2.6 Trace metal anal y sis
2.6.1 Digestion procedures
The digestion procedure commonly used at the Institute F.-A Forel for ICP-AES measurements was adapted from the technique developed by BOUCETTA and FRITSCHE (1979). Dry sediment was digested with H202 and strong acids such as HCL04, HCL and HF and the acid solution was heated at 150°C.
Despite the severe chemical and physical conditions, an alumino-silicated unattacked residue was still
observed in the solution at the end of the procedure. The solution obtained by using this procedure was not adapted for the ICP-MS used for the trace metal analysis for two main reasons: the solution contains both hydrofluoric acid which could corrode the instrument components made of glass and undissolved particles which could plug the nebulizer and the tubing.
After the examination of procedures existing in the literature, two digestion methods were chosen and adapted. The first one is a strong acid digestion with 2M HN03 (ANDERSSON, 1976; AITANG and HANI,
1983) and presents the advantage of being fast, which is appreciated when a large amount of samples must be analyzed. It is used routinely for the Swiss soil monitoring since it is recommended by the Swiss Federal Office for the Protection of the Environment in the soil pollution order (OSOL, 1987). A large fraction of trace metals are released in the acid solution, as most secondary minerais are dissolved.
Primary minerais such as quartz, muscovite and feldspars are not digested (AITANG and HANI, 1983). In order to estimate the fraction of trace metals released by 2M HN03, a total digestion of sorne sediment samples were performed to compare the trace metal concentrations in total digestion and in partial extraction.
Partial extraction with 2M HN03
The partial extraction procedure described below is slightly modified from the method described in the OSOL (1987):
• weigh 2g of dry and homogenized sediment in a clean Teflon bomb,
• gently add 20 ml 2M HN03 in the bomb and mix the solutions,
• heat the sealed bomb during 1 night at 100°C in a drying oven,
• pour the mixture in a clean centrifugate tube and centrifuge for 20 minutes at 4000 rpm.
• collect the floating solution in a clean polyethylene flask. The final dilution is about 1110.
The Teflon bomb was weighed before and after the heating in order to check that evaporation of the solution is minimal.
When necessary, the solution collected after the centrifugation was filtered through a clean 0.45 f.!m pore filter to remove all suspended particles which were not deposited during the centrifugation.
Total digestion method
The total digestion method uses strong acids such as perchloric acid and hydrofluoric acid to release the total metal content from sediments. Two different instruments were used to heat the bombs in which sediment was digested, a microwave and a heating aluminum block (Futronic). The digestion procedures are slightly different for the two instruments as the method using the microwave is realized in sealed Teflon bombs, while it is performed at ambient pressure in the heating block. The two methods are described below.
Total digestion with a microwave
The microwave digestion system used in this study is a Milestone 1200 (MLS GmbH) belonging to the department of Pedology of the Swiss Federal Institute of Technology at Lausanne. It contains two modules; the first one contains high-pressure vessels for the digestion of sediments and the second one is designed for the evaporation of the acid mixture used for the digestion. The acid digestion of geological and biological standard reference materials with a Milestone instrument showed that it is a powerful tool for rapid and reliable sample preparation in trace element analysis (NOL1NER et al., 1989). The procedure described below, in particular the evaporation phase, was tested on several samples until the optimal power and time were found.
-21-Chapter 2: Methods and Procedures
The first step of the procedure is the pressurized phase:
• weigh 200 mg of dry and homogenized sediment in a clean Teflon bomb,
• add 3ml deionized H20, 5 ml HN03 65%, 2 ml HCL 40% and 2 ml HF 40% and mix the solution,
• insert the Teflon bombs in the high-pressure vessels and run the power program given in table 2.1a.
The second step is the evaporation phase:
• after cooling and pressure release, open the bombs and add 2 ml deionized H20 and 3 ml HCL04
70%,
• insert the bombs in the evaporation vessel and run the power pro gram given in table 2.1 b,
• when the residue is dry, add 0.4 ml HN03 65% to dissolve the residue and then 20 ml deionized H20. The final dilution is about 1/100.
During the pressurized phase, a temperature sensor inserted in a bomb recorded the maximal temperature, 130°C, during the pressure step at 600 W. The evaporation phase was problematic because of the high evaporation point of the acid mixture. As the Teflon bombs were narrow and long (12 cm) and the hole for gas exhaust very small, the acid vapors condensed at the top of the bombs and fell again on the digested sediment. The power pro gram given in table 2.1 b provided the optimal time and pressure steps. An increase of power (600 W) would make the sediment splashed up on the bomb wall and sucked in the gas exhaust, while an increase of ti me with high power (500 W) would bum the residue.
Table 2.1: Program with power and time for the pressurized digestion (A.) and the evaporation phase (B.) for lake and overbank sediments and for peat samples.
Lake and overbank sediments Peat sam pies
A. pressurized B. evaporation A. pressurized B. evaporation
Ti me Power Ti me Power Ti me Power Ti me Power
[min] [Watt] [min] [Watt] [min] [Watt] [min] [Watt]
Step 1 20 250 15 250 5 250 15 250
Step 2 8 600 15 400 6 600 10 400
Step 3 15 250 5 500 4 450 5 500
Step 4 35 250 4 350 15 250
Step 5 5 250
Ventilation 20 15 20 15
Total 63 85
44
60The digestion procedure for peat samples (WEISS and SHOTYK, 1996) is different from the procedure used for lake and overbank sediments. It is briefly described below:
• weigh 200 mg of dry milled peat,
• add 2 ml deionized H20, 5 ml HN03 65%, 3 ml H202 30% and 1 ml HF 40%,
• run the power program for the pressurized phase and then for the evaporation phase (table 2.1).
Total digestion using the Futronic
The Futronic is an aluminum block with eight holes in which can be placed the Teflon bombs. By
heating the block, the digestion and the evaporation processes occur simultaneously and acid vapors are sucked in the tubes attached to the bombs and led to a neutralizing bath.
The total digestion procedure using the Futronic is the following:
• weigh 200 mg of dry and homogenized sediment in a clean Teflon bomb,
• add 3 ml deionized H20, 5 ml HCL04 and 2 ml HF,
• heat the bombs in the Futronic for 16 hours at 170°C,
• when the residue is dry, add 0.4 ml HN03 65% to dissolve the residue and then rnix with 20 ml deionized H20.
Partial extraction versus total digestion
The comparison between the average concentrations of trace metals measured in partial extractions and in total digestions indicates that a large fraction of trace metals (between 78 and 98% ), except Cr and V, are extracted from sediment with the partial extraction procedure (2M HN03) (table 2.2). Only 39% Cr and 33% V are extracted with 2M HN03.
Table 2.2: Comparison between average concentrations (mean ± 1 standard deviation) of trace metals measured in partial extractions (159 samples) and in total digestions (52 samples) and percentage of trace metals extracted by partial extraction.
Trace metal [J.lg/g] Partial extraction Total digestion Partial extraction [%]
As 11 ± 2
Cd 0.6 ± 0.1 0.7 ± 0.1 86
Co 12 ± 1 13 ± 1 92
Cr 29 ± 5 74±9 39
Cu 26 ± 3 31 ± 3 84
Hg [ng/g] 64 ± 7
Mn 463 ± 65 474 ± 71 98
Ni 37 ±4 43 ± 5 86
Pb 14 ± 2 18 ± 2 78
v
52± 11 156 ± 16 33Zn 125 ± 13 138 ± 13 91
Digestion method for Hg measurements
To measure Hg contained in sediment, the sample is digested by strong acids in an oxidizing milieu, releasing Hg2+. SnC12 is used to reduce Hg2+ in the volatile form Hg0 which is measured by the atornic absorption spectrometer.
The digestion method for the anal y sis of Hg is the following:
• weigh 1 g of dry and homogenized sediment,
• add 5 ml HN03 65% and 2 ml HCL 37%,
• heat the solutions in a "bain-marie" for 90 m.,
• add KMN04 until the purple color remains,
-23-Chapter 2: Methods and Procedures
• add 5 ml hydroxylamine,
• add 5 ml SnC12 and measure directly Hg0 . 2.6.2 Analytical methods
Most trace metals (Cd, Cu, Co, Cr, Ni, Pb, V and Zn) in lake and overbank sediments and in peat samples were analyzed by ICP-MS (inductively coupled plasma mass spectrometry). The use of ICP-MS for the trace metal analysis was preferred to the use of ICP-AES (atornic emission spectrometry) because of the low concentrations of trace metals in sediments, particularly of Cd and Pb. Fe and Mn were also measured by ICP-MS but most results are inaccurate as they were extrapolated from the calibration curve. However, the concentrations of Mn can be used in terrn of qualitative results. Mo, Tl and As concentrations measured by ICP-MS were not accurate and could not be used. Arsenic was analyzed by atornic absorption spectrometry (AAS) with hydride generation and Hg by AAS using the cold vapor technique.
Trace metal concentrations were calculated by using the calibration curves deterrnined at the beginning and at the end of a sample set. The mean concentrations of procedural blanks are subtracted to the sample concentrations.
ICP-MS
The ICP-MS used for trace metal analysis is an Ultramass 700-ICP-MS manufactured by Varian and belonging to the Swiss Federal Research Station for Agricultural Chernistry and Hygiene of Environment at Liebefeld. The sample solutions are introduced in an ultrasonic nebulizer coupled with a desolvation system (Cetac U-6000 AT+), which improves the detection limits by a factor of 5 to 50 (CETAC, 1995).
Table 2.3: Internai standard (JS) usedfor each analyte element (AE) with respective mass to charge ratio (mlz).
Analyte element rn/z of AE Internai standard rn/z of IS
(AE) (IS)
As 75 Ge 72
Cd 111 Rh 103
Co 59 Ge 72
Cr 52 Ge 72
Cu 65 Ge 72
Fe 57 Ge 72
Hg 202 Au 197
Mn 55 Ge 72
Mo 98 Rh 103
Ni 60 62 Ge 72
Pb 206 207 208 Au 197
Tl 205 Au 197
v
51 Ge 72Zn 66 67 Ge 72
Just before the trace metal analysis, samples are diluted with deionized water (Millipore) by a factor of 500 for partial extractions and of 100 for total digestions (50 for peat samples). Considering the first dilution performed during the digestion procedure, the final dilution for partial extractions is 5000 (10 x 500) and 10,000 (100 x 100) for total digestions (100 x 50 for peat samp1es). Large dilution factors were used to limit the blockage of the entrance aperture of the sampling cone, which may occur when the solutions con tain high concentrations of salts of low volatility (V ANDECASTEELE and BLOCK, 1993).
However, large dilution factors have the disadvantage that the sensitivity is decreased.
An internai standard containing Au, Be, Rh and Ge was used to correct the drift or random fluctuations of the signal. It is used also to correct systematic variations of the analytical signal in samples and standards due to matrix effects (V ANDECASTEELE and BLOCK, 1993). The accurate correction for matrix effects is possible only if the internai standard is chosen with a mass number as close as possible to that of the analyte elements (table 2.3).
The calibration curve contains two points, the calibration blank, which is ultrapure water, and one standard containing the analyte elements, prepared also with ultrapure water. The concentrations of the elements in the standard are given in table 2.4.
Five replicates were performed for each sample, blank and standard. The quality control tests are presented in chapter 2.6.3.
Table 2.4: Concentrations of trace elements contained in the standard used for the calibration, and acceptable range of concentrations for the quality control standards (/CV and CCV) expressed in %of the standard concentration. See chapter 2.6.3 for explanations.
Element Standard ICV [%]
ccv[% ]
concentration [ppb] Min Max Min Max
As 110 90 110 95 105
Cd 20 97 103 95 105
Co 20 90 110 95 105
Cr 20 97 103 95 105
Cu 20 90 110 95 105
Fe 110 90 110 90 110
Hg 20 no1 no1 no' no1
Mn 20 90 110 90 110
Mo 10 90 110 90 110
Ni 20 97 103 95 105
Pb 20 97 103 95 105
Tl 20 90 110 90 110
v
20 90 110 90 110Zn 110 97 103 95 105
1 no quality control performed on Hg
The detection limit was calculated by multiplying three times the standard deviation of the background (VANDECASTEELE AND BLOCK, 1993). The detection limits are different in each sample series as the blank concentrations depend on the signal intensity which may decrease over time because
-25-Chapter 2: Methods and Procedures
of the encrustation of cones. The blank concentrations may also be higher than expected because of possible contamination of the sample introduction system. The detection lirnits given in table 2.5 are mean detection lirnits and were calculated by using ail preparation blanks measured in the four sample series. Detection lirnits are usually sirnilar for partial extractions and total digestions, except for Mo, Ni, Pb and V which have higher detection lirnits in total digestions (table 2.5). The detection lirnit of Fe is particularly high in preparation blanks. It is lower (about 14 ppb) in blanks used for the quality controls (calibration verification, see chapter 2.6.3).
Table 2.5: Detection limits in diluted solutions of partial extractions ( 30 blanks) and total digestions (23 blanks).
The calibration li ne was established by using a Hg standard at different concentrations (0, 50, 100 and 150 j.lg/1). The detection limit obtained by this technique amounted to 5 ng mercury.
AAS-hydride generator
The introduction of sodium borohydride as reductant in the acidified digested sample transforms arsenic in a gaseous hydride, AsH3- which is then atornized in a heated quartz tube, located in a radiation bearn ÀAs= 193.7 nm) of an AAS.
The sodium borohydride is introduced through a capillary into the bottom of the conical vessel containing the acidified sample. Through the violent reaction of the alkaline reactant with the acidified sample solution and the conical shape of the vessel, turbulent rnixing takes place, resulting in a rapid and complete reaction (WELZ, 1985).
The calibration line was established with one blank and one As standard at different concentrations (20, 40 and 60 J..Lg/1) fitting the range of optimal absorbance. Before the analysis of As, the partial extraction solutions (with 2M HN03) were diluted with deionized water by a factor of 50. The detection limit is 2 ng/g.
2.6.3 Quality control
Several tests were performed during the trace metal analysis, particularly with the ICP-MS, to check the analytical quality of the results. The description and the results of the tests are presented in the following sections.
Quality control of analysis obtained by ICP-MS
Three types of tests were performed during the trace metal analysis to estimate the quality of the results:
1. a quality control test run regularly by the instrument, 2. the anal y sis of reference materials,
3. the analysis of different digestion solutions from the same sediment sample (digestion and instrumental reproducibility ).
The three tests are described below.
Quality control test
The analysis of samples was performed automatically by the instrument. To ensure a sufficient quality of results, quality control tests, introduced in the analysis program, were run automatically between each set of 18 samples. The quality test checked the concentrations of the blank and the standard used for the calibration. If the concentrations were higher or lower than the acceptable concentration lirnits, a new calibration was completed and the set of samples preceding the quality test were analyzed again.
Directly after the measurement of the calibration line, an initial calibration verification (ICV) was performed to check the concentrations of the standard used for the calibration. The concentration lirnits of the quality control standard are given in table 2.4. For example, Cd, Cr, Ni, Pb and Zn concentrations must not be lower than 97% of the standard concentration and not higher than 103% (table 2.4). An initial calibration blank (ICB) was then analyzed to check the validity of the calibration blank. The lirnit of the ICB is defined as a multiple of the detection lirnit entered in the specifications of the analysis pro gram.
The quality control tests performed between sets of samples contained a continuing calibration verification (CCV) followed by a continuing calibration blank (CCB). The CCV is a quality control standard used to check the validity of the analytical calibration, and the range of the concentration lirnits are a little bit larger than those for the ICV (table 2.4). The CCB is used to check the blank of the calibration blank. lt is useful also to detect possible contamination of the sample introduction system.
Analysis of reference mate rial
The quality control tests performed by the instrument are based on synthetic samples (standard). It is important to test the accuracy and precision of results with natural samples as the matrix is very different from the standard matrix. Chosen reference materials (LYNCH, 1990), which are not certified but analyzed by 31 laboratories, contain approximately the same geochernical composition (LYNCH, 1990) as lake and overbank sediments collected in the Mackenzie Delta. The material LKSD1, collected in two Canadian lakes (LYNCH, 1990), was digested (partial and total digestion) and then analyzed four or five times in each sample series (4 series of 60 partial extraction samples approximately) to check the
-27
-Chapter 2: Methods and Procedures
reproducibility of the instrument and to compare the results with the values given by LYNCH ( 1990). The material STSD1, a stream sediment sample collected in Canada, was analyzed 7 times in different series to check the accuracy of the instrument.
The comparison between the concentrations obtained by ICP-MS and provided by LYNCH (1990) are usually similar, specially for Cd, Co, Cr, Cu, Ni and V (table 2.6). Zn and Pb concentrations determined by ICP-MS are too low in LKSD1 but they are correct for STSD1 which has smaller concentrations of these 2 elements. The determination of Mo is correct when concentrations are around 10 ppm but the results cannat be used when concentrations of Mo are small (2 ppm). Fe and Mn concentrations are correct if concentrations in sediments are lower than 2% and 500 ppm respectively, like in LKSDl. The high concentrations of Fe and Mn in STSD 1 were not correct! y determined by !CP-MS.
Trace metal concentrations of LKSD 1 were also determined in total digestion solutions. The conclusions are similar to those for partial extractions.
Table 2.6: Comparisons of concentrations (mean conc. ± 1 standard deviation) of LSKD1 and STSD1 measured by 1CP-MS and provided by LYNCH ( 1990). Relative standard deviation (RSD) of 19 analysis of LSKD 1 and 7 analysis of STSD 1. Al! results come from partial extraction.
Measured RSD Lynch- Measured RSD [%]
Lynch-LKSDl [%] LKSDl STSDl STSDl
Cd [~-tg/g] 1.4 ± 0.0 2.9 1.2 ± 0.3 0.9 ± 0.1 8.9 0.8 ± 0.2
Co [~-tg/g] 8±0 4.1 9 ± 1 14 ± 0 2.9 14 ± 2
Cr [~-tg/g] 14 ± 1 5.7 12 ± 2 27 ± 1 2.8 28 ± 3
Cu [~-tg/g] 42 ± 1 3.1 44 ± 5 33 ± 1 3.2 36 ± 2
Fe[%] 1.7 ± 0.2 11.9 1.8 ± 0.3 2.4 ± 0.1 5.3 3.5 ± 0.2
Mn [~-tg/g] 446 ± 17 3.8 460 ± 60 2040 ± 630 30.9 3740 ± 430
Mo [~-tg/g] 11 ± 1 5.2 12 ± 2 0.7 ± 0.2 34.3 2±0.5
Ni [~-tg/g] 14 ± 1 6.1 11 ± 1 20 ± 1 5.1 18 ± 3
Pb [~-tg/g] 74 ± 7 9.0 84 ± 10 34±4 4.0 34 ±4
v
[~-tg/g] 25± 2 6.0 27 ± 3 45 ±2 4.0 47 ± 11Zn [~-tg/g] 311 ± 11 3.7 337 ± 11 161 ± 7 4.5 165 ± 8
Digestion and instrumental reproducibility
The relative standard deviations of LKSD1 measurements presented in table 2.6 indicate that the digestion and instrument reproducibility are good, considering that the 19 measurements of LKSD 1 were performed from six different digestion solutions included in different sample series during a period of 4 months. The more variable elements are Fe and Pb.
The same test of reproducibility was performed with 4 different partial extractions of the same lake sample which was analyzed 10 times. The relative standard deviation of concentrations is lower than 5%
for most elements (Cd, Cu, Mn, Ni, Pb, V, and Zn) and between 5 and 6% for Co and Cr (table 2.7). The most variable element is Fe, which is explained by the high concentrations of Fe in solutions, resulting in
detector saturation. The relative standard deviation of trace metal concentrations presented in table 2.7 reflects the instrumental and digestion reproducibility. Considering that these analysis were performed from 4 different digestion solutions included in different sample series during a period of 4 months, the instrumental and digestion reproducibility are satisfactory.
The relative standard deviation given in table 2.7 reflects the relative uncertainty (± 1 RSD) attached to the trace metal concentrations in lake and overbank sediments. It includes the instrumental uncertainty and the procedural (sample preparation) uncertainty.
Table 2. 7: Mean concentrations (mean conc. ± 1 standard deviation) of one lake sample analyzed JO times with relative standard deviation. Results co ming from 4 different partial extractions
Element reproducibility of the method (cold vapor technique). The chosen materials were Canadian stream sediments (STSD1 and STSD2 in LYNCH, 1990), having approximately the same Hg concentrations as the lake and overbank sediments.
The comparison between the measured concentrations and those given by LYNCH (1990) indicates th at the accuracy of the method is good for STSD2 and sufficient for STSD 1 (table 2.8).
Table 2.8: Hg concentrations (mean conc. ± 1 standard deviation, N= number of samples) in reference materials (STSD1 and STSD2) with relative standard deviation (RSD) and comparison with values given by LYNCH ( 1990).
-29-Chapter 2: Methods and Procedures
overbank sediments.
Quality control of As analysis
Two reference materials were analyzed four times to check the accuracy of As analysis and the reproducibility of the method (hydride technique). The chosen materials were Canadian stream and lake sediments (STSD4 and LKSD2 respectively in LYNCH, 1990), having approximately the same As concentrations as the lake and overbank sediments.
The comparison between the measured concentrations and tho se deterrnined by LYNCH ( 1990)
The comparison between the measured concentrations and tho se deterrnined by LYNCH ( 1990)