Sample preparation for mercury determination

The sample preparation varies with the complexity of the matrix, but most complex samples require decomposition of the matrix and reduction of the mercury to its elemental form. It should be noted that, for mercury analysis in different sample matrices a careful quality control/quality assurance of the obtained data should be practice in order to validate the result which should include simultaneous determination of suitable certified reference materials (CRMs).

Currently, the CRMs prepared for the quality control/quality assurance of analytical values for mercury as well as methylmercury in various biological and environmental matrices are commercially available from several organizations, including the IAEA (International Atomic Energy Agency, Analytical Quality Control Services) (IAEA, 2003), NIST (National Institute of Standards and Technology, Office of Standard Reference Materials, USA) (Klobes et al., 2006), NRCC (National Research Council of Canada) (Barcelo, 1993) NIES (National Institute for Environmental Studies, Japan) (Suzuki et al., 2004), IRMM (Institute for Reference Materials and Measurements, European Commission) (IRMM, 2010) and SABS (South African Bureau of Standards) (www.sabs.ro). These CRMs may be used as needed.

3.4.1 Total mercury

The reactive mercury in water samples is determined by selective reduction with tin chloride (SnCl2), which forms Hg0. The stable complexes of Hg(II) and the organomercurials are not reduced (Stoichev et al., 2006). For measuring HgTOT by this method it is necessary to oxidize all chemical forms of mercury in the water before the reduction step. The oxidation can be done in acid media with permanganate or bromate (Logar et al., 2001). If preconcentration of Hg0 is used, lower LODs are obtained (Puk and Weber, 1994).

Environmental solid samples are generally made into a solution with wet digestion methods and analyzed by compatible instrumental techniques.

Most of the conventional digestion procedures are not only laborious and time- consuming, but also lack sufficient efficiency and reliability. Other extraction methods, such as sonication, distillation or soxhlet extraction, also have the above drawbacks, even though reliable results are usually achieved (Tseng et al., 1998).

Innovative techniques such as supercritical fluid extraction (SFE) and microwave-assisted extraction (MAE) have been developed and are a substantial advance. However, SFE potentially has technological limitations and shows insufficient extraction efficiency, usually depending on sample matrix and analyte polarity (Tseng et al., 1998). Besides, the expensive equipment required increases the cost of the analysis.

Two different approaches in microwave extraction procedures are the use of a closed system (pressurized with a closed vessel) or an open system (non-pressurized with an open vessel) (Stoichev et al., 2006, also see figure 3.7). They have different characteristics and application. The main advantages of the MAE technique are absence of inertia, rapidity of heating, reduction of extraction time, better reproducibility and reliability, ease of automation, and good ability for selective leaching and total digestion in a wide array of sample matrices. Thus, the application of this technique to sample preparation has been widely investigated in various fields of the environmental and analytical chemistry (Tseng et al., 1998).

Figure 3.7 Closed and open microwave assisted extraction systems (Amouroux, 2007)

The extraction of HgTOT from sediments is performed with concentrated HNO3 or acid mixtures under efficient reflux or bomb decomposition. Sometimes additional oxidants are added, such as H2O2, KMnO4, etc. (Varekamp et al., 2000).

3.4.2 Advances in sample preparation for GC-based hyphenated techniques

In order to extract the mercury species intact, several types of milder are used, such as citrate buffer and extraction with dithizone/chloroform, KBr/CuSO4/H2SO4 (Lambertsson et al., 2001), HCl/HNO3 mixtures (Stoichev et al., 2006), etc.

Solid sample preparation by acid or alkaline extraction with different heating sources (sonication, stream distillation, etc.) requires from 2 to 24 hours for complete recovery of mercury species whereas the microwave extraction of the mercury species is an extremely fast method (2-10 min) (Rodriguez Martin-Doimeadios et al., 2003). Both open and closed systems are used for alkaline or acid extractions of mercury species from the sediments. This method should, however, be used with caution since it may also suffer, as for other techniques such as distillation-based methods, from artifact formation of CH3Hg+ (Stoichev et al., 2006; Bloom et al., 1997); Liang et al., 2004).

The increased use of microwave-assisted extraction techniques in speciation analysis has been also reflected with regard to GC –ICP-MS (Lobinski et al., 1998; Slaets et al., 1999).

Volatile forms such as Hg0 and Me2Hg can be directly analyzed after fast desorption from the sampling traps (gold trap, carbotrap or Tenax) and preconcentration without derivatization. But, in most of the cases, it is necessary to derivatize the ionic mercury species in order to convert them to volatile forms, which are then separated by GC and detected by specific atomic detectors.

During the hydride generation (HG) with sodium borohydride (NaBH4), the Hg2+ is transformed to Hg0, while MeHg forms MeHgH. The derivatization should be applied in inert atmosphere and should start at pH 1-2 because otherwise MeHg is reduced to Hg0. The HG can therefore be directly applied for sea and estuarine waters (Tseng et al., 1998).

The most important recent advances in sample preparation included the introduction of NaBPr4 for the derivatization of organometallic species (De Smaele et al., 1998), and the use of headspace solid-phase micro extraction (SPME) (De Smaele et al., 1999; Aguerre et al., 2000; Mester et al., 2001), stir bar sorptive extraction (Vercauteren et al., 2001) and purge and capillary trapping for analyte recovery and preconcentration (Wasik et al., 1998).

3.4.3 Derivatization techniques

The position of tetraalkylborates allowing the derivatization in the aqueous phase, such as sodium tetraethylborate (NaBEt4), for organomercury and organotin speciation analysis and the latest introduced sodium tetrapropylborate (NaBPr4) for organolead has been well established. Synthesis of NaBPr4 was described in detail and the possibility of the simultaneous determination of Sn, Hg and Pb following the propylation was demonstrated (De Smaele et al., 1998).

Two careful comparison studies are worth-noting. In one of them three derivatization approaches, namely anhydrous butylation using a Grignard reagent, aqueous butylation by means of NaBEt4 and aqueous propylation with NaBPr4 were compared for mercury

speciation (Kotrebai et al., 1999). The absence of transmethylation during the sample preparation was checked using a 97% enriched 202Hg inorganic standard (Fernandez et al., 2000).

During the ethylation with NaBEt4 the Hg2+ is transformed to HgEt2 , while MeHg forms MeHgEt as follow:.

Hg2+ + 2NaB(C2H5)4 → Hg(C2H5)2 + 2Na+ + 2”B(C2H5)3” (3.4) CH3Hg+ + NaB(C2H5)4 → CH3HgC2H5 + Na+ + “B(C2H5)3” (3.5)

The ethylation is motivated by the higher stability of MeHgEt compared to MeHgH. In salty water one part of MeHg is reduced to Hg0 during the ethylation. Thus, this procedure can be directly applied only to fresh waters (Ceulemans and Adams, 1996). As mentioned above, isotope dilution ICP-MS is a new powerful approach to solve the problems with the matrix and non-quantitative derivatization. A drawback of the ethylation procedure is the impossibility to distinguish between Hg2+ and EtHg+, both species that often coexist in the environment (Cai et al., 1997). An alternative is the use of the propylation as a derivatization technique with NaBPr4 as derivatizing agent which is more tolerant to interferences from chlorides (Demuth and Heumann, 2001). However, it was found that the propylation of extracts from soil samples suffers also from artifact formation of MeHg and especially of ethylmercury during the derivatization (Huang, 2005). Derivatization techniques for GC were reviewed (Liu and Lee, 1999).

3.4.4 Solid-phase micro-extraction

Solid-phase micro-extraction (SPME) is a preconcentration technique based on the sorption of analytes present in a liquid phase or, more often, in a headspace gaseous phase, on a microfiber coated with a chromatographic sorbent and incorporated in a microsyringe. The analytes sorbed in the coating is transferred to a GC injector for thermal desorption. SPME is an emerging analytical tool for elemental speciation in environmental and biological samples (Mester et al., 2001). This solvent-free technique

offers numerous advantages such as simplicity, the use of a small amount of liquid phase, low cost and the compatibility with an on-line analytical procedure.

SPME is based on the equilibrium between the analyte concentrations in the headspace and in the solid phase fiber coating. Low extraction efficiencies are hence sufficient for quantification but the amount of the analyte available may be very small. Therefore, it is of interest to combine SPME with the high sensitivity of GC – ICP-MS.

The first work SPME - GC – ICP-MS concerned speciation of organomercury, -lead and - tin compounds ethylated in-situ with NaBEt4 and sorbed from the headspace on a poly(dimethylsiloxane)-coated fused silica fiber (Moens et al., 1997).

A detection limit of 2 pg l-1 was reported for an aqueous standard but a value of 125 pg l-1 was given for the sample extract corresponding to a DL in the low ng g-1 range (dry weight) (Vercauteren et al., 2000).