Analysing Sr isotopes in low‐Sr samples such as single insects with inductively coupled plasma tandem mass spectrometry using N₂O as a reaction gas for in‐line Rb separation

Abstract

Rationale Strontium isotopes are valuable markers of provenance in a range of disciplines. Limited amounts of Sr in low‐mass samples such as insects mean that conventional Sr isotope analysis precludes their use for geographic origins in many ecological studies or in applications such as biosecurity. Here we test the viability of using inductively coupled plasma tandem mass spectrometry (ICP‐MS/MS) with N2O as a reaction gas for accurately determining Sr isotopes in insects with Sr < 100 ng. Methods Strontium isotopes were determined in solution mode using ICP‐MS/MS with 0.14 L/min N2O as a reaction gas to convert Sr+ into SrO+ for in‐line separation of 87Sr from 87Rb. The Sr isotope reference standards NIST SRM 987, NIST SRM 1570a and NIST SRM 1547 were used to assess accuracy and reproducibility. Ten insect species collected from the wild as a proof‐of‐principle application were analysed for Sr concentration and Sr isotopes. Results Using ICP‐MS/MS we show for the first time that internal mass bias correction of 87Sr16O/86Sr16O based on 88Sr16O/86Sr16O works to give for NIST SRM 987 a 87Sr/86Sr ratio of 0.7101 ± 0.012 (RSD = 0.17%) and for NIST SRM 1570a a 87Sr/86Sr ratio of 0.7100 ± 0.009 (RSD = 0.12%), which are within error of the accepted values. The first 87Sr/86Sr ratio of NIST SRM 1547 is 0.7596 ± 0.0014. Strontium analyses were run on 0.8 mL of 0.25–0.5 ppb Sr, which equates to 2–4 ng of Sr. Strontium isotope analysis with a precision of >99.8% can be achieved with in‐line separation of 87Sr from 87Rb at least up to solutions with 25 ppb Rb. Conclusions A minimum of 5 mg of insect tissue is required for Sr isotope analysis. This new ICP‐MS/MS method enables Sr isotope analysis in single insects, allowing population‐scale studies to be feasible and making possible applications with time‐critical uses such as biosecurity.

Trends and Advances in Inductively Coupled Plasma Tandem Quadruple Mass Spectrometry (ICP-QMS/QMS) With Reaction Cell

“…5, N2O, CO2, and CH4 were also used in multiple reports instead of the default cell gases. By using N2O as the cell gas, the production rate of the oxide ions of (Mn, Fe, Ni, Cu, Zn, and Se), 1 Sr, 3,68 Br, 8 Ba, 23 (Ge, Eu, Yb) 29 and Ra, 28,45 is greatly improved in comparison to those obtained by using O2 as the cell gas. This feature provides a higher sensitivity and/or a better separation of spectral interference.…”

“…Many references used He, H2, O2, or NH3, which are usually included as the default cell gas for ICP-QMS/QMS. 4,22,26,44,46,[62][63][64][65][66] There are also multiple references using N2O, 3,23,28,45,68 CO2, 24,27,65 or CH4 43 as the cell gas to improve the separation of spectral interferences. For single element solution, the typical relative uncertainty of isotopic ratio obtained by ICP-QMS/QMS is roughly in the range from 0.1% to 0.2%, which is inferior to that can be achieved by multiple collector (MC-) ICP-MS by 3 to 4 orders of magnitude.…”

“…This is a noteworthy advantage in comparison to MC-ICP-MS, which usually requires mathematical correction of spectral interferences for the analyzing real sample of complicate matrix and results in deterioration of analytical precision and accuracy. 3,28,45 On the other hand, highly precise analysis of the Cs isotopes ( 135 Cs + and 137 Cs + ) was achieved by complete conversion of the interfering ions, i.e., 135 Ba + and 137 Ba + , to oxide ions using N2O as the cell gas and resulted in the complete elimination of the spectral inteferences. 23,68 Regardless of the fact that CO2 as the cell gas did not improve the sensitivity for isotope analysis of U at the mass-shift mode by ICP-QMS/QMS, effective separation of the spectral interferences was achieved for the measurement of Pu + at the on-mass mode by improving the conversion of UH + to UO + .…”

Natal origin of the invasive biosecurity pest, brown marmorated stink bug (Halyomorpha halys: Penatomidae), determined by dual‐element stable isotope‐ratio mass spectrometry

“…Looking forward, the development of analytical methods for these markers in restricted mass biological samples is possible 66,67 and this specific application is being considered. 68 This later endeavour holds the potential to greatly facilitate the wider application and adoption of biogeochemical tracking in several divergent areas such as criminal forensics, ecology and pest management.…”

A bioavailable strontium (87Sr/86Sr) isoscape for Aotearoa New Zealand: Implications for food forensics and biosecurity

“…Strontium isotope analysis as a tool in forensic tracing has received limited attention to date in Aotearoa mostly due to the cost of analysis when scaled to very large numbers of samples, and the lack of a bioavailable 87 Sr/ 86 Sr baseline. Recent efforts have gone some way to tackling these issues, with technical advances to address the former [ 63 ] and with Duxfield et al [ 64 ] aggregating published geological data to create a 87 Sr/ 86 Sr baseline. The recent baseline [ 64 ] summarized the variation in geological 87 Sr/ 86 Sr for Aotearoa and did not predict the bioavailable 87 Sr/ 86 Sr produced through the complex environmental system that 87 Sr/ 86 Sr cycles through.…”