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Matrix effects originating from coexisting minerals and accurate determination of stable silver isotopes in silver deposits Qi Guo, Haizhen Wei, Shao-Yong Jiang, Simon Hohl, Yibo Lin, Yi-Jing Wang, and Yin-Chuan Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04212 • Publication Date (Web): 18 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Analytical Chemistry

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Accurate ɛ Ag in metal ore-deposits 106

108

Pd- Pd-Cu interference 104 Pd-105Pd-Fe interference 104 Pd_105Pd-Zn interference 105 Pd-108Pd-Pb interference

ACS Paragon Plus Environment


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1

2

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Matrix effects originating from coexisting minerals and

4

accurate determination of stable silver isotopes in silver

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deposits

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7

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Qi Guo a, Hai-Zhen Wei a*, Shao-Yong Jiang a,b *, Simon Hohl a, Yi-Bo Lin a,

9

Yi-Jing Wang a, Yin-Chuan Li a

10 11

a

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Engineering, Nanjing University, Nanjing 210023, PR China

13

b

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Resources, China University of Geosciences, Wuhan 430074, PR China

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences and

State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth

15 16 17 18 19 20

* Author to whom correspondence should be addressed:

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Profs. Hai-Zhen Wei, Shao-Yong Jiang

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Department of Earth Sciences and Engineering, Nanjing University

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163 Xianlin Avenue, Nanjing, Jiangsu, 210023 PRChina.

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Phone: +86 (25) 89681617; Fax: +86 (25) 89682393.

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Email address: [email protected]; [email protected]


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ABSTRACT

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Except for extensive studies in core formation and volatile-element depletion

28

processes using radiogenic Ag isotopes (i.e. the Pd-Ag chronometer), recent research

29

has revealed that the mass fractionation of silver isotopes is in principle controlled by

30

physico-chemical processes (e.g. evaporation, diffusion, chemical exchange etc.)

31

during magmatic emplacement and hydrothermal alteration. As these geologic

32

processes only produce very minor variations of δ109Ag from -0.5 ‰ to +1.1‰, more

33

accurate and precise measurements are required. In this work, a robust linear

34

relationship between instrumental mass discrimination of Ag and Pd isotopes was

35

obtained at the Ag/Pd molar ratio of 1:20. In Au-Ag ore-deposits, silver minerals have

36

complex paragenetic relationships with other minerals (e.g. chalcopyrite, sphalerite,

37

galena, and pyrite etc.). It is difficult to remove such abundant impurities completely

38

because the other metals are tens to thousands of times richer than silver. Both

39

quantitative

40

chromatography were carried out to deal with the problems. Isobaric inferences (e.g.

41

65

42

dramatically shift the measured δ109Ag values. The selection of alternative Pd

43

isotope-pairs is effective in eliminating spectral matrix effects, so as to ensure

44

accurate analysis under the largest possible ranges for metal impurities, which are

45

Cu/Ag ≤ 50:1, Fe/Ag ≤ 600:1, Pb/Ag ≤ 10:1, Zn/Ag ≤ 1:1 respectively. With the

46

modified procedure, we reported silver isotope compositions (δ109Ag) in geological

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standard materials and typical Au-Ag ore deposit samples varying from -0.029 ‰ to

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+0.689 ‰ with external reproducibility of ± 0.009 - ± 0.084 ‰. A systemic survey of

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δ109Ag (or ε109Ag) variations in rocks, ore-deposits, and environmental materials in

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nature is discussed.

evaluation

of

matrix

effects

and

modification

of

chemical

Cu40Ar+ to 105Pd, 208Pb2+ to 104Pd, and 67Zn40Ar+ to 107Ag+) and space charge effects

51

52

Keywords: Silver isotope, MC ICP-MS, Internal standard normalization, Matrix

53

effects, Silver ore deposits



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INTRODUCTION Silver has two stable isotopes with the following abundances: 107Ag: 51.8392(51)

55

109

56

and

57

as δ109Ag as per mil (‰) deviation (Eq. 1) or as ε109Ag as per ten thousand (ε)

58

deviation respectively (Eq. 2), relative to the NIST 978a, the certified stable silver

59

isotope reference material in the silver isotope community.

60

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Ag: 48.1608(51) (atom %) [1]. Silver isotope composition is usually reported

Ag = (

( / )

( / )

Ag = (( / )

− 1)×1000

( / )

− 1)×10000

(Eq. 1) (Eq. 2)

62

Pioneering silver isotope measurements used thermal ionization mass spectrometry

63

(TIMS), yielding relatively poor precision of ±1-2 ‰ [2]. Precision at this level was

64

insufficient for application of the Pd-Ag chronometers at moderate to low Pd/Ag

65

ratios (< 10,000) and in other terrestrial processes that produce variation of only a few

66

per mil in δ109Ag [3]. Recently, the analytical precision of silver isotope

67

measurements has been improved by an order of magnitude using multiple-collector

68

inductively coupled plasma mass spectrometry (MC-ICP-MS). The instrumental mass

69

discrimination in the presence of matrix elements with dry plasma vs. wet plasma has

70

been evaluated by Schönbächler [3]. They also established a three-stage ion exchange

71

procedure to separate Ag from matrix elements such as Ti and Fe, resulting in a

72

technique with an external reproducibility of ± 0.05 ‰, suitable for terrestrial basalts

73

and stony meteorites [3]. Efficient purification of Ag from environmental samples

74

was obtained using a two-column ion exchange procedure by Yang and Luo [4,5],

75

whose method yields an external reproducibility of ± 0.04 ‰ in commercial Ag

76

products and better than ± 0.015 ‰ in environmental materials.

77

In the last decade, the understanding of the fractionation of nontraditional stable

78

isotope has become a powerful tool in the earth and planetary sciences [6]. In early

79

research, silver isotope compositions were of particular interest to the study of volatile

80

depletion in the early solar system because the extinct radionuclide 107Pd decays to


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81

107

82

therefore show a good correlation between excess

83

demonstrating the presence of

84

short-lived nuclide [8,9]. The Pd-Ag chronometer has also been successfully applied

85

to study core formation and volatile-element depletion processes in the Earth [10,11],

86

where the high precision Pd-Ag isotope data show that Earth’s mantle has similar

87

δ109Ag to primitive, volatile-rich chondrites, suggesting that the Earth accreted a

88

considerable amount of material rich in moderately volatile elements [12].

Ag (half-life of 6.5 Ma) [7]. Many iron meteorites of the group II, II and IV

107

107

Ag and Pd/Ag ratios,

Pd in the early solar system and in situ decay of the

89

Apart from materials with extreme radiogenic107Ag/109Ag ratios up to 10 ‰ [9],

90

terrestrial samples have δ109Ag values within the range -0.6 - +0.6 ‰ [8], showing

91

that mass dependent stable isotope fractionation is dominant at low temperature and

92

that silver isotopes might be a useful geochemical tracer for ore deposits and

93

hydrothermal geochemistry studies. In recent years, the demand for gold and silver

94

has led to a sharp rise in geological prospecting. Au-Ag ore-deposits mainly occur as

95

native gold or native silver converted by supergene processes, and in other cases as

96

intergrown Au-Ag-Te minerals. Therefore, studies of silver isotopes, or of isotopes of

97

other major ore components are of great potential interest for constraining the

98

formation and evolution of precious metal deposits, and are likely to provide more or

99

different insights than other traditional stable isotope approaches.

100

In addition, silver particles are also well-known environmental pollutants arising

101

from rapid advances in the use of nanosilver products as an antimicrobial agent [13].

102

It has been recognized that the increased release of nanosilver to the environment

103

might cause potential toxic effects for aquatic organisms, such effects eventually

104

progressing up the food chain to humans [14]. Silver isotopes could provide a

105

sensitive forensic tool for fingerprinting the source of Ag in environmental cycling

106

[5].

107

Silver most commonly occurs in nature as a univalent cation, and is a moderately

108

volatile element that displays both siderophile and chalcophile behavior [8]. Therefore,

109

about two-thirds of the ore resources of silver deposits have complex paragenetic



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110

relationships with other minerals, such as chalcopyrite, sphalerite, galena, and pyrite

111

etc. The distribution stable silver isotopes in the major genetic types of silver ore

112

deposits and the isotope fractionation mechanisms accompanying various geological

113

processes have not been well understood to date. Recently, our preliminary

114

investigation showed that it is difficult to completely remove impurities (e.g. Cu2+/1+,

115

Fe2+/3+, Zn2+, Cd2+, Pb2+ etc.) originating from host or coexisting minerals of silver

116

deposits by using ion-exchange procedures (as shown in Figure S2). In order to

117

precisely constrain the minor natural variations of δ109Ag values in geological

118

processes, a new high-precision approach for stable silver isotope analysis has been

119

developed in this work, including the establishment of an optimal conditions for the

120

mass bias correction, the quantitative evaluation of matrix effects from impurities, the

121

modification on sample pretreatment. In addition, we report a general survey of silver

122

isotope distribution in natural samples.

123

124

EXPERIMENTAL

125

Preparation of NIST 978a standard solution and geological samples

126

The NIST SRM 978a is the only universally available Ag isotope standard. It has a 109

Ag/107Ag = 0.92904 ± 0.00022 [15], and the

127

certified absolute isotopic ratio,

128

nuclide masses of 107Ag and 109Ag are given to be 106.905095 and 108.904754 [16].

129

Milli-Q water (Resistivity, 18.2 MΩ·cm) was used throughout the experiments, and

130

concentrated HCl and HNO3 were prepared through twice sub-boiling distillation of

131

commercial acids (AR), using Savillex distillers. A stock solution containing 1000

132

µg⋅g-1 of Ag was prepared by quantitative dissolution of NIST SRM 978a (in AgNO3

133

form) in 2% (v/v) HNO3 solution, and was diluted to 100 ng⋅g-1 (i.e. 100 ppb) for

134

isotope analysis. All geological standard samples in silver sulfide form were dissolved

135

in 6.0 M HNO3, and diluted to 100 ppb after being separated from solid sulfur by

136

immediate centrifugation.


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Dissolution of Au-Ag ore-deposit samples

138

About 0.01-0.07g of Au-Ag ore-deposit sample powder was weighed and digested

139

in a mixture of 2 mL HF and 1 mL HNO3 in a closed Savillex Teflon beaker on a

140

hotplate at 110 0C for 2 days. After the sample had dried, 6 M HNO3 was added, and

141

the sample was left for a day to dry again on a hot plate. Subsequently, 6M HCl was

142

added and heated overnight to complete the dissolution. After a further drying, the

143

residues were re-dissolved in 30 mL of 0.5 M HCl. With the procedure, all geological

144

standards and skarn-type ore-deposit samples were totally dissolved and few amount

145

of black insoluble residues in orogenic-type ore-deposit samples were identified to be

146

organic matters without containing any silver. The solutions were centrifuged for a

147

few minutes before loading the supernatant into the column for matrix separation.

148

Ion exchange chromatography

149

A two-column ion-exchange procedure modified from the methods of

150

Schönbächler [3] and Luo [5] was used for the separation and purification of trace Ag

151

from minerals. The details are given in Table S1, and the mean Ag recovery obtained

152

in this study was 96.47 ± 2.51% (2SD, n≥5).

153

Preparation of internal standard palladium solution

154

In our MC ICP-MS silver isotope setup, a palladium solution was usually used as

155

an internal standard for mass bias correction to dope the samples prior to analysis.

156

Naturally occurring palladium (Pd) is composed of six stable isotopes,

157

105

Pd, 106Pd, 108Pd, and 110Pd. Except for two theoretically unstable ones (i.e. 102Pd and

158

110

Pd), the respective isotope compositions of other stable ones are

159

105

Pd (0.2233 (8)),

160

similar isotope abundances of all the stable Pd isotopes make any isotope-pairs (e.g.

161

104

162

mass bias correction in the absence of isobaric interferences.

Pd-105Pd,

104

106

Pd (0.2733 (3)),

Pd-106Pd,

104

Pd-108Pd,

105

108

104

102

Pd,

104

Pd,

Pd (0.1114(8),

Pd (0.2646 (9)) [17]. Theoretically, the

Pd-106Pd,

105

Pd-108Pd,


106

Pd-108Pd) suitable for


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Previous studies used a fixed Pd isotope-pair for correcting instrumental mass

163

107

Ag/109Ag. For example, the

108

Pd-105Pd isotope pair was selected

164

fractionation of

165

for silver isotope analysis in iron meteorites, sulfide minerals and terrestrial basalts by

166

Schönbächler [3] and the mass bias was verified independently using

167

106

168

isotope analysis in commercial products and environmental materials. In our

169

preliminary experiment on silver ore samples, Cd ions could not be completely

170

removed by using the ion-exchange chemistry (Figure S2), and the small amount of

171

Cd remaining would induce possible isobaric interference of 108Cd on 108Pd, and 106Cd

172

on

173

the applicability of other isotope pairs has also been evaluated. The standard material

174

of NIST SRM 3138 (PdCl2) was prepared as the internal standard for the

175

instrument-induced mass bias correction (Eq. 3, 4) [18]:

Pd/105Pd [3]. Yang [4] and Luo [5] used

106

Pd. Therefore, the isotope pair

# %$ " &$ β = ln " '(1% )* " &)* "

176

177

!

70/,

=

104

+,-./0,*

+,-./0,*

106

Pd/108Pd and

104

110

Pd/105Pd and

Pd/105Pd for silver

Pd-105Pd was used in this study initially, and

4 3 6 3 /ln ( % ) 6& 3 3

(Eq. 3)

2

× (6 )9 6

(Eq. 4)

178

where subscripts Measured and True indicate measured and corrected isotope ratios,

179

respectively; β is the mass bias correction factor; m107, m109 are the absolute masses of

180

the nuclides 106.905095 (107Ag), and 108.904754 (109Ag) [19]; iPd and jPd are the

181

individual isotopes in

182

106

183

(104Pd), 104.90509 (105Pd), 105.90349 (106Pd), 107.90389 (108Pd) respectively [19].

184

The absolute isotopic abundance ratios of the isotope pairs in SRM 3138 (Lot

185

No.090629, NIST Analytical Chemistry Division) are derived from the newly issued

186

IUPAC Technical Report [20].

104

Pd-105Pd,

104

Pd-106Pd,

104

Pd-108Pd,

105

Pd-106Pd,

105

Pd-108Pd,

Pd-108Pd pairs and mj and mi are the absolute masses of the nuclides 103.90404


Page 9 of 27

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187

A 800 µg⋅g-1 Pd working standard solution was freshly prepared for each analytical

188

session by quantitative dilution of a stock solution in 2% (v/v) HNO3. The Pd solution

189

was added to both sample and Ag standard solutions as a common doping matrix and

190

as an internal standard for mass bias correction. Mass discrimination and instrument

191

drift were corrected by a combination of internal normalization with Pd and

192

standard-sample bracketing.

193

Silver isotope analysis

194

A Neptune Plus MC ICP-MS (Thermo Fisher Finnigan) with an ESI PFA 50

195

µL/min nebulizer in a quartz cyclonic spray chamber was used for the measurement

196

of silver isotopic composition. The ions

197

Faraday Cups C and H2, and

198

Faraday cups L3, L2, L1 and H1, with 1011 Ω amplifiers in all cases. The mass bias

199

was corrected by sample-standard-bracketing procedure (SSB) following the

200

procedure proposed by Luo [5]. The silver content in both the sample solution and the

201

NIST 978a standard solution was kept at around 100 ppb, resulting in ~1.2V signal on

202

107

203

the inlet system with 3% HNO3 and Milli-Q water in sequence for ~10 minutes

204

between measurements to reduce the signals to ~3 mV. All reproducibilities described

205

in this work are quoted from repeated measurements of the samples (n ≥ 5, 2 S.D., 95%

206

confidence limits). The average internal analytical precision (n = 40, 4 blocks × 10

207

cycles) of the measured 109Ag/107Ag ratios of 100 ng mL-1 NIST 978a is ± 0.02 ‰ and

208

the external reproducibility varied from ± 0.006 ‰ to ± 0.009 ‰ (n ≥ 10). Typical

209

operating conditions are summarized in Table S2.

104

Pd+,

105

107

Ag+ and

Pd+,

106

Pd+,

109

Ag+ were detected using

108

Pd+ were detected using

Ag+ with the conventional H-skimmer cone. To avoid memory effects, we washed

210

211

RESULTS AND DISCUSSION

212

Optimal condition of internal standard for mass bias correction



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213

Even though Pd has been used as an internal standard for mass bias correction in

214

silver isotope analysis by MC-ICP-MS in previous studies [3,4,5], the relationship

215

between instrumental mass discrimination of Ag and Pd isotopes has not been

216

reported. To obtain the optimal condition of the internal standard for mass bias

217

correction, a series of solutions containing 100 ppb of Ag with different Ag/Pd molar

218

ratios from 1:2 to 1:20 were compared (Figure 1). Clearly, there is no direct

219

correlation in the plot of ln(109Ag/107Ag) vs. ln(105Pd/104Pd) when the Ag/Pd ratios are

220

higher than of 1:5. The data converge to linear trends as Ag/Pd ratios decrease, and

221

the best linear correlation (R2 = 0.995) is obtained for a Ag/Pd ratio of 1:20. Besides,

222

as shown in Figure S1, the Pd-corrected Ag isotopic compositions (δ109Ag) of NIST

223

SRM 978a deviate from the true value at the Ag/Pd molar ratios of 1:2 and 1:5, and

224

gradually reach to the accurate one with increasing of the Pd/Ag molar ratios.

225

Nonlinear relationships between Ag and Pd isotopes for measurements at higher

226

Ag/Pd ratios (i.e. Ag/Pd ratios of 1:2 and 1:5) might be attributed to the higher first

227

ionization energy of Pd (804 kJ⋅mol-1) compared to that of Ag (731 kJ⋅mol-1) [21],

228

resulting in the ionization of a smaller fraction of the Pd than of the Ag in the plasma

229

torch .

230 231 232

Figure 1. Correlations of ln(109Ag/107Ag) vs. ln(105Pd/104Pd) measured in solutions containing 100 ppb of Ag with different Ag/Pd molar ratios.

233 234

In order to evaluate the validity of the optimal internal standard concentration with

235

the Ag/Pd ratio of 1:20, the correlations of ln(105Pd/104Pd) vs. ln(109Ag/107Ag) obtained


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236

from different analytical sessions over several months have been compared in Figure

237

2. If the same mass bias effect acts on 105Pd-104Pd and 109Ag-107Ag isotope pairs, then

238

the slopes of the linear arrays will be identical. The slopes are closely similar,

239

showing that the linear relationship between instrumental mass discrimination of Ag

240

and Pd isotopes is pretty robust in our instrumental setup. In order to avoid any

241

non-spectral matrix effect from over-doping of the internal standard Pd, a common

242

doping matrix produced by adding 2 µg⋅g-1of Pd as an internal standard into both the

243

standard and sample solutions containing 100 ng⋅g-1 of Ag (corresponding to Ag/Pd =

244

1:20) is optimal for mass bias correction. Matrix effects are negligible in such

245

solutions, as discussed below. It is worth of noting that, different optimal Ag/Pd

246

conditions were suggested among different laboratories, such as the Ag/Pd molar ratio

247

of 1:20 by Luo et al. [5], and 1:0.5 to 1:7.4 for dry plasma measurement by Woodland

248

et al. [8] and that of 1:1.85 for the wet plasma analyses by Schönbächler et al. [3],

249

which might suggest the mass fractionation of Ag and Pd depends on both

250

instrumentation and sample introduction conditions (e.g. wet/dry plasma).

251 252 253 254

Figure 2. Long-term reproducibility of ln(109Ag/107Ag) vs. ln(105Pd/104Pd) measured in doping matrix solutions with a fixed Ag/Pd ratio of 1:20 in different analytical sessions.

255 256

Matrix effects from originated coexisting minerals in silver deposits



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Page 12 of 27

257

Both spectral (e.g. isobaric interferences) and non-spectral matrix effects have

258

been identified in MC-ICP-MS analysis routines. Generally, the latter kind was

259

induced by differences between natural (multi-elemental) samples and synthetic

260

(monoelemental) standards, such as presence of matrix elements, analyte

261

concentration mismatching, analyte oxidation state mismatching and presence of

262

organic matter [22]. These interferences are of particular concern for stable isotope

263

systems because they may cause mass-dependent biases. Previous investigations

264

revealed that matrix effects of Na, K, W and Sn ions could be rendered negligible by

265

mass bias corrections based on standard-sample bracketing and internal normalization

266

with Pd isotopes [4]. The matrix effects from Ir, Sm and Rb caused the Pd-corrected

267

Ag isotopic composition to increase by approximately 0.01%, which has been well

268

demonstrated by Carlson et al. [23]. In typical samples in metal-ore deposits, other

269

metals are tens to thousands of times more abundant than silver (Table S3). It is

270

difficult to completely remove such abundant matrices by a single ion-exchange

271

procedure, as observed in our preliminary experiments, because of the similar

272

coordination behavior Cu2+/1+, Fe2+/3+, Zn2+, Pb2+ and Ag+ with Cl- ions in the anion

273

exchange column (AG1-X8, Cl--Form) (Figure S2). In order to better evaluate the

274

matrix effect resulting from residual cations of coexisting base metals, impurities of

275

Fe2+, Cu2+, Pb2+ and Zn2+ were doped into the NIST SRM 978a Ag standard solution,

276

and these doped standards were then analyzed as samples.

277

In accordance with the usual mineral components of typical silver ore-deposits,

278

different ranges of Ag/metal ratios have been chosen. As shown in Table S4 and

279

Figure 3, the presence of matrix Cu2+ causes a dramatic negative shift in the measured

280

δ

281

value of -1.74 ‰ when the Cu/Ag ratio reaches 500:1. A linear regression of δ109Ag

282

vs. [Cu]/[Ag] gives a slope of -0.00034 (R2=0.977) (Figure 3a). By contrast, the

283

109

284

for NIST 978a when the Fe/Ag molar ratio is as high as 600:1, which implies the

285

matrix

109

Ag values. They decrease linearly with increasing of Cu/Ag molar ratios, to a

Ag/107Ag ratios measured are consistent with a mean value of 0.91836 ± 0.00030

effect

of

Fe2+

could

be

adequately

corrected


by

using

our

Page 13 of 27

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286

standard-sample-bracketing and Pd internal normalization (Figure 3b). A positive

287

shift of +0.10 ‰ in δ109Ag is induced for higher Fe/Ag molar ratios from 1000:1 to

288

2000:1. Compared with the dramatic matrix effects from Cu2+ and the minor effects

289

from Fe2+ cations matrix, the effect of Zn2+ is slight,

290

+0.07 ‰ in δ109Ag that are present for Zn/Ag ratios as low as 5:1. Significant positive

291

shifts of δ109Ag values were observed with doping of Pb ions, and the δ109Ag increases

292

to +2.70 ‰ when the Pb/Ag ratio reaches 200:1. A linear regression on the plot of

293

δ

109

positive shifts of +0.05 to

Ag vs. [Pb]/[Ag] produces a slope of +0.0143 (R2=0.982) (Figure 3d).

The major interferences for silver isotope analysis via the SSB-Pd internal

294

106

Cd+,

108

Cd+ and

295

normalization approach can be attributed to the monatomic ions

296

208

297

shown in Table 1. The SSB-Pd internal normalization approach shows a larger

298

tolerance for matrix Fe cations, and still could provide accurate silver isotope

299

measurements for the Fe/Ag molar ratios as high as 600:1. Theoretically, the

300

contribution of isobaric interference of 67Zn40Ar+ to 107Ag+ should result in a negative

301

109

302

be significant because of the low isotopic abundance of

303

nature [20], and the low formation efficiency of 67Zn40Ar+. The slight positive offsets

304

due to matrix zinc cations can be interpreted as space charge effects in the skimmer

305

cone, resulting in preferential transmission of heavier ions [24]. The contribution of

306

65

307

Cu2+ concentrations, causing the linear negative δ109Ag offsets observed in Figure 3a.

308

By contrast, the accumulation of

309

from the initial linear Ag-Pd mass bias resulting in the linear decrease shown in

310

Figure 4a, leading to linear positive offsets in δ109Ag with increasing Pb2+ ions

311

(Figure 3d).

Pb2+ (m/z=104) and polyatomic ions 65Cu40Ar+, 66Zn40Ar+, 67Zn40Ar+, 68Zn40Ar+ as

Ag/107Ag shift. However, the spectral matrix effect from the isobaric ion would not

Cu40Ar+ to

105

Pd makes

67

Zn+ (i.e. 0.0404 (16)) in

105

Pd/104Pd ratios increase proportionally with increasing

208

Pb2+ (m/z=104) shifts the

105

Pd/104Pd ratio away

312

In order to avoid polyatomic isobaric interferences from Pb and Cu matrices on the

313

internal palladium standard, alternative Pd isotope pairs were used for mass bias

314

correction. As shown in Figures 4 and 5, using 106Pd-108Pd for high Pb in the matrix,



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Page 14 of 27

315

and 105Pd-108Pd for high Cu in the matrix, led to the elimination of the linear offsets of

316

δ109Ag values. The tolerances for matrix copper and lead cations were extended to a

317

Cu/Ag molar ratio of 50:1 and a Pb/Ag molar ratio of 10:1 by combining the SSB-Pd

318

internal normalization with appropriate Pd isotope pairs (Table 2).

319

Table 1. Possible isobaric interferences on silver and palladium isotopes Mass charge ratio Isotopes Isotopic mass (u)

104

105

104

106

105

Pd

106

Pd

103.90403 6(4)

107

Pd

104.90508 5(4)

105.90348 6(4)

106.90509 7(5)

Cd+

65

108

Pd

107.90389 2(4)

109 109

Ag

108.90475 2(3)

108

Cd+

105.9065

Interferences from Cu ions (m/z)

108

Ag

106

Interferences from Cd ions (m/z)

107.9042

Cu40Ar+

104.8921 66

Interferences from Zn ions (m/z) Interferences from Pb ions (m/z)

107

Zn40Ar+

105.8884

67

Zn40Ar+

106.8895

68

Zn40Ar+

107.8872

208

Pb2+

103.9883

320 321

Table 2. Optimal isotope pairs of internal palladium standard for mass bias

322

correction Cogenetic/coexisting

Appropriate Pd isotope pair

Tolerance of impurities (molar ratios)

minerals Chalcopyrite

106

Pd-108Pd

Cu/Ag ≤ 50:1

Pyrite

104

Fe/Ag ≤ 600:1

Galena

105

Pb/Ag ≤ 10:1

Sphalerite

104

Zn/Ag ≤ 1:1

Pd-105Pd Pd-108Pd Pd-105Pd

323


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324 325 326

Figure 3. Matrix effects of Cu, Zn, Pb and Fe cations on the measured values.

109

Ag/107Ag

327 328 329 330

Figure 4. (a) Isobaric interference of Pb2+ on 104Pd and resultant Ag-Pd mass bias decoupling in the presence of lead ions; (b) Deviation of δ109Ag in NIST 978a with doping lead ions, measured using the 108Pd-105Pd isotope pair.

331



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

332 333 334 335 336

Figure 5. (a) Isobaric interference of 65Cu40Ar+ on 105Pd and resultant Ag-Pd mass bias decoupling in the presence of copper ions; (b) Correlation of ln(109Ag/107Ag) vs. ln(108Pd/106Pd) in the presence of copper ions; (c) Deviation of δ109Ag in NIST 978a with doping copper ions, measured using the 108Pd-106Pd isotope pair.

337 338

In order to check the analytical accuracy of this modified approach using an

339

appropriate Pd isotope pairs for mass bias correction, we measured δ109Ag values of

340

NIST 978a solution after doping with base metals, using a fixed and an optimal Pd

341

isotope pair (Table 2). Except for a systematic positive δ109Ag shift of +0.02 ‰ in the


Page 16 of 27

Page 17 of 27

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342

NIST 978a solution containing a little amount of Zn impurity (Zn:Ag ≤ 1:1), the

343

alternative Pd isotope-pairs directly avoids the isobaric interferences to produce

344

accurate δ109Ag data of 0.000 ± 0.013 ‰ for NIST 978a solution when the matrix

345

impurities are less than their individual tolerances (Table 3).

346

In summary, non-spectral matrix effects are adequately corrected by using the SSB

347

bracketing and Pd internal normalization approach, taking the matrix of iron ions as

348

an example. However, isobaric interferences of polyatomic ions (e.g.

349

66

350

coexisting minerals in silver deposits could induce dramatic shifts of δ109Ag from the

351

normal variation of silver isotopes in nature. Therefore, complete removal of matrix

352

ions via appropriate column chemistry procedures is mandatory for silver isotope

353

analysis in ore-deposits, especially for Zn2+ ions from sphalerite. The selection of

354

isotope pairs of internal palladium standard is optional, depending on the major

355

mineral components as we discussed above.

65

Cu40Ar+,

Zn40Ar+, 67Zn40Ar+, 68Zn40Ar+) and doubly-charged ions (e.g. Pb2+) originating from

356

Table 3. Measurements of δ109Ag for NIST 978a after doping different matrix

357

ions: results of changing Pd isotope pair Metal/Ag molar

Fixed Pd

ratios

isotope pair

Appropriate Pd

δ109Ag (‰, 2σ σ)

isotope pair

Cu/Ag: 0:1-50:1

104

0.055 ± 0.035

106

0.000 ± 0.012

Fe/Ag: 0:1-600:1

104

0.001 ± 0.006

104

0.001 ± 0.006

Pb/Ag: 0:1-10:1

104

0.076 ± 0.081

105

0.000 ± 0.013

104

+0.020 ± 0.056

Zn/Ag: 0:1-1:1

Pd-105Pd

δ109Ag (‰, 2σ σ)

Pd-105Pd Pd-105Pd

Pd-108Pd Pd-105Pd Pd-108Pd Pd-105Pd

358 359

Variation of silver isotopes in rocks, ore deposits and environmental

360

materials

361

Considering that the chemical compositions of ore-deposit samples are dramatically

362

different from those of terrestrial rocks or environmental materials, the two-step



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

363

ion-exchange procedure was modified on the basis of previous contributions [3,4,5] in

364

order to achieve full recovery of Ag and complete separation from other metals. As

365

shown in Table S5 and Table S6, a single run of separation/purification is insufficient

366

to remove such abundant matrix ions in ore-deposit samples (such as Fe, Zn and Cd

367

etc.), and the second-run of column separation could make the molar ratios of

368

metal/Ag less than 0.15 mol/mol generally (Table S6). It is worth noting that the

369

Zn/Ag is still as high as 2.29 mol/mol after the second run of ion-exchange separation

370

for the selected orogenic type Au-Ag ore-deposit sample with abundant sphalerite

371

mineral (i.e. ZK23-12-H32). The systematic positive shifts of +0.02 - +0.06 ‰ in

372

δ109Ag could be corrected directly if a little amount of Zn matrices (Zn/Ag ≤ 5:1) is

373

still left over in such kinds of samples.

374

Using the optimized analysis procedure, the silver isotopes in geological standard

375

materials and Ag-Au ore-deposit samples were precisely determined (Table 4). The

376

external reproducibilities of ±0.006 - ±0.009 ‰ for 100 ng mL-1 NIST 978 and ±0.005

377

- ±0.086 ‰ for geological standards and silver ore-deposit samples are obtained from

378

this work, which are equivalent to those reported by Luo [5] (i.e. ± 0.015 ‰ in

379

environmental materials) and Schönbächler [3] (i.e. ± 0.05 ‰ in terrestrial rocks). The

380

minor differences, ranging from -0.029 ‰ to +0.020 ‰, in δ109Ag values among

381

geological standard materials are consistent with the δ109Ag values of 0.00 ± 0.04 ‰

382

to -0.01 ± 0.05‰ in commercial inorganic silver products. Significant variations in

383

δ109Ag values from -0.014 ‰ to +0.698 ‰ were observed among different types of

384

Au-Ag ore deposits (e.g. ZK23-12-H32 (orogenic ore deposit), 16RJW-45, 16RJW-49,

385

160U-09 and160U-33 (Skarn-type ore deposit)).

386

In addition, the silver isotope compositions (given in ε109Ag) in rocks, ore-deposits,

387

and environmental materials were compared (Figure 6). The silver isotopic

388

composition of the Earth’s mantle is similar to primitive, volatile-rich chondrites, and

389

an ε109Ag value of +2.2 ± 0.7 (2σ) was estimated for the bulk silicate Earth (BSE)

390

relative to NIST 978a on the basis of mantle-derived samples [25]. The first survey of

391

ε109Ag in terrestrial rocks and meteorites demonstrated that silver isotope


Page 18 of 27

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392

fractionation did not occur within the inner solar system during condensation of the

393

solar nebular and chondrite parent body formation, while larger isotope fractionations

394

are associated with the redistribution and transportation of Ag between metal and

395

sulfide [8]. Further systematic investigations in various chondrites implied that a large

396

stable isotope fractionation up 1 ‰ in ordinary chondrites was most likely imposed by

397

redistribution and (or) loss of volatile elements during metamorphism, while

398

relatively limited ε109Ag variations of -0.8 to +2.1 in carbonaceous chondrites,

399

covering the chondritic meteorites Allende and Abee with ε109Ag values of +1.1 ± 1.8

400

and +0.5 ± 2.23, respectively [8], suggested that nebular processes and accretion did

401

not lead to a significant silver isotope fractionation [26].

402

Consistent ε109Ag results of +9.35 ± 0.68 (2σ) and +10.46 ± 0.25 (2σ) for the

403

Hawaiian basalt (KOO49-1) were obtained by Schönbächler [3] and Woodland [8]

404

respectively, which is the most positive ε109Ag end-member in nature so far. The

405

ε109Ag in the USGS standard rock SCO-1 (Cody Shale) was proved to be +1.0 ± 2.1

406

[8], and +0.12 ± 0.59 [27]. The silver isotope compositions of native silver and

407

electrum from well-known mines (Colorado, Michigan, Arizona, Canada, Germany,

408

Mexico, Italy, Chile, and one “synthetic” Ag crystal) revealed significant ε109Ag

409

variations ranging from -5.3 to +1.1 [28]. Native silver occurring in hydrothermal

410

base and noble metal deposits varies in ε109Ag values from -3.3 to +4.6, and the most

411

positive ε109Ag values from Cu-Mo porphyry and Ag epithermal deposits were

412

recognized to be predominantly derived from a mantle source, while the most

413

negative values were reported in sediment-hosted polymetallic deposits as derived

414

from crustal sources of ore metals [29]. The ε109Ag value of +1.1 in a volcanic hosted

415

massive sulfide deposit is close to the BSE value. The silver isotopic compositions in

416

ore deposits from Japan, including Au-Ag epithermal vein type deposits, Pb-Zn vein

417

deposits, polymetallic vein deposits and Kuroko type deposits, vary in ε109Ag from

418

-1.5 to +7.1 [29]. These deposits have similar geological settings, origins, and

419

depositional ages (14-13 Ma), and their large Ag isotope variations may reflect



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

Page 20 of 27

420

fractionation arising from differences in temperature of formation and host

421

mineralogy.

422

In addition, silver isotope composition in environmental processes was explored by

423

Luo [5]. The ε109Ag values varied from -0.44 in industrial sludge (SRM 2781) to

424

+0.25 in sediments (CRM PACS-2) and +0.61 in domestic sludge (SRM 2782). It is

425

worth noting that a larger positive value of +2.84 was observed in a fish liver (CRM

426

DOLT-4), implying significant silver isotope fractionation in biological uptake

427

processes. The silver isotope fractionation in environmental processes reveal that

428

precipitates (e.g. Ag nanoparticles) can be enriched in heavy Ag isotopes while the

429

residual Ag+ ions were enriched in lighter ones presumably during reduction, and the

430

adsorption of Ag+ resulted in enrichment of the heavy isotope

431

[31].

109

Ag in solid phase

432

Table 4. Silver isotope compositions in geological standards and ore-deposit

433

samples

Sample No IAEA-S-1a

Sources

δ109Ag (‰, 2σ σ, n≥4 )

Sample No

Sources

δ109Ag (‰, 2σ σ, n≥4 )

IAEA/Silver sulfide

+0.020 (0.011)

16RJW-45 b

Skarn-type

+0.472 (0.086)

deposits IAEA-S-2 a

IAEA/ Silver sulfide

-0.029 (0.005)

160U-33 b

Skarn-type deposits

+0.552 (0.027)

IAEA-S-3 a

IAEA/ Silver sulfide

-0.027 (0.006)

160U-09 b

Skarn-type deposits

+0.689 (0.010)

GBW04414 a

GBW/ Silver sulfide

-0.025 (0.007)

16RJW-49 b

Skarn-type deposits

+ 0.141 (0.084)

GBW04415 a

GBW/ Silver sulfide

-0.009 (0.017)

Ag metal c

Johnson Matthey

0.00 (0.04)

ZK23-12-H32

Orogenic ore deposit

-0.014 (0.029)

Ag nano powder c

Sigma-Aldrich

-0.01 (0.05)

b

434 435

a

: Samples not subjected to ion exchange; b: Samples subjected to ion exchange; c: The values of δ109Ag in Ag metal and Ag nano powder in the footnote (c) are cited from Luo [5]

436


Page 21 of 27

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437 438 439 440 441 442

Figure 6. Variations of ε109Ag in chondrites, rocks, ore-deposits, and environmental materials in nature, including ε109Ag values from previous contributions [3,4,5,8,27,28,29,30,32] and that in geological standards and orogenic and skarn-type Ag ore-deposit samples obtained in this work. All samples were analyzed in duplicate, and the average values are plotted.

443

444

CONCLUSIONS

445

Recent studies of mass-dependent fractionation of silver isotopes indicate several

446

areas of potential application, such as prospecting for noble metals, pollutant risk

447

assessment, and nanomaterial sciences. More accurate and precise silver isotope

448

analysis protocols are required to distinguish the small mass dependent silver isotope

449

fractionation in nature.

450

Mass discrimination and instrument drift were adequately corrected by a

451

combination of internal Pd normalization and standard-sample bracketing. An

452

identical mass bias for Pd and Ag was obtained for a Ag/Pd concentration ratio of

453

1:20, which not only maintains a linear relationship between instrumental mass

454

discrimination of Ag and Pd isotopes, but also makes non-spectral matrix effects



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

Page 22 of 27

455

negligible. The major matrix effects from paragenetic minerals in silver ore-deposits

456

are as follows: (i) A larger tolerance for matrix Fe ions was observed because iron

457

produces no direct interference from isobars. The SSB-Pd internal normalization

458

approach providea accurate silver isotope compositions fro samples with Fe/Ag molar

459

ratios up to 600:1; (ii) The isobaric interference of

460

(m/z=104) with

461

isotope pairs perform better as internal standards for Ag dissolved from copper and

462

lead enriched minerals, even

463

removal of matrix zinc ions is mandatory because of the direct isobaric interference of

464

67

104

65

Cu40Ar+ with

Pd induced linear shifts of δ109Ag,

106

105

Pd and

Pd-108Pd and

105

208

Pb2+

Pd-108Pd

after chemical separation/purification; (iii) Complete

Zn40Ar+ with 107Ag+.

465

Acceptable separation/purification of Ag from base-metal matrices can be achieved

466

by a two-step ion-exchange procedure that reduces the ratios of metal/Ag to less than

467

0.15 mol/mol, permitting precise and accurate silver isotope analysis in ore deposits.

468

With our optimized analysis procedure, δ109Ag in geological standard materials and

469

typical Au-Ag ore deposit samples varied from -0.029 ‰ to +0.689 ‰. A systemic

470

summary of δ109Ag (or ε109Ag) variations in rocks, ore-deposits, and environmental

471

materials in nature revealed that a wider δ109Ag variation range in ore-deposits and

472

environmental materials implies significant mass dependent silver isotope

473

fractionation in geological processes (e.g. magmatic exsolution and hydrothermal

474

alteration) and in physico-chemical and biological processes, including redox reaction,

475

photo-reduction, adsorption-dissolution, and biological cycling etc. Future acquisition

476

of Ag isotope data will enhance the understanding of ore-deposit formation.

477 478

Acknowledgements:

479

We are grateful to the editor Prof. R. Niessner and two reviewers for their valuable

480

scientific comments. This research was supported by the National Natural Science

481

Foundations of China (Grants Nos. 41673001, 41422302 and 41473042), the

482

Fundamental Research Funds for the Central Universities (Grant No. 20620140380),


Page 23 of 27

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483

and the State Key Laboratory of Geological Processes and Mineral Resources (Grant

484

No. GPMR201507).

485 486

Appendix A. Supporting information

487

Supplementary data associated with this article can be found in the online version.

488

489

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490 491

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[32] A. V. Chugaev, I. V. Chernyshev, High-noble measurement of

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Ag/

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Ag in

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For Table of Contents Use Only

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Manuscript title: Matrix effects originating from coexisting minerals and accurate

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determination of stable silver isotopes in silver deposits

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Name(s) of all other authors: Qi Guo, Hai-Zhen Wei, Shao-Yong Jiang, Simon

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Hohl, Yi-Bo Lin, Yi-Jing Wang, Yin-Chuan Li

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Matrix Effects Originating from Coexisting Minerals ... - ACS Publications

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