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Improved Precision and Accuracy of Quantification of Rare Earth Element Abundances via Medium-Resolution LA-ICP-MS

  • Rebecca Funderburg
  • Ricardo ArevaloJr.
  • Marek Locmelis
  • Tomoko Adachi
Research Article

Abstract

Laser ablation ICP-MS enables streamlined, high-sensitivity measurements of rare earth element (REE) abundances in geological materials. However, many REE isotope mass stations are plagued by isobaric interferences, particularly from diatomic oxides and argides. In this study, we compare REE abundances quantitated from mass spectra collected with low-resolution (m/Δm = 300 at 5% peak height) and medium-resolution (m/Δm = 2500) mass discrimination. A wide array of geological samples was analyzed, including USGS and NIST glasses ranging from mafic to felsic in composition, with NIST 610 employed as the bracketing calibrating reference material. The medium-resolution REE analyses are shown to be significantly more accurate and precise (at the 95% confidence level) than low-resolution analyses, particularly in samples characterized by low (<μg/g levels) REE abundances. A list of preferred mass stations that are least susceptible to isobaric interferences is reported. These findings impact the reliability of REE abundances derived from LA-ICP-MS methods, particularly those relying on mass analyzers that do not offer tuneable mass-resolution and/or collision cell technologies that can reduce oxide and/or argide formation.

Graphical Abstract

Keywords

Precision Laser ablation Magnetic sector LA-ICP-MS Rare earth elements 

Introduction

The rare earth elements (REE; i.e., La through Lu), share a common electronic structure: [Xe] 6s2 5d x 4f y , where 1 ≥ x ≥ 0 and 0 ≥ y ≥ 14. Consequently, under many environmental conditions found on Earth (e.g., pressure, temperature, and importantly oxygen fugacity), these elements form trivalent cations (i.e., M3+, commonly from the loss of one 4f and two 6s electrons) chemical behaviors of which are controlled primarily by systematic variations in their respective ionic radii. In the Earth sciences, the predictable geochemical behaviors of the REE can be and is commonly exploited to identify magma source contributions, discern geographical provenances, and characterize environmental conditions and geological processes, including (but not limited to):
  • Distinction of respective sources of mantle- and crustal-derived materials through proxies like Ce/Pb, Ti/Eu, Y/Ho, and Sr/Nd ratios (e.g., Hofmann, 2003 [1]; Workman and Hart, 2005 [2]; Arevalo and McDonough, 2010) [3], and identification of source lithology and/or specific mineralogy via diagnostic chemical signatures, such as the characteristic depression in the normalized abundances of heavy REE observed in derivatives of garnet-bearing peridotite (White, 2013) [4];

  • Depletion of incompatible elements (e.g., light REE relative to heavy REE) due to multiple episodes of partial melting and melt extraction (e.g., Hofmann, 1988) [5], or enrichment in incompatible elements via low degrees of melting (e.g., Sun and McDonough, 1989) [6], crustal contamination (McDonough, 1990) [7] and/or metasomatic overprinting (McKenzie, 1989) [8];

  • Classification of calcium aluminum inclusions (CAIs) and other refractory meteorite phases from patterns of normalized REE abundances (MacPherson et al. 1988) [9], and fractionation of refractory and volatile solar system materials during planetary accretion (Dauphas et al. 2015) [10];

  • Determination of oxidation states, and reconstructions of local oxygen fugacity (fO2) levels, based on Ce and Eu concentration anomalies preserved in magmatic zircons, caused by redox-sensitive valence states of Ce (3+/4+) and Eu (2+/3+) (Trail et al. 2012) [11]; and,

  • Identification of economically viable ores of REE, which are commonly exploited for catalysts, such as automotive catalytic converters; battery alloys; high-strength permanent magnets; glassmaking and polishing compounds; and light-emitting diodes (LEDs) (Goonan 2011) [12].

The measurement and quantification of REE abundances in geological and/or planetary materials with high precision and accuracy are essential to addressing these science objectives with confidence, particularly as most rocks and minerals only contain a few μg/g (or less) levels of these elements.

REE abundances are routinely measured on quadrupole inductively coupled plasma mass spectrometer (ICP-MS) equipment. However, these instruments offer only limited mass resolving powers (typically m/Δm < 500 at 5% peak intensity), restricting their ability to distinguish targeted mass peaks from potential isobaric interferences that occur when two or more atomic and/or molecular species have overlapping mass-to-charge (m/z) ratios. During the analysis of REE abundances in common geological materials, such isobaric interferences occur primarily as elemental interferences, such as 142Nd at mass station 142Ce; diatomic oxides, such as 126Te16O at 142Ce; and diatomic argides, such as 102Ru40Ar at 142Ce.

Double-focusing sector field instruments that offer adjustable mass discrimination may isolate these potential interferences, albeit at the expense of sensitivity. Collision induced dissociation (CID) cells reduce oxide and argide formation with varying degrees of efficacy, but CID cannot differentiate competing elemental isobars or double-charged interferences. Further, CID cells are normally only found on quadrupole ICP-MS instruments; sector field instruments are rarely equipped with this hardware. Thus, the method pioneered here could be used in concert with or in lieu of a CID cell.

As such, the primary objective of this study is to determine if medium mass-resolution (m/Δm > 500 at 5% peak intensity) is required for the highest precision/accuracy measurements of REE in a range of mantle-derived materials.

Instrumentation and Operating Conditions Employed for This Study

A suite of geological reference materials (Figure 1), described further below, was analyzed at NASA Goddard Space Flight Center using a Nu Instruments AttoM single-collector ICP-MS. This instrument offers tuneable mass-resolution from m/Δm = 300 up to 10,000 (measured at 5% of the maximum peak intensity), and was coupled to a Photon Machines Ultra Short Pulse Analyte G2 (ArF excimer) laser system that generates 193 nm radiation with energy densities up to >15 J/cm2, pulse widths of <4 ns, and repetition rates up to 300 Hz. Elemental abundances were measured using low- and medium-resolution mass discrimination (i.e., m/Δm = 300 and m/Δm = 2500, respectively). All other parameters (e.g., laser fluence/shots, gas flow rates, forward power, etc.; Appendix I) were held constant in order to unambiguously identify and isolate potential isobaric interferences on each monitored isotope mass station (Table 1). Multiple mass stations for each element were analyzed in order to verify terrestrial isotope ratios in the absence of obvious isobars.
Figure 1

Compositional characterization of the reference materials analyzed here, which span several orders of magnitude in trace element abundances (as represented by Th). According to the International Union of Geological Sciences (IUGS), the USGS glasses BHVO-2G and BIR-1G are classified as basalts, BCR-2G as a basaltic andesite, and the GSC, GSD, and GSE glasses as trachyandesites. According to the same standards, NIST SRM 610 is rhyolitic in composition

Table 1

Measured Isotope Mass Stations, Abundances, and Potential Isobaric Interferences During Nominal LA-ICP-MS Analysis

Element

Isotope

Mass station (Da)

Relative abundance

Hydrides (MH)

Oxidesa (M16O)

Argides (M36Ar)

Argides (M38Ar)

Argides (M40Ar)

Monoatomic interferences

La

138La

137.91

0.1%

137Ba

122Sn, 122Te

102Ru, 102Pd

100Mo

98Mo, 98Ru

138Ba, 138Ce

139La

138.91

99.9%

138Ce, 138La, 138Ba

123Sb, 123Te

103Rh

101Ru

99Ru

 

Ce

140Ce

139.91

88.5%

139La

124Sn, 124Te, 124Xe

104Ru, 104Pd

102Ru, 102Pd

100Mo, 100Ru

 

142Ce

141.91

11.1%

141Pr

126Te, 126Xe

106Pd

102Ru, 102Pd

102Ru, 102Pd

142Nd

Pr

141Pr

140.91

100.0%

140Ce

125Te

105Pd

103Rh

101Ru

 

Nd

142Nd

141.91

27.2%

141Pr

126Te, 126Xe

106Pd

104Ru, 104Pd

102Ru, 102Pd

142Ce

144Nd

143.91

23.8%

143Nd

128Te, 128Xe

108Pd, 108Cd

106Pd

104Ru, 104Pd

144Sm

146Nd

143.91

17.2%

145Nd

130Te, 130Ba, 130Xe

110Pd, 110Cd

108Pd, 108Cd

106Pd

 

Sm

147Sm

146.91

15.0%

146Nd

131Xe

110Pd, 110Cd

109Ag

107Ag

 

152Sm

151.92

26.8%

151Eu

136Xe, 136Ba, 136Ce

116Cd, 116Sn

114Cd, 114Sn

112Cd, 112Sn

152Gd

154Sm

153.92

22.8%

153Eu

138Ba, 138La, 138Ce

118Sn

116Cd, 116Sn

114Cd, 114Sn

154Gd

Eu

151Eu

150.92

47.8%

150Sm

135Ba

115In, 115Sn

113Cd, 113In

111Cd

 

153Eu

152.92

52.2%

152Sm

137Ba

117Sn

115In, 115Sn

113Cd, 113In

 

Gd

156Gd

155.92

20.5%

155Gd

140Ce

120Sn, 120Te

118 Sn

116Cd, 116Sn

156Dy

158Gd

157.92

24.8%

157Gd

142Ce, 142Nd

122Sn, 122Te

120Sn, 120Te

118Sn

158Dy

Tb

159Tb

159.93

100.0%

158Gd

143Nd

123Sb, 123Te

121Sb

119Sn

 

Dy

162Dy

161.93

25.5%

161Dy

146Nd

126Te, 126Xe

124Sn, 124Te, 124Xe

122Sn, 122Te

162Er

163Dy

162.93

24.9%

162Dy, 162Er

147Sm

127I

125Te

123Sb, 123Te

 

164Dy

163.93

28.2%

163Dy

148Nd, 148Sm

128Te, 126Xe

126Te, 126Xe

124Sn, 124Te, 124Xe

164Er

Ho

165Ho

164.93

100.0%

164Dy, 164Er

149Sm

127I

127I

125Te

 

Er

166Er

165.93

33.6%

165Ho

150Nd, 150Sm

130Te, 130Ba, 130Xe

128Te, 126Xe

126Te, 126Xe

 

167Er

166.93

22.9%

166Er

151Eu

131Xe

129Xe

127I

 

168Er

167.93

26.8%

167Er

152Sm, 152Gd

132Ba, 132Xe

130Te, 130Ba, 130Xe

128Te, 126Xe

 

Tm

169Tm

169.94

100.0%

168Er

153Eu

133Cs

131Xe

129Xe

 

Yb

171Yb

170.94

14.3%

170Er

155Gd

135Ba

133Cs

131Xe

 

172Yb

171.94

21.8%

171Yb

156Gd, 156Dy

136Xe, 136Ba, 136Ce

134Xe, 134Ba

132Ba, 132Xe

 

173Yb

172.94

16.1%

172Yb

157Gd

137Ba

135Ba

133Cs

 

Lu

175Lu

174.94

97.4%

174Yb

159Tb

139La

137Ba

135Ba

 

176Lu

175.94

2.6%

175Lu

160Gd, 160Dy

140Ce

138Ba, 138La, 138Ce

136Xe, 136Ba, 136Ce

176Yb

Hf

177Hf

176.94

18.6%

176Hf

161Dy

141Pr

139La

137Ba, 137La, 137Ce

 

178Hf

177.94

27.3%

177Hf

162Dy, 162Er

142Ce, 142Nd

140 Ce

138Ba, 138La, 138Ce

 

180Hf

179.95

35.1%

179Hf

164Dy

144Sm, 144Nd

142Ce, 142Nd

140Ce

 

a Diatomic oxides M17O and M18O represent much lower risks of serving as isobaric interferences due to their limited isotopic abundances (i.e., 17O/16O ≈ 10-4, 18O/16O ≈ 10–3), and the low oxide production rates (i.e., ThO/Th < 0.20%) supported by the method described here

Low mass resolving powers (i.e., m/Δm = 300) are commonly associated with commercial quadrupole mass spectrometers, and considered to be the default for sector field instruments. However, a higher mass resolving power of m/Δm = 2500 was chosen for this study in order to enable the separation of key isobaric interferences (e.g., those with high abundances and/or high molecular formation efficiencies) while preserving maximum ion transmission/signal intensity, and by extension, instrument sensitivity. This specific mass resolving power was determined to be the lowest acceptable setting given probabilities of oxide and argide formation (typically <0.20% and <0.01%, respectively), and expected cation concentrations found in common geological materials (Table 2). Potential isobars that remain irresolvable at a mass resolving power of m/Δm = 2500 (5% peak height) are generally characterized by expected abundances several orders of magnitude below those expected across the explored range of terrestrial compositions (Appendix II). For reference, given prototypical peak shapes/kurtosis at m/Δm = 2500 at 5% of the maximum peak height, competing isobars may be resolved at an equivalent resolution of m/Δm ≈ 3300 at full width of half maximum (FWHM) of the peak height, enabling high-fidelity spectral quantitation via peak top comparisons and/or smaller peak area integration windows (Table 2; Appendix III).
Table 2

Recommended Mass Stations for REE Analysis via LA-ICP-MS Methods, and Model Compositions of Common Geological Materials to Which This Method May Be Applied

Preferred isotope

Relative abundance

Interferencesa at resolution (m/Δm = 300, 5%)

Interferencesa at resolution (m/Δm = 1500, 5%)

Interferencesa a resolution (m/Δm = 2500, 5%)

Relevant interferences a,b above m/Δm = 2500 (5%)

Representative compositions of common geological materials (in μg/g)

CI Chondritec

MORBd

OIBe

BCCf

Basalt Zircong

Granitoid Zircong

Kimberlite Zircong

Lamproite Zircong

Carbonatite Zircong

139La

99.9%

138CeH, 138LaH, 138BaH, 123Sb16O, 123Te16O, 99Ru40Ar

138CeH, 138LaH, 138BaH, 123Sb16O, 123Te16O, 99Ru40Ar

138CeH, 138LaH, 138BaH, 123Te16O

None

0.25

3.8

37

20

-

12

<0.09

<0.20

0.17

140Ce

88.5%

139LaH, 124Se16O, 124Te16O, 124Xe16O, 100Mo40Ar, 100Ru40Ar

139LaH, 124Se16O, 124Te16O, 124Xe16O, 100Mo40Ar, 100Ru40Ar

139LaH, 124Se16O, 124Te16O, 124Xe16O

None

0.64

12

80

43

2.1

61

0.87

7.9

4.5

141Pr

100%

140CeH, 125Te16O, 101Ru40Ar

140CeH, 125Te16O, 101Ru40Ar

140CeH, 125Te16O

None

0.10

1.7

9.7

4.9

0.08

8

<0.08

0.15

0.54

144Nd

23.8%

143NdH, 128Te16O, 128Xe16O, 104Ru40Ar, 104Pd40Ar, 144Sm

143NdH, 128Te16O, 128Xe16O, 104Ru40Ar, 104Pd40Ar, 144Sm

143NdH, 128Te16O, 128Xe16O, 144Sm

144 Sm

0.47

9.8

39

20

0.90

45

0.39

1.8

6.2

154Sm

22.8%

153EuH, 138Ba16O, 138La16O, 138Ce16O, 114Cd40Ar, 114Sn40Ar, 154Gd

153EuH, 138Ba16O, 138La16O, 138Ce16O, 114Cd40Ar, 114Sn40Ar, 154Gd

153EuH, 138Ba16O, 138La16O, 138Ce16O,154Gd

138 Ba 16 O, 138 Ce 16 O, 154 Gd

0.15

3.3

10

3.9

1.8

22

0.53

3.3

6.6

153Eu

52.2%

152SmH, 137Ba16O, 113Cd40Ar, 113Ir40Ar

152SmH, 137Ba16O, 113Cd40Ar, 113Ir40Ar

152SmH, 137Ba16O

137 Ba 16 O

0.06

1.2

3.0

1.1

1.6

2.1

0.36

0.50

4.0

158Gd

24.8%

157GdH, 142Ce16O, 142Nd16O, 118Sn40Ar, 158Dy

157GdH, 142Ce16O, 142Nd16O, 118Sn40Ar, 158Dy

157GdH, 142Ce16O, 142Nd16O, 158Dy

1142 Ce 16 O, 142 Nd 16 O, 158 Dy

0.20

4.4

7.6

3.7

11

58

1.5

15

20

159Tb

100%

158GdH, 143Nd16O, 119Sn40Ar

158GdH, 143Nd16O, 119Sn40Ar

158GdH, 143Nd16O

143 Nd 16 O

0.04

0.74

1.1

0.60

-

-

-

-

-

163Dy

24.9%

162DyH, 147Sm16O, 123Sb40Ar, 123Te40Ar, 158Dy

162DyH, 147Sm16O, 123Sb40Ar, 123Te40Ar, 158Dy

162DyH, 147Sm16O, 158Dy

147 Sm 16 O

0.25

5.1

5.6

3.6

35

225

4.1

37

56

165Ho

100%

164DyH, 164ErH, 149Sm16O, 145Te40Ar

164DyH, 164ErH, 149Sm16O, 145Te40Ar

164DyH, 164ErH, 149Sm16O

149 Sm 16 O

0.06

1.1

1.1

0.77

15

85

1.3

14

15

167Er

22.9%

166ErH, 151Eu16O, 127I40Ar

166ErH, 151Eu16O, 127I40Ar

166ErH, 151Eu16O

151 Eu 16 O

0.17

3.2

2.6

2.1

65

378

4.3

63

53

169Tm

100%

168ErH, 153Eu16O, 129Xe40Ar

168ErH, 153Eu16O, 129Xe40Ar

168ErH, 153Eu16O

153 Eu 16 O

0.03

0.45

0.35

0.28

-

-

-

-

-

172Yb

21.8%

156Gd16O, 156Dy16O, 132Ba40Ar, 132Xe40Ar

156Gd16O, 156Dy16O, 132Ba40Ar, 132Xe40Ar

156Gd16O, 156Dy16O

156 Gd 16 O

0.17

3.0

2.2

1.9

118

769

5.6

126

64

175Lu

97.4%

174YbH, 159Tb16O, 135Ba40Ar

174YbH, 159Tb16O, 135Ba40Ar

174YbH, 159Tb16O

159 Tb 16 O

0.03

0.45

0.30

0.30

25

150

1.1

28

11

178Hf

27.3%

177HfH, 162Dy16O, 162Er16O, 138Ba40Ar, 138La40Ar, 138Ce40Ar

177HfH, 162Dy16O, 162Er16O, 138Ba40Ar, 138La40Ar, 138Ce40Ar

177HfH, 162Dy16O, 162Er16O

162 Er 16 O

0.11

2.4

7.8

3.7

6100

12800

13100

10000

10400

a Listed interferences cannot be resolved at either 5% of the maximum peak height or full width at half maximum (FWHM) of the peak intensity

b Irresolvable monoatomic interferences and diatomic oxides and argides with expected cation abundances within a factor of 100 of the targeted analyte (given the range of terrestrial compositions provided in this table).

c CI carbonaceous chondrite composition from Palme and Jones [13]

d Global mid-ocean ridge basalt (MORB) composition from Arevalo and McDonough [3]

e Ocean island basalt (OIB) composition from Sun and McDonough [14]

f Bulk continental crust (BCC) model composition from Rudnick and Gao [15]

g Median compositions of zircons from a range of rock types from Belousova et al. [16]

Prior to each daily run of analyses, the ion lenses and the position of the torch of the ICP-MS were tuned to maximize signal (based on 238U spectra) and minimize oxide production (ThO/Th < 0.20%) first in low-resolution (m/Δm = 300, 5% peak height), then in medium-resolution (m/Δm = 2500, 5% peak height). A three-stage mass calibration technique was used to calibrate the magnet for accurate peak scanning: the first calibration was completed in low-resolution with a wide search window, then in medium-resolution with a wide search window, and finally in medium-resolution with a narrow search window. Peak shapes were verified individually and the scanning deflectors were calibrated prior to each run. The spot size for both low- and medium-resolution scans was held constant (diameter = 150 μm) in order to circumvent disproportionate laser-induced elemental fraction (LIEF) associated with laser pit aspect ratios (driven by the efficiency by which particles can be extracted from the ablation site), and ultimately allow for a direct comparison between low- and medium-resolution techniques.

The complete list of mass stations monitored in this study is provided in Table 2, along with the most likely monoatomic and diatomic isobaric inferences introduced during laser processing of oxygen-rich geological materials in an ambient He environment, and sample injection into an argon plasma torch. Elemental abundances were quantified for 138, 139La, 140, 142Ce, 141Pr, 142, 144, 146Nd, 147, 152, 154Sm, 151, 153Eu, 156, 158Gd, 159Tb, 162, 163, 164Dy, 165Ho, 166, 167, 168Er, 169Tm, 172, 173Yb, 175, 176Lu, and 177, 178, 180Hf, with 178Hf serving as the internal standard. Detection parameters are outlined in the Appendix.

Prior to each analysis, a blank signal was collected for 30 s with the laser firing but the shutter closed to collect a background signal with all sources of electronic noise present, including Johnson, flicker, and shot varieties. The background signal was subtracted from the analyte signal for data quantitation. Helium was used as the carrier gas at a flow rate of 1.00 L/min because of its higher ionization potential relative to other carrier gas options (Russo 1995) [17]; all ablations occurred in an ambient He atmosphere within the laser cell; the gas dynamics at the ablation site, as driven by the gas flow rate and the HelEx high-performance two-volume sample cell, achieved >99% signal washout within less than a single s, thereby maximizing spatial resolution (including depth profiling). Between the ablation of each sample and reference material, the ablation system was flushed with He gas for 30 s to statistically eliminate any memory effects and/or hysteresis.

Data Processing

For all analyses, NIST 610 served as the standard; values for the major, minor, and trace element composition of this material were taken from Jochum et al. [18]. Raw data were collected using the Nu AttoM time sesolved analysis (TRA) software package. The software was used to identify background and active signals, as well as to calculate first-order statistics, including count rate averages, standard deviations, and statistical outliers (e.g., outside 3× the interquartile range). Subsequently, abundances were quantitated manually by correcting for instrumental drift (assuming a linear function) and converting count rates to concentrations, assuming 178 Hf as the internal standard.

Results

Low- Versus Medium-Resolution

Fractionation plots for each geological sample analyzed in low- (m/Δm = 300, 5% peak height) and medium-resolution (m/Δm = 2500, 5% peak height) are shown in Figure 2. In this type of diagram, discrepancies between published values of REE abundances in the suite of geological materials investigated here (Jochum et al. 2005) and those derived from this study are shown as deviations from unity (y = 1). Data points that plot above unity in any single sample material indicate a problematic isobar at that particular mass station in the analyte under those analytical conditions (i.e., mass discrimination). Conversely, data points that plot below unity suggest an isobar on that particular mass station in the bracketing reference material. See Appendix IV for complete results.
Figure 2

Comparison of quantitated REE abundances in the USGS reference glasses measured via low-resolution (m/Δm = 300 at 5% peak height) and medium-resolution (m/Δm = 2500 at 5% peak height) mass discrimination and compared with published values (Jochum et al. 2005). Significantly more scatter and larger absolute deviations in the data collected in low-resolution indicate isobaric interferences on the monitored mass stations (some of which is also observed in medium-resolution)

As shown in Figure 2, deviations between the quantitated values derived here and those found in the literature can vary by up to an order of magnitude in both low- and medium-resolution measurements at specific mass stations. These mass stations are plagued by isobaric interferences that require even greater mass resolving powers than those evaluated in this study, such as those outlined in Table 2. In nearly all cases, however, low-resolution measurements of the monitored mass stations deviate more intensely than those collected in medium-resolution. In particular, quantitated values of REE abundances collected in medium-resolution for GSC-1 g, BHVO-2 g, and BCR-2 g are significantly better aligned with the accepted published values for these elements. Some mass stations, such as 138 La, deviate significantly from the published values due to low relative abundances. However, for the majority of the mass stations, the low-resolution data set for these reference materials show variations between 0.3 and 3× published values, whereas the medium-resolution data generally fall within a few tens of percent.

Both low- and medium-resolution mass discrimination generate highly corroborative data in accord with published values for GSD-1 g and GSE-1 g. Because these samples are of the same lithology as GSC-1 g, the determined REE abundances of which are shown to be more sensitive to mass resolution, the uniform behavior of GSD-1g and GSE-1 g is likely due to their compositions enriched in incompatible trace elements, including the REE; elevated abundances of REE equate to higher counting rates, and by extension, reduced vulnerability to isobaric interferences. In contrast, significant scatter is seen with both resolving powers for BIR-1 g, the sample with the lowest concentrations of REE. In the case of BIR-1 g and other similarly depleted samples, counting statistics due to low REE abundances and limited count rates may ultimately limit the precision/accuracy of these measurements.

Ultimate Accuracy and Preferred Mass Stations

Medium-resolution mass discrimination is shown in Figure 2 to generate data that substantiate published values with equal or higher frequency compared with data collected via low-resolution techniques. In order to establish a more quantitative comparison, however, the absolute accuracies of both methods implemented here are evaluated in Figure 3. Best-fit bivariate linear regression statistics, which account for the uncertainties in both x- and y-coordinates (as opposed to conventional univariate least-squares methods), are provided for both the low- and medium-resolution data. In these types of graphical comparisons, a linear regression with a slope equal to unity (i.e., m = 1.0, within analytical uncertainty) indicates statistical uniformity between the data derived here and previously published data. As seen qualitatively (data point proximity to 1:1 line) and quantitatively (regression statistics) in these plots, data derived from medium-resolution mass discrimination correlate more closely with the REE abundances established in the literature compared with low-resolution data. More specifically, within analytical uncertainty (95% confidence envelope), the medium-resolution data acquired here are statistically indistinguishable from those determined by Jochum et al. (2005), which include measurements of multiple splits of both glass and powder of each material analyzed by LA-ICP-MS, isotope dilution ICP-MS, and isotope dilution thermal ionization mass spectrometry (TIMS). Although many of the linear regression analyses for the low-resolution data also coincide with a slope of m = 1 within uncertainty, the probability of these trend lines coinciding with a slope of unity is statistically less likely than the medium-resolution trend lines.
Figure 3

Accuracy of abundances measured here in low and medium-resolution. Error bars in the y-axis represent 2 sigma uncertainties on the external precision of four replicate measurements; error bars on the x-axis represent 95% confidence levels in the published values (Jochum et al. 2005). Statistics reported in each panel represent bivariate linear regression analyses, which incorporate analytical uncertainties in both x- and y-axis variables (unlike conventional univariate regressions). In this graphical representation, accuracy is manifest as a slope of unity (m = 1.0). Table I. LA-ICP-MS operational parameters employed during this study

Based primarily on the fractionation plots discussed above (Figure 2) and bivariate linear regression statistics (Figure 3), in Table 2 we have derived a list of preferred mass stations to quantitate the abundance of each REE via LA-ICP-MS. Preference was given to mass stations with the highest isotopic abundance in order to emphasize higher counting rates and by extension achievable internal precision (Poisson statistics). Table 2 also provides representative compositions of an array of geological materials commonly analyzed for REE abundances, including chondritic materials, oceanic basalts, continental crust, and zircons of different genetic origins, in order to enable risk assessment for potential isobaric interferences.

Conclusions

In situ LA-ICP-MS offers spatially resolved measurements of μm-size targets, such as individual minerals grains, while supporting minimal analytical blanks and low limits of detection. Moreover, such techniques avoid contamination risks associated with sample processing (e.g., high-temperature acid digestion, column chemistry, etc.) and consume orders-of-magnitude smaller quantities of sample (i.e., μg) compared with traditional solution techniques (i.e., mg). However, LA-ICP-MS methods are often complicated by isobaric interferences introduced during the ablation of the sample matrix, such as diatomic argides and oxides, particularly during the measurement of trace elements (i.e., μg/g level concentrations).

Here, we analyzed multiple isotopes of each REE with low- (m/Δm = 300, 5% peak intensity) and medium-resolution (m/Δm = 2500) mass discrimination in a suite of geological reference materials (i.e., BCR-2G, BHVO-2G, BIR-1G, and GSC/GSD/GSE-1G) in order to assess if higher mass resolving powers than those offered by quadrupole ICP-MS instruments are required for the highest precision/accuracy data. In summary, REE abundances quantitated from medium-resolution mass spectra are shown to be statistically equal (with 95% statistical confidence) to values established in the literature [18]; in contrast, data collected in low-resolution show significant deviations from published values, indicating spectral susceptibility to isobaric interferences (Figure 4).
Figure 4

A representative spectra for 141Pr is shown. The low-resolution (m/Δm = 300) collects the highest signal, but contains many unresolved isobaric interferences. Analyses at m/Δm = 2500 resolve many of these isobaric interferences without sacrificing as much signal as analyses at m/Δm = 4000

Notes

Acknowledgements

Funding for this study was provided through the NASA Science Innovation Fund and Center for Research and Exploration in Space Science and Technology (CRESST). R.F. acknowledges financial support from the Universities Space Research Association. R.F. also thanks Megan Elwood Madden for serving as a faculty sponsor, which enabled earning academic credit. R.A. Jr. acknowledges support from the NASA Astrobiology Institute (NAI) Cooperative Agreement Notice Cycle 7 (CAN7)

Supplementary material

13361_2017_1745_MOESM1_ESM.xlsx (12 kb)
Appendix I Operating parameters for low- and medium-resolution LA-ICP-MS analysis. List of resolving power required to resolve isobaric interferences. (XLSX 12 kb)
13361_2017_1745_MOESM1_ESM.xlsx (12 kb)
Appendix II List of resolving power required to resolve isobaric interferences.
13361_2017_1745_MOESM2_ESM.jpg (36 kb)
Appendix IIIa Representative spectra illustrating the 5% peak intensity and 50% full width half maximum resolving powers for low- and medium-resolutions measured. (JPG 35 kb)
13361_2017_1745_MOESM3_ESM.jpg (36 kb)
Appendix IIIb (JPG 35 kb)
13361_2017_1745_MOESM4_ESM.xlsx (38 kb)
Appendix IV Table of results. (XLSX 38 kb)

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Copyright information

© American Society for Mass Spectrometry 2017

Authors and Affiliations

  • Rebecca Funderburg
    • 1
  • Ricardo ArevaloJr.
    • 2
  • Marek Locmelis
    • 3
  • Tomoko Adachi
    • 2
  1. 1.Texas Christian UniversityFort WorthUSA
  2. 2.NASA Goddard Space Flight CenterGreenbeltUSA
  3. 3.Department of Geosciences and Geological and Petroleum EngineeringMissouri University of Science and TechnologyRollaUSA

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