Qingxia YAN, Ye TIAN, Ying LI, Hong LIN, Ziwen JIA, Yuan LU, Jin YU, Chen SUN, Xueshi BAI, Vincent DETALLE. Detection and quantification of Pb and Cr in oysters using laser-induced breakdown spectroscopy[J]. Plasma Science and Technology, 2023, 25(4): 045509. DOI: 10.1088/2058-6272/aca504
Citation:
Qingxia YAN, Ye TIAN, Ying LI, Hong LIN, Ziwen JIA, Yuan LU, Jin YU, Chen SUN, Xueshi BAI, Vincent DETALLE. Detection and quantification of Pb and Cr in oysters using laser-induced breakdown spectroscopy[J]. Plasma Science and Technology, 2023, 25(4): 045509. DOI: 10.1088/2058-6272/aca504
Qingxia YAN, Ye TIAN, Ying LI, Hong LIN, Ziwen JIA, Yuan LU, Jin YU, Chen SUN, Xueshi BAI, Vincent DETALLE. Detection and quantification of Pb and Cr in oysters using laser-induced breakdown spectroscopy[J]. Plasma Science and Technology, 2023, 25(4): 045509. DOI: 10.1088/2058-6272/aca504
Citation:
Qingxia YAN, Ye TIAN, Ying LI, Hong LIN, Ziwen JIA, Yuan LU, Jin YU, Chen SUN, Xueshi BAI, Vincent DETALLE. Detection and quantification of Pb and Cr in oysters using laser-induced breakdown spectroscopy[J]. Plasma Science and Technology, 2023, 25(4): 045509. DOI: 10.1088/2058-6272/aca504
The quantitative determination of heavy metals in aquatic products is of great importance for food security issues. Laser-induced breakdown spectroscopy (LIBS) has been used in a variety of foodstuff analysis, but is still limited by its low sensitivity when targeting trace heavy metals. In this work, we compare three sample enrichment methods, namely drying, carbonization, and ashing, for increasing detection sensitivity by LIBS analysis for Pb and Cr in oyster samples. The results demonstrate that carbonization can remove a significant amount of the contributions of organic elements C, H, N and O; meanwhile, the signals of the metallic elements such as Cu, Pb, Sr, Ca, Cr and Mg are enhanced by 3–6 times after carbonization, and further enhanced by 5–9 times after ashing. Such enhancement is not only due to the more concentrated metallic elements in the sample compared to the dried ones, but also the unifying of the matter in carbonized and ashed samples from which higher plasma temperature and electron density are observed. This condition favors the detection of trace elements. According to the calibration curves with univariate and multivariate analysis, the ashing method is considered to be the best choice. The limits of detection of the ashing method are 0.52 mg kg−1 for Pb and 0.08 mg kg−1 for Cr, which can detect the presence of heavy metals in the oysters exceeding the maximum limits of Pb and Cr required by the Chinese national standard. This method provides a promising application for the heavy metal contamination monitoring in the aquatic product industry.
Heavy metals are accumulative pollutants that are widely present in the ecological environment and are transferred and enriched in the food chain step by step. Aquatic products have a strong enrichment capacity for heavy metals, especially shellfish organisms. With rapid industrialization and urbanization in China, heavy metal pollution in aquatic products has become a serious issue [1]. According to the Chinese national standard GB2762-2017, the maximum limits of Pb and Cr in shellfish samples are 1.5 mg kg−1 and 2.0 mg kg−1, respectively. The accumulation of heavy metals in vital organs in the human body can result in numerous serious health effects such as neurotoxic and carcinogenic effects [2]. Excessive intake of Pb and Cr can lead to pathological changes in organs and the central nervous system, as well as various adverse lung diseases [3]. Conventional methods for heavy metal detection in aquatic products include atomic absorption spectrometry [4], atomic fluorescence spectrometry [5], and inductively coupled plasma optical emission spectrometry (ICP-OES) or mass spectrometry (ICP-MS) [6, 7]. Although these methods are effective, they are in general laborious and time-consuming, associated with expensive equipment or a certain amount of expertise. Due to complex organic components of seafood samples, substantial quantities of chemical reagents or solvents are usually required for pretreatment. Therefore, a rapid and environmentally friendly method needs to be developed for the identification of the heavy metal content in oysters and other related aquatic products [8].
Laser-induced breakdown spectroscopy (LIBS) is an optical emission spectroscopy technique that utilizes a high-energy laser pulse to generate a transient plasma on the analyzed sample surface, and the laser-induced plasma is used as the spectroscopic emission source for elemental analysis [9]. With the advantages of multi-elemental and rapid quantitative analytical capabilities, LIBS has been shown to be an attractive technique for food analysis [10–12], and has been applied in various food matrixes such as milk [13], flour [14], wines [15], and tea samples [16]. In particular, several studies have reported elemental detection and quantification with LIBS for seafood or meat analysis, for example, the quantification of P in seafood [17], the quantification of heavy metals in Sargassum fusiforme [18] and Tegillarca granosa [19], the determination of Ca, Mg, K and Na in bovine and chicken meat [20], and the determination of Cr in pork [21] and Cu in beef [22].
Although LIBS has demonstrated its versatility in many food-related fields, a big challenge affecting analytical performance is the relatively low detection sensitivity for heavy metal detection and quantification of food samples [11]. The level of heavy metals allowed in food samples is usually sub-ppm or even ppb. The limit of detection (LOD) of conventional LIBS can barely reach this level; therefore, signal enhancement methods are required in this application. To date, the most frequent sample preparation method of food samples for LIBS analysis consists of drying and pressing the powder samples into pellets [16–22]. However, they cannot sufficiently improve the detection sensitivity of heavy metals. Yang et al proposed a solid–liquid–solid transformation (SLST) sample preparation method for the determination of toxic elements in rice, and the LODs of Cd and Pb were 2.8 and 43.7 μg kg−1, respectively [23]. This method gives high sensitivity but relies on the addition of hydrochloric acid solution together with an ultrasound-assisted extraction approach, and a glass slide is required as the solid substrate, which may cause interference of the spectral lines. Other methods using dual-pulse LIBS (DP-LIBS) or microwave-assisted LIBS (MA-LIBS) can also effectively improve the sensitivity of LIBS for food analysis [24, 25], but complex experimental setups and higher costs are required.
In this work, we aim to develop a highly sensitive sample preparation method for the determination of heavy metals in seafood with the standard LIBS setup. Oysters are chosen to study in this work because they are the most dominant mollusk species among shellfish organisms [26], and are subject to a non-selective diet (all parts except the shell are consumed) compared to other aquatic organisms. Fresh oysters are treated by three types of sample enrichment methods, namely drying, carbonization, and ashing. Water content accounting for ~80% of the total weight of fresh oysters can be removed after the drying process. The organic matter can be further destroyed by heating the sample to an elevated temperature during the carbonization and ashing processes, and the associated metallic elements are generally transformed to carbonate or oxide forms [27]. The dry ashing method offers the advantages of requiring few or no reagents, utilizing a simple apparatus, lending itself to batch sample preparation and requiring limited operator attention, and has been commonly used for the mineralization of organic materials, biological tissues, foodstuffs, etc [28]. Moreover, pellet samples can be easily obtained after the drying, carbonization, and ashing processes, and are suitable for LIBS measurements due to their flat and homogeneous surfaces. A series of oyster samples with different concentrations of Pb and Cr are prepared, and the signal enhancements of Pb and Cr as well as the quantification ability are compared between the sample enrichment methods of drying, carbonization, and ashing.
2.
Materials and methods
2.1
Sample preparation
The oyster samples used in this work were purchased from a local supermarket in Qingdao, China. The fresh oyster samples were washed with distilled water and shelled before being stirred in a meat mixer to form the homogenate. The meat mixer was cleaned with antibacterial detergent and distilled water carefully and dried before use. Each sample was immersed in standard solutions containing different concentrations of Pb and Cr, shaken for 5 min and placed in a glassware. A total of ten standard solutions were prepared by weighing different masses of lead acetate and chromium chloride and dissolving them into deionized water. The concentrations of Pb in the prepared solutions were 0, 50, 100, 200, 300, 400, 500, 600, 700, and 800 mg l−1, while the concentrations of Cr were 0, 20, 50, 100, 150, 200, 250, 300, 400, and 500 mg l−1. The mass ratio between the fresh oyster sample and the water solution was 10:1. After that, the samples were dried in an electric blast dryer at 65 ℃ until the sample weight was constant. The Pb and Cr concentrations in the dried samples were previously determined by the standard ICP-MS method. Then, the Pb and Cr concentrations in the fresh samples were obtained by inversion of the wet-to-dry ratio of the measured concentrations of the dried samples, as shown in table 1, where sample No. 1 was a fresh oyster without contamination.
Table
1.
Reference concentrations of Pb and Cr in the fresh oyster samples (mg kg−1).
After drying, the samples were further enriched by carbonization and ashing. The dried sample was weighed and placed in a resistance furnace with sufficient oxygen at 450 ℃ for 45 min to obtain a carbonized sample. The carbonized sample was then weighed and placed in a muffle furnace with insufficient oxygen at 600 ℃ for 45 min to obtain an ashed sample. The temperatures used for carbonization and ashing were carefully chosen to avoid the volatilization of metallic elements in the samples during the pyrolysis process [29]. To form laser-ablated samples with good hardness, the dried, carbonized, and ashed samples were ground into powders, and the microcrystalline cellulose [(C6H10O5)n] was mixed as a binder at a mass ratio of 3:2 (oyster powder : microcrystalline cellulose) to form the pellet samples. The mixture powder was put into a shaker for vigorous shaking for 5 min to ensure the homogeneity of the mixtures. The pellet samples were finally obtained by pressing 0.35 g of the mixture powder under a pressure of 30 MPa for 2 min. The resultant pellets had a diameter of 13 mm (shown in figure 1(b)), and these pellets were prepared with different Pb and Cr concentrations for the following LIBS measurements.
Figure
1.
(a) Schematic of the experimental setup, and (b) photographs of the pellet samples with different enrichment methods of drying, carbonization, and ashing. Each crater on the sample surface resulted from ten consequent laser pulses.
A schematic of the experimental setup used in this work is shown in figure 1(a). A Nd: YAG pulsed laser (Beamtech Optronics, Dawa-200) with a wavelength of 1064 nm, a pulse width of 8 ns (full width at half maximum), and a repetition rate of 10 Hz was used as the laser ablation source. The laser energy delivered to the sample was 80 mJ/pulse. The laser beam was expanded by a combination of a concave lens and convex lens, and then focused on the sample by a plano-convex lens (f=100 mm). The laser focus plate was 1 mm below the sample surface and the laser spot on the surface was about 200 μm in diameter, corresponding to a laser fluence of 250 J cm−2. The plasma emission was coupled to a spectrometer (AvaSpec-ULS2048-USB2) with a spectral resolution of 0.1 nm and a wavelength range of 200–800 nm through an optical fiber probe. The spectrometer was triggered by a delay generator (Stanford Research Systems, DG 645) that was synchronized with the laser pulse signal by a photodiode (PIN). The optimized spectral detection delay was set as 1.5 μs and the integration time was 1.05 ms, which are the same as the values we used in our previous work [17]. During the measurements, the sample was translated using a motorized X–Y stage to provide a fresh surface to each laser burst, and the distance between the sample surface and the focusing lens was kept constant by using a monitoring complementary metal–oxide–semiconductor camera together with a laser probe. For each sample, 10 replicate spectra were acquired, and each spectrum was an accumulation of 200 laser shots distributed in 20 craters with each of them ablated by 10 consequent laser pulses. Photographs of the laser-ablated pellets with different enrichment methods of drying, carbonization, and ashing are shown in figure 1(b).
3.
Results and discussion
3.1
Spectral features with different sample enrichment methods
Typical LIBS spectra obtained from an oyster sample prepared with different enrichment methods of drying, carbonization, and ashing are first shown in figure 2. The intensities of C, H, N, and O as the elements from organic materials (shown in figures 2(a) and (b)) all became clearly lower after the carbonization than after drying. This is consistent with the fact that the organic elements can be highly decomposed during the carbonization process, leading to a significant decrease in the spectral line intensities. For the ashing process, it makes the decomposition of organic elements more complete; however, in figures 2(a) and (b), the intensities of C, H, N, and O all become higher after ashing. For C, N and O in particular, their intensities are quite similar between drying and ashing. This can be attributed to the altered matrix after the ashing process, which promotes laser ablation and corresponds to an intense plasma and stronger LIBS signals. Such an improvement caused by the matrix effect is further shown in the following section by calculating the plasma temperature and electron density. Note that the observed N and O emission lines could also be contributed by the ambient air since the experiment was conducted in an atmospheric environment.
Figure
2.
Typical LIBS spectra containing the organic elements of C, H (a), and N, O (b), and the metallic elements of Cu, Pb, Sr (c), and Ca, Cr, Mg (d) in oyster samples prepared with different enrichment methods.
For the inorganic elements such as Cu, Pb, Sr, Ca, Cr, and Mg shown in figures 2(c) and (d), we can see that all these spectral intensities were clearly improved after carbonization, and were further improved after ashing. This indicates that on the basis of enrichment by water evaporation with the drying process, carbonization and ashing can further effectively enrich the metallic elements by destroying the organic matter. This not only enhances the LIBS signals of metallic elements including the heavy metals of Pb and Cr, but also reduces the spectral interference from the organic matter and moisture content in the oyster samples.
The spectral intensities of the organic elements and metallic elements were extracted from the spectra via different enrichment methods, and the signal enhancement factors of carbonization (ICarbonization/IDrying) and ashing (IAshing/IDrying) were calculated and are shown in figure 3. We can see that the enhancement factors of carbonization and ashing are all around one for the organic elements of C, H, N, O, indicating that the LIBS signals of organic elements were lower or at the same level after the carbonization and ashing processes. For the metallic elements of Cu, Pb, Sr, Ca, Cr, and Mg, the LIBS signals were enhanced by 3–6 times after carbonization, and further by 5–9 times after ashing. For the two heavy metals of Pb and Cr analyzed in this work, the enhancement factors of carbonization are 5.4 and 4.4, respectively, and the enhancement factors of ashing are 8.9 and 7.3, respectively. This means that the LIBS signals of heavy metals in oysters can be significantly enhanced using the carbonization and ashing methods compared with the dried samples. In addition, we also measured the real concentrations of Pb and Cr in one of the dried, carbonized, and ashed samples by ICP-MS. The real concentrations of Pb and Cr were 139 and 130 mg kg−1 in the dried sample, 284 and 261 mg kg−1 in the carbonized sample, and 546 and 552 mg kg−1 in the ashed sample, respectively. We found that the concentrations of Pb and Cr were increased by ~2 times after carbonization, and further increased by ~4 times after ashing. Therefore, the LIBS signal enhancement factors of carbonization and ashing are clearly higher than the concentration-increasing ratios, suggesting that the enhancement is contributed by not only the enriched concentrations in the sample but also the improved sample matrix that benefits the LIBS analysis compared to the dried samples.
Figure
3.
Signal enhancement factors of the organic elements and metallic elements after carbonization and ashing.
We also calculated the relative standard deviations (RSDs) of the organic elements and metallic elements to show the signal stability with the different enrichment methods (figure 4). For each sample, its RSD (standard deviation of the line intensities divided by their mean value) was first calculated based on the ten replicate spectra of each sample. Then the RSD value shown in figure 4 is an average from the ten samples shown in table 1. We can see that for the organic elements, the RSDs are generally higher after carbonization compared with drying, due to the reduced signal intensities caused by the decomposition of organic elements in the carbonized samples. For the metallic elements, the RSDs are generally lower after carbonization. We emphasize that for both the organic elements and metallic elements, the ashing method provides the lowest RSDs compared with the drying and carbonization methods. This means that the stability of LIBS signals can also be clearly improved by the ashing method. Visually checking the ablated sample surface shown in figure 1(b), the carbonized sample has a larger laser ablation crater with irregular shapes, corresponding to a stronger sample sputtering effect during the laser ablation process. For the ashed sample, the laser ablation craters are much smaller with smoother edges, indicating a higher laser ablation efficiency of the ashed sample. Therefore, the altered sample matrix with the ashing method can improve the quality of LIBS signals both in terms of signal intensity and stability.
Figure
4.
Relative standard deviations (RSDs) of the organic elements and metallic elements with different enrichment methods.
3.2
Plasma properties with different sample enrichment methods
Based on the spectral results above, we further calculated the plasma property parameters including the electron density and plasma temperature to verify the enhancement mechanism between different sample enrichment methods. The electron density was estimated from the Stark-broadened profile of the Ca I 422.7 nm line with Lorentzian fit, and a broadening coefficient value of 6.30×10−4 nm for the Ca I 422.7 nm line was used [30]. The Stark broadening was corrected by subtracting the instrumental broadening measured by a standard low-pressure Hg lamp. The plasma temperature was calculated using the Saha–Boltzmann plot method [31], based on the line intensities of Ca including the Ca I lines at 422.6, 430.3, 430.8, 431.9, 445.4, 643.9, 646.3 nm, and the Ca II lines at 315.9, 317.9, 373.7, 393.4, 396.9 nm. The needed spectroscopic data of the Ca lines were taken from the NIST database. The obtained plasma temperature and electron density with different sample enrichment methods are shown in table 2. We can see that, compared with drying, both the plasma temperature and electron density were clearly increased after carbonization, with a plasma temperature value of 7700 K versus 6900 K, and an electron density value of 1.3×1017 cm−3 versus 0.4×1017 cm−3. After ashing, the plasma temperature and electron density were further increased, with values of 8000 K and 1.6×1017 cm−3 respectively. The variations of plasma parameters are caused by the matrix effect of the dried, carbonized and ashed samples, and the improvements in the plasma temperature and electron density correspond to stronger LIBS signals. This proves again that the signal enhancement observed above is contributed by not only the enriched concentrations but also the improved matrix effect after carbonization and ashing.
Table
2.
Plasma temperature and electron density with different enrichment methods.
Quantification of Pb and Cr was performed in this section with univariate analysis to assess the detection sensitivity of the enrichment methods for heavy metal analysis in oysters. Calibration curves were built with the Pb I 405.8 nm line and the Cr I 427.5 nm line, and the Ca I 422.7 nm line was used as the internal reference for spectral normalization. The obtained calibration curves of Pb and Cr are shown in figures 5(a) and (b), where the data points correspond to the average line intensities after normalization and the error bars correspond to the standard deviations deduced from the ten replicate spectra. A linear regression was used and the correlation coefficient R2 expresses the degree of correlation of the experimental data to the regression curve. The theoretical LODs were also calculated and are shown in the figures according to the definition LOD=3σa/b, where σa is the standard deviation of the spectrum background and b is the slope of the regression curve.
Figure
5.
Calibration curves of Pb I 405.8 nm line (a) and Cr I 427.5 nm line (b) obtained with different enrichment methods. The R2 and LOD determined from each curve are indicated in the figures.
From figure 5, we can see that there is a relatively good linear relationship between the LIBS intensities and the reference concentrations of Pb and Cr, with the R2 values all above 0.95. For the calibration curve of Pb shown in figure 5(a), an R2 value of 0.9783 was obtained using the ashing method, which is higher than those from the drying method (0.9657) and carbonization method (0.9537). This is consistent with the RSD results shown in figure 4 that the ashed samples have a higher stability of LIBS signal due to the altered sample matrix, and therefore correspond to a better correlation of the calibration curve. The LOD is the key factor to evaluate the detection sensitivity of the heavy metals in this work. We can see that the LOD of Pb using the drying method is 3.37 mg kg−1, while it is significantly reduced to 0.80 mg kg−1 using the carbonization method and is further reduced to 0.52 mg kg−1 using the ashing method. This means the detection sensitivity of Pb can be greatly improved by using the carbonization and ashing methods compared with drying, which is also consistent with the signal enhancement results shown in figures 2 and 3.
Similar results were obtained for the calibration curve of Cr shown in figure 5(b). The R2 value of the ashing method is 0.9912, which is higher than those of the drying method (0.9814) and carbonization method (0.9534). The LOD of Cr with the drying method is 0.47 mg kg−1, and it is significantly reduced to 0.11 mg kg−1 using the carbonization method and is further reduced to 0.08 mg kg−1 using the ashing method. In addition to the calculated LOD values, we also give the LIBS spectra obtained from the samples with low concentrations of Pb and Cr, to verify the detection ability of trace heavy metals of the different enrichment methods. Figure 6(a) shows the spectra of sample No. 1 which contains 0.57 mg kg−1 Pb and 0.24 mg kg−1 Cr, and figure 6(b) shows the spectra of sample No. 2 which contains 5.94 mg kg−1 Pb and 1.80 mg kg−1 Cr. We can see in figure 6(a) that only the ashing method has observable Pb and Cr lines, while in figure 6(b), both the carbonization and ashing methods have clear Pb and Cr lines. The observed spectra with low concentrations of Pb and Cr are in good agreement with the LOD values calculated based on the calibration curves shown in figure 5. We conclude that compared with the drying method, the detection sensitivity is enhanced by more than 4 times that with the carbonization method, and more than 6 times that with the ashing method. According to the Chinese national standard GB2762-2017, the maximum limits of Pb and Cr in oyster samples are 1.5 mg kg−1 and 2.0 mg kg−1, respectively. Therefore, both the carbonization and ashing methods meet the detection limits of Pb and Cr required by the Chinese national standard.
Figure
6.
LIBS spectra of Pb I 405.8 nm and Cr I 425.4, 427.5 nm obtained with different enrichment methods. Sample No. 1 contains 0.57 mg kg−1 Pb and 0.24 mg kg−1 Cr (a), while sample No. 2 contains 5.94 mg kg−1 Pb and 1.80 mg kg−1 Cr (b).
Multivariate analysis with the partial least squares (PLS) method was applied for the quantification of Pb and Cr in this section to further assess the analytical performances including detection accuracy and precision with the different enrichment methods. The main idea of PLS is to project the raw input variables into new latent variables with the maximal variations, which has been widely used in LIBS-based qualitative and quantitative studies [32–36]. For each enrichment method, three samples (Nos. 4, 6, 9 shown in table 1) were selected as the validation set, while six samples (Nos. 2, 3, 5, 7, 8, 10 shown in table 1) were used as the calibration set to construct the PLS model. Sample No. 1 was not used in the construction of the model because its contents of Pb and Cr were so low that the emission lines were not observable for the drying method (figure 6(a)). For each sample, ten replicate spectra were used to draw the error bars of the model-predicted concentrations. To reduce the complexity of the data, two wavelength coverages from 366.4 to 370.1 nm (containing a Pb I 368.4 nm line) and 404.8 to 406.1 nm (containing a Pb I 405.8 nm line) were selected as the input variables for Pb, and two wavelength coverages from 423.6 to 428.0 nm (containing Cr I 425.4 and 427.5 nm lines) and 519.4 to 522.3 nm (containing Cr I 520.6 and 520.8 nm lines) were selected as the input variables for Cr. The optimal number of latent variables of PLS was determined based on the root mean square error of cross-validation (RMSECV) calculated by the leave-one-out cross-validation method. To evaluate the quantitative performances of the established model, the parameters of figures-of-merit were extracted from the PLS predicted results including the correlation coefficient (R2), root mean square error of calibration (RMSEC), root mean square error of prediction (RMSEP), and RSD. The PLS model-predicted concentration as a function of reference concentration and the corresponding fitting curves of Pb and Cr with different enrichment methods are shown in figure 7. The validation samples are represented in the figures with colored dots that are not involved in the construction of the model.
Figure
7.
PLS model-predicted concentration as a function of reference concentration with the drying method for Pb (a) and Cr (b), with the carbonization method for Pb (c) and Cr (d), and with the ashing method for Pb (e) and Cr (f).
Table 3 sums up the figures-of-merit of the quantification performances of Pb and Cr with the different enrichment methods. We can see that compared with the reference concentration values provided by the standard ICP-MS technique, the RMSEP values of Pb and Cr are 5.86 and 1.29 mg kg−1 for the drying method, 4.70 and 4.60 mg kg−1 for the carbonization method, and 3.49 and 1.28 mg kg−1 for the ashing method, respectively. This indicates a good prediction accuracy of LIBS with the enrichment methods. Comparing to the drying method, the carbonization method has larger RMSEC and RMSEP values for Cr, and the RSD value also becomes higher for the element of Pb. This indicates a decrease in the accuracy and precision degrees after carbonization, which may be caused by the stronger sample sputtering effect during the laser ablation of the carbonized sample. As shown in figure 1(b), the laser ablation craters of the carbonized sample are much larger and with irregular shapes. This corresponds to the stronger sputtering effect and is due to the fact that the hardness of the carbonized sample surface is lower when pressing the carbonized powders into pellets, compared with the dried and ashed samples. Another reason for the decreased accuracy and precision of carbonization may be due to the incomplete combustion of the carbonized sample during the carbonization process, leading to a less homogeneous carbonized pellet compared with the dried and ashed ones. For the ashing method, it gives the lowest RMSEC, RMSEP, and RSD values both for the elements of Pb and Cr, indicating that the quantification ability is higher than those for the drying and carbonization methods. Therefore, considering the detection sensitivity as well as the accuracy and precision, the ashing method is the best choice for heavy metal detection and quantification in oyster samples. We remark that this method relies on a sample preparation process with tolerable complexity, and does not need complex experimental setups and high costs to achieve a significant signal enhancement. It can also be extended to the highly sensitive LIBS analysis of heavy metals for a variety of organic materials such as the biological tissues, agricultural products, and organic products used in culture heritage [37, 38].
Table
3.
Comparisons of figures-of-merit between different enrichment methods.
In this work, we applied LIBS for heavy metal detection and quantification in oysters with three sample enrichment methods, namely drying, carbonization, and ashing. The water content in fresh oysters can be removed after drying, while the organic matter can be further destroyed after carbonization and ashing. Oyster samples with different concentrations of Pb and Cr were prepared into pellets, and the signal enhancements as well as the quantification ability were compared between the different enrichment methods. It was shown that for the organic elements of C, H, N, and O, the spectral intensities are clearly lower after carbonization. For the metallic elements such as Cu, Pb, Sr, Ca, Cr, and Mg, the spectral intensities are greatly enhanced by 3–6 times after carbonization, and further enhanced by 5–9 times after ashing. Such enhancement is contributed by not only the enriched concentrations of metallic elements in the sample, but also the improved matrix effect of the carbonized and ashed samples where higher plasma temperature and electron density were observed. From the calibration curves of univariate analysis, the LODs of the drying method are 3.37 and 0.47 mg kg−1 for Pb and Cr, while they are significantly reduced to 0.80 and 0.11 mg kg−1 using the carbonization method, and further reduced to 0.52 and 0.08 mg kg−1 using the ashing method, which are sufficiently lower than the maximum limits of Pb and Cr required by the Chinese national standard. From the calibration curves of multivariate analysis with PLS, the ashing method is considered to be the best choice compared with the drying and carbonization methods, due to its high detection sensitivity as well as good accuracy and precision in quantitative analysis. This work could be potentially applied to heavy metal contamination monitoring in the aquatic product industry.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (No. 2019YFD0901701), National Natural Science Foundation of China (Nos. 12174359 and 61975190), and Provincial Key Research and Development Program of Shandong, China (No. 2019GHZ010).
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