1. Introduction 3. Instrumentation 2. Objectives 4. Results
Mercury is an environmental pollutant of global concern. It occurs in nature in different forms, divided into inorganic mercury and organic mercury. All species have severe toxic effects on biota, but the toxicity and physiological metabolism differs, with organic mercury known as the most toxic mercury compound. The main mercury species in nature are Hg0, Hg2+ and MeHg, and this work focuses on the speciation of MeHg and Hg2+.
Figure 1 shows the central part of the global mercury cycle.
The aim of this research was the development of an online pre-concentration system for speciation with HPLC-CV-AFS.
The Limit of detection for most analysis methods is in the high ng L-1 range which is simply not sensitive good enough for most environmental samples especially water.
We developed therefore a preconcen-tration set-up prior to the separation-detection unit:
MeHg and Hg2+ is trapped on thiol and thiourea bound to silica (see Fig. 2) from the sample and subsequently eluted with the mobile phase into the HPLC-CV-AFS.
Fig. 3
Oxidant
0.01 M Br-/BrO3- in 1.2 M HCl
3 ml/min
Reductant
2 % SnCl2 in 1.2 M HCl
5 ml/min
Mobile phase
1.5 mM Ammonium pyrrolidine dithiocarb-amate in 78 % methanol
1 mL/min
Table 2
The main part of the system is a Millennium Merlin System coupled to a UV cracker and a cooling module (PSA S570U100 & S570C100).
Two basic HPLC pumps are used (one pumps the sample!)
Sample is loaded (in principle any volume) onto a preconcentration column
The valve is switched after the preconcentration
The mobile phase elutes the mercury species from the pre-conc. column onto the main separation column
Fig. 2
Preconcen
-tration
material
Fig. 1
Mercury enters the atmosphere as Hg0,
is oxidized and enters the aquatic
environment as Hg2+, is methylated
by microorganisms into the more
toxic MeHg which accumulates
throughout the aquatic food
web and affects in the end
us as human beings.
4. The solvent stream is mixed with an oxidant stream to oxidise MeHg+ to Hg2+;
UV supports the oxidation
The solvent stream is mixed
with reductant to produce Hg0
Argon gas carries Hg0 as a vapour into the AFS detec-tion chamber
4. Results
4.1 Performance
The parameters defining the pre-
concentration were analysed first.
Crucial points are:
(a) pH of the sample
(b) Flow-rate of sample for preconc.
Fig. 5
Analysis of sediment
ERM®-CC580 after 2
different extraction procedures
The flow-rate of the sample for the preconcentration in the range of 2 to 10 mL min-1 has no influence on the analysis. No break-through of mercury was observed.
4.5 Comparison with other methods
We analysed various pilot-whale tissues (liver, muscle, kidney) with SS-ID-ICP-MS and this method and Figure 6 shows the correlation between the results. A slope of could be obtained which makes this method a reliable method for speciation.
4.4 Water, fish, lobster, hair and sediment samples
Various samples with different and challenging sample matrices were analysed and the results are presented in table 2. The samples range from spiked samples to Certified reference materials.
Overall, the recovery is in the range of 91.6 – %. Only sediment ERM®-CC580 showed a lower recovery with 87.5 %., however here a a selective extraction of the MeHg was used.
In this sediment, the MeHg:Hg2+ ratio is 1:1786. A huge Hg2+ peak occurred next to the MeHg peak (see figure 5) but a selective extraction for MeHg reduced the peak to 0.28 ± 0.22 % of the original Hg2+ concentration.
Fig. 4
Fig 4 shows the pH dependency of a 100 ng L-1 MeHg solution (10 mL preconcentrated). There is a broad stability from a pH of 1 to 6 with an intensity increase for pH 0, a decrease towards pH 7 and no possibility for separation after pH 7. Samples need no exact buffering, a rough acidifcation with HCl brings the sample into the suitable pH range.
4.2 Figures of merit
Fig. 6
The method has a broad linear range from pg L-1 range to at least 1 µg L-1. An LOD of 0.04 ng L-1 for MeHg can be achieved when 200 mL sample are pre-concentrated. The linearity for a MeHg calibration is defined by an R2 of
Samples
Certified or spiked concentration for MeHg
Measured concentration
Recovery for MeHg as Hg (%)
Water samples
Sea water
10.0 ng L-1
9.91 ± 5.5 ng L-1
99.1 ± 5.5 %
River water
9.70 ± ng L-1
97.0 ± 2.6 %
Crude seawage
9.25 ± 0.14 ng L-1
92.5 ± 1.4 %
Spiked Urine
50.0 ng L-1
49.9 ± 2.4 ng L-1
99.9 ± 4.8 %
Estuarine sediment
ERM®-CC580
75.5 ± 3.7 µg kg-1
77.8 ± 5.7 µg kg-1
103.7 ± 7.6 %
65.6 ± 4.7 µg kg-1*
87.5 ± 6.3 %*
Fish and lobster CRM
NRCC-TORT-2
152 ± 13 µg kg-1
154.6 ± 5.8 µg kg-1
102 ± 3.8 %
NRCC-DOLT-2
693 ± 53 µg kg-1
756 ± 39 µg kg-1
109.1 ± 5.7 %
NRCC-DOLT-4
1.33 ± 0.12 mg kg-1
1.21 ± mg kg-1
91.6 ± 3.3 %
NRCC-DORM-3
355 ± 56 µg kg-1
362 ± 9 µg kg-1
101.9 ± 2.6 %
Hair
NIES No.13
3.8 ± 0.4 mg kg-1
3.91 ± 0.22 mg kg-1
± 5.8 %
IAEA-085
21.9 ± 2.0 mg kg-1
21.92 ± 1.09 mg kg-1
100.1 ± 5.0 %
5. Conclusion
Our method is an easy and cost effective speciation approach for MeHg based on the preconcentration of the mercury species on thiol/thiourea material and the subsequent analysis with HPLC-CV-AFS. The method works with good recoveries for a broad variety of samples and even challenging sample matrices have no effect on the preconcentration or the analysis.
4.3 Sample preparation
Water samples are filtered, acidified with HCl (final concentration of 0.37 % (v/v)).
Sediments were acid leached with 18.5 % (v/v) HCl for 30 min under ultrasound. A 1:50 dilution was done for analysis.
A selective extraction for MeHg was performed based on the extraction of MeHg into dichloromethane from the acid leachate, and a back-extraction into water by evaporating the dichloromethane with a stream of air.
The biological samples were digested in alkaline medium in the microwave, ultracentrifuged, acidified, diluted and analysed.
6. Acknowledgement
This PhD was funded by PS Analytical and the University of Aberdeen. A big thank-you therefore to Dr. Warren T. Corns and Dr. Bin Chen, and to Dr. Eva Krupp and Prof. Jörg Feldmann.
Table 2
* Selective extraction for MeHg with dichloromethane