FATE OF MERCURY IN THE ARCTIC Michael Evan ... - COGCI

F. Roos-Barraclough et al. / The Science of the Total Environment 292 (2002) 129–139
2.1. Determination of a suitable peat analysis
program for Hg analysis using the Leco AMA 254
The Leco AMA 254 is fully compliant with
EPA (1998) method 7473.Samples contained in
nickel sample holders enter a sealed drying and
combustion furnace, where they are dried in a
stream of oxygen before being thermally decomposed.Gases
from the thermally decomposed sample
are swept in the stream of oxygen through a
catalyst furnace at 750 8C, which fully decomposes
the gases and traps NO 2, SO2 and halogens.Mercury
is trapped on a gold amalgamator situated at
the end of the furnace.Waste gases are removed
from the system by the gas stream.The amalgamator
is then heated to 500 8C to release the
mercury, which is measured by atomic absorption
spectrometry.The detection limit of the instrument
is 0.01 ng Hg and the working range is 0.05–600
ng Hg, with repeatability being -1.5%.
The instrument was calibrated using liquid standards
prepared from Merck mercury standard solu-
y1
tion, 1000 mg l .Solutions of concentration 10
y1
and 1000 ng ml were used to dose the instru-
ment.A 10-point calibration was made from 0.00
to 29 ng Hg.The equation used to calculate the
calibration curve is:
1
kŽ NST. s µ AjiŽ NST. ymji∂
8ji n
Where k is the constant of proportionality, NST is
the relative non-absorbable radiation flux, A is the
corrected absorbance and m is the quantity of
mercury in the cell.For the calibration obtained,
slope (k)s42.64"0.62 ng.
To test the effect of increased drying time (and
therefore increased Hg passing through the apparatus
from oxygen supply), blanks were run on
the Leco AMA 254 using drying times of 9 and
500 s (other parameters being kept constant:
decomposition time 150 s, waiting time 45 s).
The validity of the recommended drying time
was also tested w0.7=vol.water (ml)x s.Ten
samples of the in-house peat standard OGS 1878
P were moistened (made up to 95% water,
RH OG18
V) and analysed using the recommend-
2
ed drying time.Ten dry samples were also analysed
using a drying time of 20 s.
131
A suitable decomposition time was established
by measuring Hg in nine samples of OGS 1878 P
at 100, 125, 150 and 175 s decomposition time
(drying and waiting time being constant at 30 and
45 s, respectively).One coal and five plant-derived
certified standard reference materials (SRMs) were
analysed using this program.
2.2. Effects of sample preparation: drying times
and temperatures
Homogenisation of the sample is difficult if the
peat is wet but can easily be carried out by hand
using dry peat.However, drying peat, particularly
samples from cold regions, could results in a loss
of Hg by volatilisation.For this reason, tests of
the effect of air-drying peat at room temperature
on Hg content were carried out.The Hg profile of
an Arctic peat core was measured twice; once
using samples which had been kept frozen since
collection and once using samples which had been
air-dried overnight in a class 100 clean-air cabinet.
All samples were measured in duplicate using the
Leco AMA 254 program recommended here.Bulk
density was determined for each sample as
described below and from this, the volumetric
y3
concentrations (ng cm ) were calculated.
To investigate the effect of prolonged drying at
high temperatures and the effect of fertilisation
upon Hg losses upon drying, bulk samples of peat
from the ombrotrophic, pre-anthropogenic section
(88–234 cm) of the EGR2A core was homogenised
using a food mixer.The mixture was divided
into five sections of 150 g each.Three sections
were artificially fertilised with NH NO (1 mM),
4 3
Ca (PO ) (1mM), KCO (1 mM), respectively,
3 4 2 2 3
and a fourth was fertilised with a mixture of the
three above additives (1 mM each).The fifth
section was left unfertilised.These sections were
then subsampled into five parts, which were dried
at the following temperatures; RT, 30, 60, 90 and
105 8C for the following times; 0, 19, 24, 48, 120,
168 and 336 h.The drying was carried out in
acid-cleaned, Teflon bowls, each containing 3.5 g
of peat slurry.After drying, the samples were
stored frozen, sealed in air-tight plastic bags until
AAS Hg analysis using the Leco AMA 254.