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A green and fast laser fusion technique for bulk silicate rock analysis by laser ablation ICP-MS Chenxi Zhang, Zhaochu Hu, Wen Zhang, Yongsheng Liu, Keqing Zong, Ming Li, Haihong Chen, and Shenghong Hu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02471 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016
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Analytical Chemistry
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A green and fast laser fusion technique for bulk silicate rock analysis
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by laser ablation ICP-MS
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Chenxi Zhang,† Zhaochu Hu,*,†, Wen Zhang†, Yongsheng Liu†, Keqing Zong†, Ming
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Li†, Haihong Chen†, Shenghong Hu†
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†
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Geosciences, Wuhan 430074, China
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‡
State Key Laboratory of Geological Processes and Mineral Resources, China University of
‡
The Beijing SHRIMP Center, Institute of Geology Chinese Academy of Geological Sciences,
Beijing 102206, P.R. China
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*Author to whom correspondence should be sent. E-mail:
[email protected] Tel.: +86 27 61055600, Fax: +86 27 67885096
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Submitted to Analytical Chemistry
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Analytical Chemistry
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ABSTRACT:
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Sample preparation of whole-rock powders is the major limitation for their
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accurate and precise elemental analysis by laser ablation ICP-MS. In this study, a
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green, efficient, and simplified fusion technique using a high energy infrared laser
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was developed for major and trace elemental analysis. Fusion takes only tens of
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milliseconds for each sample. Compared to the pressed pellet sample preparation, the
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analytical precision of the developed laser fusion technique is higher by an order of
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magnitude for most elements in granodiorite GSP-2. Analytical results obtained for
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five USGS reference materials (ranging from mafic to intermediate to felsic) using the
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laser fusion technique generally agree with recommended values with discrepancies
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of less than 10% for most elements. However, high losses (20–70%) of highly volatile
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elements (Zn and Pb) and the transition metal Cu are observed. The achieved
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precision is within 5% for major elements and within 15% for most trace elements.
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Direct laser fusion of rock powders is a green and notably simple method to obtain
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homogeneous samples, which will significantly accelerate the application of laser
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ablation ICP-MS for whole-rock sample analysis.
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INTRODUCTION
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Accurate determination of major and trace element concentrations and isotopic
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ratios in geological samples is a prerequisite for most geological investigations.
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Dissolution is usually required for whole-rock elemental and isotope ratio analyses
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using modern instrumental techniques, such as inductively coupled plasma mass
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spectrometry (ICP-MS) and multi-collector ICP-MS. However, dissolution is tedious
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and time-consuming and is thus the limiting factor for high sample throughput,
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especially for geological sample analysis.1, 2 Many hazardous digestion solvents (e.g.,
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HF, HNO3, HCl) are used in analytical geochemistry laboratories, which may cause
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risk to the operator and result in pollution of the environment. Laser
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ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provides
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several advantages,3,
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sample preparation, low blanks, and high sample throughput. This method has thus
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become a popular technique, not only for in situ microanalysis6-8 but also for bulk
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analysis of geological materials.9-19 However, LA-ICP-MS is a micro-sampling
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technique, and the need for preparation of stable, homogeneous, and mechanically
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resistant targets prior to the analysis of whole-rock powdered samples has been a
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major drawback hindering accurate and precise measurements using this technique.
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Three sample preparation methods are generally used for this purpose: (a) pressed
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powder pellets,12,
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lithium-borate fusion glasses.14, 16, 29, 30
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4
including the ability for multi-elemental analysis, simple
18, 20-23
(b) flux-free fusion glasses,11,
15, 17, 19, 24-28
and (c)
Pressed powder pellet is the first technique used in sample preparation for 4
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whole-rock LA-ICP-MS analysis and is usually used for X-ray fluorescence (XRF)
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analysis. Several binders have been used for enhancing the mechanical resistance of
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pellets and analytical sensitivity, including PVA,12,
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powder,33 and vanillic acid;34 however, the ablation yields still vary greatly between
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individual spots. The analytical precision (RSD) of this method is not sufficiently high
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(approximately 10-20%) due to the inhomogeneity of the pressed pellets and the
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dilution of trace elements by the binder. Grain-size has been recognized as an
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important factor for producing homogeneous and cohesive undiluted pressed pellets.18,
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22, 23, 35, 36
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powder tablets without the addition of a binder by applying wet-milling protocols in
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an aqueous suspension, using a high power planetary ball mill and agate tools. They
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found that the precision was in the same range of 55%). Homogeneity in such samples can only be obtained by applying high melting
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temperatures (1700-1800℃) and long fusion times (60-120 sec or higher), resulting in
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the preferential loss of highly volatile elements (e.g., Pb, Zn, and Cs) and some
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transition metal elements (e.g., Cr, Ni, and Cu) by alloying with the metal heater. An
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alternative approach to realize fast fusion and sample homogeneity is the dilution of
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high SiO2 concentrations by the addition of high-purity MgO.27,
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However, this
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introduces other problems, such as contamination and a tedious sample preparation
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process. In addition, the molten glass has to be quenched immediately to avoid the
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formation of mineral crystals. In spite of this, crystallization during quenching is
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sometimes unavoidable, especially for Fe- and Ni-rich samples (olivine crystals
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formed upon quenching in komatiitic15).
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Another technique, lithium-borate fusion glass formation, reduces the
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temperature required for fusion of the rock powder by mixing it with a flux agent
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(generally LiBO2 or LiB4O7).9, 14, 29, 38 Consequently, the loss of volatile elements is
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suppressed, as has been extensively used for XRF analysis for years. This method is
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suitable for a wide variety of bulk compositions including mafic, intermediate, and
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silicic rocks. Yu et al.30 fused seventeen reference materials, with SiO2 contents
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ranging from 50% to 77%, with a lithium borate flux (sample: flux =1:3). Agreement 6
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of the LA-ICP-MS results is less than 10% relative with published reference values
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and the analytical precision based on replicate analyses typically has RSD better than
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5% when using a matrix-matched calibration strategy. However, the sample is diluted
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because of the addition of a large amount of flux agent (the flux to sample ratio
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commonly being 3-5:1), leading to increased limits of detection.30, 39 Additionally, the
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flux introduces inevitable contamination into the samples (e.g., La and Ce)
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the ICP-MS instrument (e.g., Li and B), due to which a longer rinsing time is required
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between samples.30
16, 40
and
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In this study, a green, cost-efficient, and simplified direct fusion technique was
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established to prepare homogenous fused glasses for routine LA-ICP-MS whole-rock
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analysis. Briefly, we used a high energy infrared laser to achieve rapid melting of
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samples and instant cooling of glasses in tens of milliseconds, bypassing the use of
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any chemical reagent or melting container. To test the melting capabilities of the
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infrared laser for silicate rocks (especially for zircon-bearing granitic rocks and other
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felsic samples), both major and trace elemental contents of five powdered
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international
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basalt-andesite-rhyolite) were determined simultaneously by LA-ICP-MS after fusion.
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The reliability of this technique is demonstrated by the satisfactory accuracy and
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precision of LA-ICP-MS data for most elements.
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EXPERIMENTS
reference
materials
(spanning
the
compositional
range
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Geological Materials. A series of RMs ranging from mafic (basalts BHVO-2
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and BCR-2) to intermediate (andesite AGV-2 and granodiorite GSP-2) to felsic 7
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(rhyolite RGM-2) composition were analyzed to assess the performance of the method
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for which reliable compilation values are available from the geochemical GeoReM
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database (http://georem.mpch-mainz.gwdg.de).41,
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difficult because of their high melt viscosities, which hinders rapid homogenization of
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the melt during fusion. In addition, it is also difficult to completely dissolve the
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frequently present refractory accessory minerals, such as zircon, in felsic rock.
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Therefore, the rhyolite RMs USGS RGM-2 and granodiorite RM USGS GSP-2,
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whose SiO2 contents and refractory mineral contents are sufficiently high, were
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selected to optimize and evaluate the new fusion technique.
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Fusion of felsic rocks is very
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Sample Preparation Procedure. Prior to sample fusion, the powdered reference
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material was prepared into pressed pellets using a hand operated hydraulic pressure
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pelleter (TP40, Herzog, Germany). Approximately 0.5 g powder sample, without
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further grinding (200 mesh), was compressed at a pressure of 240 MPa for 5 min with
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the help of a circular plastic resin ring (sample holder) to retain the form of the
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powdered sample (Figure 1a). The pressed pellet was then fused at ambient
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temperature and pressure. A high energy infrared laser (JHM-1GY200E, Wuhan
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Chutian Industry Laser Equipment Co., LTD, China), consisting of optical maser,
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laser power supply, cooling system, laser target designate system, light guide focusing
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system, computer-control system and workbench, was used as the heat producer to
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achieve complete fusion, which is widely used for metal welding in industry (Figure
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S1). The pressed pellet was put under the laser focus position by 20 cm (Figure 2),
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high-power and long-pulse mode of the laser was set during sample fusion, with a 8
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repetition rate of 2 Hz. Under defocus and relatively lower repetition rate conditions
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of infrared laser was used to ensure sample melting instead of ablation. Details of the
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apparatus and operating conditions are listed in Table S1. Figure 1b shows the
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momentary state of molten glass (GSP-2). After several pulses, the top layer
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(approximately 1 mm thick) of the sample was fused, and natural cooling immediately
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embedded in the pellet. A visual representation of this procedure is shown in Video S1.
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As shown in Figures 1c and 1d, GSP-2 glasses contained more bubbles than BHVO-2
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glasses because the discharge of bubbles was hindered by the high viscosity of the
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glass melt. Nonetheless, a large amount of pure glass was still available.
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Instrumentation. Experiments were conducted on an Agilent 7500a ICP-MS
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(Agilent Technology, Tokyo, Japan) in combination with a 193 nm ArF excimer LA
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system (GeoLas 2005, Lambda Physik, Göttingen, Germany) owned by the State Key
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Laboratory of Geological Processes and Mineral Resources, China University of
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Geosciences (Wuhan). The 193 nm excimer laser is installed with an optical
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configuration that produces a fairly flat-topped lateral energy distribution, leading to
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pan-shaped ablation pits on the sample. Helium was chosen as the ablation cell gas as
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it has been found to consistently enhance the signal 2 folds compared to argon gas
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with the 193 nm excimer laser.43 The carrier gas flows were optimized by ablating
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NIST SRM 610 to obtain maximum signal intensity for U+, while keeping the ThO/Th
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ratio
