food hygiene & safety 17
endorses the provisional tolerable weekly intake of
1.6mg/kg of methyl mercury. The European Commission
has introduced the EC 1881/2006 regulation, setting
maximum levels for certain contaminants in foodstuffs.
Atomic absorption (AA) spectroscopy is an ideal tool
for the measurement of low levels of mercury in fish. For
laboratories interested in total mercury measurements
this technique provides fast and accurate analysis of
samples with detection limits below 0.07 ppb (µg/L) in
solution. This equates to 0.014 mg/kg in the original
fish sample, based on a 0.5g in 100 mL preparative
method. For laboratories analysing methyl mercury, AA
spectrometers provide an excellent screening tool. Their
cost-effectiveness and ease-of-use makes them a perfect
partner to more complex and expensive techniques,
such as HPLC-ICP-MS or GC-ICP-MS.
AA case study
For this particular application a Thermo Scientific iCE
3500 AA spectrometer and a Thermo Scientific VP100
vapour generation accessory were used. The continuous
flow of reagents ensures that the system is self-cleaning,
reducing memory effects and increasing sample
throughput. The VP100 is entirely controlled by Thermo
Scientific SOLAAR software, meaning that setting up a
Fig. 1. The procedure for preparing samples, sample spikes and
method and running an analysis is extremely simple.
matrixmatched standards for the analysis of mercury in fish.
A mercury cell (provided as standard with the VP100) using a mortar and pestle. After drying, portions of
was also used. This accessory provides an increased approximately 0.5 g should be accurately weighed out
pathlength compared to a normal vapor cell and gives for digestion. For wet weight measurements the fresh
exceptionally low detection limits. fish should be homogenised in a food processor and
The sample preparation procedure is shown in a portion of approximately 0.5 g should be accurately
Fig. 1. There are four main sections: sample drying, weighed and placed in a microwave digestion vessel. This
sample preparation, sample digestion and mercury provides a representative fish sample.
reduction. The drying section may not be applicable for Following preparation in this manner, 1 mL of
all situations, as it is only necessary if the final mercury 1000 ppb Hg standard solution was added to half
concentration is needed as a dry weight value, for example of the salmon and sardine samples. This spike gave
mg/kg dry weight. Most countries and official regulatory a concentration of 10 ppb Hg in the final 100 mL
bodies concentrations of mercury in a wet weight of sample. The other half of the samples did not have
sample. mercury added to them to allow the calculation of spike
recoveries. The microwave digestion vessels containing
1. Sample drying phase is not necessary if the final
the samples were placed in a fume extraction hood
concentration of mercury is needed for a wet-weight
before adding 10 mL concentrated HNO3. The vessels
sample.
were left for at least 30 minutes without their lids on to
2. Refer to the manufacturers guidelines when designing
allow gases to escape. After this time the vessels were
a digestion programme.
placed into a microwave digestion system.
3. CARE: The reaction is exothermic and the flask
After digestion the samples were transferred to
may become hot. Also, make sure to add the
a 100 mL graduated flask and 60 mL of six per cent
hydroxylamine chloride slowly, otherwise the solution
potassium permanganate solution was added. The
may foam and eject some sample from the flask.
sample vessels were left for at least two hours to ensure
Three different types of fish sample were used during that all the mercury in the sample was reduced to Hg2+.
the evaluation of this method: fresh fish (salmon), It is very important to check that the vessels are not
canned fish (sardine), and DORM-2 certified reference sealed during this stage, as gases are produced that could
material (National Research Council of Canada, Institute cause pressure to build up.
for National Measurement Standards, Ottawa, Canada). After the mercury was reduced, 15 mL of 20 per cent
If dry weight measurements are needed then the fish hydroxylamine chloride solution was added to remove
samples should be homogenised and dried in an oven the excess potassium permanganate. Care was taken
at 80°C until they reach a constant weight. Alternatively, during the addition of the hydroxylamine chloride, as
the fish tissue can be freeze-dried and homogenised this produces an exothermic reaction and the vessel may
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