Analytica Chimica Acta 851 (2014) 64–71
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On-line double isotope dilution laser ablation inductively coupled plasma mass spectrometry for the quantitative analysis of solid materials Beatriz Fernández a, *, Pablo Rodríguez-González a , J. Ignacio García Alonso a , Julien Malherbe b , Sergio García-Fonseca c, Rosario Pereiro a , Alfredo Sanz-Medel a a
Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Clavería, 8, Oviedo 33006, Spain Chemical Sciences Division, Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8391, Gaithersburg, MA 20899, USA c ISC Science–Innovative Solutions in Chemistry, Oviedo, Spain b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
Development of a double IDMS strategy for direct solid analysis by LA-ICP-MS. The proposed method requires the sequential analysis of the sample and a standard. The previous characterization of the spike solution is not required in double IDMS. Quantitative bulk analysis of Sr, Rb and Pb were performed in silicate glasses and powdered samples. Powdered samples were analyzed as pressed pellets and glasses prepared by fusion.
Development and validation of a new on-line double IDMS methodology to achieve an accurate, precise, and time-effective strategy for direct determination of trace elements in solid samples by LA-ICP-MS.
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 June 2014 Received in revised form 4 August 2014 Accepted 9 August 2014 Available online 13 August 2014
We report on the determination of trace elements in solid samples by the combination of on-line double isotope dilution and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The proposed method requires the sequential analysis of the sample and a certified natural abundance standard by on-line IDMS using the same isotopically-enriched spike solution. In this way, the mass fraction of the analyte in the sample can be directly referred to the certified standard so the previous characterization of the spike solution is not required. To validate the procedure, Sr, Rb and Pb were determined in certified reference materials with different matrices, including silicate glasses (SRM 610, 612 and 614) and powdered samples (PACS-2, SRM 2710a, SRM 1944, SRM 2702 and SRM 2780). The analysis of powdered samples was carried out both by the preparation of pressed pellets and by lithium borate fusion. Experimental results for the analysis of powdered samples were in agreement with the certified values for all materials. Relative standard deviations in the range of 6–21% for pressed pellets and 3–21% for fused solids were obtained from n = 3 independent measurements. Minimal sample preparation, data treatment and consumption of the isotopically-enriched isotopes are the main advantages of the method over previously reported approaches. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Laser ablation inductively coupled plasma mass spectrometry Isotope dilution mass spectrometry Quantification methodology Powdered samples
* Corresponding author. Tel.: +34 985103000x5365. E-mail address: [email protected] (B. Fernández). http://dx.doi.org/10.1016/j.aca.2014.08.017 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.
B. Fernández et al. / Analytica Chimica Acta 851 (2014) 64–71
1. Introduction Laser ablation (LA) coupled to inductively coupled plasma mass spectrometry (ICP-MS) is one of the most powerful and versatile techniques for the elemental and isotopic analysis of solid materials. LA-ICP-MS provides fast and sensitive analyses of different types of solids (e.g., geological, environmental, and biological matrices), including microanalysis and depth profiling with good lateral and depth resolution [1,2]. While qualitative and semi-quantitative analysis by LA-ICP-MS is almost routine today, LA-ICP-MS is still far from being completely accepted for quantitative analyses, mainly due to fractionation effects and the persistent lack of adequate reference materials for the wide variety of samples of interest. Several calibration strategies for LA-ICP-MS have been developed so far, based on matrix-matched direct solid ablation [3], dual introduction of sample and standard [4] or direct liquid ablation [5]. LA-ICP-MS can be also used in combination with isotope dilution mass spectrometry (IDMS). IDMS is internationally regarded as a primary measurement method that is directly traceable to the International System of Units. It is widely employed in different fields to obtain accurate and precise determinations because the analytical result is not affected by signal drifts, matrix effects or analyte losses [6,7]. Providing a complete mixing of the isotopically-enriched spike and the analyte, the combination of IDMS and LA-ICP-MS may correct for common fractionation and matrix effects that cannot be controlled using other calibration techniques. So far, the combination of LA-ICP-MS and IDMS has been applied for the direct analysis of solids using two different strategies. Becker and co-workers [8,9] proposed an on-line isotope dilution analysis performed during the laser ablation of the sample by introducing a dry aerosol of a nebulized isotopicallyenriched spike solution into the ablation chamber. Such methodology is able to correct for errors derived from the ICP-MS detection step, but it is unable to correct those derived from the ablation processes itself. A different quantification methodology, based on the addition of the corresponding isotope-enriched spike solutions to the powdered sample with subsequent drying and pressing of the isotope-diluted sample, was investigated by Heumann and co-workers [10,11]. Also, Fernández et al. developed a solid spiking methodology [12] for the analysis of powdered samples and an in-cell IDMS strategy [13]. Liquid [10,11] and solid spiking [12] approaches allowed the matrix-matched quantification without any external standard correcting for all signal variations during the analysis, either derived from the instrumental drift or from the variation of the mass ablation rates. Nevertheless, the required addition of the liquid spike solutions to solid powders and the required homogenization and drying steps increase considerably the total analysis time, particularly when a large number of samples has to be analyzed. Moreover, due to the lack of homogeneity of the blends, the number of measurements required to obtaining reasonable results in terms of accuracy and precision is high. For example Fernández et al. [12,13] reported that around 40 independent analyses were required to obtain precisions lower than 15% RSD. To overcome this problem, Reid et al. [14] and, more recently, Malherbe et al. [15] applied LA-ID-ICP-MS to the elemental analyses of soil and sediment samples prepared by fusion with lithium borate. A better homogeneity of the blends was obtained compared to that showed for pressed pellets resulting in RSD values below 3%; nevertheless, this strategy requires a careful characterization of the isotopically-enriched spike solution, both in isotopic abundances and concentration. The characterization of the concentration of the isotopicallyenriched spike required in conventional or “single” IDMS can be
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avoided by using “double” IDMS. In this mode, the determination of the analyte in the sample by the single IDMS procedure is complemented by a reverse IDMS experiment in which the same spike is mixed with a certified natural abundance standard [16]. In this case, the sought mass fraction of the analyte in the sample can be referred directly to the selected natural abundance standard. Almost no information regarding the spike is required for the double IDMS equation (with the exception of the isotopeabundance ratio in the spike). In addition, no mass bias corrections need to be applied assuming same mass bias for sample and natural standard [16]. Double IDMS has not been evaluated yet for the direct analysis of solids by LA-ICP-MS. We present here the development and validation of an on-line double IDMS methodology to achieve a fast, accurate and precise quantification of trace elements in solid samples by LA-ICP-MS. As a proof of concept, we determine Sr, Rb and Pb in several certified reference materials of different matrices, including silicate glasses (SRM 610, 612 and 614) and powdered samples (MESS-2, PACS-2, SRM 2710a, SRM 2711a, SRM 1944, SRM 2702 and SRM 2780). Powdered samples were analyzed applying two different sample preparation strategies: pressed pellets and lithium borate fusion. The advantages of the proposed methodology in comparison with previously reported approaches are discussed. 2. Experimental 2.1. Standards, reagents and samples The NIST glass reference materials SRM 610, 612 and 614 (National Institute of Standards and Technology, Gaithersburg, USA) were used for the development of the on-line double LA-IDICP-MS methodology. MESS-2 (estuarine sediment) and PACS-2 (marine sediment) from the National Research Council of Canada (Ottawa, Canada) were used in the form of pressed pellets. Five standard reference materials (SRMs) from the National Institute of Standards and Technology (Gaithersburg, USA) were employed in the form of lithium borate glasses: SRM 2710a (Montana I soil), SRM 2711a (Montana II soil), SRM 1944 (New York/New Jersey waterway sediment), SRM 2702 (marine sediment) and SRM 2780 (hard rock mine waste). The materials in powdered form were handled according to the instructions provided in the corresponding certificates of analysis. The isotopically-enriched spike solutions of 86Sr (96.30 0.01% enrichment), 87Rb (98.40 0.01% enrichment) and 207Pb (94.60 0.01% enrichment) were from ISC Science (Oviedo, Spain). The concentration of the multi-elemental isotopically-enriched spike solution was selected to obtain optimum isotope ratios in the mixtures according to the random error propagation theory [16]. High-purity deionized water from a Milli-Q system (18.2 MV cm Millipore, Bedford, MA, USA) and sub-boiled nitric acid were employed for the preparation of all solutions. 2.2. Sample preparation Pressed pellets of 5 mm in diameter were prepared in a laboratory press (2 t for 5 min) for MESS-2 and PACS-2 reference materials. Alternatively, glasses of SRM 2710a, SRM 2711a, SRM 1944, SRM 2702 and SRM 2780 were prepared by lithium borate fusion by mixing an accurately weighted amount of SRM (approximately 0.75 g) with an accurately weighted amount (ca. 4 g) of lithium borate flux (67% Li2B4O7–33% LiBO2) from Spex (New Jersey, USA) in a 30 mL capacity platinum crucible. The fusion was performed by the Perl'x31 induction-heated machine (PANalytical, Almelo, The Netherlands) using a seven steps fusion program: (1) heating at 850 C for 1.5 min; (2) heating at 975 C for 5 min; (3)
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Table 1 Operating conditions of the ICP-MS and laser ablation system. ICP-MS
7500ce Agilent Technologies
RF power Cooling gas flow Make up gas flow Nebulizer gas flow Isotopes Dwell time
1500 W 15.5 L min1 0.25 L min1 0.6 L min1 84 Sr, 85Rb, 86Sr, 10 ms
LA system Laser energy Repetition rate Spot size Scan speed Ablation mode Carrier gas flow (He)
CETAC LSX-213 75% (4.2 mJ) 10 Hz 200 mm 20 mm s1 Single line scan 0.6 L min1
87
Rb,
88
Sr,
207
Pb,
208
Pb
heating at 975 C for 7 min with mixing by rocking the crucible; (4) cooling to room temperature (with mixing); (5) manual addition of one drop of a 20% LiI (aq) non-wetting agent; (6) heating at 975 C for 4 min (with mixing); and (7) casting into a Pt dish (1 min 40 s at 975 C) followed by forced-air cooling from under the dish (30 s). A 3 cm diameter glass was then obtained in the casting dish resulting in an approximate 1:6 dilution of the reference material. A previous study [17] demonstrated that no analyte loss occurred during the fusion process. 2.3. Instrumentation Element detection was carried out using a quadrupole ICP-MS (7500ce, Agilent Technologies) coupled to a CETAC LSX-213 laser system (Cetac Technologies, Omaha, NE, USA). The optimized conditions used for LA-ICP-MS measurements are listed in Table 1. Fig. 1 shows the experimental set-up employed in this work. The LA-ICP-MS coupling was carried out in wet plasma conditions using a home-made Y-piece of glass. The laser-generated aerosol was transported through a high-purity tube (Tygon1 tubing) into the ICP torch by the He carrier gas of the ablation cell. Before the introduction into the plasma, the laser-generated aerosol was mixed with a liquid aerosol nebulized by means of a Meinhard nebulizer coupled to a cyclonic spray chamber. This dual-flow introduction system enables a complete and easy optimization of
the LA-ICP-MS coupling by nebulizing a 1 ng g1 tuning solution as well as a complete mixing of the ablated sample with the isotopically-enriched spike. In addition, the LA-ICP-MS coupling was optimized daily using a SRM NIST 612 glass standard, searching for high sensitivity, reduced background intensity, and a 238U/232Th signal ratio close to 1 to ensure a low fractionation effect. 248ThO/232Th signal ratio was also measured for controlling oxide formation. During laser ablation analyses, the plasma was always kept under wet conditions by the continuous nebulization either of a nitric acid solution or the isotopically-enriched spike solution. 2.3.1. Confocal laser scanning microscopy High-resolution 3D images from sample surface (SRM 610, 612 and 614) analyzed by LA-ICP-MS were generated by sequential acquisition using a Leica TCS SP2 AOBS spectral confocal microscope. The instrument is equipped with an Ar/Kr laser, at 488 nm in reflection mode and a Plan Apochromat 10/0.40CS objective lens operated in dry conditions, without band-pass filter. Stacks of images were collected every 1 mm along the z-axis. 3D images and crater profiles were generated with Leica Confocal Software. 2.4. On-line double LA-ID-ICP-MS The proposed double IDMS strategy requires not only the analysis of the sample but also the analysis of a certified natural abundance standard. Thus, two solids (the sample and a natural abundance standard) were placed together in the ablation cell as depicted in Fig. 1. The laser-generated aerosol, coming from the ablation of the sample or the ablation of the standard, is mixed with the nebulized isotopically-enriched spike solution (or the nitric acid solution) in the Y-piece located before the ICP torch. The final mass faction of the element in the sample can be calculated according to the conventional double IDMS equation [16]: mn 1Rm2 Rs Rm1 Rt (1) ws ¼ wn ms 1Rm1 Rs Rm2 Rt In this equation the subscript “s” indicates the sample, “n” the natural abundance standard, “t” the isotopically-enriched spike solution, “m1” the mixture sample/spike and “m2” the mixture standard/spike. Thus, ws is the mass fraction of the element in the sample, wn the mass fraction of the element in
Fig. 1. Schematic diagram of the experimental set-up employed.
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the standard, mn and ms the masses ablated from natural abundance standard and sample, and R indicates the corresponding isotope ratios experimentally measured. Note that in Eq. (1), Rs is reciprocal to Rm and Rt with respect to the isotopes in the numerator and the denominator. One of the limiting factors in some “single” IDMS methodologies applied to LA-ICP-MS is to achieve an accurate determination of the mass taken from sample and spike. The proposed methodology assumes that the ablation efficiency, and therefore the mass ablated from the sample and the standard, is the same. Thus, mn and ms are removed from Eq. (1) leading to Eq. (2): 1Rm2 Rs Rm1 Rt (2) ws ¼ wn 1Rm1 Rs Rm2 Rt Hence, the mass fraction of the element in the sample can be directly determined from four experimentally measured isotope ratios and the mass fraction of the certified reference material (wn). The measurement sequence designed for the proposed online double LA-ID-ICP-MS analysis is represented in Fig. 2 and includes six steps to determine the different isotope ratios required by Eq. (2): (1) nitric acid nebulization (blank); (2) nitric acid nebulization during standard ablation (Rn); (3) spike nebulization during standard ablation (Rm2); (4) spike nebulization (Rt); (5) spike nebulization during sample ablation (Rm1); and (6) nitric acid nebulization during sample ablation (Rs). In this work, the isotope ratios measured were 208Pb/207Pb and 88Sr/86Sr. The determination of Rb required the measurement of all Sr and Rb isotopes and a mathematical correction based on the application of isotope pattern deconvolution to avoid the isobaric interference of 87Sr. Applying the measurement procedure explained in Fig. 2, the blank correction can be carried out by subtracting the signal obtained when only HNO3 is nebulised. Also, assuming same mass bias during the analytical run for sample and standard the application of any mass bias correction is not required. In any case, the detector dead time of the instrument must be properly corrected as in any other isotope ration measurement procedure. Finally, it is worth stressing that the determination of elements presenting a wide variation of isotope abundances in nature such as Pb Sr and Rb is facilitated as the natural isotopic composition of the element can be experimentally determined within the same analytical run.
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3. Results and discussion 3.1. Analysis of glass samples NIST glass series SRM 610–616 are widely employed for the calibration of laser ablation methodologies. Thus, three glass standards (SRM 610, 612 and 614) were selected as model samples to start with the development of the double IDMS methodology. In all the experiments, SRM 612 was employed as the natural abundance standard. Fig. 3a shows the LA-ICP-MS profile obtained for two Pb isotopes when SRM 612 was used both as the sample and the standard. During the analysis, nitric acid and the isotopically-enriched spike solution (10 mg kg1 86Sr, 87Rb and 207 Pb) were alternatively nebulized and mixed with the lasergenerated aerosol. As can be seen in Fig. 3a, 208Pb and 207Pb ion signals intensities showed different values along the time profile, according to the proposed laser ablation strategy. The correct application of Eq. (2) requires the measurement of the isotope ratios of the sample, standard, spike and their mixtures to calculate the final mass fraction of Sr, Rb and Pb in the sample. As an example, Fig. 3b shows the 208Pb/207Pb isotope ratio in the different regions. To avoid potential memory effects (both from the spray chamber and the ablation process) isotope ratios were calculated as the average of the selected intervals enclosed with dashed lines in Fig. 3b. The precision obtained in the isotope ratio measurements was typically lower than 10%. The observed results for the determination of Sr, Rb and Pb in SRM 612 by on-line double LA-ID-ICP-MS are summarized in Table 2. As expected, the obtained mass fractions were well in agreement with the corresponding certified values, within the given uncertainties, and the precision of Sr, Rb and Pb determinations was in the range of 6–12% relative standard deviation (RSD) obtained from n = 3 independent replicates. Once the potential of on-line double LA-ID-ICP-MS was proved when using the same material (SRM 612) as sample and standard, the applicability of this methodology was evaluated for the analysis of glasses SRM 610 and 614, using in both cases SRM 612 as the standard. The three glasses present different element concentration levels (500 mg kg1 SRM 610, 50 mg kg1 SRM 612 and 1 mg kg1 SRM 614) as well as different colors (dark blue, light blue and colorless, respectively). The results obtained are also
Fig. 2. Illustration of the ablation strategy designed for on-line double LA-ID-ICP-MS analysis.
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Fig. 3. ICP-MS profile obtained for the analysis of SRM 612 (sample and standard) using the laser ablation strategy designed for on-line double LA-ID-ICP-MS methodology. The different steps, both for the ablation and the nebulization, are indicated with arrows in the figure. (a) 208Pb and 207Pb ion signals intensities; (b) 208Pb/207Pb isotope ratio.
Table 2 Determination of Sr, Rb and Pb mass fraction (in mg kg1) in SRM 610, 612 and 614 by double on-line LA-ID-ICP-MS using SRM 612 as standard in all cases. Uncertainty is expressed as the standard deviation of n = 3 independent analyses. Sample
Standard
SRM 612 Sr Rb Pb
SRM 612
SRM 610 Sr Rb Pb
SRM 612
SRM 614 Sr Rb Pb
SRM 612
Certified concentration
Double on-line LA-ID-ICP-MS concentration
Ratio (experimental/certified)
78.4 0.2 31.4 0.4 38.57 0.2
78.5 4.6 32.7 3.2 38.6 4.5
1.0 0.06 1.04 0.10 1.0 0.12
515 0.5 425.7 0.8 426 1
400.3 12.7 286.9 38.5 269.7 17.6
0.78 0.02 0.67 0.11 0.63 0.04
57.1 1.9 1.4 0.1 3.9 0.1
1.3 0.1 1.6 0.2 1.7 0.1
45.8 0.1 0.855 0.005 2.32 0.04
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given in Table 2. As can be observed, important deviations from the certified values were obtained for Sr, Rb and Pb in SRM 610 and 614, particularly in the case of SRM 610. These results can be attributed to the different matrix of sample and standard. The different color and opacity of the glasses may lead also to a different amount of mass ablated in the sample and standard and hence to significant errors in the elemental determinations. In order to investigate this point, microscopy measurements on the crater profiles after samples ablation were performed to measure the crater depth in each case (SRM 610, 612 and 614). The craters formed in the samples after LA-ICP-MS analysis were subsequently visualized by confocal laser scanning microscopy generating 3D-images of the samples surface. The depth of those craters and their shape were also measured to evaluate ablation efficiencies depending on the matrix composition. Experimental results showed different depths for the craters obtained in SRM 610, 612 and 614. Table 2 shows that approximately the same recovery is obtained for the three elements in each material (0.7 and 1.5 for SRM 610 and SRM 614, respectively). This demonstrates that, under the same LA experimental conditions, the mass ablated from sample and standard was different. Thus, the methodology fails when the standard and the sample have a different matrix even for the case of glass samples. One important consideration here is that the experimental results indicate that glasses SRM 610, 612 and 614 show different ablation rates and that the proposed methodology could be able to quantify the relative ablation efficiency between similar matrix samples. 3.2. Analysis of powdered samples as pressed pellets The proposed methodology was next applied to the analysis of samples in powdered form. First, simple pelletizing was used as the sample preparation mode. Two powdered sediment reference materials were selected: MESS-2 (estuarine sediment) as standard and PACS-2 (marine sediment) as sample. The subboiled nitric acid solution and the isotopically-enriched spike solution (6 mg kg1 86Sr, 87Rb and 207Pb) were alternatively mixed with the laser-generated aerosol in the Y-piece. As shown in Table 3, Sr, Rb and Pb mass fractions determined in PACS-2 by online double LA-ID-ICP-MS were in good agreement with certified values. The precision and accuracy obtained were similar than those previously reported using other LA-ID-ICP-MS quantification approaches. Tibi and Heumann [10] reported a sample
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preparation procedure of 2–3 h and Fernández et al. [12,13] decreased the sample preparation time to several minutes by applying a solid spiking and in-cell approaches. Our approach is comparable to those reported by Fernández et al. in terms of analysis time, but here the previous and time-consuming preparation and characterization of the solid enriched material is not required. Concerning the number of replicates, Tibi and Heumann [10] reported RSD values of 1–27% obtained from three independent blends and Fernández et al. [12,13] RSD values of 2–17% using a solid spiking technique and in the range of 8–15% using an in-cell spiking, both obtained from n = 39 measurements. In contrast, applying a minimal sample preparation (just the pressed pellet preparation) we obtain here precisions between 6 and 21% (RSD) calculated from the standard deviation of n = 3 independent replicates. The higher uncertainty values obtained in powdered samples compared to those obtained with the glass samples could be attributed to the heterogeneity of the pressed pellets. 3.3. Analysis of powdered samples prepared by lithium borate fusion So far, two main limitations of the proposed methodology have been found: (i) the different ablation efficiency between sample and standard due to the sample matrix composition, and (ii) the heterogeneity of the pressed pellets. To overcome both problems a glass fusion approach [18] was applied to extend the on-line double IDMS methodology to the analysis of a wider range of matrices. The glass fusion approach ensures not only the homogeneity of the sample but also a similar matrix for all of the samples since an approximate 1:6 dilution of the powders with the lithium borate is carried out. The combination of the glass fusion approach with the proposed on-line double LA-ID-ICP-MS methodology was evaluated by analyzing five standard reference materials from NIST (including two soils, two sediments and a hard rock mine waste). As an example, Fig. 4 collects the LA-ICP-MS profile obtained for 207 Pb and 208Pb isotopes (Fig. 4a) and the 208Pb/207Pb isotope ratio (Fig. 4b) when SRM 2711 was used as the standard and SRM 1944 as the sample. Table 3 collects Sr, Rb and Pb mass fractions obtained for the analysis of SRM 2710a, SRM 1944, SRM 2702, and SRM 2780 compared with the corresponding certified values. The soil SRM 2711a was selected as the standard in all cases because its Sr and Rb mass fraction values of 242 10 mg g1 and 120 3 mg g1,
Table 3 Determination of Sr, Rb and Pb mass fraction (in mg kg1) in PACS-2 (using MESS-2 as standard) and SRM 2710a, SRM 1944, SRM 2702 and SRM 2780 (using SRM 2711a as standard). PACS-2 and MESS-2 were prepared as pressed pellets for laser ablation analysis whereas SRMs were prepared as glasses following a fusion process. Uncertainty is expressed as 2 s standard deviation of n = 3 independent analyses (95% confidence interval). Standard
Sample
Element
Pressed pellets
MESS-2 Estuarine sediment
PACS-2 Marine sediment
Sr Rb Pb
276 30 – 183 8
236.6 98.8 68.1 4.4 194.2 49.0
Glasses by fusion
SRM 2711a Montana II soil
SRM 2710a Montana I soil
SRM 2711a Montana II soil
SRM 1944 Waterway sediment
SRM 2711a Montana II soil
SRM 2702 Marine sediment
SRM 2711a Montana II soil
SRM 2780 Hard rock mine waste
Sr Rba Pb Src Rba Pb Sr Rb Pb Sra Rbb Pb
255 7 117 3 5520 30 136.8 1.5 75 2 330 48 119.7 3 127.7 8.8 132.8 1.1 217 18 175 5770 410
277.6 36.6 123.8 16.2 6071.4 664.2 133.0 11.4 64.9 7.0 286.6 41.2 104.6 27.6 102.8 20.8 102.7 16.4 203.3 84.0 182.2 11.0 5358.0 586.6
a b c
Reference value. Information value. Not certified but determined by ID-ICP-MS.
Certified concentration
Experimental concentration
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Fig. 4. ICP-MS profile obtained for the analysis of SRM 1944 (SRM 2711 as the standard) using the laser ablation strategy designed for on-line double LA-ID-ICP-MS methodology. (a) 208Pb and 207Pb ion signals intensities; (b) 208Pb/207Pb isotope ratio.
respectively, are in the range of the other SRMs, and the Pb mass fraction has an intermediate value (1400 10 mg g1). The concentration of the isotopically-enriched spike solution employed for these measurements was 6 ng g1 of 86Sr, 87Rb and 207 Pb. In all cases, n = 3 independent replicates were performed for each sample. Table 3shows that the mass fractions obtained by on-line double LA-ID-ICP-MS were in agreement with the certified values at 95% confidence interval except for the certified value of Pb in SRM 2702 and the reference value of Rb in SRM 1944. It should be highlighted that Sr and Rb mass fractions were in the same range for the five SRMs (120–255 mg g1 and 75–175 mg g1, respectively), but a wide concentration range was successfully investigated for Pb (132–5770 mg g1). The results indicate that the differences in the matrix composition do not affect the accuracy of the results. Moreover, except for Sr in SRM 2702 and SRM 2780, the precision of Sr, Rb and Pb determinations was in the range of 3–21% RSD for only three independent replicates. These RSD values were similar to those obtained in the analysis of the glass samples, therefore, the fusion approach seems to
overcome the heterogeneity problem of the powder samples. Additionally, the RSD values obtained are in the range of those obtained by Reid et al. [14] and Malherbe et al. [15] who employed the fusion approach after spiking the solid samples with an isotopically-enriched spike solution. An important advantage of the proposed method in comparison with those previously reported is the lower amount of enriched material which is required. For example, in the analysis of the same SRM materials, Malherbe et al. [15] employed for each blend between 130 and 440 mg of enriched Sr and between 580 and 7200 mg of enriched Pb, whereas less than 50 mL of a 6 ng g1 solution was employed in this work. 4. Conclusions We propose here on-line double LA-ID-ICP-MS as an accurate, precise, and time-effective strategy for direct determination of trace elements in solid samples. Unlike previously reported approaches combining LA-ICP-MS and IDMS, the previous characterization of the isotopically-enriched material is not
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required. Excepting the mass fraction of the certified solid standard, all parameters involved in the double IDMS equation can be experimentally determined within the same analytical run. This includes the isotopic composition of the isotopically-enriched element in the spike solution and the natural abundance elements in the sample. Therefore, the determination of elements presenting a wide variation of isotope abundances in nature such as Pb or Sr is facilitated. Also, assuming same mass bias during the analytical run for sample and standard avoids the application of any mass bias correction. The amount of isotopically-enriched material and the total analysis time is also minimized in comparison with previously published approaches. The methodology has been successfully applied to the analysis of powdered samples. However, it should be stated that experimental results demonstrated that the methodology cannot be applied when the standard of natural abundance and the sample have a different matrix even for the case of glass samples (e.g., SRM 612 as standard and SRM 610 or 614 as sample). This limitation can be easily tackled for powdered samples prepared following a fusion process since the possible differences in the ablation efficiency between the standard and the samples will be minimized by the flux with lithium borate. The precision obtained in the isotope ratio measurement in glasses and with the fusion approach was typically lower than 10%. Such precision depends on the concentration level and limits the overall precision of the method. Further optimization of the measurement procedure, the coupling of specific devices to obtain more homogeneous aerosols or the use of a different ICP-MS instrument capable of more precise isotope ratio measurements like MC-ICP-MS would significantly improve the results. Disclaimer Certain commercial equipment, instruments or materials are identified in this work to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for this purpose. Acknowledgments Financial support from “Plan Nacional de I + D + I” (Spanish Ministry of Science and Innovation and FEDER Program) through MAT2010-20921-C02-01 and PCTI Asturias through the project IE13-024 is gratefully acknowledged. The authors are also grateful for financial support from the Spanish Ministry of Economy and Competitiveness through Project Ref. CTQ2012-36711. P.R.-G. acknowledges his research contract RYC-2010-06644 to the
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Spanish Ministry of Economy and Competitiveness through the Ramón y Cajal Program. References [1] J. Pisonero, D. Günther, Femtosecond laser ablation inductively coupled plasma mass spectrometry: fundamentals and capabilities for depth profiling analysis, Mass Spectrom. Rev. 27 (2008) 609–623. [2] J. Koch, D. Günther, Review of the state-of-the-art of laser ablation inductively coupled plasma mass spectrometry, Appl. Spectrosc. 65 (2011) 155A–162A. [3] V. Mozna, J. Pisonero, M. Hola, V. Kanicky, D. Günther, Quantitative analysis of Fe-based samples using ultraviolet nanosecond and femtosecond laser ablation-ICP-MS, J. Anal. At. Spectrom. 21 (2006) 1194–1201. [4] L. Halicz, D. Günther, Quantitative analysis of silicates using LA-ICP-MS with liquid calibration, J. Anal. At. Spectrom. 19 (2004) 1539–1545. [5] D. Günther, R. Frischknecht, H.J. Muschenborn, C.A. Heinrich, Direct liquid ablation: a new calibration strategy for laser ablation-ICP-MS microanalysis of solids and liquids, Fres. J. Anal. Chem. 359 (1997) 390–393. [6] K.G. Heumann, Isotope-dilution ICP-MS for trace element determination and speciation: from a reference method to a routine method? Anal. Bioanal. Chem. 378 (2004) 318–329. [7] P. Rodríguez-González, J.M. Marchante-Gayon, J.I. García-Alonso, A. Sanz-Medel, Isotope dilution analysis for elemental speciation: a tutorial review, Spectrochim. Acta Part B 60 (2005) 151–207. [8] J.S. Becker, C. Pickhardt, W. Pompe, Determination of Ag, Tl, and Pb in few milligrams of platinum nanoclusters by on-line isotope dilution in laser ablation inductively coupled plasma mass spectrometry, Int. J. Mass Spectrom. 237 (2004) 13–17. [9] C. Pickhardt, A.V. Izmer, M.V. Zoriy, D. Schaumlöffel, J.S. Becker, On-line isotope dilution in laser ablation inductively coupled plasma mass spectrometry using a microflow nebulizer inserted in the laser ablation chamber, Int. J. Mass Spectrom. 248 (2006) 136–141. [10] M. Tibi, K.G. Heumann, Isotope dilution mass spectrometry as a calibration method for the analysis of trace elements in powder samples by LA-ICP-MS, J. Anal. At. Spectrom. 18 (2003) 1076–1081. [11] S.F. Boulyga, J. Heilmann, T. Prohaska, K.G. Heumann, Development of an accurate sensitive and robust isotope dilution laser ablation ICP-MS method for simultaneous multi-element analysis (chlorine, sulfur, and heavy metals) in coal samples, Anal. Bioanal. Chem. 389 (2007) 697–706. [12] B. Fernández, F. Claverie, C. Pecheyran, O.F.X. Donard, Solid-spiking isotope dilution laser ablation ICP-MS for the direct and simultaneous determination of trace elements in soils and sediments, J. Anal. Atom. Spectrom. 23 (2008) 367–377. [13] B. Fernández, F. Claverie, C. Pecheyran, J. Alexis, O.F.X. Donard, Direct determination of trace elements in powdered samples by in-cell isotope dilution femtosecond laser ablation ICPMS, Anal. Chem. 80 (2008) 6981–6994. [14] J.E. Reid, I. Horn, H.P. Longerich, L. Forsythe, G.A. Jenner, Determination of Zr and Hf in a flux-free fusion of whole rock samples using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) with isotope dilution calibration, Geostand. Newslett. 23 (1999) 149–155. [15] J. Malherbe, F. Claverie, A. Alvarez, B. Fernández, R. Pereiro, J. Molloy, Elemental analyses of soil and sediment fused with lithium borate using isotope dilution laser ablation-inductively coupled plasma-mass spectrometry, Anal. Chim. Acta 793 (2013) 72–78. [16] J.I. García Alonso, P. Rodríguez-González, Isotope Dilution Mass Spectrometry, RSC Publishing, 2013, pp. 41–76 Printed in the United Kingdom by CPI Group (UK) Ltd., Croydon CR0 4YY, UK (Chapter 2). [17] F. Claverie, J. Malherbe, N. Bier, J.L. Molloy, S.E. Long, Putting a spin on LA-ICPMS analysis combined to isotope dilution, Anal. Bioanal. Chem. 405 (2013) 2289–2299. [18] B. Stoll, K.P. Jochum, K. Herwig, M. Amini, M. Flanz, B. Kreuzburg, D. Kuzmin, M. Willbold, J. Enzweiler, An automated iridium-strip heater for LA-ICP-MS bulk analysis of geological samples, Geostand. Geoanal. Res. 32 (2008) 5–26.
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
On-line double isotope dilution laser ablation inductively coupled plasma mass spectrometry for the quantitative analysis of solid materials Beatriz Fernández a, *, Pablo Rodríguez-González a , J. Ignacio García Alonso a , Julien Malherbe b , Sergio García-Fonseca c, Rosario Pereiro a , Alfredo Sanz-Medel a a
Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Clavería, 8, Oviedo 33006, Spain Chemical Sciences Division, Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8391, Gaithersburg, MA 20899, USA c ISC Science–Innovative Solutions in Chemistry, Oviedo, Spain b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
Development of a double IDMS strategy for direct solid analysis by LA-ICP-MS. The proposed method requires the sequential analysis of the sample and a standard. The previous characterization of the spike solution is not required in double IDMS. Quantitative bulk analysis of Sr, Rb and Pb were performed in silicate glasses and powdered samples. Powdered samples were analyzed as pressed pellets and glasses prepared by fusion.
Development and validation of a new on-line double IDMS methodology to achieve an accurate, precise, and time-effective strategy for direct determination of trace elements in solid samples by LA-ICP-MS.
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 June 2014 Received in revised form 4 August 2014 Accepted 9 August 2014 Available online 13 August 2014
We report on the determination of trace elements in solid samples by the combination of on-line double isotope dilution and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The proposed method requires the sequential analysis of the sample and a certified natural abundance standard by on-line IDMS using the same isotopically-enriched spike solution. In this way, the mass fraction of the analyte in the sample can be directly referred to the certified standard so the previous characterization of the spike solution is not required. To validate the procedure, Sr, Rb and Pb were determined in certified reference materials with different matrices, including silicate glasses (SRM 610, 612 and 614) and powdered samples (PACS-2, SRM 2710a, SRM 1944, SRM 2702 and SRM 2780). The analysis of powdered samples was carried out both by the preparation of pressed pellets and by lithium borate fusion. Experimental results for the analysis of powdered samples were in agreement with the certified values for all materials. Relative standard deviations in the range of 6–21% for pressed pellets and 3–21% for fused solids were obtained from n = 3 independent measurements. Minimal sample preparation, data treatment and consumption of the isotopically-enriched isotopes are the main advantages of the method over previously reported approaches. ã 2014 Elsevier B.V. All rights reserved.
Keywords: Laser ablation inductively coupled plasma mass spectrometry Isotope dilution mass spectrometry Quantification methodology Powdered samples
* Corresponding author. Tel.: +34 985103000x5365. E-mail address: [email protected] (B. Fernández). http://dx.doi.org/10.1016/j.aca.2014.08.017 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.
B. Fernández et al. / Analytica Chimica Acta 851 (2014) 64–71
1. Introduction Laser ablation (LA) coupled to inductively coupled plasma mass spectrometry (ICP-MS) is one of the most powerful and versatile techniques for the elemental and isotopic analysis of solid materials. LA-ICP-MS provides fast and sensitive analyses of different types of solids (e.g., geological, environmental, and biological matrices), including microanalysis and depth profiling with good lateral and depth resolution [1,2]. While qualitative and semi-quantitative analysis by LA-ICP-MS is almost routine today, LA-ICP-MS is still far from being completely accepted for quantitative analyses, mainly due to fractionation effects and the persistent lack of adequate reference materials for the wide variety of samples of interest. Several calibration strategies for LA-ICP-MS have been developed so far, based on matrix-matched direct solid ablation [3], dual introduction of sample and standard [4] or direct liquid ablation [5]. LA-ICP-MS can be also used in combination with isotope dilution mass spectrometry (IDMS). IDMS is internationally regarded as a primary measurement method that is directly traceable to the International System of Units. It is widely employed in different fields to obtain accurate and precise determinations because the analytical result is not affected by signal drifts, matrix effects or analyte losses [6,7]. Providing a complete mixing of the isotopically-enriched spike and the analyte, the combination of IDMS and LA-ICP-MS may correct for common fractionation and matrix effects that cannot be controlled using other calibration techniques. So far, the combination of LA-ICP-MS and IDMS has been applied for the direct analysis of solids using two different strategies. Becker and co-workers [8,9] proposed an on-line isotope dilution analysis performed during the laser ablation of the sample by introducing a dry aerosol of a nebulized isotopicallyenriched spike solution into the ablation chamber. Such methodology is able to correct for errors derived from the ICP-MS detection step, but it is unable to correct those derived from the ablation processes itself. A different quantification methodology, based on the addition of the corresponding isotope-enriched spike solutions to the powdered sample with subsequent drying and pressing of the isotope-diluted sample, was investigated by Heumann and co-workers [10,11]. Also, Fernández et al. developed a solid spiking methodology [12] for the analysis of powdered samples and an in-cell IDMS strategy [13]. Liquid [10,11] and solid spiking [12] approaches allowed the matrix-matched quantification without any external standard correcting for all signal variations during the analysis, either derived from the instrumental drift or from the variation of the mass ablation rates. Nevertheless, the required addition of the liquid spike solutions to solid powders and the required homogenization and drying steps increase considerably the total analysis time, particularly when a large number of samples has to be analyzed. Moreover, due to the lack of homogeneity of the blends, the number of measurements required to obtaining reasonable results in terms of accuracy and precision is high. For example Fernández et al. [12,13] reported that around 40 independent analyses were required to obtain precisions lower than 15% RSD. To overcome this problem, Reid et al. [14] and, more recently, Malherbe et al. [15] applied LA-ID-ICP-MS to the elemental analyses of soil and sediment samples prepared by fusion with lithium borate. A better homogeneity of the blends was obtained compared to that showed for pressed pellets resulting in RSD values below 3%; nevertheless, this strategy requires a careful characterization of the isotopically-enriched spike solution, both in isotopic abundances and concentration. The characterization of the concentration of the isotopicallyenriched spike required in conventional or “single” IDMS can be
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avoided by using “double” IDMS. In this mode, the determination of the analyte in the sample by the single IDMS procedure is complemented by a reverse IDMS experiment in which the same spike is mixed with a certified natural abundance standard [16]. In this case, the sought mass fraction of the analyte in the sample can be referred directly to the selected natural abundance standard. Almost no information regarding the spike is required for the double IDMS equation (with the exception of the isotopeabundance ratio in the spike). In addition, no mass bias corrections need to be applied assuming same mass bias for sample and natural standard [16]. Double IDMS has not been evaluated yet for the direct analysis of solids by LA-ICP-MS. We present here the development and validation of an on-line double IDMS methodology to achieve a fast, accurate and precise quantification of trace elements in solid samples by LA-ICP-MS. As a proof of concept, we determine Sr, Rb and Pb in several certified reference materials of different matrices, including silicate glasses (SRM 610, 612 and 614) and powdered samples (MESS-2, PACS-2, SRM 2710a, SRM 2711a, SRM 1944, SRM 2702 and SRM 2780). Powdered samples were analyzed applying two different sample preparation strategies: pressed pellets and lithium borate fusion. The advantages of the proposed methodology in comparison with previously reported approaches are discussed. 2. Experimental 2.1. Standards, reagents and samples The NIST glass reference materials SRM 610, 612 and 614 (National Institute of Standards and Technology, Gaithersburg, USA) were used for the development of the on-line double LA-IDICP-MS methodology. MESS-2 (estuarine sediment) and PACS-2 (marine sediment) from the National Research Council of Canada (Ottawa, Canada) were used in the form of pressed pellets. Five standard reference materials (SRMs) from the National Institute of Standards and Technology (Gaithersburg, USA) were employed in the form of lithium borate glasses: SRM 2710a (Montana I soil), SRM 2711a (Montana II soil), SRM 1944 (New York/New Jersey waterway sediment), SRM 2702 (marine sediment) and SRM 2780 (hard rock mine waste). The materials in powdered form were handled according to the instructions provided in the corresponding certificates of analysis. The isotopically-enriched spike solutions of 86Sr (96.30 0.01% enrichment), 87Rb (98.40 0.01% enrichment) and 207Pb (94.60 0.01% enrichment) were from ISC Science (Oviedo, Spain). The concentration of the multi-elemental isotopically-enriched spike solution was selected to obtain optimum isotope ratios in the mixtures according to the random error propagation theory [16]. High-purity deionized water from a Milli-Q system (18.2 MV cm Millipore, Bedford, MA, USA) and sub-boiled nitric acid were employed for the preparation of all solutions. 2.2. Sample preparation Pressed pellets of 5 mm in diameter were prepared in a laboratory press (2 t for 5 min) for MESS-2 and PACS-2 reference materials. Alternatively, glasses of SRM 2710a, SRM 2711a, SRM 1944, SRM 2702 and SRM 2780 were prepared by lithium borate fusion by mixing an accurately weighted amount of SRM (approximately 0.75 g) with an accurately weighted amount (ca. 4 g) of lithium borate flux (67% Li2B4O7–33% LiBO2) from Spex (New Jersey, USA) in a 30 mL capacity platinum crucible. The fusion was performed by the Perl'x31 induction-heated machine (PANalytical, Almelo, The Netherlands) using a seven steps fusion program: (1) heating at 850 C for 1.5 min; (2) heating at 975 C for 5 min; (3)
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Table 1 Operating conditions of the ICP-MS and laser ablation system. ICP-MS
7500ce Agilent Technologies
RF power Cooling gas flow Make up gas flow Nebulizer gas flow Isotopes Dwell time
1500 W 15.5 L min1 0.25 L min1 0.6 L min1 84 Sr, 85Rb, 86Sr, 10 ms
LA system Laser energy Repetition rate Spot size Scan speed Ablation mode Carrier gas flow (He)
CETAC LSX-213 75% (4.2 mJ) 10 Hz 200 mm 20 mm s1 Single line scan 0.6 L min1
87
Rb,
88
Sr,
207
Pb,
208
Pb
heating at 975 C for 7 min with mixing by rocking the crucible; (4) cooling to room temperature (with mixing); (5) manual addition of one drop of a 20% LiI (aq) non-wetting agent; (6) heating at 975 C for 4 min (with mixing); and (7) casting into a Pt dish (1 min 40 s at 975 C) followed by forced-air cooling from under the dish (30 s). A 3 cm diameter glass was then obtained in the casting dish resulting in an approximate 1:6 dilution of the reference material. A previous study [17] demonstrated that no analyte loss occurred during the fusion process. 2.3. Instrumentation Element detection was carried out using a quadrupole ICP-MS (7500ce, Agilent Technologies) coupled to a CETAC LSX-213 laser system (Cetac Technologies, Omaha, NE, USA). The optimized conditions used for LA-ICP-MS measurements are listed in Table 1. Fig. 1 shows the experimental set-up employed in this work. The LA-ICP-MS coupling was carried out in wet plasma conditions using a home-made Y-piece of glass. The laser-generated aerosol was transported through a high-purity tube (Tygon1 tubing) into the ICP torch by the He carrier gas of the ablation cell. Before the introduction into the plasma, the laser-generated aerosol was mixed with a liquid aerosol nebulized by means of a Meinhard nebulizer coupled to a cyclonic spray chamber. This dual-flow introduction system enables a complete and easy optimization of
the LA-ICP-MS coupling by nebulizing a 1 ng g1 tuning solution as well as a complete mixing of the ablated sample with the isotopically-enriched spike. In addition, the LA-ICP-MS coupling was optimized daily using a SRM NIST 612 glass standard, searching for high sensitivity, reduced background intensity, and a 238U/232Th signal ratio close to 1 to ensure a low fractionation effect. 248ThO/232Th signal ratio was also measured for controlling oxide formation. During laser ablation analyses, the plasma was always kept under wet conditions by the continuous nebulization either of a nitric acid solution or the isotopically-enriched spike solution. 2.3.1. Confocal laser scanning microscopy High-resolution 3D images from sample surface (SRM 610, 612 and 614) analyzed by LA-ICP-MS were generated by sequential acquisition using a Leica TCS SP2 AOBS spectral confocal microscope. The instrument is equipped with an Ar/Kr laser, at 488 nm in reflection mode and a Plan Apochromat 10/0.40CS objective lens operated in dry conditions, without band-pass filter. Stacks of images were collected every 1 mm along the z-axis. 3D images and crater profiles were generated with Leica Confocal Software. 2.4. On-line double LA-ID-ICP-MS The proposed double IDMS strategy requires not only the analysis of the sample but also the analysis of a certified natural abundance standard. Thus, two solids (the sample and a natural abundance standard) were placed together in the ablation cell as depicted in Fig. 1. The laser-generated aerosol, coming from the ablation of the sample or the ablation of the standard, is mixed with the nebulized isotopically-enriched spike solution (or the nitric acid solution) in the Y-piece located before the ICP torch. The final mass faction of the element in the sample can be calculated according to the conventional double IDMS equation [16]: mn 1Rm2 Rs Rm1 Rt (1) ws ¼ wn ms 1Rm1 Rs Rm2 Rt In this equation the subscript “s” indicates the sample, “n” the natural abundance standard, “t” the isotopically-enriched spike solution, “m1” the mixture sample/spike and “m2” the mixture standard/spike. Thus, ws is the mass fraction of the element in the sample, wn the mass fraction of the element in
Fig. 1. Schematic diagram of the experimental set-up employed.
B. Fernández et al. / Analytica Chimica Acta 851 (2014) 64–71
the standard, mn and ms the masses ablated from natural abundance standard and sample, and R indicates the corresponding isotope ratios experimentally measured. Note that in Eq. (1), Rs is reciprocal to Rm and Rt with respect to the isotopes in the numerator and the denominator. One of the limiting factors in some “single” IDMS methodologies applied to LA-ICP-MS is to achieve an accurate determination of the mass taken from sample and spike. The proposed methodology assumes that the ablation efficiency, and therefore the mass ablated from the sample and the standard, is the same. Thus, mn and ms are removed from Eq. (1) leading to Eq. (2): 1Rm2 Rs Rm1 Rt (2) ws ¼ wn 1Rm1 Rs Rm2 Rt Hence, the mass fraction of the element in the sample can be directly determined from four experimentally measured isotope ratios and the mass fraction of the certified reference material (wn). The measurement sequence designed for the proposed online double LA-ID-ICP-MS analysis is represented in Fig. 2 and includes six steps to determine the different isotope ratios required by Eq. (2): (1) nitric acid nebulization (blank); (2) nitric acid nebulization during standard ablation (Rn); (3) spike nebulization during standard ablation (Rm2); (4) spike nebulization (Rt); (5) spike nebulization during sample ablation (Rm1); and (6) nitric acid nebulization during sample ablation (Rs). In this work, the isotope ratios measured were 208Pb/207Pb and 88Sr/86Sr. The determination of Rb required the measurement of all Sr and Rb isotopes and a mathematical correction based on the application of isotope pattern deconvolution to avoid the isobaric interference of 87Sr. Applying the measurement procedure explained in Fig. 2, the blank correction can be carried out by subtracting the signal obtained when only HNO3 is nebulised. Also, assuming same mass bias during the analytical run for sample and standard the application of any mass bias correction is not required. In any case, the detector dead time of the instrument must be properly corrected as in any other isotope ration measurement procedure. Finally, it is worth stressing that the determination of elements presenting a wide variation of isotope abundances in nature such as Pb Sr and Rb is facilitated as the natural isotopic composition of the element can be experimentally determined within the same analytical run.
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3. Results and discussion 3.1. Analysis of glass samples NIST glass series SRM 610–616 are widely employed for the calibration of laser ablation methodologies. Thus, three glass standards (SRM 610, 612 and 614) were selected as model samples to start with the development of the double IDMS methodology. In all the experiments, SRM 612 was employed as the natural abundance standard. Fig. 3a shows the LA-ICP-MS profile obtained for two Pb isotopes when SRM 612 was used both as the sample and the standard. During the analysis, nitric acid and the isotopically-enriched spike solution (10 mg kg1 86Sr, 87Rb and 207 Pb) were alternatively nebulized and mixed with the lasergenerated aerosol. As can be seen in Fig. 3a, 208Pb and 207Pb ion signals intensities showed different values along the time profile, according to the proposed laser ablation strategy. The correct application of Eq. (2) requires the measurement of the isotope ratios of the sample, standard, spike and their mixtures to calculate the final mass fraction of Sr, Rb and Pb in the sample. As an example, Fig. 3b shows the 208Pb/207Pb isotope ratio in the different regions. To avoid potential memory effects (both from the spray chamber and the ablation process) isotope ratios were calculated as the average of the selected intervals enclosed with dashed lines in Fig. 3b. The precision obtained in the isotope ratio measurements was typically lower than 10%. The observed results for the determination of Sr, Rb and Pb in SRM 612 by on-line double LA-ID-ICP-MS are summarized in Table 2. As expected, the obtained mass fractions were well in agreement with the corresponding certified values, within the given uncertainties, and the precision of Sr, Rb and Pb determinations was in the range of 6–12% relative standard deviation (RSD) obtained from n = 3 independent replicates. Once the potential of on-line double LA-ID-ICP-MS was proved when using the same material (SRM 612) as sample and standard, the applicability of this methodology was evaluated for the analysis of glasses SRM 610 and 614, using in both cases SRM 612 as the standard. The three glasses present different element concentration levels (500 mg kg1 SRM 610, 50 mg kg1 SRM 612 and 1 mg kg1 SRM 614) as well as different colors (dark blue, light blue and colorless, respectively). The results obtained are also
Fig. 2. Illustration of the ablation strategy designed for on-line double LA-ID-ICP-MS analysis.
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Fig. 3. ICP-MS profile obtained for the analysis of SRM 612 (sample and standard) using the laser ablation strategy designed for on-line double LA-ID-ICP-MS methodology. The different steps, both for the ablation and the nebulization, are indicated with arrows in the figure. (a) 208Pb and 207Pb ion signals intensities; (b) 208Pb/207Pb isotope ratio.
Table 2 Determination of Sr, Rb and Pb mass fraction (in mg kg1) in SRM 610, 612 and 614 by double on-line LA-ID-ICP-MS using SRM 612 as standard in all cases. Uncertainty is expressed as the standard deviation of n = 3 independent analyses. Sample
Standard
SRM 612 Sr Rb Pb
SRM 612
SRM 610 Sr Rb Pb
SRM 612
SRM 614 Sr Rb Pb
SRM 612
Certified concentration
Double on-line LA-ID-ICP-MS concentration
Ratio (experimental/certified)
78.4 0.2 31.4 0.4 38.57 0.2
78.5 4.6 32.7 3.2 38.6 4.5
1.0 0.06 1.04 0.10 1.0 0.12
515 0.5 425.7 0.8 426 1
400.3 12.7 286.9 38.5 269.7 17.6
0.78 0.02 0.67 0.11 0.63 0.04
57.1 1.9 1.4 0.1 3.9 0.1
1.3 0.1 1.6 0.2 1.7 0.1
45.8 0.1 0.855 0.005 2.32 0.04
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given in Table 2. As can be observed, important deviations from the certified values were obtained for Sr, Rb and Pb in SRM 610 and 614, particularly in the case of SRM 610. These results can be attributed to the different matrix of sample and standard. The different color and opacity of the glasses may lead also to a different amount of mass ablated in the sample and standard and hence to significant errors in the elemental determinations. In order to investigate this point, microscopy measurements on the crater profiles after samples ablation were performed to measure the crater depth in each case (SRM 610, 612 and 614). The craters formed in the samples after LA-ICP-MS analysis were subsequently visualized by confocal laser scanning microscopy generating 3D-images of the samples surface. The depth of those craters and their shape were also measured to evaluate ablation efficiencies depending on the matrix composition. Experimental results showed different depths for the craters obtained in SRM 610, 612 and 614. Table 2 shows that approximately the same recovery is obtained for the three elements in each material (0.7 and 1.5 for SRM 610 and SRM 614, respectively). This demonstrates that, under the same LA experimental conditions, the mass ablated from sample and standard was different. Thus, the methodology fails when the standard and the sample have a different matrix even for the case of glass samples. One important consideration here is that the experimental results indicate that glasses SRM 610, 612 and 614 show different ablation rates and that the proposed methodology could be able to quantify the relative ablation efficiency between similar matrix samples. 3.2. Analysis of powdered samples as pressed pellets The proposed methodology was next applied to the analysis of samples in powdered form. First, simple pelletizing was used as the sample preparation mode. Two powdered sediment reference materials were selected: MESS-2 (estuarine sediment) as standard and PACS-2 (marine sediment) as sample. The subboiled nitric acid solution and the isotopically-enriched spike solution (6 mg kg1 86Sr, 87Rb and 207Pb) were alternatively mixed with the laser-generated aerosol in the Y-piece. As shown in Table 3, Sr, Rb and Pb mass fractions determined in PACS-2 by online double LA-ID-ICP-MS were in good agreement with certified values. The precision and accuracy obtained were similar than those previously reported using other LA-ID-ICP-MS quantification approaches. Tibi and Heumann [10] reported a sample
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preparation procedure of 2–3 h and Fernández et al. [12,13] decreased the sample preparation time to several minutes by applying a solid spiking and in-cell approaches. Our approach is comparable to those reported by Fernández et al. in terms of analysis time, but here the previous and time-consuming preparation and characterization of the solid enriched material is not required. Concerning the number of replicates, Tibi and Heumann [10] reported RSD values of 1–27% obtained from three independent blends and Fernández et al. [12,13] RSD values of 2–17% using a solid spiking technique and in the range of 8–15% using an in-cell spiking, both obtained from n = 39 measurements. In contrast, applying a minimal sample preparation (just the pressed pellet preparation) we obtain here precisions between 6 and 21% (RSD) calculated from the standard deviation of n = 3 independent replicates. The higher uncertainty values obtained in powdered samples compared to those obtained with the glass samples could be attributed to the heterogeneity of the pressed pellets. 3.3. Analysis of powdered samples prepared by lithium borate fusion So far, two main limitations of the proposed methodology have been found: (i) the different ablation efficiency between sample and standard due to the sample matrix composition, and (ii) the heterogeneity of the pressed pellets. To overcome both problems a glass fusion approach [18] was applied to extend the on-line double IDMS methodology to the analysis of a wider range of matrices. The glass fusion approach ensures not only the homogeneity of the sample but also a similar matrix for all of the samples since an approximate 1:6 dilution of the powders with the lithium borate is carried out. The combination of the glass fusion approach with the proposed on-line double LA-ID-ICP-MS methodology was evaluated by analyzing five standard reference materials from NIST (including two soils, two sediments and a hard rock mine waste). As an example, Fig. 4 collects the LA-ICP-MS profile obtained for 207 Pb and 208Pb isotopes (Fig. 4a) and the 208Pb/207Pb isotope ratio (Fig. 4b) when SRM 2711 was used as the standard and SRM 1944 as the sample. Table 3 collects Sr, Rb and Pb mass fractions obtained for the analysis of SRM 2710a, SRM 1944, SRM 2702, and SRM 2780 compared with the corresponding certified values. The soil SRM 2711a was selected as the standard in all cases because its Sr and Rb mass fraction values of 242 10 mg g1 and 120 3 mg g1,
Table 3 Determination of Sr, Rb and Pb mass fraction (in mg kg1) in PACS-2 (using MESS-2 as standard) and SRM 2710a, SRM 1944, SRM 2702 and SRM 2780 (using SRM 2711a as standard). PACS-2 and MESS-2 were prepared as pressed pellets for laser ablation analysis whereas SRMs were prepared as glasses following a fusion process. Uncertainty is expressed as 2 s standard deviation of n = 3 independent analyses (95% confidence interval). Standard
Sample
Element
Pressed pellets
MESS-2 Estuarine sediment
PACS-2 Marine sediment
Sr Rb Pb
276 30 – 183 8
236.6 98.8 68.1 4.4 194.2 49.0
Glasses by fusion
SRM 2711a Montana II soil
SRM 2710a Montana I soil
SRM 2711a Montana II soil
SRM 1944 Waterway sediment
SRM 2711a Montana II soil
SRM 2702 Marine sediment
SRM 2711a Montana II soil
SRM 2780 Hard rock mine waste
Sr Rba Pb Src Rba Pb Sr Rb Pb Sra Rbb Pb
255 7 117 3 5520 30 136.8 1.5 75 2 330 48 119.7 3 127.7 8.8 132.8 1.1 217 18 175 5770 410
277.6 36.6 123.8 16.2 6071.4 664.2 133.0 11.4 64.9 7.0 286.6 41.2 104.6 27.6 102.8 20.8 102.7 16.4 203.3 84.0 182.2 11.0 5358.0 586.6
a b c
Reference value. Information value. Not certified but determined by ID-ICP-MS.
Certified concentration
Experimental concentration
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Fig. 4. ICP-MS profile obtained for the analysis of SRM 1944 (SRM 2711 as the standard) using the laser ablation strategy designed for on-line double LA-ID-ICP-MS methodology. (a) 208Pb and 207Pb ion signals intensities; (b) 208Pb/207Pb isotope ratio.
respectively, are in the range of the other SRMs, and the Pb mass fraction has an intermediate value (1400 10 mg g1). The concentration of the isotopically-enriched spike solution employed for these measurements was 6 ng g1 of 86Sr, 87Rb and 207 Pb. In all cases, n = 3 independent replicates were performed for each sample. Table 3shows that the mass fractions obtained by on-line double LA-ID-ICP-MS were in agreement with the certified values at 95% confidence interval except for the certified value of Pb in SRM 2702 and the reference value of Rb in SRM 1944. It should be highlighted that Sr and Rb mass fractions were in the same range for the five SRMs (120–255 mg g1 and 75–175 mg g1, respectively), but a wide concentration range was successfully investigated for Pb (132–5770 mg g1). The results indicate that the differences in the matrix composition do not affect the accuracy of the results. Moreover, except for Sr in SRM 2702 and SRM 2780, the precision of Sr, Rb and Pb determinations was in the range of 3–21% RSD for only three independent replicates. These RSD values were similar to those obtained in the analysis of the glass samples, therefore, the fusion approach seems to
overcome the heterogeneity problem of the powder samples. Additionally, the RSD values obtained are in the range of those obtained by Reid et al. [14] and Malherbe et al. [15] who employed the fusion approach after spiking the solid samples with an isotopically-enriched spike solution. An important advantage of the proposed method in comparison with those previously reported is the lower amount of enriched material which is required. For example, in the analysis of the same SRM materials, Malherbe et al. [15] employed for each blend between 130 and 440 mg of enriched Sr and between 580 and 7200 mg of enriched Pb, whereas less than 50 mL of a 6 ng g1 solution was employed in this work. 4. Conclusions We propose here on-line double LA-ID-ICP-MS as an accurate, precise, and time-effective strategy for direct determination of trace elements in solid samples. Unlike previously reported approaches combining LA-ICP-MS and IDMS, the previous characterization of the isotopically-enriched material is not
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required. Excepting the mass fraction of the certified solid standard, all parameters involved in the double IDMS equation can be experimentally determined within the same analytical run. This includes the isotopic composition of the isotopically-enriched element in the spike solution and the natural abundance elements in the sample. Therefore, the determination of elements presenting a wide variation of isotope abundances in nature such as Pb or Sr is facilitated. Also, assuming same mass bias during the analytical run for sample and standard avoids the application of any mass bias correction. The amount of isotopically-enriched material and the total analysis time is also minimized in comparison with previously published approaches. The methodology has been successfully applied to the analysis of powdered samples. However, it should be stated that experimental results demonstrated that the methodology cannot be applied when the standard of natural abundance and the sample have a different matrix even for the case of glass samples (e.g., SRM 612 as standard and SRM 610 or 614 as sample). This limitation can be easily tackled for powdered samples prepared following a fusion process since the possible differences in the ablation efficiency between the standard and the samples will be minimized by the flux with lithium borate. The precision obtained in the isotope ratio measurement in glasses and with the fusion approach was typically lower than 10%. Such precision depends on the concentration level and limits the overall precision of the method. Further optimization of the measurement procedure, the coupling of specific devices to obtain more homogeneous aerosols or the use of a different ICP-MS instrument capable of more precise isotope ratio measurements like MC-ICP-MS would significantly improve the results. Disclaimer Certain commercial equipment, instruments or materials are identified in this work to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for this purpose. Acknowledgments Financial support from “Plan Nacional de I + D + I” (Spanish Ministry of Science and Innovation and FEDER Program) through MAT2010-20921-C02-01 and PCTI Asturias through the project IE13-024 is gratefully acknowledged. The authors are also grateful for financial support from the Spanish Ministry of Economy and Competitiveness through Project Ref. CTQ2012-36711. P.R.-G. acknowledges his research contract RYC-2010-06644 to the
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