Article pubs.acs.org/ac
Microtrace Analysis of Rare Earth Element Residues in Femtogram Quantities by Laser Desorption and Laser Postionization Mass Spectrometry Zhibin Yin,† Zhouyi Xu,† Rong Liu,† Wei Hang,*,†,‡ and Benli Huang† †
Department of Chemistry and the MOE Key Lab of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, and ‡State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, Fujian 361005, China S Supporting Information *
ABSTRACT: A newly developed laser desorption and laser postionization time-of-flight mass spectrometer (LD-LPITOFMS) for the direct microtrace determination of rare earth elements (REEs) in residues has been presented. Benefiting from spatially and temporally separated processes between desorption and ionization, LD-LPI-TOFMS plays a dual role in alleviating the barriers of deteriorating spectral resolution at high irradiance, serious matrix effects and elemental fractionation effects at low irradiance. Compared with the conventional laser desorption/ionization (LDI) method, this technique offers unambiguous full-elemental determination of 15 REEs with more uniform relative sensitivity coefficients (RSCs) ranging from 0.5 to 2.5 for all REEs investigated, satisfying the semiquantitative analysis criteria. More importantly, a highly sensitive analysis of REEs with very little consumption was achieved by getting the utmost out of desorbed neutral atoms instead of increasing the amount of the sample, resulting in outstanding relative and absolute limits of detection (LODs and ALODs) of ∼ng/mL and ∼femtogram. The results presented here indicate that LD-LPI-TOFMS offers great potential in microtrace determination for elements in solution samples with minor sample preparation.
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judicious choice of methods for REEs determination, ICPMS is renowned for its extremely low LOD, multielemental detection, and wide linear dynamic range.7−9 In some cases, special consideration should be given in the case of severe matrix effects and extremely low concentration, for example, reaction/ collision cells, preconcentration, and matrix separation.10−12 As useful methods complementary to the digestion-based methods, the proliferating use of lasers in optical and mass spectrometry has given rise to a vast number of techniques for direct solid analysis. Some extensively used methods involve laser-induced breakdown spectrometry (LIBS),13−15 laser ablation inductively coupled plasma mass spectrometry (LAICPMS),16−19 and laser ablation/ionization mass spectrometry (LAI-MS).20−22 Compared to conventional analysis of solution, these techniques offer high throughput and fast screening without complicated matrix separation for the determination of trace and ultratrace elements in liquid samples by dried droplets, drastically reducing the amount of the sample required to perform the measurements. Evaporating a microdroplet solution to form the residue has proven to be a practical
s valuable indicators in ocean, earth, and environment science, rare earth elements (REEs) have garnered significant attention in marine geochemistry, climate change, petrogenetic evolution, and anthropogenic pollution.1−6 To fully exploit these applications, a comprehensive understanding of the distribution of REEs in seawater and marine deposits is imperative. The acronym “REEs” mostly signifies a synonym for the lanthanide series elements, consisting of stable elements from lanthanum (La, Z = 57) to lutetium (Lu, Z = 71). Yttrium (Y, Z = 39), a Group IIIB transition metal rather than a lanthanide, is also considered a part of the lanthanide series because of its similar chemical properties and coexistence with them in the lanthanides. Additionally, the considerably low concentration (below μg/mL, ppm) of REEs and the complicated matrix resulting from high salt content in the natural environment has a negative effect upon their accurate determination. Faced with these problems, several solventbased analytical techniques have been widely used for the determination of REEs. Among these solvent-based techniques, inductively coupled plasma coupled to either optical emission spectrometry (ICPOES) or mass spectrometry (ICPMS) has moved to the forefront. Severe spectral interference in the mixture of REEs and relatively high limit of detection (LOD) has hindered the further applications of ICPOES. As the most © XXXX American Chemical Society
Received: March 21, 2017 Accepted: June 14, 2017 Published: June 14, 2017 A
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EXPERIMENTAL SECTION Laser Desorption and Laser Postionization Source. A reflectron time-of-flight mass spectrometer was built in-house for both ionization sources, LDI-TOFMS and LD-LPITOFMS. A schematic diagram of the apparatus is elaborated in Figure 1. Briefly, the ionization source consists of two
and easy strategy if sample amounts are limited, especially for invaluable samples with minimal amounts or samples with a complicated matrix.23 In an effort to quantify trace elements with very low sample volumes, an appropriate strategy for achieving low LODs concomitant with less sample consumption is highly preferred.19,24,25 Absolute limit of detection (ALOD), commonly referring to minimal consumption of a sample amount for a detectable signal, has been one of the most significant figures of merit for the evaluation of given analytical techniques. Equally important as relative limit of detection (defined as the lowest concentration of analyte required for detection), ALOD has been highlighted in diverse laser-based methods.19,21,26 The integrated features between LA and ICPMS have made LAICPMS the most attractive technique for dried residue analysis with low LOD down to ∼ng/mL (ppb) yet relatively poor ALOD of ∼pg due to aerosol loss during atmosphere transportation.19,24 As a well-established solid analytical method, the analogous ALOD down to ∼pg and relatively high LOD of tens of μg/mL (ppm) were reported for LIBS.13,26 Profiting from microprobe analysis and compatible vacuum of ion source and mass analyzer, LAI-MS is capable of offering excellent ALOD in some special cases as well as LOD at subppm.20,27 However, the resulting poor mass resolution and multicharged ions at the high power density of the laser (>109 W/cm2) could not be obviated. To circumvent these problems, the laser postionization of sputtered neutral atoms has been proposed.28−30 By using this method, a greater degree of control over each step of the desorption and ionization processes spatially and temporally can be provided, making it possible to eliminate the discrimination between the matrix effect at low or medium irradiance and the deteriorating spectral resolution at high irradiance. Additionally, atom utilization, the ratio of the detected ions and consumed atoms, can be greatly improved with minimum sample consumption.31,32 Atom utilization is a key point, especially where a high sensitivity is desired. Therefore, high atom utilization is a particular prerequisite for achieving low LODs and ALODs concurrently. Recently, two-step laser mass spectrometry has been used for direct qualitative and quantitative analysis of small molecules in a variety of matrices.33−37 Unfortunately, only a few reports associated with laser ablation and laser postionization have been reported for direct solid analysis. The huge potential of the analogous combination to render microtrace analysis of all the REEs with LOD of ∼ppb as well as ALOD down to ∼10−15 g attainable has not yet been explored.38 In this manuscript, a laser desorption and laser postionization source (LD-LPI) coupled to TOFMS has been developed, shedding light on the capability of ultrasensitive determination of all REEs in liquid samples. Using a preparation that involves simple dried droplets, this approach affords full-elemental determination of the 15 REEs without the degraded spectral resolution that is commonly observed in high irradiance LDIMS. More specifically, LODs down to ∼ppb and ALODs at the femtogram level can be achieved by LD-LPI-TOFMS. LD-LPITOFMS facilitates microtrace determination of REEs, especially with a very scarce sample of a complicated matrix. Aside from these traits, remarkable relative sensitivity coefficients (RSCs) from 0.5 to 2.5 with terbium as the internal standard element were achieved for REEs, fulfilling the requirement for semiquantitative analysis.
Figure 1. Schematic diagram of the LD-LPI-TOFMS.
identical Nd:YAG lasers (Minilite II, Continuum Inc., U.S.A.) with 5 ns pulse duration and 10 Hz repetition rates except for wavelengths, which were implemented for laser desorption (532 nm) and postionization (266 nm), respectively. The distinction between these two modes lies in whether the frequency-quadruple laser is activated or not. The desorption laser is considered as a heat source for REE atoms to be vaporized, and the laser-induced thermal desorption process of elements is insensitive to the wavelength. In this work, the visible wavelength of 532 nm was adopted due to the easy observation of CCD system and visual laser adjustment. Considering that all the ionization energies (IEs) of 15 REEs are between 5.4 and 6.3 eV, the postionization wavelength of 266 nm with 4.67 eV photon energy was chosen so that twophoton ionization of a neutral atom process occurs efficiently.39 The desorption laser was ill-focused to ∼150 μm by a planoconvex lens (f = 60 mm) for uniform sampling and achieving a low power density. With the optimized desorption laser energy of 75 μJ at the irradiated area for LD-LPI-MS, the laser irradiance was 8.5 × 107 W/cm2. To maximize the detection efficiency and minimize LODs and ALODs, the maximum laser energy up to 8 mJ, corresponding to the power density of 8.2 × 108 W/cm2, was employed for postionization through the whole series of experiments unless otherwise stated. In theory, higher irradiance can be acquired by further reducing the postionization laser spot; however, a lower extraction yield of neutral atoms can be foreseen at the smaller laser spot due to the spatial spread and velocity dispersion of atoms. Furthermore, in a very small ionization volume, poor spectral resolution derived from the space charge effect can be inevitable.40,41 There is a trade-off in implementing desirable sensitivity and reasonable spectral resolution. Thus, a focused spot size of ∼500 μm in diameter was utilized for “capturing” neutral atoms. A distance of ∼1 mm (ΔZ in Figure 1) was chosen, corresponding to the delay time of 5.6 μs between the two lasers (τ in Figure 1), for achieving the lower LODs and ALODs. The experimental details can be further found in the Supporting Information. Time-of-Flight Mass Spectrometer. A pulsed extraction voltage was switched on for pulling the postionized ions into B
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distinguish the 15 REEs from each other when 175 μJ of laser energy was used. In the conventional high irradiance LDI technique for salt residues, worse mass resolution at laser irradiances greater than 108 W/cm2 was achieved due to the large kinetic energy dispersal (KED) and space-charge effect.44,45 The poor mass resolution can be ascribed to ion yields that are highly nonlinear, depending upon laser intensity. For high irradiance (typically ≥109 W cm−2), a thick and hyperthermal plasma is formed for the subsequent ablation and ionization, while laser-induced thermal vaporization process is dominant under laser irradiance of 106−108 W cm−2. In the former case, although full-element analysis can be achieved, large KED of ions is inevitable, resulting in severely deteriorated spectral resolution. On the other hand, only elemental species with low ionization energy are generated under low laser irradiance (106−108 W cm−2), thus, the detected elemental species and their intensities strongly depend on the laser irradiance, and the dependence is nonlinear at low irradiance. As a consequence, either no detectable signal is observed at low irradiance or abundant high KED ions are generated at high irradiance with the plasma ignited.46,47 Although time-lag focusing and second-order correction techniques are proposed for ameliorating detrimental impacts, this is of little help if high irradiance (>109 W/cm2) is employed. At medium irradiance, a compromise between detectable elemental species and spectral resolution will be acquired, so highly nonuniform RSCs are unavoidable. In sharp contrast, an unambiguous spectrum of 10 ppm REE residue can be achieved by LD-LPI-TOFMS (red line in Figure 3a) under a low desorption laser energy of 75 μJ. We can rationalize that only Na+ and K+ signals were obtained by the
acceleration in accordance with the end of the postionization laser. The energy dispersal of ions was focused by a double stage reflector.42 A digital oscilloscope (42MXs, LeCroy, U.S.A.) was applied for recording the output signal. A selfcompiled program written in LabVIEW (National Instruments, Austin, TX) was used for data processing. Both the ionization source and mass analyzer were kept in the same vacuum chamber pumped with a turbomolecular pump to a pressure of 3 × 10−5 Pa. To facilitate high throughput and time-saving analysis, a direct insertion probe (DIP) was employed for rapid sample exchange without venting the vacuum of the chamber. And the target with the residues deposited was mounted on a two-dimensional translation stage (SLC-17, SmartAct GmbH, Germany). The typical operation parameters can be found in Table S-1 of the Supporting Information. Sample Preparation of Dried Droplet Residues from the 15 REEs. A standard solution (GSB 04−1789−2004) consisting of the 15 REEs with yttrium and 14 lanthanide series elements from La to Lu was purchased from the National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials (Beijing, China). The original stock solution concentration of REEs was 100 μg/mL (ppm) dissolved in HNO3, and the solution was diluted into a series of concentrations ranging from 0.1−20 ppm for further investigation. The concentrations of all the residues referred to in this manuscript point to mass concentration (μg/mL) in the solution before drying. The residues were prepared by evaporating a 500 nL solution of REEs on the stainless steel target via a hot drier. The metal target was employed for a higher desorption efficiency instead of a quartz glass plate because of its high absorbance of laser irradiance and better thermal conductivity.43 One issue to consider is that the constituents of the metal target are also determined, but it should be interference-free for REE determination.
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RESULTS AND DISCUSSION Comparison of RSCs between LDI-MS and LD-LPI-MS for REEs. Prior to LD-LPI analysis, 10 ppm REE residue at diverse laser irradiance was carried out by LDI-TOFMS, and the results are depicted in Figure 2. No obvious signal other
Figure 2. LDI-MS spectra of REEs at different laser energies from 75 to 175 μJ, resulting in desorption irradiance of 0.85−2.0 × 108 W/cm2. All the spectra were accumulated with 500 laser shots.
than contaminating elements of Na+ and K+ could be generated using laser energy below 100 μJ, whereas weak signals of REEs could be acquired at an elevated laser energy of 125 μJ (inset in Figure 2) along with the emergence of constituent elements in the stainless steel target (Cr+ and Fe+). As the laser energy increases, the intensities of REEs are incremental but at the expense of deteriorating spectral resolution. It is difficult to
Figure 3. (a) Comparative mass spectra of LDI-TOFMS (black line) and LD-LPI-TOFMS (red line) and (b) the enlarged mass spectra at the mass ranges from 135 to 193 amu. The same desorption laser energy (and irradiance) of 75 μJ (and 8.5 × 107 W/cm2) was employed for both techniques. All the spectra were accumulated with 500 laser pulses. To avoid overcrowding in the mass spectrum, some oxides of REEs are not displayed. C
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Analytical Chemistry LDI technique at such low irradiance (black line in Figure 3a). Once the postionization laser was ignited, all the REEs contained in the solution, elemental constituents of the stainless steel substrate (Cr+, Mn+, Fe+, Ni+), as well as contaminating alkali ions, can be observed distinctly. Some background signals of residual gas by LD-LPI within the lowmass range can also be visible, similar to the previous reports.38,48 Relatively abundant hydrocarbons remain in the vacuum chamber, even though under the pressure of 3 × 10−5 Pa. The enlarged spectra reporting the 14 REEs from La+ to Lu+ are displayed in Figure 3b. All the REEs in the solution and their isotopes can be clearly distinguished with high signal-tonoise ratio (SNR), facilitating the spectral interpretation and follow-up quantitative analysis. To avoid ambiguities between REE mixtures and adventitious hydrocarbons, the analysis of single REE was conducted at the early stage. Just as shown in Figure 3a, hydrocarbon interference peaks are all below 90 amu, which is interference-free for REEs determination. Owing to the extremely proximal atomic masses of the REEs, most isotopes of the REEs are susceptible to mutual interference by the adjacent elements and relevant oxides, which is also common for the ICPMS method because of polyatomic ions and isobaric interferences. Even so, each REE owns its specific isotope, which is interference-free for calculating signal intensity by means of corresponding isotopic abundance. The REE intensities used for RSC values were superimposed from the intensities of the oxides with a considerable contribution, especially for LaO+, which makes a major contribution to the veritable intensity of La+. Whether oxides exist or not was judged by matching the calculated and experimental isotope abundances. On closer inspection of the comparative results in Figure 3, the LD-LPI technique was found to be capable of efficiently utilizing the original “wasteful” neutral atoms generated by low LD irradiance. This efficient utilization is of paramount significance for increasing atom utilization and sensitivity, which is an indispensable prerequisite for minuscule ALODs and ultrahigh sensitivity.49,50 The term “relative sensitivity coefficient (RSC)”, which is also called relative instrument response or relative sensitivity factor, is common used in direct solid analysis, especially in laser-based mass spectrometry (e.g., LA-ICPMS).20 It not only depends on the incident laser irradiance but is closely bound up with the sample properties (e.g., reflectivity, optical absorptivity, thermal conductivity and capacity) and IEs of species. To further demonstrate the analytical capability of the LD-LPI technique, a comparison between LDI and LD-LPI techniques on RSCs of 10 ppm REE residue is shown in Figure 4. Tb+ was chosen as the internal reference element because of its moderate mass number and physical properties. Each RSC value was acquired by the duplication of at least three parallel tests. For LDI-MS, the RSC values range from 0.01 to 7 at the laser irradiance of 1.4 × 108 W/cm2. This anticipated result is reasonable because LDI-MS has been severely hampered in the quantitative capability because of the highly nonlinear dependence upon laser intensity with desorption and ionization processes taking place in one step. Another undesirable aspect is that the matrix effects and the elemental fractionation effects can be inevitable at a low laser irradiance, whereas deteriorating spectral resolution occurs when employing a high irradiance laser. Both the factors make a contribution to RSCs with a higher fluctuation for LDI-MS. For example, an extremely low RSC value for La+ can be associated with the relatively high boiling point (3737 K) and enthalpy change of vaporization
Figure 4. Comparative diagram of RSCs for all the REEs via LDI-MS (in blue) and LD-LPI-MS (in pink). The residue concentrations are 10 ppm for both techniques.
(ΔHvap) as high as 400 kJ/mol. With desorption irradiance and postionization irradiance of 8.5 × 107 W/cm2 and 8.2 × 108 W/ cm2, respectively, the RSC values for all the REEs lie between 0.5 and 2.5, indicating that the elemental fractionation effects has been significantly mitigated compared to LDI-MS. The capability of the LD-LPI technique for decoupling of the laser desorption and the postionization processes is mainly responsible for the highly uniform RSCs of the 15 REEs with similar yet somewhat different physical properties, such as melting point (MP), boiling point (BP), ΔHvap, and IEs.29 More importantly, there is not a resonant pathway at 266 nm wavelength for all REEs, which is another key factor for uniform RSCs due to similar photoionization efficiencies.51 Resonance enhanced multiphoton ionization has proven to be an ultratrace analysis and highly selective technique for the determination of extremely low elemental species, whereas nonresonant pathway provides more outstanding quantitative ability of multielemental mixtures with more uniform RSCs.52 Giving insight into the relatively uniform values of RSC within 1 order of magnitude for all the REEs acquired by LD-LPI, this method is of great benefit for the semiquantitative analysis of REEs. Reproducibility and Universality of LD-LPI-MS for REEs. To evaluate the reproducibility and universality of this method, the homogeneity of the residue at different sampling spots was considered. Relatively uniform elemental distribution of all the REEs from 10 ppm residues at the ambilateral of the residue center could be achieved with relative standard deviations (RSDs) better than 25% (see Figure S-1 of the Supporting Information). A dissimilarity of signal intensities for the 15 REEs can be contemplated because of the discrepancies in elemental properties and ionization cross sections. This dissimilarity is in agreement with different RSC values for REEs displayed in Figure 4. This technique performs a rapid and convenient determination of REEs, irrespective of a tailor-made target by chemical modification. Four concentrations of 100 ppb, 500 ppb, 5 ppm, and 20 ppm were chosen for the reproducible determination of REEs by the LD-LPI technique. Each concentration was reduplicated by three parallel experiments. As shown in Figure 5, all 15 REEs can be obtained explicitly with high SNR, even at the concentration of 100 ppb. The data were acquired with a D
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the thickness of the 100 ppb residue was so thin that signal instability was increased. LODs and ALODs of LD-LPI-TOFMS for 15 REEs. A noteworthy aspect of this study is that it inspects the analytical performance of the LD-LPI technique, for example, LOD and ALOD. Microtrace analysis demands the analysis of samples with very limited consumption. To this end, ALOD is also another concern other than low LOD. Proposed by Yu et al., the desorbed mass of a certain element can be calculated according to eq 1 with a relatively homogeneous elemental distribution in the center of the residue.21 mdesorption = (Sdesorption /Sresidue) × mresidue Figure 5. Typical spectrum of REE residue at two distinct mass ranges of 85−110 amu (for Y+ and YO+) and 136−188 amu (from La+ to TmO+) at the concentration of 100 ppb. The signal intensities of the mass spectrum were integrated until a single irradiated area was sampled completely. Some oxides of REEs are not marked to avoid overcrowding in the mass spectrum.
(1)
where mdesorption is the specific elemental mass by laser desorption until no elemental signal is detected, and mresidue is the corresponding total mass of the specific element in the residue dependent upon solution volume and concentration. Sdesorption and Sresidue represent the area of laser desorption and the deposited residue, respectively. The volume of 500 nL and concentrations ranging from 0.1 to 20 ppm were used for calculating deposited masses. To facilitate explicit inspection of the desorption spot size, the 20 ppm residue was adopted. Two desorption spots on the residue can be clearly observed in Figure 7a. The area of Sdesorption and Sresidue is 3.8 mm2 and 7.5 × 10−3 mm2, respectively. Their corresponding mass spectra are displayed in Figure 7b.
single irradiated area finished by consecutive laser shots of 15 pulses. The signal stability of all the REEs was within 30% in parallel experiments. A further experiment at 10 ppb residue was also performed, but the signals of the REEs was unstable for different sampling spots because that the residue thickness was too thin to sample uniformly. More specifically, 10 ppb REE residue derived from 500 nL solvent corresponds to 5 pg for each deposited REE amount. Given that the coverage diameter of the residue was approximately 2.4 mm, nanometer scale thickness of the residue can be foreseeable.19 Apart from the signal stability, the influence of different concentrations on RSC values by the LD-LPI method was also investigated. The results in Figure 6 reveal that the RSC values
Figure 6. RSC values of the 15 REEs via LD-LPI-MS at different concentrations from 0.1 to 20 ppm. Each RSC value was acquired by the duplication of three parallel tests.
of all the 15 REEs have a relatively small fluctuation for all those concentrations. RSDs for 0.5−20 ppm concentrations are better than 15% for all the REEs, suggesting that sample concentration has little effect on RSC values. However, with integration of 100 ppb, the RSDs enlarged to ∼30% were obtained for some REEs. A plausible explanation might be that
Figure 7. (a) Image of two desorbed spots at 20 ppm residue on the stainless steel target, which were acquired until no ion signals were attainable. (b) Individual spectrum from Spot 1 and Spot 2, respectively at two distinct mass ranges of 87−108 amu (for Y+ and YO+) and 135−190 amu (from La+ to TmO+). E
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Analytical Chemistry The incremental concentrations from 0.1 to 20 ppm were used to calculate the LODs and ALODs of all 15 REEs. Good linearity of calibration curves and excellent correlation coefficients (R2) better than 0.95 can be acquired for all the REEs whether LODs or ALODs, and four representatives are shown in Figure 8. And the calibration curves of the remaining
Compared with the conventional LA-ICPMS, the LD-LPI technique affords analogically low LODs of dozens of ppb but much lower ALODs by more than 2 or 3 orders of magnitude. Although ALODs below the femtogram level have been reported in some special cases for laser microprobe mass spectrometry (LMMS), such low ALODs for all the REEs in the solution mixture are rarely referred. Lower LODs and ALODs can also be acquired by the LD-LPI method with a higher postionization efficiency. Therefore, future efforts might be taken to achieve higher atom utilization by postionization at the higher laser irradiance.
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CONCLUSIONS In this work, the figures of merit of analytical methodology for rapid REE residue analysis have been demonstrated via a newly constructed LD-LPI-TOFMS. The results herein established the LD-LPI technique as a viable option of great potential for microtrace determination of REEs with LODs down to ∼ppb and, more importantly, with ALODs at the femtogram level. This approach enables the separation of desorption and ionization events temporally and spatially, providing independent optimization of both processes. Compared to conventional LDI-MS, LD-LPI-MS offers advantages, such as alleviation of the matrix effects and the elemental fractionation effects at low and medium irradiance, and improved mass spectral resolution at high laser irradiance. The relatively uniform RSCs for all the REEs can be achieved in range of 0.5−2.5, satisfying the criterion of semiquantitative analysis. With these merits, a practical and simple strategy can be proposed for multielemental determination in solution by drying a droplet into residue. The method affords microtrace analysis with comprehensive consideration of sensitivity and consumption amount, especially in the frontier occasions such as microscale imaging and single cell analysis. Furthermore, since femtosecond LAI opens up the possibility for elemental quantification in alloy and semiconductor with elemental fractionation and thermal effect significantly reduced,54−56 making use of femtosecond laser could be an important direction to pursue in our future LD-LPI-MS work.
Figure 8. Signal intensities of the representative REEs for (a) Y, (b) Gd, (c) Tb, and (d) Lu vs the residue concentration (black lines of left half of diagrams) and ablated mass (red lines of right half of diagrams).
REEs are shown in Figure S-2 (Supporting Information). Integrating the relatively uniform RSCs mentioned above and the results of the calibration curves acquired here indicate that the LD-LPI technique performs a good semiquantitative capability for trace REEs. Summarized in Table 1 are LODs and ALODs of all the REEs that were calculated based on the slopes of the calibration curves and 3σ of background noise.19,53 Table 1. Averaged RSCs, R2 of Calibration Curves, LODs and ALODs for REEs via LD-LPI-TOFMS element
RSC
R2 (LOD and ALOD)
LODa (ppb)
ALODa (fg)
Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
2.11 0.59 1.21 2.16 1.15 1.12 1.17 2.27 1.00 1.10 0.65 1.04 1.11 1.06 1.04
0.9654 0.9660 0.9584 0.9832 0.9678 0.9706 0.9866 0.9874 0.9600 0.9625 0.9546 0.9980 0.9591 0.9733 0.9865
14 58 26 16 28 29 30 15 32 28 51 30 30 28 31
14 58 26 16 28 29 30 15 32 28 51 30 30 28 31
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b01033. Operating parameters of instrument, residue homogeneity, calibration curves and correlation coefficients (R2) of 15 REEs by LD-LPI-TOFMS (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wei Hang: 0000-0002-9145-3181
a
The 3-fold standard deviation (3σ) value for the 15 REEs is averaged for five concentrations from 0.1 to 20 ppm. LODs and ALODs of 15 REEs are calculated by the individual slopes of the calibration curves and 3σ. The authentic intensities of 15 REEs are converted by their monoisotope intensities and corresponding elemental abundance. In some cases, the intensities of the corresponding oxides (La, Ce, Pr, etc.) are also superimposed on REE intensities due to strong peaks of the oxides.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Natural Science Foundation of China Financial (21427813). This work is also supported by the Foundation for Innovative F
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Research Groups of the National Natural Science Foundation of China (21521004).
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DOI: 10.1021/acs.analchem.7b01033 Anal. Chem. XXXX, XXX, XXX−XXX
