Article pubs.acs.org/ac
Total Rock Dissolution Using Ammonium Bifluoride (NH4HF2) in Screw-Top Teflon Vials: A New Development in Open-Vessel Digestion Wen Zhang,† Zhaochu Hu,*,† Yongsheng Liu,† Haihong Chen,† Shan Gao,† and Richard M. Gaschnig‡ †
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China Department of Geology, University of Maryland, College Park, Maryland 20742, United States
‡
S Supporting Information *
ABSTRACT: Complete sample digestion is a prerequisite for achieving reproducible and accurate analytical results for geological samples. Open-vessel acid digestions successfully dissolve mafic samples, but this method cannot achieve complete dissolution of felsic samples, because of the presence of refractory minerals such as zircon. In this study, an efficient and simplified digestion technique using the solid compound NH4HF2 in a screw-top vial has been developed for multielement analysis of different types of rock samples. NH4HF2 has a higher boiling point (239.5 °C) than conventional acids such as HF, HNO3 and HCl, which allows for an elevated digestion temperature in open vessels, enabling the decomposition of refractory phases. Similar to HF, HNO3 and HCl, ultrapure NH4HF2 can be produced using a conventional PFA sub-boiling (heating and cooling) purification system. A digestion time of 2−3 h for 200 mg NH4HF2 in a Savillex Teflon vial at 230 °C is sufficient to digest 50 mg of the felsic rock GSP-2, which is ∼6 times faster than using conventional closed-vessel acid digestion at 190 °C (high-pressure PTFE digestion bomb). The price of a Savillex Teflon vial is far less than the price of a high-pressure PTFE digestion bomb (consisting of a PTFE inner vessel and an outer stainless steel pressure jacket). Moreover, the NH4HF2−open-vessel acid digestion is not hampered by the formation of insoluble fluorides. We have successfully applied the NH4HF2−open-vessel acid digestion to the digestion of a series of international geological reference materials, including mafic to felsic igneous rocks and shales. This method provides an effective, simple, economical, and comparatively safe dissolution method that combines the advantages of both the open- and closed-vessel digestion methods.
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ultramafic rocks4), because of the incomplete dissolution of refractory minerals such as zircon. In addition, this technique requires long reaction times and the use of large amount of acids, which increases the cost and potential for sample contamination.7−9 Microwave digestion with open or closed vessels has been demonstrated to dissolve a range of materials with dramatically shorter dissolution times. However, several authors have found that microwave digestion does not always allow for the accurate analyses of elements such as Cr, Zr, Hf, and heavy REEs, because of the relatively short digestion times (a maximum of one hour) in a microwave system, especially when refractory minerals (e.g., zircon, chromite, rutile, corundum, cassiterite) are present.8−10,13−18 Alkali fusion and borate fusion are effective techniques for the decomposition of rocks that contain acid-resistant accessory minerals.6,8,9 However, fusion methods are not used regularly for trace element measurements in geological samples, because
omplete sample digestion is a prerequisite for achieving reproducible and accurate analytical results for geological samples.1,2 Complete recovery of trace elements, particularly the rare-earth elements (REEs), Zr, and Hf, in felsic rocks, is difficult, because of the presence of refractory minerals such as zircon.3 Refractory phases are also present in ultrabasic rocks (komatiites) and have led to difficulties in the accurate quantification of Zr and Hf.4 Many digestion methods for the decomposition of geological samples have been developed, and these include open-vessel acid digestion, microwave digestion, alkali fusion, sintering with Na2O2, and high-pressure acid digestion.5−10 Here, open-vessel acid digestion refers to mixedacid attack in open vials6,7 or Savillex screw-top Teflon vials (low pressure)9,11,12 placed on a hot plate. Open-vessel acid digestion has long been a popular and simple method for the digestion of inorganic and organic materials in chemical laboratories. However, the maximum digestion temperatures for open-vessel acid digestion are limited by the ambient pressure boiling point of the corresponding acid or acid mixture (e.g., boiling point of 38.3% HF = 112 °C, 68% HNO3 = 122 °C, and 20.24% HCl = 110 °C). Open-vessel acid digestion is successful when applied to most fine-grained mafic rocks, but it is problematic when applied to felsic rocks (and even some © 2012 American Chemical Society
Received: August 17, 2012 Accepted: November 26, 2012 Published: November 26, 2012 10686
dx.doi.org/10.1021/ac302327g | Anal. Chem. 2012, 84, 10686−10693
Analytical Chemistry
Article
the NH4HF2−open-vessel acid digestion method is elimination of the use of very corrosive and toxic HF, although NH4HF2 does decompose, forming HF, so safety issues are still a concern. Moreover, the high boiling point of NH4HF2 (239.5 °C) allows an increased digestion temperature in Savillex screwtop Teflon vials. NH4HF2-open vessel acid digestion method thus effectively dissolves refractory minerals. Note that the excess of the digestion reagent (NH4HF2) is removed by the evaporation process. This characteristic reduces the TDS content, which is also a distinct advantage over alkali and borate fusions for NH4HF2 digestion method. We report a systematic investigation of the decomposition capabilities of NH4HF2 for felsic rock GSP-2 using an openvessel digestion (Savillex screw-top Teflon vials). The NH4HF2−open-vessel acid digestion method was also used successfully to determine trace elements in a series of international rock RMs.
of its generally high blank levels and high total dissolved solids (TDS) content.9 Sintering followed by the separation of the FeTi hydroxides offers the possibility of eliminating the added salts. However, it is restricted to the measurement of only the elements coprecipitated by the Fe-Ti hydroxides.9 Most geological laboratories employ high-pressure acid digestion (closed-vessel digestions) to decompose samples that may contain phases that are resistant to acid attack for multielement analyses that include REE and high-field-strength elements (HFSEs) (e.g., felsic rocks, ultramafic rocks, sediments, soils).6,7,9,17,19−28 Digestions performed in closed vessels can reach higher temperatures because the boiling point of the reagents is raised by the pressure generated within the vessel. Conventional closed acid digestion bombs consist of a PTFE beaker with a lid that fits tightly into an outer stainless steel pressure jacket (a “bomb” is usually used to describe these closed vessels in geology laboratories). The outer jacket has a screw-top lid, which, when tightened, forms a gas-tight highpressure seal between the beaker and its lid. These bombs produce very high pressures (7−12 MPa) when subjected to high temperatures (110−250 °C).7,17 Such increased temperatures and pressures can significantly shorten sample decomposition times and allow the digestion of refractory phases.7,28 In comparison with open-vessel acid digestion, the disadvantages of bomb dissolutions are the tedious operating procedures, the high cost of the bomb-jacket set, and increased danger due to the higher pressure produced within the vessel. Hydrofluoric acid (HF) is the most effective acid for breaking the strong Si−O bond, and it has been used widely in the digestion of geological samples using procedures for both open and closed vessels.6−8 However, HF is extremely corrosive and toxic, creating significant safety issues. Recently, efforts have been made to explore new, safe, and effective digestion methods for geological materials utilizing ammonium compounds. Mariet et al.29 reported that the digestion capacity of a mixture of HNO3 + H2O2 + NH4F was similar to that of HNO3 + HF + HClO4 in conventional openvessel acid digestion by dissolving and analyzing three certified reference materials (RMs): Lichen 336, Basalt BE-N, and Soil 7. Hu et al.28 demonstrated that a combination of NH4F and HNO3 in high-pressure digestion is effective in dissolving igneous rocks, ranging from mafic to felsic. In this study, we describe a new digestion reagent, ammonium bifluoride (NH4 HF2 ), which achieves total dissolution in Savillex screw-top Teflon vials without the use of HF. Previous studies have attempted to use NH4HF2 for silicate minerals and rock digestion in the past two decades. Kolikova et al.30 reported interactions between NH4HF2 and silicate minerals (albite, biotite, diopside, forsterite, hedenbergite, hornblende, kaolinite, plagioclase) by stirring solid NH4HF2 with powdered minerals at ambient temperatures, resulting in partial decomposition of the silicates. Drying of the activated samples at 110 ± 5 °C accelerated solid-phase reactions. In metallurgy, Rimkevich et al.31 investigated fluoride processing of nonbauxite aluminum ores and developed the NH4HF2 method for the complex extraction of alumina, aluminum fluoride, amorphous silica, and other useful components. Medkov et al.32 also developed a treatment using NH4HF2 for gold-containing graphite-bearing ores to concentrate gold and to extract accompanying useful components. However, to our knowledge, NH4HF2 has not been used routinely to dissolve geological samples for an accurate determination of trace elements. A clear advantage of
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EXPERIMENTS Instrumentation. Experiments were carried out using an Agilent 7500a ICP-MS instrument (Agilent Technologies, Japan) equipped with a Micro Flow nebulizer (0.10 mL min −1 , PFA) and a double-path spray chamber. The spectrometer was optimized to obtain good signal intensities for Li, Y, Ce, and Tl, while keeping the CeO+/Ce+ and Ce2+/ Ce+ ratios below 1.2%. The optimum instrument conditions are summarized in Table S1 in the Supporting Information. The oxides and hydroxides of Ba and the light lanthanides can cause interference on the medium lanthanides.33,34 Interferences by oxides and hydroxides were calculated and corrected for the samples. Typical interference factors were as follows: 0.05% 137 16 Ba O + 136Ba16OH on 153Eu; 0.6% 142Nd16O and 0.7% 142 Ce 16 O on 158 Gd; 0.7% 143 Nd 16 O on 159 Tb; 0.04% 146 Nd16OH and 0.08% 147Sm16O on 163Dy; 0.6% 150Nd16O and 0.1% 150Sm16O on 166Er. Details of the operating conditions and correction procedures are presented in the Supporting Information. Reagents. Ultrapure water with a resistivity of 18.0 MΩ/cm was obtained from a Milli-Q water purification system (Millipore, Bedford, MA, USA). Commercially available nitric acid (HNO3, 68%, GR grade) and hydrofluoric acid (HF, 40%, GR grade) were further purified in a sub-boiling distillation system. Analytical calibration standards (1, 10, and 50 ng g−1 for all elements) were prepared by gravimetric serial dilution from 10 μg g−1 certified stock multielement solutions (SPEX CertiPrep, USA) in 2% (v/v) HNO3. The In internal standard solution was prepared from 1.0 mg g−1 of single-element standard solutions (National Center for Analysis and Testing of Steel Materials, China). The internal standard concentration of In was constant at 10 ng g−1 in all the final sample solutions, calibration standards, and blanks. NH4HF2 (98% metals basis) was purchased from Sigma−Aldrich and purified using a 120mL PFA Sub-boiling system (Savillex, Eden Prairie, MN, USA), because of the high concentration levels of Cr and Zn in the unpurified commercial ammonium bifluoride. The sub-boiling still (Savillex, Eden Prairie, MN, USA), which seems to be efficient at removing metallic or cationic impurities,35 consists of two 120-mL PFA bottles connected at right angles by a threaded PFA block. NH4HF2 was placed into the feed bottle and heated by two heat lamps to maintain a temperature of ∼140−150 °C. At this temperature, NH4HF2 changed from a solid to a liquid (melting point = 124.6 °C) and evaporated 10687
dx.doi.org/10.1021/ac302327g | Anal. Chem. 2012, 84, 10686−10693
Analytical Chemistry
Article
slowly, with the vapor condensing and recrystallizing in the water-cooled collecting bottle. Approximately 50 g NH4HF2 (filled in the feed bottle) were purified to 25 g (generally, 12 h were needed). All solutions were stored in high-density polyethylene (HDPE) bottles. Polyethylene bottle and pipet tips were cleaned in 10% (v/v) HNO3 for 24 h and rinsed five times with Milli-Q water before use. All operations were performed at a clean bench. Geological Materials. Felsic rocks are known to be difficult to dissolve, because of the presence of refractory minerals such as zircon. Therefore, the granodiorite RM USGS GSP-2, whose Zr content (550 μg g−1) is among the highest known for rock RMs36 and, consequently, should also represent one of the more challenging samples to dissolve, was chosen to evaluate the efficacy of NH4HF2 dissolution. To further assess the performance of the method, a series of RMs covering the compositional spectrum of igneous rocks, ranging from mafic (basalts BHVO-2 and BCR-2) to intermediate (andesite AGV2; granodiorite GSP-2) to felsic (granite G-2 and GSR-1). Two sedimentary rocks (shales SCo-1 and GSR-5) were also analyzed. Decomposition Procedure. The test of digestion parameters, including the digestion temperature and time, the amounts of NH4HF2, and the mass portion of sample, was designed and carried out to optimize the NH4HF2−open-vessel acid digestion procedures through analyses of trace elements in GSP-2. The optimized sample digestion method was ultimately used for the analyses of the eight international RMs described above. The recommended NH4HF2 digestion method is as follows: (1) 200 mg of NH4HF2 powder and 50 mg of rock powder (grain size of
