Investigating the feasibility of ICP-MS/MS for differentiating NIST salmon reference materials through determination of Sr and S isotope ratios
Graphical abstract
Introduction
Salmon is one of the highest value species group for US commercial fisheries, aquaculture production and trade [1]. Consumer-driven preferences and perceptions, along with market-based data and advertising, can all influence the commodity price of salmon [2]. Wild-caught fisheries products may be considered as healthier, more natural products that taste better compared to aquaculture-farmed products. Or in the reverse argument, scientific advancement and control helps ensure that farm-raised fisheries are more sustainable and produce fish freer of environmental contaminants, without any bycatch. Verifying provenance (country or region or origin) and quantifying any misrepresentation of products within global seafood supply chains is of paramount concern to protect consumer health, salmon fisheries of all types, and the environment. This has a direct benefit to consumers in that greater assurance is afforded to the authenticity and cost of products in the marketplace. Globally, salmon is a representative species of concern for illegal, unregulated and unreported (IUU) fishing [3]. Such activities include illegal harvesting (overfishing, undersized fish collection), over-quota farming practices, and product substitution [4]. Chemical profiling using DNA, stable isotope ratios, organic constituents and elemental fingerprints all can aid in combating these efforts [2,5,6].
NIST is developing two new reference materials (RM), NIST RM 8256 Wild-Caught Coho Salmon and NIST RM 8257 Aquacultured Coho Salmon as food fraud standards, with certified data for genetics, fatty acids and stable isotope ratios. They will also potentially serve as method evaluation materials for comprehensive chemical analysis. This will help provide matrix-similar reference materials that are well-characterized chemically, fostering more powerful seafood fraud analysis paradigms based on fused chemical data. In the case of the current study, these salmon materials were sampled opportunistically for an initial evaluation of strontium (Sr) and sulfur (S) isotopic signatures using tandem ICP-MS/MS. The elements were chosen based on their efficacy to potentially differentiate the materials based on isotopic composition, by leveraging the differences in the marine/estuarine (NIST RM 8256) and riverine (NIST RM 8257) source locations of the materials, and expected differences in wild versus aquaculture feed diets. Strontium and S isotopic compositions are difficult to measure using the same sample preparation and instrument workflows, but tandem ICP-MS/MS offers one possible path to accomplish this.
It is widely recognized that the family of multicollector-based mass spectrometries, employing thermal ionization [7] or inductively coupled plasma sources [8] for the measurement of numerous element systems, or gas sources for light elements [9], can provide very precise and accurate isotope ratio data when the techniques are correctly applied. There is no acceptable alternative to the use of sample matrix reduction and analyte element purification techniques in combination with multicollector mass spectrometry for the study of elements such as transition metals that feature very small natural isotopic variations [10]. For certain element systems that exhibit large natural isotopic variations like Sr and S, single collector techniques such as sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) and even quadrupole ICP-MS have been utilized to measure isotope ratios in environmental [11] and food sample matrixes [12]. Element-specific or ion exchange columns are usually needed to mitigate spectrochemical inferences from concomitant elements present in samples, but new technologies are emerging to automate these processes for elements like Sr and Pb [13] that should be of benefit to food fraud studies. Collision cell ICP-MS instruments using collision gases such as He or reactive gases such as H2 can help to reduce spectrochemical interferences, but not eliminate them altogether [14]. Physical resolution can be used in SF-ICP-MS for the same purpose, but sometimes chemical separations still need to be employed to achieve the most accurate results. For example, with SF-ICP-MS it is routine to measure 34S/32S isotope ratios in the presence of oxygen isotopes without prior chemical separation [15,16]. This requires using only physical ion beam resolution at reduced sensitivity (i.e., medium resolution (4000) or high resolution (10 000) modes [17]). However, the isobaric overlap is too tight for measuring 87Sr/86Sr isotope ratios in the presence of 87Rb, requiring a mass resolving power of approximately 3 X 105 [18].
The inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) instrumental technique [19] can be used to effectively mitigate spectrochemical interferences using a completely different mechanism that can be thought of as orthogonal to chemical separation of solution samples based on column chromatography. Instead, utilizing the selectivity of the first (selector) quadrupole (Q1) and employing a reactive gas such as oxygen (O2) in the reaction cell (typically a quadrupole, hexapole or octopole cell), an ICP-MS/MS instrument can effect a gas phase chemical separation, wherein the product ions formed in the reaction zone are shifted to alternate mass-to-charge ratios that can be passed to the analyzer quadrupole, with no measurable isotopic mass discrimination from in-cell chemical reactions [19]. Taking the elements S and Sr as examples, due to their utility in food forensics studies [2], 34S/32S would be measured as the molecular analogue 34S16O+/32S16O+, with both S isotopes shifted by 16 u, which would eliminate numerous unreactive polyatomic interferences derived from nitrogen and oxygen species [20]. The 31P–1H hydride interference on 32S and the 33S–1H interference on 34S would also be mitigated. Similarly, 87Sr/86Sr would be measured as 87Sr16O+/86Sr16O+, and the 87Rb and 86Kr, and matrix ion interferences such as 44Ca43Ca+ would be eliminated based on instability in the Q3 field, or unique chemical reactions occurring inside the reaction cell [21]. Liu and coworkers have recently shown encouraging ICP tandem MS/MS data for Sr isotope ratios in rock materials obtained with and without prior analyte element purification, using O2 as the reaction gas [22]. Vanhaecke and coworkers have also reported on the successful use of the reaction gas methyl fluoride (CH3F) with tandem ICP-MS/MS in both solution and laser ablation modes to measure Sr isotopes in geological materials, with the Sr isotopes shifted by 19 u, as SrF+ species [18,23]. Murphy and coworkers recently reported on the use of nitrous oxide (N2O) reaction gas to perform successful strontium isotope ratio measurements in the presence of Rb using tandem ICP-MS/MS [24].
Our study applied a simple microwave decomposition protocol without subsequent analyte purification steps for either Sr or S. This was followed by employing tandem ICP-MS/MS with O2 reaction gas to generate Sr and S isotopic data of requisite quality to chemically differentiate the two NIST salmon reference materials. More generally, this feasibility study tried to apply a high precision measurement strategy to begin to characterize the achievable precision of ICP-MS/MS via evaluation of sources of uncertainty, and through comparison of control data for Sr and S isotopic reference materials to literature values. The application of tandem ICP-MS/MS to Sr and S isotope ratio measurements may provide laboratories with fast (but fit for purpose) food fraud detection and sample flagging capabilities. It could also be of interest to laboratories that cannot afford to invest nearly $1 M in multicollector ICP-MS instrumentation and the requisite additional costs for space, infrastructure and expertise.
Section snippets
Materials
The following materials were secured for the experiments. NIST SRM 987 Strontium Carbonate Isotopic Standard [25] and NIST RM 8555 IAEA-S-2 (Sulfur Isotopes in Silver Sulfide) [26] were used as isotopic bracketing standards, respectively, for Sr and S isotope ratio measurements. Traceability to the mole for 87Sr/86Sr isotope ratio measurements in the test samples was achieved by comparative measurements of SRM 987 Strontium Carbonate Isotopic Standard, for which the absolute 87Sr/86Sr amount
Consideration of Rb/Sr mass fraction ratios in test materials
The Sr bracketing standard (SRM 987) is a pure material (strontium carbonate) with a negligible amount of Rb. The method evaluation materials JCp-1 [38] (Rb/Sr ≈ 1X10−5) and NASS-6 [39] (Rb/Sr ≈ 1.5X10−2) have low Rb content relative to strontium, as these materials are derived from oceanic sources. However, wild salmon life history includes freshwater incursions into rivers, and different aquaculture practices could potentially introduce sources of Rb into farmed salmon. In a comprehensive
Conclusions
The CGM approach developed by Dronov et al. [32,33] for improving the precision of isotope ratio measurements using quadrupole ICP-MS was tested using ICP-MS/MS with the calibration method of standard-sample bracketing. The oxygen mass shift mode was successfully used to mitigate interferences for the Sr and S isotope systems by leveraging tandem peak jumping and the gas phase chemistry occurring in the octopole reaction cell. In combination with the calibration approach applied, ICP-MS/MS
Author contributions statement
All authors contributed to sample preparation, analytical measurements, data analysis and writing efforts for this manuscript. Specifically Davis and Ellisor prepared the salmon samples via microwave digestion and contributed to data interpretation and writing of the manuscript. Ellisor cleared the work internally to perform analyses on vertebrate samples. Christopher designed the tandem ICP-MS/MS experiments, contributed to data interpretation, literature review and manuscript writing,
NIST disclaimer
Any mention of commercial products is to specify adequately the analytical procedures used. It does not imply recommendation or endorsement by NIST, or that the products mentioned are necessarily the best available for the intended purpose. Employment of the sample digestion procedures outlined would expose analysts to nitric acid; appropriate personal protective equipment should be used if these methods are adopted for sample preparation.
Code availability
The R statistical package Methcomp [37] was used for orthogonal least squares regression.Empty Cell
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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