DEVELOPMENT OF AN ANALYTICAL PROTOCOL FOR THE DETERMINATION OF MERCURY CONCENTRATIONS IN SOLID PEAT SAMPLES: EFFECTS OF SAMPLE PREPARATION

 

F. Roos, Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland        

Tel. ++41 (31) 631 8761         fax. ++41 (31) 6314843  email fiona.roos@geo.unibe.ch

H. Biester, Institute of Environmental Geochemistry, University of Heidelberg, Germany

M.E. Goodsite, Department of Chemistry, Odense University, Odense, Denmark

A. Martinez-Cortizas, Department of Soil Science, University of Santiago di Compostela, Santiago, Spain

W. Shotyk, Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland

 

Abstract: Traditional peat sample preparation methods such as drying at high temperatures and milling may be unsuitable for Hg analysis in peats due to the possible presence of volatile Hg(0), which could be lost during drying.  Factors such as variation in bulk density, water content and Hg variation in individual peat slices must also be taken into account during the analytical procedure. Here, the variation of water content, bulk density and Hg concentration in an ombrotrophic peat core were quantified in order to determine their relative importance as sources of analytical error. The effects of drying and grinding peat samples are investigated.  These results form an important part of the development of an analytical protocol for Hg in peat samples.

 

Introduction: Records of net accumulation of atmospheric Hg are well-preserved in peat cores from ombrotrophic bogs (Benoit et al 1998, Shotyk et al 2000).  By measuring the concentrations of Hg in peat extending back in time to pre-anthropogenic periods, natural “background values” and their climate-related variations can be quantified and used to identify the effects of recent increases due to human activities (Martinez-Cortizas et al, 1999). During traditional acid digestion of peat samples, various sources of contamination must be considered and clean techniques such as those employed by Weiss et al (1999) used.  Analysis of solid samples by combustion and subsequent trapping of Hg on gold before AAS analysis (Salvato et al 1994) not only avoids several possible sources of contamination (acids, digestion vessels) but is also safer and allows a higher sample throughput rate.  The ash from the combusted samples can be recovered and used for further analysis, for example trace metal determination by XRF.  However, the volatility of Hg(0) combined with the natural variability of peat structural components (water content, bulk density, ash content) mean that care must also be taken to achieve accurate results when analyzing solid samples.

 

Methods: 

A peat monolith from Schoepfenwaldmoor (SWM), Switzerland, was used to investigate the variation of water content, bulk density and Hg concentration in a typical ombrotrophic peat.  Water content and bulk density variation were studied both throughout 17cm of the core and within an individual peat slice. Variation in Hg concentrations within a single slice was also recorded. The investigation of the effect of air drying at room temperature was also done using samples from this core. Determination of water content and its variation in air dried peat “microcores” or “plugs” was done using samples from Etang de la Gruere (EGR) in the Swiss Jura mountains, also an ombrotrophic bog. The effect of grinding dry samples was studied using a homogenized in-house peat standard, which had previously been dried at 105°C.

 

Variation of bulk density and water content.

The monolith was collected at SWM (coordinates CH 631.250, 177.000) on 28.8.91 using a Wardenaar corer.  The monolith obtained (10cm x 10cm x 100cm) was stored at -18°C after collection.  It was then cut (frozen) into 1cm slices and the individual slices were again stored at -18°C until analysis.  The top 17 slices were used in the following analyses. A stainless steel tube with a sharpened end, of diameter 16mm was used to remove three plugs from each 1cm slice. The height of these plugs was measured and the volume of each calculated (v = pr2h) in cm3.  The wet weight of each plug was recorded and the plugs were then dried to constant weight in an oven at 105°C. The weights of the dry plugs were recorded.  The dry plugs were then stored in air-tight plastic boxes which had been soaked for 1hour in 10% HNO3 and rinsed 6 times with 18W water.

 

Variation of Hg in one 10x10x1cm slice and effect of air drying on Hg content of peat.

Slice 15 (i.e.15cm from surface) of SWM core was used to investigate the variation of Hg in one 10cm x 10cm x 1cm slice of peat.  Sixteen plugs (four rows of four) were extracted as described above, at even distance from one another. The wet weights of these plugs were recorded. Half the plugs were left to dry in open, clean plastic boxes in a class 100 clean air cabinet overnight.  The other half of the plugs were sealed in clean plastic boxes and placed in the freezer at -18°C overnight. All the samples were analyzed for Hg using the LECO AMA 254 AAS Advanced Mercury Analyzer.  Wood was removed from the samples before analysis. The drying time used for the wet samples was [0.7 x volume of water] seconds, with water volume calculated from water content and mass of wet sample. The drying time used for air-dried samples was 70s.  Decomposition time for all samples was 200s. The theoretical dry weight of the samples was calculated using their wet weights and the average water content of slice 15, previously determined to be 93.04+/-0.38%.  The Hg concentration was calculated and expressed as ng g-1 in dry weight of peat.

 

Variation of water content in air-dried plugs

The EGR core (coordinates CH 570.525, 232.150) was collected on 26.8.91.  The core was taken using a Livingston corer, and gave a cylindrical core in 1m sections (total length 692cm, d=8cm).  The core was sliced frozen into 2cm slices.  The slices were stored in individual plastic bags at -18°C until analysis. Slices 490cm-592cm were allowed to thaw before plugs were taken. Four plugs were taken from each slice.  One plug from each slice was weighed wet.  All the plugs were allowed to air dry in a class 100 clean air cabinet for 20 hours. The air-dried plugs were re-weighed after 20 hours and then placed in an oven at 105°C until constant weight was obtained.  The percentage water remaining in the air-dried plug was then calculated from the air-dried weight and the constant dry weight of the plugs.  This value for water content of air dried samples was used to calculate the theoretical dry weight of the remaining air dried plugs, which were subsequently analyzed for Hg.

 

Effect of grinding samples

Five samples of an in-house peat reference material were ground using a coffee mill.  These samples were analyzed for Hg using the LECO AMA-254 and the values obtained were compared to those obtained using 5 unground samples of the same material.

 

Results and Discussion: Variation of bulk density and water content

The average water content of the 17 slices analyzed was 94.01%, with a standard deviation of 1.60%.  The average relative standard deviation of the percentage water in a single slice was 0.97%.  These results indicate that water content variation is negligible in the part of the core studied.

However, the average bulk density for all the samples studied was 0.05g cm-3, with a standard deviation of 0.02 and the average relative standard deviation of bulk density within a slice was 15.63%.  This is a significant variation, indicating that bulk density variation should be taken into account when comparing Hg values from different parts of a core.

Hg variation in a slice

Hg concentrations within a slice were found to be very variable (Table 1):

Sample Treatment

Calc.dry mass sample (g)

ng Hg

Hg conc (ng g-1)

Dry

126.70

12.51

98.7

Dry

122.10

7.08

58.0

Dry

97.67

6.84

70.0

Dry

118.98

6.29

52.8

Dry

92.00

6.03

65.6

Dry

118.40

7.10

67.5

Dry

124.40

9.23

74.2

Dry

119.22

9.56

80.2

Wet

125.00

11.35

90.8

Wet

110.30

5.85

53.1

Wet

117.01

4.85

41.4

Wet

41.19

3.48

84.6

Wet

50.03

3.40

67.9

Wet

17.91

1.25

69.9

Wet

28.30

1.99

70.4

Wet

14.03

0.98

69.7

Table 1

These results indicate the importance of either homogenizing the peat prior to measurement (difficult if peat is wet) or analyzing several samples from each slice and averaging the results  in order to obtain representative values.

The average Hg concentration for dried samples was 70.9 ± 14.2 ng g-1( n=8).  The average Hg concentration for wet samples was 68.5 ± 15.8 ng g-1 (n=8).  It is therefore concluded that Hg was not measurably lost on air drying the samples.

 

Variation of water content in air dried plugs.

 The average water content of the plugs left to air dry for 20 hours was 10.9% after drying, with a standard deviation of 1.6%. Water content of samples to be analyzed can thus be calculated by comparison with a similarly sized and shaped sample, which has been allowed to dry for the same length of time and is subsequently dried to constant weight.

Effect of Grinding samples

Sample treatment

Mass analyzed (mg)

Hg (ng g-1)

Unground

118

47.1

Unground

102

46.4

Unground

158

48.6

Unground

85

49.3

Unground

63

47.8

Ground

61

43.5

Ground

61

45.0

Ground

98

46.3

Ground

108

46.3

Ground

92

45.0

 Table 2

The average Hg concentration for unground samples was 47.9 ± 1.2 ng g-1 (n=5) and the average for ground samples was 45.2 ±1.2 ng g-1 (n=5) (Table 2).  It is therefore possible that a slight loss of Hg occurs on grinding.  This step, which promotes homogenization of the sample and also allows a greater mass of sample to be analyzed at one time, by fitting more material into each sample vessel, can be replaced by simply crushing the dried samples by hand in a plastic sample bag.  In this way an acceptably fine-grained powder can be obtained, which gives greater homogeneity than the unground samples (relative standard deviation in Hg concentration results using hand-ground samples was 3%).

 

References:

Benoit JM, Fitzgerald WF, Damman AWH (1998), Environmental Research Sect A 78:118-133

 

Martinez-Cortizas A, Pontvedra-Pombal X, Garcia-Rodeja E, Novoa-Munoz JC, Shotyk, W (1999) Science 284:939-942

 

Salvato N, Pirola C (1994) Ind.Aliment 33 332:1229-1238

 

Shotyk W, Goodsite ME, Lohse C, Hansen TS, Roos F, Biester H, Cheburkin AK, Frei R, Heinemeier J, Appleby PG, Reese S, Asmund G (2000) Continuous 3000 year record of atmospheric Hg in Southern Greenland and Denmark, submitted.

 

Weiss D, Shotyk W, Schaefer H, Loyall U, Grollimund E, Gloor M (1999)  Fresenius J Anal Chem 363:300-305