Thermal desorption analysis for mercury is a method to obtain information about different mercury fractions in solid samples by separating them based on their desorption temperature.
Background:
Thermal desorption atomic absorption analysis is a technique for the direct analysis of solid samples for their total mercury content. The sensitive technique requires little if any sample preparation and can be calibrated with simple standards. Mercury is liberated from the solid sample material by heating the materials placed in a quartz furnace, transported by a carrier gas towards a gold trap, concentrated on the gold trap and finally released by heating the gold trap and determining by flameless atomic absorption spectrometry. By using the gold amalgamation step for collecting the mercury, its later release from the trap makes the signal generation independent of the desorption kinetics of the mercury from the original sample material. While in this way a reliable determination of total mercury from different solid sample materials is possible, no information on the presence of different mercury species is available.
Mercury "speciation analysis":
The concept of using thermal desorption analysis for speciation analysis is based on the observation that mercury is released from the sample material over a wide range of temperatures, with showing different peaks at different temperatures when raising the temperature slowly. The hope is that such peaks within the thermogram can be assigned to different mercury species. Temperature ranges for the release of different mercury species obtained by different authors are shown in Table I.
Table 1: Temperature ranges for the release of different mercury species from soils and ores
Mercury species
| Mercury release
temperature
| Reference
|
Hg°
| < 80
| Watling, Davis et al, 1973
|
| | < 150
| Bombach et al, 1994
|
| Hg2Cl2 | 170 | Watling, Davis et al. 1973 |
| HgCl2 | <250
| Koksoy, Bradshaw and Tooms, 1967 |
| 220 | Watling, Davis et al., 1973 |
| HgO | 270-535
| Koksoy, Bradshaw and Tooms, 1967 |
| 160-495
| Watling, Davis et al. 1973 |
HgS
| 300 | Watling, Davis et al. 1973
|
| 280-400
| Biester, 1994 |
| 210-340 | Koksoy, Bradshaw and Tooms, 1967 |
| 400 | Bombach et al, 1994 |
| Hg in Pyrite | 450 | Watling, Davis et al, 1973 |
| Hg in Sphalerite | 600 | Watling, Davis et al, 1973 |
| Hg-humic substance | 200-300
| Biester, 1994 |
It can be seen that species with low oxidation states, i.e., Hg ° and Hg1+ are released at lower temperature. It can also be observed, that the release temperature of different species overlap with each other. Even further, when looking at the thermograms of different species, it can be observed that several species have more than one peak in the thermogram. It can be seen that there is no direct correlation between the physicochemical properties of the pure mercury compounds, i.e., melting and sublimation point and vapour pressure, with the temperature range in which they are released from the soil matrices.
Unfortunately, the release kinetics of mercury from solid sample materials is dependent on many factors:
a) the mercury species
b) its interaction with the solid sample material
c) the particle size of the solid material
d) the operating parameters of the system (e.g. heating rate, carrier flow rate, carrier gas)
The release behaviour of the Hg-species, as demonstrated by the thermograms, can be in part a reflection of the kinetics, the thermodynamics and mechanisms of the vaporisation processes. These characteristics under non-equilibrium conditions are different from those in equilibrium (Somorjai and Lester, 1967), resulting in different volatilisation rates. Therefore, vaporisation behaviour, in non-equilibrium conditions, cannot be predicted from equilibrium measurements.
Also, since the sample mass that can be accepted by the furnace is rather limited, reproducibility is dictated by the homogeneity of the sample. Species transformation during heating can be observed, depending on the composition of the matrix. For example, the oxidation of elemental mercury to Hg(II) can be observed, causing double peaks in the mercury release curve. Wallschläger et al. (1998) observed that upon addition of Hg model compounds to a sediment matrix, all species were transformed to the same new speciation pattern, regardless of their original speciation.
While most authors claim that the thermal release analysis of soil and ore samples was suitable to differentiate mercury species, quantitative results for different species are seldom reported. Observed thermograms of real samples are most often qualitatively discussed and compared, and related species are suggested rather than identified. Different fractions are often qualitatively reported as "main" and "secondary" fraction and characterized by terms such as "mobile", "semi-mobile" and "non-mobile" rather than true species identification. In cases where quantitative results are assigned to different species, authors are very vague in describing which part of the thermogram was assigned to which species and how signal integration was performed. For example, Tersic et al. (2011) "quantified" non-cinnabar Hg by doubling the first half of the peak occurring at 150–250 °C and then calculated the content of cinnabar as the difference between non-cinnabar Hg and total Hg.
The thermal release of Hg(II) is quite different for each mercury compound. Reported temperatures for HgCl2 are widely dispersed. Thus, the assignment of the peak at 250°C is not clear, there may be an overlap with the peak of Hg0, and it can theoretically be confused with the peak of Hg(I). Also, the desorption range of mercury bound to humic substances is partially overlapped with the temperatures of synthetic and natural cinnabar. Validation of the methods is often performed by comparing the total mercury content, indicating accuracy in the range of +- 20%.
While the technique has been used now for more than 20 years, there is hardly any development observable with respect to instrumentation and methodology. According to the definition of "speciation analysis" by IUPAC, thermal desorption analysis is not a method for speciation analysis but a fractionation technique. The separation power of the technique is simply not sufficient to differentiate between distinct species but can only separate more volatile fractions from less volatile fractions. Anyhow, in absence of more powerful methodology for the true speciation analysis for mercury in solid sample materials, the information provided by thermal desorption analysis might still be highly welcomed.
Related studies
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Further chapters on techniques and methodology for speciation analysis:
Chapter 1:
Tools for elemental speciation Chapter 2: ICP-MS - A versatile detection system for speciation analysis Chapter 3: LC-ICP-MS - The most often used hyphenated system for speciation analysis Chapter 4: GC-ICP-MS- A very sensitive hyphenated system for speciation analysis Chapter 5: CE-ICP-MS for speciation analysis Chapter 6: ESI-MS: The tool for the identification of species Chapter 7: Speciation Analysis - Striving for Quality Chapter 8: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis Chapter 9: Gas chromatography for the separation of elemental species Chapter 10: Plasma source detection techniques for gas chromatography Chapter 11: Fractionation as a first step towards speciation analysis Chapter 12: Flow-injection inductively coupled plasma mass spectrometry for speciation analysis Chapter
13: Gel electrophoresis combined with laser ablation inductively
coupled plasma mass spectrometry for speciation analysis Chapter 14: Non-chromatographic separation techniques for speciation analysis
last time modified: November 12, 2024
