The microwave digestion vessels containing the samples were placed in a fume extraction hood before adding 10 mL concentrated HNO3. The vessels were left for at least 30 minutes without their lids on to allow gases to escape. The vessels were subsequently digested using a microwave digestion system. Alternatively, digestion could have been performed using a hotblock.
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Following digestion, the samples were transferred to a 100 mL graduated flask and 60 mL of 6% potassium permanganate solution was added. Sample vessels were left for at least two hours to ensure that all the mercury in the sample was reduced to Hg2+. It was very important to make sure that the vessels were not sealed at that stage, as gases produced could cause pressure to build up.
After the mercury was reduced, 15 mL of 20% hydroxylamine chloride solution was added to remove the excess potassium permanganate. It was essential to add the hydroxylamine chloride slowly during that stage and to gently mix the solution during the addition. The solution was then allowed to cool and deionized water was added to increase the volume up to 100 mL.
We prepared standards from a 1,000 parts per million (ppm; mg/L) mercury standard solution, which was first diluted to produce a 1,000 ppb (mg/L) stock solution to allow simple preparation of a range of standards. To demonstrate the linear range of AA spectrometry, we used a wide range of standards (1 to 100 ppb). The standards were matrix matched and prepared in the same order as the samples.
The VP100 requires both a reductant and an acid solution to perform the reactions that form the gaseous mercury. For this application, the reductant was a solution of 7.5% stannous chloride (SnCl2) stabilized in 10% HCl. The acid solution was 50% HCl.
The analysis was performed using the most sensitive absorption wavelength for mercury at 253.7 nm. Five replicate analyses were used, with each replicate taking four seconds, to assess thoroughly the short-term stability of the instrument during the development of this method. For normal use, three replicates would be adequate. Deuterium background correction was employed throughout the analysis. The VP100 and spectrometer parameters are shown.
The calibration curve showed excellent linearity up to 100 ppb, which is equivalent to 20 mg/kg in a fish sample weighing 0.5 g. The R2 value was 0.9989, demonstrating the superb performance of AA spectrometry over a wide concentration range. This calibration is equivalent to concentrations of 0 mg/kg to 20 mg/kg mercury in the original fish samples, assuming a sample mass of 0.5 g. The percent RSDs for each of the standards were less than 2.5%, demonstrating the excellent stability of both the spectrometer and the VP100 accessory.
We calculated the method detection limit (MDL) and characteristic concentration using the automated instrument performance wizard in the SOLAAR software. This feature is able to guide analysts through the steps necessary to quantify the performance of a method. It also automates all of the data processing, making the entire procedure quick and easy. The method was found to have a detection limit of 0.068 ppb (mg/L) in solution. This equates to a MDL of 0.014 mg/kg in the original fish sample, assuming a sample mass of 0.5 g. The MDL provides a measure of the noise and stability of the system. A lower detection limit allows for confident determination of lower mercury concentrations. The characteristic concentration is related to the sensitivity of the method. The characteristic concentration of this method was found to be 0.724 ppb in solution. This would be the equivalent of 0.145 mg/kg in the initial fish sample, assuming a sample weight of 0.5 g.