Elemental concentrations in fish from lakes in Northwest England

Fish were collected in 2012 and 2013 from Windermere, Bassenthwaite Lake and Derwent Water in the English Lake District. Species collected were: Roach (Rutilus rutilus), Perch (Perca fluviatilis), Ruffe (Gymnocephalus cernuus), Brown trout (Salmo trutta), Pike (Esox lucius) and Vendace (Coregonus albula). Samples were frozen to minus 20 degrees Centigrade as soon as possible after collection. Fish samples were thoroughly defrosted, weighed and gutted prior to the whole-fish being ashed at 500°C. The ash was then weighed. Ashed fish samples were digested using an aqua regia reagent (fresh 3:1 mixture of concentrated HNO3 and HCl aristar grade) at 175°C for 12 minutes using a microwave oven (CEM, MarsXpress). Analysis of the ashed fish digests was then conducted by ICPMS (Perkin Elmer, NeXION 300D) or ICPOES (Perkin Elmer, 7300 DV). ICPMS measurements were made using two types of instrument operation, either quantitative or semi-quantitative (using Perkin Elmer Total Quant software). The quantitative measurements were made using external calibration after spiking the digests with 10 microgram per litre Ga, In and Re as internal standards to compensate for instrumental drift and matrix effects. The semi-quantitative measurements were made using the ICPMS instrument operating in kinetic energy discrimination (KED) mode with the addition of helium gas. The semi-quantitative and quantitative modes differed as the former used an element concentration to intensity response curve to extrapolate concentrations over a wide mass range. ICPMS and ICPOES instrument limits of detection (LOD) were calculated using mean and standard deviation measurements for the digestion reagent blank (yb, plus sb where yb is blank concentration and sb is three standard deviations of the blank). Method LODs were then calculated taking into account the dilution arising from the digestion procedure. Note that for some elements determined using the semi-quantitative ICPMS method it was not possible to determine instrument LODs from the analysis of a digestion reagent blank and in these cases the instrument LODs were estimated from an assessment of instrument sensitivity and assigned a values of 0.001 microgram per litre (equivalent to a mean method LOD of 0.001 milligrams per kilogram). Results are reported as milligrams per kilogram fresh weight (converted from ash weight using fresh to ash weight ratio for each sample).