Green and Fast Laser Fusion Technique for Bulk Silicate Rock

Sep 15, 2016 - Green and Fast Laser Fusion Technique for Bulk Silicate Rock Analysis by Laser Ablation-Inductively Coupled Plasma Mass Spectrometry ...
0 downloads 8 Views 2MB Size
Subscriber access provided by UNIVERSITY OF LEEDS

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

1

A green and fast laser fusion technique for bulk silicate rock analysis

2

by laser ablation ICP-MS

3

Chenxi Zhang,† Zhaochu Hu,*,†, Wen Zhang†, Yongsheng Liu†, Keqing Zong†, Ming

4

Li†, Haihong Chen†, Shenghong Hu†

5



6

Geosciences, Wuhan 430074, China

7 8



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

9 10 11 12 13 14 15

*Author to whom correspondence should be sent. E-mail:

[email protected]

Tel.: +86 27 61055600, Fax: +86 27 67885096

16 17 18 19 20 21 22

Submitted to Analytical Chemistry

23 1

ACS Paragon Plus Environment

Analytical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

24

For Table of Contents Only

25 26 27

2

ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

28

ABSTRACT:

29

Sample preparation of whole-rock powders is the major limitation for their

30

accurate and precise elemental analysis by laser ablation ICP-MS. In this study, a

31

green, efficient, and simplified fusion technique using a high energy infrared laser

32

was developed for major and trace elemental analysis. Fusion takes only tens of

33

milliseconds for each sample. Compared to the pressed pellet sample preparation, the

34

analytical precision of the developed laser fusion technique is higher by an order of

35

magnitude for most elements in granodiorite GSP-2. Analytical results obtained for

36

five USGS reference materials (ranging from mafic to intermediate to felsic) using the

37

laser fusion technique generally agree with recommended values with discrepancies

38

of less than 10% for most elements. However, high losses (20–70%) of highly volatile

39

elements (Zn and Pb) and the transition metal Cu are observed. The achieved

40

precision is within 5% for major elements and within 15% for most trace elements.

41

Direct laser fusion of rock powders is a green and notably simple method to obtain

42

homogeneous samples, which will significantly accelerate the application of laser

43

ablation ICP-MS for whole-rock sample analysis.

44 45

3

ACS Paragon Plus Environment

Analytical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

46

Page 4 of 24

INTRODUCTION

47

Accurate determination of major and trace element concentrations and isotopic

48

ratios in geological samples is a prerequisite for most geological investigations.

49

Dissolution is usually required for whole-rock elemental and isotope ratio analyses

50

using modern instrumental techniques, such as inductively coupled plasma mass

51

spectrometry (ICP-MS) and multi-collector ICP-MS. However, dissolution is tedious

52

and time-consuming and is thus the limiting factor for high sample throughput,

53

especially for geological sample analysis.1, 2 Many hazardous digestion solvents (e.g.,

54

HF, HNO3, HCl) are used in analytical geochemistry laboratories, which may cause

55

risk to the operator and result in pollution of the environment. Laser

56

ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provides

57

several advantages,3,

58

sample preparation, low blanks, and high sample throughput. This method has thus

59

become a popular technique, not only for in situ microanalysis6-8 but also for bulk

60

analysis of geological materials.9-19 However, LA-ICP-MS is a micro-sampling

61

technique, and the need for preparation of stable, homogeneous, and mechanically

62

resistant targets prior to the analysis of whole-rock powdered samples has been a

63

major drawback hindering accurate and precise measurements using this technique.

64

Three sample preparation methods are generally used for this purpose: (a) pressed

65

powder pellets,12,

66

lithium-borate fusion glasses.14, 16, 29, 30

67

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

ACS Paragon Plus Environment

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

68

whole-rock LA-ICP-MS analysis and is usually used for X-ray fluorescence (XRF)

69

analysis. Several binders have been used for enhancing the mechanical resistance of

70

pellets and analytical sensitivity, including PVA,12,

71

powder,33 and vanillic acid;34 however, the ablation yields still vary greatly between

72

individual spots. The analytical precision (RSD) of this method is not sufficiently high

73

(approximately 10-20%) due to the inhomogeneity of the pressed pellets and the

74

dilution of trace elements by the binder. Grain-size has been recognized as an

75

important factor for producing homogeneous and cohesive undiluted pressed pellets.18,

76

22, 23, 35, 36

77

powder tablets without the addition of a binder by applying wet-milling protocols in

78

an aqueous suspension, using a high power planetary ball mill and agate tools. They

79

found that the precision was in the same range of 55%). Homogeneity in such samples can only be obtained by applying high melting

95

temperatures (1700-1800℃) and long fusion times (60-120 sec or higher), resulting in

96

the preferential loss of highly volatile elements (e.g., Pb, Zn, and Cs) and some

97

transition metal elements (e.g., Cr, Ni, and Cu) by alloying with the metal heater. An

98

alternative approach to realize fast fusion and sample homogeneity is the dilution of

99

high SiO2 concentrations by the addition of high-purity MgO.27,

37

However, this

100

introduces other problems, such as contamination and a tedious sample preparation

101

process. In addition, the molten glass has to be quenched immediately to avoid the

102

formation of mineral crystals. In spite of this, crystallization during quenching is

103

sometimes unavoidable, especially for Fe- and Ni-rich samples (olivine crystals

104

formed upon quenching in komatiitic15).

105

Another technique, lithium-borate fusion glass formation, reduces the

106

temperature required for fusion of the rock powder by mixing it with a flux agent

107

(generally LiBO2 or LiB4O7).9, 14, 29, 38 Consequently, the loss of volatile elements is

108

suppressed, as has been extensively used for XRF analysis for years. This method is

109

suitable for a wide variety of bulk compositions including mafic, intermediate, and

110

silicic rocks. Yu et al.30 fused seventeen reference materials, with SiO2 contents

111

ranging from 50% to 77%, with a lithium borate flux (sample: flux =1:3). Agreement 6

ACS Paragon Plus Environment

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

112

of the LA-ICP-MS results is less than 10% relative with published reference values

113

and the analytical precision based on replicate analyses typically has RSD better than

114

5% when using a matrix-matched calibration strategy. However, the sample is diluted

115

because of the addition of a large amount of flux agent (the flux to sample ratio

116

commonly being 3-5:1), leading to increased limits of detection.30, 39 Additionally, the

117

flux introduces inevitable contamination into the samples (e.g., La and Ce)

118

the ICP-MS instrument (e.g., Li and B), due to which a longer rinsing time is required

119

between samples.30

16, 40

and

120

In this study, a green, cost-efficient, and simplified direct fusion technique was

121

established to prepare homogenous fused glasses for routine LA-ICP-MS whole-rock

122

analysis. Briefly, we used a high energy infrared laser to achieve rapid melting of

123

samples and instant cooling of glasses in tens of milliseconds, bypassing the use of

124

any chemical reagent or melting container. To test the melting capabilities of the

125

infrared laser for silicate rocks (especially for zircon-bearing granitic rocks and other

126

felsic samples), both major and trace elemental contents of five powdered

127

international

128

basalt-andesite-rhyolite) were determined simultaneously by LA-ICP-MS after fusion.

129

The reliability of this technique is demonstrated by the satisfactory accuracy and

130

precision of LA-ICP-MS data for most elements.

131

EXPERIMENTS

reference

materials

(spanning

the

compositional

range

132

Geological Materials. A series of RMs ranging from mafic (basalts BHVO-2

133

and BCR-2) to intermediate (andesite AGV-2 and granodiorite GSP-2) to felsic 7

ACS Paragon Plus Environment

Analytical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

134

(rhyolite RGM-2) composition were analyzed to assess the performance of the method

135

for which reliable compilation values are available from the geochemical GeoReM

136

database (http://georem.mpch-mainz.gwdg.de).41,

137

difficult because of their high melt viscosities, which hinders rapid homogenization of

138

the melt during fusion. In addition, it is also difficult to completely dissolve the

139

frequently present refractory accessory minerals, such as zircon, in felsic rock.

140

Therefore, the rhyolite RMs USGS RGM-2 and granodiorite RM USGS GSP-2,

141

whose SiO2 contents and refractory mineral contents are sufficiently high, were

142

selected to optimize and evaluate the new fusion technique.

42

Fusion of felsic rocks is very

143

Sample Preparation Procedure. Prior to sample fusion, the powdered reference

144

material was prepared into pressed pellets using a hand operated hydraulic pressure

145

pelleter (TP40, Herzog, Germany). Approximately 0.5 g powder sample, without

146

further grinding (200 mesh), was compressed at a pressure of 240 MPa for 5 min with

147

the help of a circular plastic resin ring (sample holder) to retain the form of the

148

powdered sample (Figure 1a). The pressed pellet was then fused at ambient

149

temperature and pressure. A high energy infrared laser (JHM-1GY200E, Wuhan

150

Chutian Industry Laser Equipment Co., LTD, China), consisting of optical maser,

151

laser power supply, cooling system, laser target designate system, light guide focusing

152

system, computer-control system and workbench, was used as the heat producer to

153

achieve complete fusion, which is widely used for metal welding in industry (Figure

154

S1). The pressed pellet was put under the laser focus position by 20 cm (Figure 2),

155

high-power and long-pulse mode of the laser was set during sample fusion, with a 8

ACS Paragon Plus Environment

Page 8 of 24

Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

156

repetition rate of 2 Hz. Under defocus and relatively lower repetition rate conditions

157

of infrared laser was used to ensure sample melting instead of ablation. Details of the

158

apparatus and operating conditions are listed in Table S1. Figure 1b shows the

159

momentary state of molten glass (GSP-2). After several pulses, the top layer

160

(approximately 1 mm thick) of the sample was fused, and natural cooling immediately

161

embedded in the pellet. A visual representation of this procedure is shown in Video S1.

162

As shown in Figures 1c and 1d, GSP-2 glasses contained more bubbles than BHVO-2

163

glasses because the discharge of bubbles was hindered by the high viscosity of the

164

glass melt. Nonetheless, a large amount of pure glass was still available.

165

Instrumentation. Experiments were conducted on an Agilent 7500a ICP-MS

166

(Agilent Technology, Tokyo, Japan) in combination with a 193 nm ArF excimer LA

167

system (GeoLas 2005, Lambda Physik, Göttingen, Germany) owned by the State Key

168

Laboratory of Geological Processes and Mineral Resources, China University of

169

Geosciences (Wuhan). The 193 nm excimer laser is installed with an optical

170

configuration that produces a fairly flat-topped lateral energy distribution, leading to

171

pan-shaped ablation pits on the sample. Helium was chosen as the ablation cell gas as

172

it has been found to consistently enhance the signal 2 folds compared to argon gas

173

with the 193 nm excimer laser.43 The carrier gas flows were optimized by ablating

174

NIST SRM 610 to obtain maximum signal intensity for U+, while keeping the ThO/Th

175

ratio