Vulpes vulpes_tissue composition_Total mercury

Description

This data set concerns the tissue composition (water and lipid proportions) and total mercury in wet, dry and dry-lipid extracted form for red fox tissues (Vulpes vulpes), collected between 2012 and 2020. Contaminant concentrations in wildlife toxicology studies are inconsistently expressed in wet or dry mass units, or even on a lipid-normalized basis, but tissue composition is rarely reported, and the conversion between dry and wet mass units, notably, is often based on assumed empirical moisture contents in tissues. However, diverse factors (e.g., tissue, storage conditions) may affect tissue composition and render comparisons between studies difficult or potentially biased. We used these data as an illustration, to quantify the effects of diverse variables on moisture and lipid contents and their consequences on contaminant concentration in different tissues, when converting between wet and dry mass units (lipid extracted or not). We found that moisture content differed largely between organs, enough to preclude the use of a single conversion factor, and decreased rapidly yearly by 1% when stored in a -80°c freezer. Although most fox tissues have low lipid concentrations, lipid content affected water content and their extraction affected the wet to dry mass conversion factor. We thus recommend reporting tissue composition (at least water and lipid contents) systematically in toxicology studies and using tissue/species specific conversion factors to convert between dry and wet mass concentration.

Steps to reproduce

We weighed between 0.087 g and 2.177 g of wet tissue that was freeze-dried for 48 hours at -50°C (collector temperature) using a Labconco FreeZone 2.5 Liter Benchtop Freeze Dry System freeze dryer. We extracted lipids by leaving the ground freeze-dried samples 12 hours in a Soxhlet apparatus and used Petroleum ether (Ref.: 67482-540, Anachemia, vwr International, Canada) as solvent (Elliott et al. 2017). We then dried the samples for 72 hours in a Fisher Scientific Isotemp® drying oven at 60°C. Samples were weighed between each step to determine tissue composition. We measured total mercury concentration (hereafter THg) in the samples at each step (wet, freeze-dried, and dry-lipid-extracted) using the direct mercury analyzer Hydra IIc (Teledyne Leeman Laboratories, Hudson, NH) at the Center for Earth Observation Science, University of Manitoba. Hydra IIc is a fully automatized spectrometer that analyzes mercury concentration in samples, by thermal decomposition (all forms of mercury are reduced to Hg(0) in a catalyst), amalgamation (Hg(0) is collected using a gold trap), and atomic absorption spectrometry, following EPA method 7473 (US EPA 1998). With this instrument, there is no need for sample preparation. Hydra IIc was calibrated using certified reference materials MESS-3 (marine sediment, mercury concentration = 91 ± 9 ng/g), MESS-4 (marine sediment, 90±40 ng/g), and PACS-3 (marine sediment, 2980 ±360 ng/g). Quality assurance and control were tested by running the certified reference materials (CRM) MESS-3, MESS-4, TORT-2 (lobster hepatopancreas, 270 ± 60 ng/g), TORT-3 (lobster hepatopancreas, 292 ± 22 ng/g), DORM-4 (fish protein, 412 ± 36 ng/g), DOLT-5 (dogfish liver, 440±180 ng/g ng/g), NIST 2709a (soil, 900 ± 200 ng/g), NCP III-9 S2 (fish muscle, 933 ± 55.5 ng/g), and PACS-3, two to three times every 14 samples (Recovery: 8.1%±0.3; see Table S2 for details on accepted CMR runs). If a CRM concentration was outside the certified range, we ran up to two more CRM. If the extra CRM were still outside the certified range, we recalibrated the Hydra IIc. Most samples were replicated 2 to 5 times and we averaged the replicates (Average coefficient of variation: CVwet = 0.08, CVdry = 0.06, CVdry-lipid-extracted = 0.03; number of samples replicated: nwet = 165, ndry = 126, ndry-lipid-extracted = 38). However, some samples (mostly dry and dry-lipid extracted) occurred in too low quantity to be replicated, and we had to discard some more replicates after quality check, resulting in approximately 20% of our samples not being replicated. Further details on how samples were collected are available in the linked manuscript (Preprint for now).

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