Elemental Analysis using ICPMS

After the otoliths have been decontaminated, they can be analyzed for elemental composition as described under Elemental Tags. Here we describe the details of preparation subsequent to decontamination.

Digestion is carried out in pre-cleaned (acid washed) polypropylene vials (eg- 30-ml skirted polypropylene centrifuge tubes available from Sarstedt). Other vials can be used, but they should have a non-pigmented screw cap without a cap liner. Assuming the otoliths are between 100-500 mg in weight, they are digested for several hours in 2 ml of 1:1 SuperQ water (water which has been distilled, deionized, and run through reverse osmosis) and high purity nitric acid (Seastar) in the original sample containers and then warmed for 15 mins at 50-60 degrees C to complete digestion. For the larger otoliths, an additional 0.5-1.0 ml of acid is added before dilution to complete digestion. Solutions are then diluted to a final volume of 30 mL with SuperQ water. Solutions are then diluted again 1:10 in SuperQ to leave them in the final concentation of about 0.1% w/v. Rhodium is sometimes added as an internal standard to monitor changes in nebulization efficiency; since we assay most elements using isotope dilution, Cr-52 is added as the internal standard for Mn. Enriched isotopes for assay by isotope dilution ICPMS are added at this stage; all vials received the same amount of spike.

A full method blank is run which includes all materials and reagents involved in the analysis, including sample vials, volumetric tubes, pipette tips, plastic film (may be used to cover open samples), water, acids and even the toothbrush used for cleaning the specimens.

Simple dilute acid standard solutions are used for the initial instrument calibrations, but high calcium matrix standards are used to monitor (and correct for) ionization interferences and instrument drift.

The samples are prepared and analyzed in separate "batches". Two or three reagent blanks are processed with each group of specimens.

A "pool" of approximately twenty otoliths is prepared and portions of this bulk solution are analyzed within each analytical batch. A similar "pool" of otolith material from previous work is also analyzed with each batch. These are not standard materials, but were chosen to provide a means of monitoring within-run and between-run precision.

The samples are generally supplied in groups, based upon sample collection location. The samples are randomized within each group and each group is divided among the preparation batches. The samples are further randomized within each preparation batch. This is done to ensure that sequence effects do not have a pronounced effect on one particular group of samples.