Abundance and Impact of Doubly Charged Polyatomic Argon

Jun 16, 2016 - Doubly charged molecular ions of alkaline earth metals and argon could be identified as spectral interferences in an inductively couple...
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Abundance and Impact of Doubly Charged Polyatomic Argon Interferences in ICPMS Spectra Bodo Hattendorf,*,† Bianca Gusmini,† Ladina Dorta,† Robert S. Houk,‡ and Detlef Günther† †

Department of Chemistry and Applied Biosciences, Laboratory for Inorganic Chemistry, ETH Zurich, Vladimir Prelog Weg 1, 8093 Zurich, Switzerland ‡ Ames Laboratory, U.S. Department of Energy, Department of Chemistry, Iowa State University, Ames Iowa 50011, United States S Supporting Information *

ABSTRACT: Doubly charged molecular ions of alkaline earth metals and argon could be identified as spectral interferences in an inductively coupled plasma mass spectrometer. These molecular ions were found to occur at abundances reaching about 10−4 relative to the alkaline earth atomic ion abundances. They can thus substantially affect ultratrace analyses and, when present at similar concentration as the analyte elements, also isotope ratio measurements. For the case of Cu and Zn isotope ratio analyses, the same mass concentration of Sr was found to alter the measured 63Cu/65Cu and 64Zn/66Zn isotope ratios by −0.036‰ to −0.95‰ due to SrAr2+, appearing at m/Q 63 and 64. BaAr2+ can affect Sr isotope analyses, MgAr2+ may impair S isotope ratio measurements, while CaAr2+ may cause interference to Ca+ isotopes. The abundances of the doubly charged molecular ions were higher than those of the corresponding singly charged species, which is in accordance with their generally higher bond dissociation energies. The relative abundances were found to depend significantly on the inductively coupled plasma (ICP) operating conditions and generally increase with increasing carrier gas flow rates or lower gas temperature of the ICP. They also increase by about an order of magnitude when a desolvated aerosol is introduced to the ICP.

S

occurring interferences that can be caused by the sample components, their actual isotope ratio value, and the mass discrimination of the ICPMS. The most prominent spectral interferences are isobaric ions of the elements or singly charged molecular ions, consisting of isotopes of the abundant plasma components (i.e., argon, solvents) alone or containing isotopes of elements that abundantly occur in the sample material. Interferences of this type are numerous and can be formed by practically all elements in the periodic table. The abundance of these interferences is then determined by their concentration and the respective chemical properties that govern dissociation within the ICP or reactions during ion extraction via the vacuum interface. They most frequently occur as singly charged diatomic ions, but also complex molecules and/or clusters have been observed.6,7 Another important group are doubly charged ions, which are mainly formed from elements with a second ionization energy (IE) lower than 15.7 eV (1st ionization energy of Ar). Typical examples are the alkaline earth metals (excluding Be), rare earth elements, and several others, such as Y, Nb, Sn, Hf, Re, and Pb, and actinides, such as Th and U. The occurrence of multiply charged molecular ions in an ICPMS was until now only described for species combining high oxygen affinity and low second ionization energy, such as ThO2+ and ThOH2+.24

pectral interferences in inductively coupled plasma mass spectrometry (ICPMS) constitute one of the major challenges in accurate and precise element and isotope ratio analyses. For high precision isotope ratio analysis, aiming at discrimination of variations in the 10−4 to 10−5 range, this problem amplifies because even minute amounts of a spectral interference can alter the measured intensity ratio by more than the measurement uncertainty. This causes the isotope ratio to be biased in proportion to the concentration ratio of interference and analyte. Much effort has been made in ICPMS in identifying potential spectral interferences from its early days onward,1−11 and a careful chemical isolation of the element of interest usually is the key to obtain highly precise and accurate data.12−15 Matrix separation may, however, not be sufficiently effective or cannot be carried out before the analysis, most prominently when using laser ablation for spatially resolved sampling. Spectral interferences then need to be accounted for by different means. In several cases, higher mass resolving power16 can be employed when the exact isotope masses differ substantially. Spectral interferences requiring mass resolving power (m/Δm) of >12000, however, can usually not be accounted for by this, and the sensitivity loss additionally limits the attainable precision and accuracy. In a similar fashion, collision or reaction cells17−21 can be employed in specific cases. Frequently, mathematical correction of the interference signal is carried out to account for the spectral interference by measuring an (noninterfered) isotope of the interfering species and by subtracting the fraction of the interference signal via a known isotope ratio22 or using spectral deconvolution.23 This, of course, relies on a-priori knowledge of all potentially © 2016 American Chemical Society

Received: April 24, 2016 Accepted: June 16, 2016 Published: June 16, 2016 7281

DOI: 10.1021/acs.analchem.6b01614 Anal. Chem. 2016, 88, 7281−7288

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Analytical Chemistry This study had initially set out to investigate matrix-induced mass bias effects on copper and zinc isotopes in multicollector ICPMS measurements. Archer and Vance25 had previously carried out similar experiments using a multicollector ICPMS and had observed a greater variation of the measured isotope ratios of Cu and Zn when adding Sr to the solutions than for Fe. The dependency, however, could still be explained by the general mass fractionation trend of Cu vs Zn. Our measurements, however, showed distinct differences when adding Sr compared to other matrix elements, which turned out to be a result of formerly unexpected ArSr2+. It contributes specifically to the ion signal intensities measured at m/Q 63 (Cu) and 64 (Zn) in this case. Thus, a more detailed study on the dependency on sample composition and instrumental conditions was carried out, in order to investigate the effect of these species in isotope ratio analyses.

Table 1. Typical Operation Conditions Wet conditions Radio frequency power Reflected power Nebulizer Plasma gas Auxiliary gas Carrier gas flow ratea Nebulizer pressure Hot membrane gas Spray chamber gas

Dry conditions

1300 W ≤3 W MicroMIST, nominal uptake 100 μL/min 13 L/min 0.75 L/min 0.97−1.15 L/min 1.75−1.98 L/min 2−2.7 bar (30−38 psi) 0.5−2.9 bar (8−42 psi) 4.0−4.2 L/min 0.2 L/min

Gas flow rate into the ICP, determined at the nebulizer outlet using a bubble debimetre (Fisher BioBlock Scientific, Illkirch. France). a

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Table 2. m/Q Assignment of the Detector Array for the Experimentsa

EXPERIMENTAL SECTION Reagents and Standards. All solutions were prepared in cleaned polypropylene bottles. 1% HNO3 solution was prepared from ultrapure water (>18 MΩcm−1, Millipore, Bedford, USA) and nitric acid (HNO3) (Fluka, analytical grade, purified by double subboiling distillation) and was further used to dilute all other samples. Samples were prepared from 1000 mg/kg single element stock solutions. Dilutions were chosen to account for the instrument sensitivities when using the respective sample introduction systems. For the conventional nebulizer (“wet conditions”) Cu, Zn and Ga mass fractions were ≈330 μg/kg and Sr and Ba were ≈6.7 mg/kg. With the desolvated aerosol (“dry conditions”) the mass fractions were lowered to ≈50 μg/kg for Cu, Zn and Ga and ≈1 mg/kg for Sr and Ba, respectively. The effect of concomitant elements was studied by adding Sc, Co, Y, Sr and Ho to Cu, Zn and Ga at relative mass fractions of 1, 5, 10, and 20 times. Instruments. All data were obtained using a Nu Plasma HR multicollector ICPMS (Nu Instruments, Wrexham, Great Britain). Experiments were carried out with conventional nebulization using a free aspirating microconcentric nebulizer (MicroMIST, Glass Expansion, Melbourne Australia) mounted in a Peltier-cooled cyclonic spray chamber or using a desolvated aerosol using the same nebulizer mounted in a membrane desolvation unit (DSN 100, Nu Instruments, Wrexham, Great Britain). Operating parameters of the instruments were optimized daily for maximum analyte sensivity and signal stability. In order to investigate the influence of the ICP operating conditions, the nebulizer gas flow rate was varied in several experiments, while the sampling depth was held at a fixed value. The range of flow rates used included only settings where the ion signal intensities were still >10% of the maximum signal, where the highest gas flow rate was usually limited by the maximum allowable setting of the instrument. In Table 1 typical operation conditions for wet aerosol conditions and dry aerosol condition are listed. All isotope ratio and raw intensity measurements were carried out at low mass resolving power (m/Δm ≈ 300) to maximize sensitivity. Ion optics and zoom lens were optimized daily for peak parallelism and coincidence. The cup configuration for measurements of the respective isotope systems is given in Table 2. Detector gain calibration was carried out daily after instrument warm-up before starting the experiments. Measurement of the isotope ratios was carried out for 20 replicates of 10 s each. Detector dark noise was recorded

Detector 1 2 3

H6

H4

H2

Ax

L2

L3

71

69 68

68 67

67 66

66 65

65 64 88

IC1

L4

L5 63

63.5 87.5

64 63 87

86

a

Key: 1, Cu+, Zn+, and Ga+ isotope ratios; 2, Cu+, Zn+ isotope ratios and SrAr2+ intensity; 3, Sr+ isotopes and 135Ba40Ar2+ intensity. Ion signals for 84Sr+, 87Sr2+, 87Sr16O+, 135Ba2+ were recorded using Ax and 87 40 + 135 16 + Sr Ar , Ba O , and 135Ba40Ar+ via IC1

by ion beam deflection in the electrostatic sector for 30 s before signal acquisition.

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RESULTS AND DISCUSSION Matrix effects in Cu and Zn isotope ratio measurements. The variation of mass fractionation during isotope ratio measurements can be extracted from plots of the natural logarithm of the measured intensity ratio for one isotope pair against that of another pair as shown in Figure 1. As long as the data fall on a straight line, the mass fractionation of the two isotope pairs is correlated and correction of the instrumental mass fractionation is possible via one naturally invariant isotope pair. This approach was initially suggested by Longerich et al.,26 was further developed by Marechal et al.27 and is frequently used for mass bias correction in ICPMS today.28−33 For the data shown in Figure 1, solutions with identical mass fractions of Cu, Zn and Ga and spiked with increasing amounts of Sc, Co, Ho or Sr were analyzed in a sequence from the lowest toward the highest spike level. This sequence was repeated 2 times each for Sc, Co, and Ho and 4 times with Sr. It was observed that a high degree of correlation of Cu, Zn, and Ga isotope ratios existed in the case of addition of Sc, Co, and Ho, independent of the level of these elements relative to Zn and Cu. The variations in isotope ratios are represented by the spread of the data points, but overall there is little deviation of the spiked samples from the unspiked ones and the dependencies are close to linear. This indicates that the variations in the measured isotope ratios were determined by similar processes, which can originate in ion production and/or transmission, and whose underlying mechanisms essentially were mass dependent. In the case of Sr-addition on the other hand, this correlation broke down completely (Figure 2), whenever the ratios included the isotopes 64Zn or 63Cu. All other isotope ratio 7282

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Figure 1. Log−log plots of different isotope pairs in solutions containing only Cu, Zn, and Ga (black circles) and spiked with Sc, Co, or Ho at mass fractions of 1, 5, 10, and 20 times that of Cu, Zn, or Ga (gray circles).

Figure 2. Log−log plots of the isotope pairs shown as in Figure 1 in solutions containing only Cu, Zn, and Ga (black circles) and spiked with Sr at levels of 1, 5, 10, and 20 times the Cu, Zn, or Ga mass fractions. Note that ln(66Zn/64Zn) is plotted with about 4-fold wider scaling than in Figure 1. 7283

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63, 63.5, 64, 65, and 66 when aspirating 1 mg/L Sr with the DSN (dry plasma). The calculation is based on natural abundances of Cu, Zn, Sr, and Ar, and contributions from Cu and Zn were assessed via m/Q 65 and 66, respectively, while m/Q 63.5 was used for SrAr2+. In this case, the residual signals of Cu and Zn are presumably memory effects from previous analyses or contaminants in the sample analyzed, but the dominating species at m/Q 64 and 63 is SrAr2+. Similar spectra could be recorded for the other alkaline earth metals, Mg, Ca, and Ba and revealed similar, unique signals at m/Q 32.5 (25 Mg40Ar2+), 41.5 (43Ca40Ar2+), and 88.5 (137Ba40Ar2+). The respective formation rates can be related to quantum-mechanical calculations of the corresponding bond energies, which is discussed in more detail in an accompanying publication.34 Influence of ICP operating conditions on formation of MAr2+ ions. The formation of these doubly charged argide ions was further investigated as a function of ICP operating conditions. The aerosol gas flow rate was varied within a range close to the conditions yielding highest ion signal intensities for the singly charged analyte ions. These experiments were carried out for Sr and Ba, whose doubly charged argide ions are most easily detectable using wet and dry sample introduction. To simplify the evaluation, all intensity ratios were calculated relative to the odd isotopes 87Sr and 135Ba, whose signal intensities, however, were too high to be recorded directly for the mass fractions chosen. Thus, 84Sr+ was monitored and the signal of 87Sr+ inferred from natural isotope abundance, while the signal intensity for 135Ba+ was determined via the natural abundances of 130Ba+, 132Ba+, and 134Ba+ and using the exponential law for mass bias correction. The ion signal intensities of doubly charged atomic (87Sr2+, 135 2+ Ba ) and molecular ions (87Sr40Ar2+, 135Ba40Ar2+) and the corresponding singly charged ions (84Sr+, 87Sr40Ar+, 130Ba+,

pairs still showed a similarly high correlation and variability, such as when using the Sc, Co, or Ho spikes. The increase in Sr-spike level lead to stepwise shifts in ln(66Zn/64Zn) and ln(65Cu/63Cu) values (i.e., toward lower apparent 66Zn/64Zn and 65Cu/63Cu isotope ratios) and affected all isotope ratios involving 64Zn or 63Cu in a similar way. Specifically, the deviations from straight lines occurring for these isotope pairs of Zn only (i.e., ln(66Zn/64Zn) vs ln(68Zn/64Zn)) and the small variations seen for pairs not using 64Zn (i.e., ln(68Zn/66Zn) vs ln(68Zn/67Zn)) are indicative of spectral interferences.27 Mass spectra recorded for the different solutions then revealed an unexpected signal at m/Q 63.5, whose signal intensity scaled with the amount of Sr in the sample. From this point on, it was clear that only the formation of SrAr2+ can provide a reasonable explanation for the effects observed and that 86Sr40Ar2+, 88 38 2+ Sr Ar , and 88Sr40Ar2+ interfere with 63Cu+ and 64Zn+. Table 3 presents the respective contributions of the ion signals of Cu, Zn, and SrAr2+ for ion signals obtained at m/Q Table 3. Measured vs Reconstructed Signal Intensities for Cu, Zn, and SrAr2+ Isotopes for a Solution Containing 1 mg/ L Sr under “Dry” Plasma Conditionsa Measured

Contribution from

m/Q

mV

RSD

Cu

63 63.5 64 65 66

0.94 0.39 4.72 0.18 0.014

2.1% 1.4% 1.7% 1.1% 9.8%

0.39

Sum

Zn

SrAr2+

mV

0.024

0.55 0.39 4.61

0.95 0.39 4.63 0.18 0.014

0.18 0.014

a

Residual ion signals for Cu and Zn are memory effects from previous analyses and correspond to mass fractions below 10 ng/kg. Bold entries indicate the isotope signal used as reference.

Figure 3. Ion signals for Sr+ and the doubly charged ion intensity ratios, in dependence on carrier gas flow into the ICP for wet (left, 6.7 mg/kg Sr) and dry (right, 1 mg/kg Sr) sample introduction. 7284

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Figure 4. Ion signals for Ba+ and intensity ratios in dependence on carrier gas flow into the ICP for wet (6.7 mg/kg Ba, left) and dry (1 mg/kg Ba, right) sample introduction.

Figure 5. Dependence of the 64Zn/66Zn isotope ratio bias by SrAr2+ ions on the Sr/Zn mass fraction ratio using dry sample introduction. The table also lists the respective biases in the 65Cu/63Cu, 66Zn/64Zn, and Sr isotope ratio measurements with wet and dry sample introduction and mass fraction ratios of 1.

Figure 3 shows the intensity profiles obtained for Sr+ and the Sr /Sr+ and SrAr2+/Sr+ intensity ratios in dependence on gas flow rates for wet and dry sample introduction. The ratios were calculated from raw ion signals without mass bias correction. It is nonetheless obvious that the operating conditions of the ICP ion source had a significant effect on the formation of the doubly charged argide ions. The intensity ratio with wet sample introduction is, by about 1 order of magnitude, smaller and decreases with increasing gas flow rate, while a steady increase is observed under dry conditions. Wet conditions also exhibit a smaller variability in the SrAr2+/Sr2+ intensity ratio, and the SrAr2+ abundance generally seems to correlate with the abundance of Sr2+ in the spectrum. With dry sample introduction, on the other hand, the variability of the intensity ratio of the molecular ion is far more pronounced while correlation with the doubly charged atomic ions was only found for gas flow rates below 1.87 l/min. The intensity ratios for

132

Ba+, 134Ba+, 135Ba40Ar+) and oxide ions (87Sr16O+, 135Ba16O+) were acquired for various carrier gas flow rates. The latter were included because they typically serve as indicators for the robustness of the ICP and show similar dependencies for most ICPMS instruments. The potential contributions from SrOH+ and BaOH+, however, were not assessed in particular. For 87 40 + Sr Ar it turned out that the ion signal was severely overprinted by 127I+. 87 2+ 87 40 2+ 87 + 87 16 + Sr , Sr Ar , Sr , Sr O , and 87Sr40Ar+ were recorded for nebulizer gas flow rates between 0.97 L/min and 1.15 L/ min for wet conditions and from 1.72 to 1.98 L/min total gas flow rate under dry conditions. The highest ion signal intensity of 87Sr+ was obtained at 1.08 L/min and 1.85 L/min, respectively. For the corresponding Ba species, the carrier gas flow rate was varied between 0.97 and 1.12 L/min (wet) and between 1.75 and 1.98 L/min (dry) with maximum signals for 134 + Ba at 1.07 and 1.87 L/min, respectively.

2+

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Analytical Chemistry Table 4. Spectral Interferences Caused by Doubly Charged Molecular Ions of Argona Interference 24

40

Mg Ar Mg40Ar 40 Ca40Ar 44 Ca40Ar 46 40 Ti Ar 48 40 Ti Ar 50 40 Ti Ar 86 40 Sr Ar 88 40 Sr Ar 90 40 Zr Ar 92 40 Zr Ar 94 40 Zr Ar 96 40 Zr Ar 116 40 Sn Ar 118 40 Sn Ar 120 40 Sn Ar 122 40 Sn Ar 124 40 Sn Ar 134 40 Ba Ar 136 40 Ba Ar 138 40 Ba Ar 140 Ce40Ar 142 Nd40Ar 142 Ce40Ar 144 Nd40Ar 144 Sm40Ar 146 Nd40Ar 148 Sm40Ar 148 Nd40Ar 26

Rel. Ab (%) 78.6 11.1 96.6 2.08 7.97 73.5 5.38 9.82 82.3 51.2 17.1 17.3 2.77 14.6 24.2 32.3 4.58 5.58 2.41 7.82 71.4 88.1 27.0 11.0 23.7 3.09 17.1 11.3 5.74

Isotope 32

S S 40 Ca 42 Ca 43 Ca 44 Ca 45 Sc 63 Cu 64 Zn 65 Cu 66 Zn 67 Zn 68 Zn 78 Se 79 Br 80 Se 81 Br 82 Se 87 Sr 88 Sr 89 Y 90 Zr 91 Zr 91 Zr 92 Zr 92 Zr 93 Nb 94 Zr 94 Zr 33

m/Δm × 103

Interference 150

23 44 105 1134 28 74 17 11 14 12 8.9 9.6 6.7 5.4 5.9 5.2 4.9 4.9 3.6 3.2 3.2 3.1 3.1 3.0 3.0 2.9 3.0 2.9 2.8

40

Sm Ar Nd40Ar 152 Sm40Ar 154 Gd40Ar 154 Sm40Ar 156 Gd40Ar 158 Gd40Ar 160 Dy40Ar 160 Gd40Ar 162 Dy40Ar 164 Dy40Ar 164 40 Er Ar 166 40 Er Ar 168 40 Er Ar 170 Yb40Ar 170 40 Er Ar 172 Yb40Ar 174 Yb40Ar 176 40 Hf Ar 176 Yb40Ar 176 Lu40Ar 178 40 Hf Ar 180 40 Hf Ar 204 Pb40Ar 206 Pb40Ar 208 Pb40Ar 232 Th40Ar 238 40 U Ar 150

Rel. Ab. (%) 7.37 5.62 25.6 2.17 22.6 20.4 24.7 2.33 21.8 25.4 28.1 1.60 33.5 26.7 3.04 14.8 21.8 31.7 5.18 12.6 2.59 26.9 35.1 1.40 24.0 52.2 99.6 98.92

Isotope 95

Mo Mo 96 Mo 97 Mo 97 Mo 98 Mo 99 Ru 100 Ru 100 Ru 101 Ru 102 Ru 102 Ru 103 Rh 104 Ru 105 Pd 105 Pd 106 Pd 107 Ag 108 Pd 108 Pd 108 Pd 109 Ag 110 Pd 122 Sn 123 Sb 124 Sn 136 Ba 139 La 95

m/Δm × 103 2.8 2.7 2.7 2.7 2.7 2.7 2.7 2.5 2.5 2.6 2.5 2.5 2.5 2.5 2.4 2.4 2.3 2.4 2.3 2.2 2.2 2.3 2.2 1.9 1.9 1.9 1.9 1.4

a

Listed are potentially abundant (>1%) MAr2+ ions, their isotopologue abundance fractions (based on natural abundances of the M and Ar isotopes), the interfered analyte isotopes, and the mass resolving power required to separate analyte and interference. A comprehensive table also including the minor ions is available as Supporting information (Table S1). MAr2+ occurring at half nominal m/Q are generally not considered.

SrO+/Sr+ (data not shown), in comparison, always showed a similar profile with nearly steady values for low gas flow rates (8 × 10−6 wet, 1.5 × 10−6 dry) and a steep increase for gas flow rates exceeding those yielding the highest sensitivity for Sr+. The corresponding Ba species show highly similar profiles for the doubly charged molecular ion (Figure 4), albeit at slightly higher intensity ratio for BaAr2+/Ba+. Again, the dependence on gas flow rate was remarkably different for wet and dry sample introduction but similar to that of the Sr species Sr. The singly charged argide ions, on the other hand, appeared at about ten times lower intensity than the corresponding doubly charged species but had similar dependencies on gas flow rate under wet and dry conditions. Again, the BaO+/Ba+ intensity ratios (not shown) showed similar trends with variation of gas flow rate, with both sample introductions systems with initially low abundances (10−4 wet, 6 × 10−5 dry). Effects in isotope ratio measurements. The relatively low abundance of the MAr2+ molecular ions will only affect elemental analyses when M occurs as the major component in the sample (≈ at concentration ratios >10−4 relative to the analyte isotope). Their effect in isotope ratio analyses, however, can be more significant, especially when measurement uncertainties in the range of 10−4 or below shall be achieved. Figure 5 shows the effect of increasing Sr/Zn concentration ratios on 66Zn/64Zn isotope ratio data, using dry sample

introduction and the instrument optimized for the highest sensitivity of Zn+. As can be seen, the bias is directly proportional to the Sr level by about −1‰ per Sr/Zn mass fraction ratio. The table also lists the respective bias caused by Sr on Cu or Zn isotope ratio data and Ba on Sr isotope ratios for wet and dry sample introduction. 88Sr40Ar2+, causing the highest signal, already leads to a significant deviation in 64Zn+ isotope ratio measurements when the mass fraction of Sr is similar to that of Zn. The BaAr2+ species interfering with Sr isotopes, on the other hand, are far less abundant and the bias much less pronounced. Other isotopes. The alkaline earth metals belong to the elements where doubly charged atomic ions are of significant abundance in ICPMS spectra. Apart from these, however, several other elements have sufficiently low second ionization energy so that the corresponding MAr2+ ions may occur in the mass spectrum. This is especially true for the rare earth elements, the actinides, and Pb. Table 4 lists potentially relevant interferences due to MAr2+ formation together with the mass resolving power required to separate them from atomic ions of the same nominal m/Q. It can be seen that the mass resolving power of current ICPMS instruments is insufficient to separate them from atomic analyte ions in many cases. Especially for isotopes with m/Q below 65, a mass resolving power >10000 7286

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would be required. Sr isotopes could be separated from BaAr2+ at m/Δm of 4000, albeit with a substantial loss in sensitivity. It may be highlighted here that the CaAr2+ interference is of particular interest. The abundant molecular ions interfere primarily with Ca isotopes and would thus typically be included in an external calibration. Still, the comparably high abundance of CaAr2+/Ca+ can have a substantial effect on absolute Ca isotope data. For a raw abundance ratio of 2 × 10−4, as found in these experiments for dry sample introduction,34 the bias in measured 44Ca/42Ca or 43Ca/42Ca isotope ratios, for example, would amount to almost −0.6‰. Variations in formation rates due to ICP operating conditions will additionally affect the accuracy of isotope ratio analyses.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Markus Reiher for his contribution by calculating the bond dissociation energies. This work was supported by ETH Zurich.

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REFERENCES

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CONCLUSION This work has shown that doubly charged molecular ions of alkaline earth metals with argon can be formed in ICPMS and cause spectral interferences that had not been described previously. The formation of these interferences depends strongly on the operating conditions of the ICP ion source but can reach levels that are commonly observed for prominent molecular ions, such as singly charged metal oxide ions, for example. These species were identified for all alkaline earth metals within this study, but other elements with second ionization energies below 15.7 eV may likely form similar species at element specific formation rates. In most cases, however, at least one abundant isotopologue is of odd nominal m/Q and the interference may be easily identified and eventually corrected for. Still, cases exist where the interference may not be as easily detectable (e.g., CeAr2+ interference on Zr+). The magnitude of occurrence of these interferences and the influence on analyte ion signal intensities were found to be proportional to the concentration ratio of analyte and interference, and to significantly depend on the sample introduction method. Wet sample introduction was found to lead to a lower overall relative interference level than when dry sample introduction is used. Lower total carrier gas flow rate in the case of aerosol desolvation may, to some extent, also reduce the relative interference level. In any case, however, these interferences may affect elemental and particularly isotope ratio analyses to quite an extent. Conventional sample introduction using solution nebulization can, to a large extent, circumvent the problem by chemical purification of the analyte elements. In applications where such isolation is not possible, such as laser ablation (LA) sampling, for example, mathematical correction of the interference would be required. Such a correction would most conveniently be carried out by subtracting the relative contribution of the interfering ion using odd m/Q isotopologues as reference. In fact, during preparation of this manuscript, a report was published showing exactly that BaAr2+ causes a significant bias in Sr isotope ratio measurements of Barite using LA-ICMPS.35

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b01614. Full list of isotopes potentially affected by spectral interference from MAr2+ ions (PDF) 7287

DOI: 10.1021/acs.analchem.6b01614 Anal. Chem. 2016, 88, 7281−7288

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DOI: 10.1021/acs.analchem.6b01614 Anal. Chem. 2016, 88, 7281−7288