Reproductive Ecology and Body Burden of Resident ... - The Love Lab

Otolith preparation and microchemical analysis using laser-ablation ICP-MS
Each otolith was soaked in deionized water for 1-3 hours, cleaned with a soft nylon brush in a deionized
water bath, rinsed, and air dried in clean, covered plastic multiwell trays. A transverse section was cut from
each otolith using a low-speed rotary diamond blade saw and mounted on a plastic slide with metal-free
epoxy resin (Epo-Thin, Buehler). Plastic slides were cleaned in metal-free soap and rinsed in distilled water
(resistivity>2 M cm). Each section was polished to the center of the otolith core using a lapping wheel and
9, 3, and 1 μm 3M diamond polishing films. The sections were broad, 0.5-1mm between cuts, to keep the
polished surface, which was to be assayed, free of the epoxy used to bond the base of the section to the slide.
After polishing, decontamination steps described by Warner et al. (2005) were performed in a clean
laboratory. Each otolith section mounted on a slide was individually soaked in equal parts of 30% H 2
0 2
and
0.1 mol l -1 NaOH for 30 minutes, rinsed with ultrapure water (N-pure, resistivity >18.1M cm), individually
soaked in 5 separate 5-minute baths of ultrapure water, rinsed again in ultrapure water, and air-dried
in a HEPA-filtered Class 100 laminar flow hood. The otoliths were placed separately in multi-welled tissue
culturing trays and metal-free airtight bags until the microchemical analysis. All materials that came into
contact with the otoliths during the decontamination process (e.g., glassware, plastic trays, plastic instruments)
were cleaned in metal-free soap, rinsed five times in distilled water (resistivity>2 M cm), soaked in 1
N mol l -1 trace metal grade HCl overnight, and rinsed 10 times with ultrapure water.
We used laser-ablation inductively coupled plasma mass spectrometry (LA ICP-MS) to sample a discrete
portion of the sectioned otolith nearest the edge. Otolith material was vaporized by a 213 nm solid
state Nd:YAg laser ablation system (New Wave UP213). Helium carrier gas swept the sample of ablated particles
into a spray chamber where Argon gas was mixed with the aerosol before entering the ICP-MS torch.
The sample was analyzed on a Finnigan MAT Element 2-sector field ICP-MS. An imaging system was used
to focus the laser on the surface of the otolith and to delineate two sample targets, 100 µm long x 40 µm
wide, following the contour of the edge of the sectioned otolith (Figure 2). This method of sampling the
outermost otolith material corresponding to recent growth
before capture reduces the uncertainty that the trace elements
were incorporated into the otolith when the individual did not
occupied the locality were it was collected.
Otoliths were run as blocks with one specimen per site
per species randomly assigned to each block. One to five sequences
were run per day. A sequence was comprised of one to
seven otoliths. The two targeted lines on the sectioned otolith
were pre-ablated as a precautionary cleaning measure.
We initially assayed a suite of isotopes: Magnesium-24
( 24 Mg), Calcium-48 ( 48 Ca), Strontium-87 ( 87 Sr), Silver-109 Figure 2. Image of otolith with ablated sample lines.
( 109 Ag), Cadmium-114 ( 114 Cd), Cesium-133 ( 133 Cs), Barium-138
( 138 Ba), Cerium-140 ( 140 Ce), Mercury-202 ( 202 Hg), Lead-208 ( 208 Pb), and Uranium-238 ( 238 U) were
determined from the sample of the first line target by using low resolution mode (R=300); Aluminum-27
( 27 Al), Calcium-48 ( 48 Ca), Vanadium-51 ( 51 V), Chromium-52 ( 52 Cr), Manganese-55 ( 55 Mn), Iron-56 ( 56 Fe),
Cobalt-59 ( 59 Co), Copper-63 ( 63 Cu), and Zinc-66 ( 66 Zn) were determined from the sample of the second line
target in medium resolution mode (R=4000). The isotope intensities of each sample were blank-corrected
by subtracting isotope intensities of a 1% nitric acid (HNO 3
) instrument blank preceding the sample sequence.
The abundance of an element was expressed as a ratio relative to the amount of calcium to control
for the amount of material analyzed per sample spot.
We eliminated undetectable isotopes from our assay panel after 21 of 40 sample sequences were run
(50 kelp rockfish and 63 sanddab otoliths). We considered a negative blank-corrected intensity value to be
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