2.3. Distillation of natural water samples
Natural water samples analyzed in this study contained high
amounts of DOC and therefore it was necessary to separate CH3Hg+
from the interfering sample matrix prior to the instrumental determination.
For this purpose, N2-assisted distillation method in which
CH3Hg+ was separated as its chloride complex was used. The inhouse
made distillation system was equipped with N2 purging gas,
heated metal block and glass distillation vessels with silicone plugs
and Teflon tubes (one for distillate and one for N2 gas). The system
enabled the distillation of 20 samples simultaneously.
Prior to the distillation, a 50 g aliquot of an unfiltered sample
(preserved with 0.4% HCl) was accurately weighted into a 125ml FLPE
bottle. Sample was spiked with a weighted aliquot of an isotopically
enriched methyl mercury solution (50 or 100 pg CH3
201Hg+ g−1) and
allowed to equilibrate for about 20 h. After equilibration, samples
were transferred to the distillation vessels and the FLPE bottles, used
as receiving vessels, were placed in an ice bath. Samples were distilled
at the temperature of 145 °C under N2 flow rate of about 100mlmin−1.
N2 flow rate varied slightly between the vessels resulting in the variation
in the distillation time between the samples. The distillationwas stopped
when approximately 70–90% of the sample had been distilled by
removing the receiving vessel from the distillation system. The sample
vessels were capped and stored at 4 °C in the dark until analysis. The
total distillation time for a 50ml sample volume was usually between 2
and 3h. Since the spikewas added to the samples prior to the distillation
step, the amount of collected distillate was not crucial and incomplete
recovery of CH3Hg+ during distillation and in later analytical steps
could be corrected.