid substrate using a Drop Shape Analyzer (Kruss, DSA100, Hamburg, Germany) in order to evaluate the variation in signal intensity caused by the hydrophobic and hydrophilic nature of the cellulosic materials.

3 Results and discussion

The use of paper substrates for LIBS measurements has been adopted by several researchers,^19,32–34 as it provides a straightforward procedure for analysing liquid samples. In spite of the paper substrate's usefulness, the effect of its properties, such as grammage, thickness and minor components, on the LIBS signal has not systematically been studied yet. In the present work, solutions of Cu(II) and Mn(II) ions were used to verify the LIBS emission signal as a function of the paper type used as a substrate.

3.1 Performance evaluation of solid substrate type

Initially, Cu I 324.76 and Mn I 403.23 nm emission signals were evaluated by using a 10 mg per L solution of the metal ions. Fig. 1 shows the intensity for these emission lines obtained with each paper substrate. As can be seen in Fig. 1, Cu I and Mn I emission lines are found even when the paper substrate itself (blank measurement) is analysed, which can impair the determination of the ions in solution. The paper manufacturing steps include wood extraction, preparation, pulping, bleaching, sheet formation and finishing.³⁵ During the bleaching stage, side reactions may occur, causing the presence of metallic species in some of the papers used. For example, Fe, Cu and Mn can originate from fillers and/or coatings²⁵ or from different stages of the manufacturing process, generated by equipment in contact with the pulp during processing or present in other supporting compounds.³⁶ In addition, these metal ions can be linked to the active centres of natural enzymes contained in plants, such as Fe and Mn in peroxidases, and Cu in lactases.³⁷

Fig. 1 Effect of the paper substrate on the emission intensity of (a) Cu I 324.74 nm and (b) Mn I 403.23 nm. (A) Thermal paper, (B) recycled, (C) photographic, (D) tracing paper I, (E) colour set, (F) craft, (G) wood, (H) tracing paper II, (I) laid I, (J) office, (K) eggshell, (L) linen, (M) coffee filter, (N) laid II, (O) quantitative, (P) chromatographic, (Q) qualitative.

However, it is noticeable from Fig. 1 that the emission signals obtained with a 10 mg per L Cu(II) and Mn(II) solution significantly vary according to the paper substrate used. This result clearly shows that paper properties are critical for the analytical figures of merit of a given method developed for a specific analyte. In addition, it cannot be affirmed that blank signal intensities are proportional to the metal ion content, as the paper characteristics also affect them. The presence of fillers (naturally or added), such as calcium carbonate, silicates, chalk, gypsum can affect the signal, as fillers can be preferentially ablated, carrying the metal ions sorbed on them, intensifying their emission signal.²⁵

Based on the different Cu and Mn emission intensities obtained for the substrates evaluated in Fig. 1, it was considered that this behaviour could be influenced by the grammage and thickness of the paper substrates. Araya-Hermosilla et al.³⁸ compared the Whatman qualitative filter paper (grammage of 140 g m⁻²) and the M&N filter paper (grammage of 61 g m⁻²) and incorporation onto the paper surface. Authors noted that M&N filter paper, with lower grammage and a more porous surface, presented greater sorption of metal ions. Table 2 lists the signal intensities and their respective standard deviations (SD) for the Cu I and Mn I lines obtained with all the paper substrates. Paper thickness increased after the addition of the analyte, as the drying process modifies its structure and properties, causing the cellulosic material to curl and increase in thickness.³⁹ No correlation between emission intensity and grammage or thickness was found for both metal ions, as can be seen in Fig. S2. Therefore, it seems that fillers and minor components of the paper substrate can be responsible for the differences in the emission intensities of the analytes, as most of the paper substrates studied are not expected to be of high purity. For example, office paper (labelled J) provides much more intense emission signals for Cu and Mn than the quantitative paper (labelled O), which is usually used as a substrate for analyte phase transfer.^32,40

Table 2 Effect of grammage and thickness of paper substrates on the emission signals for Cu I and Mn I

Substrate	Grammage (g m^−2)	Thickness without analytes (μm)	Thickness with analytes (μm)	Intensity Cu I ± SD (×10^3 a.u.)	Intensity Mn I ± SD (×10^3 a.u.)
A	48	54	65	144 ± 37	91 ± 35
B	100	93	95	135 ± 19	66 ± 16
C	130	114	126	16 ± 4	27 ± 5
D	60	51	59	20 ± 6	8 ± 2
E	100	83	94	224 ± 25	60 ± 7
F	80	81	111	132 ± 5	55 ± 7
G	90	113	114	56 ± 5	52 ± 3
H	90	72	86	14 ± 5	8 ± 2
I	180	282	292	16 ± 2	5 ± 1
J	75	91	145	272 ± 19	48 ± 5
K	180	238	268	273 ± 15	50 ± 7
L	90	113	126	169 ± 6	29 ± 6
M	50	68	71	55 ± 5	25 ± 3
N	160	255	270	57 ± 8	28 ± 3
O	95	171	213	27 ± 3	12 ± 3
P	80	125	162	36 ± 4	17 ± 2
Q	80	117	121	27 ± 3	12 ± 2

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In order to demonstrate the effect of minor components of the paper on the signal intensity, office paper was washed with deionised water and with HNO₃ solutions at concentrations of 1.0, 5.0 and 10% (v/v). Washings were carried out by submerging the paper in deionised water for 5 min or in HNO₃ solution for 5 and 15 min. In the latter case, paper was subsequently washed with deionised water to remove the acidic solution. After the washing procedure, as the paper substrate was barely humidified with water, only 20 min was enough to dry it at 60 °C before LIBS analysis. As can be seen in Fig. 2, after washing the substrate with deionised water, a small decrease in the analytical signal was observed after deposition of 5 mg L⁻¹ Cu(II) and Mn(II) on paper. However, a significant decrease in the emission signal was observed when the paper was previously washed with HNO₃ and the 1.0% solution was enough to clean up the paper in a time interval as short as 5 min. It is important to note that washing with 1.0% HNO₃ for 5 min is appropriate because long periods of exposure in an acidic environment can cause the rupture of the paper cellulose fibres, which would affect the homogeneous distribution of the analyte on the paper surface. The acidic hydrolysis of cellulose is responsible for the deterioration of the resistance properties of the paper, as the glycosidic bonds of cellulose are broken, initially causing a small reduction in the degree of polymerization, which gradually decreases, leading to coalescence of cellulose fibrils.⁴¹

Fig. 2 Emission intensity of (a) Cu I 324.74 nm and (b) Mn I 403.23 nm as a function of the type of washing of the office paper substrate: (A) without washing (control), (B) water for 5 min, (C) 1.0% HNO₃ for 5 min, (D) 1.0% HNO₃ for 15 min, (E) 5.0% HNO₃ for 5 min, (F) 5.0% HNO₃ for 15 min, (G) 10% HNO₃ for 5 min, and (H) 10% HNO₃ for 15 min.

It was initially considered that the signal decrease was due to the removal of fillers, such as calcium carbonate, by the acidic washing, as fillers are preferably ablated and metal ions would be adsorbed on them.²⁵ However, the C I (247.91 nm) emission signal was virtually the same after paper washing, indicating that plasma characteristics did not significantly change after filler removal. Therefore, the results shown in Fig. 2 suggest that metal ions present in the paper can contribute to improving the emission signals of Cu I and Mn I. Thus, the emission spectrum of the raw office paper was analysed, with the presence of Na, Ca and Fe, among other possible minor components, being identified. While Ca(II) ions are added in the production of paper (in the form of carbonate, as a filler),²⁵ the iron present in office paper can come from different sources, such as the raw material itself (wood) and/or the cellulosic pulp bleaching stage.^37,42 As an attempt to explain the intensification of Cu and Mn emission signals, a series of measurements were performed by adding Na(I), Ca(II) and Fe(II) ions to the paper substrate previously washed with nitric acid. In addition, the effects of Cr(III), Co(II) and Ni(I) were also investigated, as they are transition metals that also belong to the first series. The signal increase due to the presence of sodium and calcium was smaller than those provided by iron, and signal intensity similar to those provided by the raw paper (not washed) was obtained when all the ions were simultaneously added to the paper substrate, as can be seen in Fig. 3. It is important to mention that C I emission increased with the addition of these metal ions, signaling a more efficient ablation and/or a higher plasma temperature. For this study, office paper was washed with a 1.0% HNO₃ solution; metal ions were added at a concentration of 0.018 mol L⁻¹ to a 5.0 mg per L Cu(II) and Mn(II) solution and 300 μL of the final solution were deposited onto the paper, which was dried for 120 min before LIBS measurements.

Fig. 3 Emission intensity of (a) Cu I 324.74 nm and (b) Mn I 403.23 nm as a function of the following chemical treatments: (A) unwashed paper (control), (B) paper washed with HNO₃, followed by the addition of (C) Na, (D) Ca, (E) Cr, (F) Fe, (G) Co, (H) Ni and (I) Na, Ca, Cr, Fe, Co, and Ni (the blank solution was prepared by adding the referred ions, without the addition of Cu(II) and Mn(II) ions).

It is important to note that experiments were performed by using metal ion solutions of high concentration (0.018 mol L⁻¹), in order to make evident the signal enhancement. Fig. 4 shows the effect of adding increasing amounts of Fe(II) ions on Cu I and Mn I emission signals. It can be noted that a proportional increase occurs as the amount of Fe(II) added to the paper increases. However, the effect caused by the addition of Fe(II) ions is far from the signal increase provided by the office paper itself.

Fig. 4 Emission intensity of (a) Cu I 324.74 nm and (b) Mn I 403.23 nm as a function of iron concentration: (A) unwashed paper (control), (B) washed with HNO₃ for 5 min and H₂O for 5 min, followed by the addition of Fe(II) solution with concentration of (C) 0.0036 mol L⁻¹, (D) 0.0072 mol L⁻¹, (E) 0.0107 mol L⁻¹, (F) 0.0143 mol L⁻¹ and (G) 0.018 mol L⁻¹.

The chemical composition and the effect of electron density are not the only relevant factors to be considered in increasing the signal intensity. Thus, the morphology of the paper microfiber was verified by electron microscopy before and after washing the office paper with 1.0% HNO₃ for 5 minutes, as shown in Fig. 5. An analysis of the contact angle of water with the paper surface before and after acidic washing was also carried out, aimed at verifying the hydrophobicity changes that would occur with this procedure and its effect on the emission signal for Cu I and Mn I.

Fig. 5 Scanning electron microscopy of office paper: (a) unwashed and (b) washed with nitric acid solution. The water contact angle is shown at the upper-right corner of each micrograph.

It is possible to observe that unwashed office paper presents clusters of chemical elements between the microfibers which are removed by washing. It is also possible to observe the fibres of the washed paper widely dispersed in different orientations, demonstrating its highly porous and rough nature. The clusters of chemical elements observed in Fig. 5a are mainly made up of carbon, oxygen and calcium as can be seen in Fig. 6, which originates from the paper manufacturing process.

Fig. 6 Energy dispersive spectroscopy analysis of unwashed office paper with a compositional map of the main chemical elements.

The difference between the contact angle of washed and unwashed office paper shown in Fig. 5 is related to the water absorption capacity, which is influenced by the arrangement of cellulosic microfibers of the paper.⁴³ Unwashed office paper has a smooth, uniform and hydrophobic surface, which results in a greater contact angle with water, meaning liquid droplets remain more rounded and are not quickly absorbed by the paper. Paper washed with HNO₃ solution has a more porous and absorbent surface, forming a smaller contact angle, increasing its wettability. Thus, the hydrophobic surface of the office paper allowed a higher amount of sample on its surface, increasing the availability of metal ions for ablation, which may have resulted in an increase of the emission signal. The hydrophobicity of the paper can be related to the presence of lignin, which is a hydrophobic component, that is, the higher the lignin content in a paper, the greater its hydrophobicity and the greater the contact angle. The capillarity of liquids occurs through hydrogen bonds between water molecules and the β-D glucopyranoside hydroxyl groups of the paper.⁴⁴ Thus, the hydrophobicity of cellulose decreases in paper washed with nitric acid solution due to the loss of hydroxyl groups, resulting in an increase in capillarity, contact angle, and water absorption capacity. It should also be considered that during the office paper manufacturing process, additives and surface coatings may be used that confer a certain hydrophobicity, such as kaolin, waxes, hydrophobic polymers such as polyethylene or polypropylene and acrylic resins, used to increase the durability of the paper. Dong et al.⁴⁵ proposed a hydrophobic substrate for LIBS measurements of Cu in solution, using an ultra-ever dry hydrophobic coating on the surface of a glass slide in magnetic confinement, obtaining an increase in sensitivity and detection of Cu at ppt levels.

The results indicate that the intensity increase of the emission signal obtained with the office paper is a synergistic effect between its chemical composition and hydrophobicity, allowing higher availability of ions for ablation on the surface of the substrate, demonstrating that office paper has great potential as a LIBS signal enhancer. Considering the results previously presented, calibration curves were obtained with the paper substrates J (office), K (eggshell), and L (linen), whose performances were superior to those of the other substrates, and then compared with those obtained with the quantitative paper (paper “O” of Table 1), which is usually employed as a paper substrate. The results are depicted in Fig. 7 and 8 for Cu(II) and Mn(II) ions, respectively, demonstrating linear responses up to 5.0 mg L⁻¹, with different sensitivities for each paper substrate. It is also evident that the lowest sensitivity is provided by the quantitative paper, probably because it is the substrate with highest purity.

Fig. 7 (a) Calibration curves for Cu(II) ions using different solid substrates and (b) LIBS emission spectra for 5 mg per L Cu(II) solution using different solid substrates.

Fig. 8 (a) Calibration curves for Mn(II) ions using different solid substrates and (b) LIBS emission spectra for 5 mg per L Mn(II) solution using different solid substrates.

Table 3 lists the parameters obtained from calibration curves shown in Fig. 7 and 8. Limits of detection (LOD) and quantification (LOQ) were estimated as recommended by the International Union of Pure and Applied Chemistry (IUPAC). LOD was taken as three times the blank standard deviation (10 replicates) divided by the slope of the calibration curves, while LOQ was calculated by taking 10 times the blank standard deviation. It should be noted that commercial papers usually provide better LOD/LOQ than quantitative paper, being a simple alternative to improve sensitivity in LIBS determinations. In addition to these results, experiments were also performed with different brands of office paper found in the Brazilian market. As expected, different results were obtained as shown in Fig. S3 and Table S2 of the SI, indicating the fabrication process will determine the performance of the substrate.

Table 3 Analytical parameters obtained from calibration curves for Cu(II) and Mn(II) shown in Fig. 7 and 8 (LOD and LOQ expressed in mg L⁻¹)

Substrate	Copper					Manganese
	Slope	Intercept	R ^2	LOD	LOQ	Slope	Intercept	R ^2	LOD	LOQ
Office	18 285	2248	0.998	0.3	1.0	2815	9801	0.987	1.1	3.6
Eggshell	17 183	−11 591	0.969	0.1	0.4	2598	8	0.986	1.0	3.3
Linen	8778	−6282	0.907	0.4	1.2	4426	7589	0.999	0.5	1.6
Quantitative	1345	469	0.977	1.0	3.3	1049	−762	0.914	1.3	4.2

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The best results for Cu(II) and Mn(II) obtained with eggshell and office papers, respectively, were compared with those already described in the literature, as shown in Table 4, demonstrating it is possible to obtain valuable results by employing ordinary papers commercialized in the market.

Table 4 Comparison of the LOD (mg L⁻¹) of other methods using LIBS for Cu(II) and Mn(II) detection

Element	Substrate or method used	LOD	Ref.
Cu(II)	Ice	2.3	7
	Ring-oven/filter paper	0.3	20
	Magnesium alloy	0.25	46
	AAOPM membrane filter	0.18	47
	Paraffin wax/glass plate	0.12	48
	Matte photographic paper	0.08	49
	Eggshell paper	0.1	This work
	Office paper	0.3	This work
Mn(II)	Aluminum platform	6.0	50
	Liquid jet	0.7	51
	Electroosmotic flow-driven platform	0.079	52
	Eggshell paper	1.0	This work
	Office paper	1.1	This work

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According to the guidelines for the quality of drinking water of the World Health Organization, the maximum limits are 2.0 mg L⁻¹ for Cu(II) and 0.08 mg L⁻¹ for Mn(II).⁵³ According to European Union directive 2020/2184, regarding the quality of water intended for human consumption, Cu must have a maximum concentration of 2.0 mg L⁻¹ while for Mn it is up to 0.05 mg L⁻¹.⁵⁴ It is worth noting that, in Brazil, the Ministry of Health in its Ordinance GM/MS no. 888, of May 4, 2021, established maximum limits of 2.0 mg L⁻¹ and 0.1 mg L⁻¹ for Cu and Mn, respectively, for the water quality for human consumption. Therefore, the performance of the paper substrates tested in the present work has the potential to satisfy the requirements of different agencies of quality water control for Cu(II) ions. However, the method for Mn presented a LOD value (1.0 mg L⁻¹) higher than the maximum limit permitted by the aforementioned legislation.

4 Conclusion

The present work evaluated the effect of different types of paper on the emission signals of Cu and Mn in LIBS measurements. It was observed that the physical characteristics of the paper, such as grammage and thickness, were not related to the emission signal variation. The sensitivity of the measurements (taken as the slope of the calibration curve) provided by using office paper as a substrate was around 14 and 2.7 times higher than those obtained with quantitative filter paper for Cu and Mn, respectively. Detection limits as low as 0.1 and 0.5 mg L⁻¹ were respectively obtained for Cu(II) and Mn(II), by using eggshell and linen paper. The improvement in analytical performance of ordinary papers can be ascribed to the presence of mineral fillers, which are soft and easy to ablate and/or the presence of minor components, such as metal ions present in fillers or originating from the paper production process. Furthermore, the hydrophobic characteristic of the paper can also allow greater availability of ions on the surface of the paper substrate for the ablation process. Therefore, the use of ordinary papers can be a feasible alternative to determine metal ions in water with improved sensitivity, providing a straightforward LIBS analytical method. As a corollary, it is important to keep in mind that comparison among results described in the literature must be performed with caution because the filler amount and minor components can be different across different paper manufacturers.

Author contributions

Adriana L. Sales: conceptualization, formal-analysis, validation, investigation, methodology, writing-original-draft, writing-review-editing. Nilvan A. Silva: methodology, writing-original-draft, writing-review-editing. Ivo M. Raimundo Jr: conceptualization, writing-original-draft, writing-review-editing, supervision, project-administration.

Conflicts of interest

There are no conflicts do declare.

Data availability

Data for this article are available at Repositório de Dados de Pesquisa da Unicamp (REDU) at https://doi.org/10.25824/redu/JWSU29.

The supplementary information file contains tables and figures that complement the main text. See DOI: https://doi.org/10.1039/d5ja00194c.

Acknowledgements

Authors acknowledge INCTAA (CNPq 465768/2014-8 and FAPESP 2014/50951-4) and the National Institute of Science and Technology of Nanomaterials for Life (INCTNanovida, CNPq 406079/2022-6) for financial support. IMRJ is thankful to CNPq for the fellowship (310915/2023-6).

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