18 food hygiene & safety
term stability of the instrument during the development
of the method. For normal use, three re-samples would
be adequate. Deuterium background correction was used
throughout the analysis.
The calibration curve showed excellent linearity up
to 100 ppb (Fig. 2), which is equivalent to 20 mg/kg in a
fish sample (assuming a sample weight of 0.5 g) with an
R
2
value of 0.9989. This shows the superb performance of
the iCE 3000 series over a wide concentration range. This
calibration is equivalent to concentrations of 0–20mg/kg
mercury in the original fish samples, assuming a sample
mass of exactly 0.5 g. The per cent relative standard
Fig. 2. The calibration curve showed excellent linearity
deviations ( per cent RSDs) for each of the standards were
up to 100 ppb.
less than 2.5 per cent. This demonstrates the excellent
stability of both the spectrometer and the VP100 accessory.
☞
become hot. It is essential to add the hydroxylamine The method detection limit (MDL) and characteristic
chloride slowly during this stage and to gently mix the concentration were calculated using the automated
solution during the addition. Without these precautions ‘Instrument Performance’ Wizard in the SOLAAR
a violent reaction may occur that could eject some software, automating all of the data processing, making
sample from the flask, leading to inaccurate results. After the entire procedure quick and easy. The method was
allowing the solution to cool, deionised water was added found to have a detection limit of 0.068 ppb (µg/L) in
to make the volume up to 100 mL. solution. This equates to an MDL of 0.014 mg/kg in the
original fish sample (assuming a sample mass of 0.5 g).
Preparation and results
The MDL provides a measure of the noise and stability
Standards were prepared from a 1000 ppm (mg/L) of the system. A lower detection limit allows you to
mercury standard solution. This standard was first confidently determine lower concentrations of mercury
diluted to produce a 1000 ppb (µg/L) stock solution in your samples. The characteristic concentration
to allow simple preparation of a range of standards. To is related to the sensitivity of the method. The
demonstrate the linear range of the iCE 3000 Series characteristic concentration of this method was found to
AA spectrometers a wide range of standards were used be 0.724 ppb in solution. This would be the equivalent
(1–100 ppb). The standards were matrix matched and of 0.145 mg/kg in the initial fish sample (assuming a
prepared in the same order as the samples. sample weight of 0.5 g).
The VP100 requires both a reductant and an acid Salmon and sardine samples were spiked with
solution (50 per cent HCl) to perform the reactions 10 ppb mercury prior to digestion and compared with
that form the gaseous mercury. For this application unspiked samples to calculate recoveries. These 10 ppb
the reductant was a solution of 7.5 per cent stannous spikes would correspond to a concentration of
chloride (SnCl
2
) stabilised in 10 per cent HCl. 2 mg/kg in normal fish samples (assuming a sample
The analysis was performed using the most sensitive weight of 0.5 g) and demonstrate the accuracy of the
absorption wavelength for mercury at 253.7 nm. Five analysis at levels appropriate to current legislation. The
re-samples were used, with each re-sample taking four agreement with expected results is excellent, with the
seconds. This was done to thoroughly assess the short- recovered values all falling within six per cent of the
expected values. This demonstrates the repeatability and
accuracy of both the sample digestion procedure and
the vapour analysis using the Thermo Scientific iCE 3000
Series AA spectrometers (Fig. 3).
To ensure the accuracy of the sample preparation,
digestion and analysis, three separate samples of the
DORM-2 standard reference material were also analysed.
The recoveries from these samples were also excellent,
with an accuracy of ±2 per cent or better. These results
show that an AA spectrometer coupled with a vapour
generation accessory offer excellent linear range,
stability and accuracy during the analysis of trace levels of
mercury in fish. p
Dr Andrew David Bowen is Applications Scientist and Hazel
Fig. 3. The repeatability and accuracy of the AA spectrometer were Dickson is Applications Chemist with Thermo Fisher Scientific,
successfully proved. Cambridge, UK. www.thermo.com/ice
www.scientistlive.com
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