Rapid Determination of Mercury in Plant and Soil Samples Using ...

RAPID DETERMINATION OF MERCURY IN PLANT AND SOIL SAMPLES USING INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION SPECTROSCOPY, A COMPARATIVE STUDY FENGXIANG X. HAN1,2,∗ , W. DEAN PATTERSON1 , YUNJU XIA1 , B. B. MARUTHI SRIDHAR3 and YI SU1 1

Diagnostic Instrumentation and Analysis Laboratory (DIAL), Mississippi State University, 205 Research Blvd., Starkville, MS 39759, USA; 2 Department of Plant and Soil Sciences, Mississippi State University, Mississippi State, MS 39762, USA; 3 Department of Forest Products, Mississippi State University, Mississippi State, MS 39762, USA (∗ author for correspondence, e-mail: [email protected]; Tel: (662) 325 7380, Fax: (662) 325 8465)

(Received 11 May 2005; accepted 9 September 2005)

Abstract. The objectives of this study were to simplify sample preparation and validate mercury detection in soil and plant samples using inductively coupled plasma atomic emission spectroscopy (ICP-AES). A set of mercury contaminated and mercury free soil and plant samples were digested and analyzed by ICP-AES, inductively coupled plasma mass spectrometry (ICP-MS), and cold vapor atomic absorption spectroscopy (CVAAS). Results show that mercury measurements in soil and plant samples using ICP-AES were in agreement with those analyzed using ICP-MS and CVAAS. The concentrations of mercury in soils and plant tissues determined by ICP-AES were 92.2% and 90.5% of those determined by CVAAS and ICP-MS, respectively. Digestion of soil samples with 4 M HNO3 and direct measurement by ICP-AES without reduction of Hg2+ to Hg0 gave a reasonable and acceptable recovery (92%) for determining Hg in soils. We conclude that ICP-AES with optimized conditions (addition of gold chloride, extension of washing time, linear working range, and selection of wavelength – 194 nm) resulted in reliable detection of mercury in environmental samples. Keywords: mercury, inductively coupled plasma atomic emission spectroscopy, inductively coupled plasma mass spectrometry, cold vapor atomic absorption spectroscopy, soil, plant

1. Introduction Mercury (Hg) has been used for at least the past 2500 years due to its unique chemical and physical properties (Crock, 1996). Mercury is released into the environment in considerable amounts by anthropogenic activities, and it has been proven to be a potent neurotoxin. The global mercury production since the beginning of the industrial revolution has been estimated at 0.64 million metric tons (Han et al., 2002). It is estimated that the annual anthropogenic input of mercury into the environment is as high as 6×106 kg/yr (Nriagu and Pacyna, 1988). Combining both anthropogenic input and natural sources, about 741 × 106 kg mercury has been released into the atmosphere, 118 × 106 kg released in water, and 806 × 106 kg released into soils (Nriagu, 1979; Andren et al., 1991). The release of large quantities of mercury Water, Air, and Soil Pollution (2006) 170: 161–171 DOI: 10.1007/s11270-006-3003-5

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into the environment has resulted in its widespread occurrence in the entire food chain (Adriano, 2001). One in every three lakes in the United States and nearly one-quarter of the nation’s rivers, contain various pollutants including mercury that consumption of fish caught in these locations should be limited or avoided (CNN, 2004). Many analytical techniques, including neutron activation analysis, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), cold vapor atomic absorption spectroscopy (CVAAS), and inductively coupled plasma atomic fluorescence spectroscopy (ICPAFS), have been used to determine mercury (Crock, 1996; Van Delft et al., 1988; Montaser and Golightly, 1992; Clevenger et al., 1997). In addition, mercury has been measured by graphite furnace-atomic absorption spectrometry (GFAAS) using Zeeman background correction and a gold-coated graphite tube (Grobenski et al., 1985; Lee et al., 1989). Most of these methods involve four steps for the analysis of mercury: sample decomposition, reduction of mercuric ion, phase separation of Hg vapor, and detection. Sample digestion converts all the forms of mercury present in samples to Hg (II) by using acids, or acid mixtures, such as HNO3 , H2 SO4 , HCl, and perchloric acid. Some oxidants including hydrogen peroxide, potassium or sodium dichromate, potassium permanganate or vanadium pentoxide are also used to digest samples (Crock, 1996). Inorganic and organo- mercury in solution are reduced to Hg0 by using tin chloride or sodium borohydride (NaBH4 ). However, elemental interferences from the precious metals (Au, Pd, Pt and Ag) and from Sb, As, Bi, Cu, I and Se have been reported. These elements can lower the Hg content by coprecipitation as well as adsorption, especially when using NaBH4 (Bartha and Ikrenyi, 1982, 1987). Matrix problems are also common to ICP-MS and ICP-AES. Cold vapor atomic absorption spectroscopy has been extensively used to determine mercury concentrations in environmental samples. The types of samples analyzed include soils, sediments and wastes. On the other hand, in most environmental samples, mercury often exists as a co-contaminant, which requires simultaneous determination of Hg with other common pollutants. With more convenience and multi-element measurement functions, ICP-AES has become an attractive instrument for simultaneously determining co-contaminants in environmental samples. Velitchkova et al. (2004) employed ICP-AES studying spectral interferences in the determination of trace elements, including mercury, in environmental materials. Unfortunately, the use of ICP-AES for the determination of mercury in various environmental samples has not been extensively compared with other methods, such as CVAAS and ICP-MS. Moreover, time-consuming sample preparation steps including the reduction of Hg2+ , warrant further simplification for the rapid determination of mercury in these samples. The objectives of this study were to simplify the sample preparation and validate mercury detection in mercury contaminated plant and soil samples using ICPAES. The optimized procedures with ICP-AES could eliminate the time-consuming

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reduction process of Hg2+ to Hg0 and determine mercury in environmental samples with minimal interferences.

2. Experimental 2.1. INSTRUMENTATION An Optima 4300 DV ICP-AES equipped with a Scott-Flow nebulizer, an ELAN 9000 ICP-MS, and a FIMS-100 CVAAS, all from Perkin Elmer Instruments (Shelton, CT), were used to determine mercury concentrations in solutions. Instrumental and spectral processing parameters as well as major spectral interferences for the ICP-AES technique are listed in Table I. 2.2. REAGENTS Trace metal grade, concentrated nitric acid and 30% H2 O2 used for the digestion of samples and the matrix-matching of calibration and quality control standards were purchased from Fisher Scientific (Pittsburg, PA). A 1000 mg/L gold standard stock solution (purchased from Inorganic Ventures: Lakewood, NJ) was used for amalgamation of mercury by spiking 1 mg/L of Au TABLE I Summary of instrument settings, spectral processing parameters and interferences Plasma conditions

Instrument conditions

Spectral peak processing parameters

Major spectral interferences

Plasma flow: 15 L/min Auxiliary flow: 0.2 L/min Nebulizer flow: 0.80 L/min RF power: 1300 watts Plasma view: axial Read/integration time: auto mode, min 5 sec – max 50 sec Delay time: 70 sec Replicates: 3 Wash time: 90 sec Pump flow rate: 1.5 ml/min Spectral lines detected: Hg 253.647 nm, Hg 194.168 nm Peak Algorithm: peak area Background correction: 2 point Hg line at 253.647 nm has a spectral interference at 253.681 due to iron Hg line at 194.168 nm is free of spectral interferences

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in all samples, calibration standards, and quality control standards. Calibration standards were prepared from a 20 mg/L stock solution of mercury purchased from Absolute Standards (Hamden, CT) and were matrix-matched with blank digestion solution and 1 mg/L of Au. Quality control and check standards were prepared from a 2 mg/L stock solution of mercury purchased from Environmental Express (Mt. Pleasant, SC) and were matrix-matched using blank digestion solution and a 1 mg/L Au addition. The blank digestion solution was prepared by diluting appropriate amounts (equal to those used for digesting the samples) of nitric acid and peroxide in deionized water. Deionized water (17.8 M-cm) was used in the dilution of samples and standards and was provided by a Nanopure Infinity ultrapure water system from Barnstead International (Dubuque, IA). The certified standard reference material used in this study (SRM 2710, Montana Soil Highly Elevated Traces) which contained mercury in soil was obtained from the National Institute of Standards and Technology (Gaithersburg, MD; http://www.nist.gov/).

2.3. SOIL

AND PLANT TISSUES

The soil used for this study was an Ora soil (fine-loamy, siliceous, thermic Typic Fragiudults) exhibiting mature pedogenic development. The region of origin for this soil is an Upper Coastal Plain where soils are formed in weathered, stratified deposits of sand, silt, clay and gravel in several geological formations. Surface soil (0 to 15 cm) was sampled from local farm land in Starkville, MS, USA. All samples were air-dried and ground to pass a 2 mm sieve. Approximately 1.5 kg of air-dried soil was weighed and transferred into plastic pots. Nitrogen, phosphorous, and potassium (N:P:K = 1:1:1) were added to the soils as base fertilizers. Chemical grade HgCl2 was used as the mercury source. Mercury treatments included a control and 250, 500 and 1000 mg/kg treatments. Each treatment consisted of five duplicates. Mercury salt was ground and gradually mixed with air-dried soil. All pots were watered and kept at field capacity moisture throughout the growing season. Due to plant uptake, some relocation, and possible biotransformation of mercury in the soil, mercury concentrations in the mercury-treated soils were in the range of 70–550 mg/kg after one growing season. For our discussions, the measured mercury concentrations in the mercury-treated soils after one growing season will be used. Chinese Brake Ferns (Pteris Vittata) 4 to 5 months old were obtained from Edenspace (Edenspace Inc. Dulles, VA). These plants were transplanted into plastic pots and were harvested 23 days after the transfer. Plant and soil samples were then collected for analyses. The harvested shoots were dried at 80 ◦ C in an oven for 48 h. Dried shoots were then ground and weighed. Plant samples (approximately 0.5 g) were digested with concentrated HNO3 and H2 O2 on a hot plate (Jackson, 1956;


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Han et al., 2004). Soil samples were digested with 4 M HNO3 at 80 ◦ C for 16 h (Sposito et al., 1982; Han and Banin, 1997). The digested solutions were filtered and then analyzed for Hg using inductively coupled plasma-atomic emission spectrometry (ICP-AES), inductively coupled mass spectrometry (ICP-MS), and cold vapor atomic absorption spectrometry (CVAAS). In addition, a standard soil reference material (SRM 2710, National Institute of Standards Technology) was also digested with 4 M HNO3 at 80 ◦ C for 16 h (Sposito et al., 1982; Han and Banin, 1997). The mercury concentration in this digested solution was measured directly by ICP-AES.

3. Results and Discussion 3.1. LIMITS

OF DETECTION AND QUANTITATION OF

ICP-AES

During the ICP-AES method development, a series of standards of decreasing concentration were run to determine the limit of detection (LOD) and the limit of quantitation (LOQ) for detecting mercury. These results were obtained using a calibration based on a calibration blank and three calibration standards (concentrations of 0.05, 0.5 and 1 mg/L) made in 5% nitric acid. The matrix-matched (in 5% nitric acid) check standards analyzed in order of decreasing concentration were 0.05, 0.03, 0.02, and 0.01 mg/L. The measured values by the instrument were 0.050, 0.030, 0.021, and 0.015 mg/L, respectively. The LOQ can be defined as the level above which quantitative results are acquired with a certain amount of confidence (Keith et al., 1983). The data above indicate an LOQ of 0.02 mg/L (or 20 μg/L) which is the level of mercury the ICP-AES method can accurately detect quantities. The detection limit can be expressed as the lowest analyte concentration that an instrument can reliably detect (MacDougall et al., 1980). According to the above data, the detection limit is approximately 0.01 mg/L (or 10 μg/L). This is the minimum concentration of mercury which the method can detect (confirmed by visual inspection of the peak spectra), however, the level of accuracy is greatly decreased. Statistically, the LOD is arbitrarily defined as three times the standard deviation or uncertainty of the measurement (or 3σ above the blank signal) while the LOQ is arbitrarily defined as ten times the standard deviation (Taylor, 1987; MacDougall et al., 1980; Keith et al., 1983). The LOQ represents the concentration where “the relative confidence in the measured value is about ±30% at the 95% probability level” (Taylor, 1987). To demonstrate the calculated LOD and LOQ based on statistics, the standard deviations were calculated from three blanks run as samples (10 replicates per sample blank). The average standard deviation for these blanks was 0.0017 (0.0014, 0.0018, and 0.0019). Therefore, the calculated LOD would be 3(0.0017) = 0.0051 mg/L or close to 5 μg/L. The LOQ would be calculated as

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10(0.0017) = 0.017 mg/L or 17 μg/L. These results closely approximate the data acquired by running various standard concentrations where the LOD was found to be approximately 10 μg/L and the LOQ was found to be 20 μg/L as discussed previously.

3.2. MEMORY /CARRYOVER

EFFECTS

To address the memory effects or carryover problems associated with analyzing mercury by ICP-AES, all blanks, standards, and sample solutions were spiked with 1 mg/L of gold (from a purchased stock solution of 1000 mg/L gold standard in 10% HCl) to amalgamate the mercury (Allibone et al., 1999; Fatemian et al., 1999). Amalgamation of mercury with gold preserves it within solutions, preventing mercury loss and accumulation in the ICP-AES sample introduction system. Furthermore, the addition of 1 mg/L of gold to a 5% HNO3 rinse/wash solution, with extended rinse times of up to 90 s between samples, helped alleviate memory effects. Gold as a preservative for drinking water samples has been reported to be more advantageous than a nitric acid-dichromate preservation mixture (Allibone et al., 1999). Instrument blanks, prepared identically yet independently of the calibration blank, were analyzed after the initial calibration and after each check standard to verify the absence of memory effects which would cause positive errors. To determine the highest concentration of mercury that could be analyzed by this method without the occurrence of memory effects, a series of standards of increasing concentration (0.01, 0.02, 0.03, 0.05, 1.0 and 5.0 mg/L) were examined followed by the analysis of instrument blanks. This allowed us to find the concentration level at which memory or carryover effects begin to influence the results. The readings of the instrument blanks were in the range of 0.002 to 0.003 mg/L after the analysis of check standard solutions with mercury concentrations of 0.01, 0.02, and 0.03 mg/L. The values of the instrument blanks increased slightly to 0.005–0.007 mg/L after the analysis of standard solutions in the range of 0.05 to 1.0 mg/L and reached 0.012 mg/L after the examination of 5.0 mg/L check standards. As indicated by the data, the memory effects were negligible up to a concentration of 1 mg/L of mercury (concentration of the highest calibration standard). At 5 mg/L, however, the concentration value of the subsequent blank was almost double the previous blank, suggesting that a small memory effect was taking place. Through various mercury analyses on a number of samples, we have observed that the memory effect actually begins to take place at concentrations slightly higher than 2 mg/L. Therefore, our linear working range is from about 20 μg/L to 2 mg/L. However, for all analyses performed, samples higher than our highest calibration standard of 1 mg/L were diluted to fall within the calibration range. This ensured that the results would not be influenced by carryover or memory effects.


3.3. PRECISION

167

AND ACCURACY

A certified reference soil standard containing mercury (NIST SRM 2710) was analyzed to test the accuracy of the method. The certified value (certified by CVAAS) of mercury in the soil was 32.6 ± 1.8 mg/kg. This certified standard was digested in triplicate and analyzed by ICP-AES. Our analyses produced an average result of 32.9 ± 0.61 mg/kg, providing a mercury recovery of 100.9% with a 0.92% relative error. This ICP-AES result is statistically the same as the certified value at the 95% confidence level. To further demonstrate the precision and accuracy of the method, various blanks and quality control standards were examined during various analyses. Measurements of 0.02, 0.5 and 1.0 mg/L check standard solutions yielded results of 0.0207 ± 0.0008, 0.516 ± 0.022, and 1.000 ± 0.008 mg/L, respectively. % RSD values for each measurement (based on three replicate readings from the instrument) ranged from 2.25 to 7.4% for the 0.02 mg/L standard and decreased to 0.32 to 1.2% for the 0.5 and 1.0 mg/L standards. These data represent quality control results typically acquired with the mercury method by ICP-AES. 3.4. ICP-AES

OPERATING PARAMETERS

Calibration parameters included the following steps: A. Standards used to calibrate: calibration blank and 0.05, 0.5 and 1 mg/L standards (matrix-matched with blank digestion solution and 1 mg/L Au solution). B. Calibration fit: Linear fit with calculated intercept C. Correlation coefficient: typically ≥0.9999. The instrument conditions, as well as the spectral processing parameters, are outlined in Table I. In addition, we found that for the mercury line at 253.647 nm there is a spectral interference at 253.681 nm due to iron. This is especially troublesome due to the fact that Fe is a major constituent in most environmental samples such as soils. Fortunately, the mercury line at 194.168 nm is free of spectral interference, therefore, this was the emission line used for quantitation throughout the analyses. As mentioned earlier, to overcome the memory effects associated with analyzing mercury by ICP-AES, all blanks, standards, and samples were spiked with 1 mg/L of gold standard to amalgamate the mercury (Allibone et al., 1999; Fatemian et al., 1999). 3.5. C OMPARISON ICP-MS

OF MERCURY ANALYSES BY

ICP-AES, CVAAS

AND

We compared mercury analysis results determined by ICP-AES and CVAAS in the same set of digested plant or soil solutions (Table II). Our results showed mercury

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TABLE II Comparisons of mercury analyses in digested plant and soil solutions determined by ICP-AES and CVAAS and in plant and soil samples by ICP-AES and ICP-MS Mercury concentrations in digested solutions (mg/L)

Sample Soils

Plants

Regression R2

ICP-AES (x)

CVAAS (y)

0.30 0.33 0.77 0.90 2.41 2.56 2.60 3.04 2.66 2.88 3.38 3.85 3.89 4.19 3.90 4.31 3.91 4.29 4.69 5.24 6.02 6.50 7.16 8.16 33.9 35.7 40.9 44.6 62.0 68.5 62.8 67.1 0.14 0.13 0.16 0.15 0.69 0.71 0.72 0.66 0.73 0.79 0.73 0.83 0.74 0.72 0.77 0.78 0.89 0.94 0.89 0.99 0.91 0.89 0.94 0.96 0.97 1.05 1.09 1.19 1.11 1.09 1.25 1.16 1.29 1.41 1.37 1.49 1.89 2.04 3.17 3.51 3.88 3.83 6.44 6.62 7.73 7.84 y = 1.084x − 0.036 0.9996

Mercury concentrations in soil/plant samples (mg/kg)

ICP-AES/ CVAAS

ICP-AES (x)

ICP-MS (y)

ICP-AES/ ICP-MS

0.93 0.86 0.94 0.86 0.92 0.88 0.93 0.90 0.91 0.90 0.93 0.88 0.95 0.92 0.91 0.94 1.03 1.07 0.97 1.08 0.92 0.88 1.03 1.00 0.94 0.90 1.02 0.97 0.92 0.92 1.02 1.08 0.91 0.92 0.93 0.90 1.01 0.97 0.99

12.6 15.1 69.9 122 64.3 155 476 407

14.5 18.6 76.4 134 73.7 172 536 454

0.87 0.81 0.92 0.91 0.87 0.90 0.89 0.90

45.5 239 45.0 735 105

55.3 265 55.1 810 123

0.82 0.90 0.82 0.91 0.85

y = 1.105x + 2.703 0.9998


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values in all digested soil and plant sample solutions determined by ICP-AES were in close agreement with those determined by CVAAS. CVAAS generally gave an average of 2% and 7.7% higher mercury values than ICP-AES for plant and soil samples, respectively. Mercury concentration ranges in the digested soil solutions were from trace to 60 mg/L while those in the digested plant solutions were from trace to 8 mg/L. This indicates that ICP-AES yields reasonable recoveries (92%) for mercury in both digested soil and plant samples over a wide range of mercury concentrations. In addition, we selected some plant and soil samples and compared mercury analysis results acquired by ICP-AES with those obtained by ICP-MS (Table II). Mercury concentrations determined by both ICP-AES and ICP-MS were in close agreement. The concentrations of mercury in contaminated soil and plant samples determined by ICP-MS were 9.5% higher than those by ICP-AES. The certified reference soil sample (NIST SRM 2710) was digested with 4 M HNO3 followed by direct mercury detection with ICP-AES. The mercury value obtained by ICP-AES was highly agreeable with the certified value acquired by CVAAS (mercury recovery 100.9% with 0.92% relative error). This indicates that a 4 M HNO3 acid digestion of soil followed by direct determination by ICP-AES yields reasonable recoveries for mercury. Furthermore, the addition of mercury standard solutions (spikes) to the 4 M HNO3 digested soil extracts yielded an average recovery of 106% (99.1–112.8%), as compared to values obtained by CVAAS (data not shown). This implies that the addition of the mercury spike solutions in sample extracts was completely recovered as detected by ICP-AES. 3.6. EFFECT

OF HOLDING TIME ON MERCURY CONCENTRATIONS IN DIGESTED SOLUTIONS

To study the effect of short-term storage on mercury concentrations in digested solutions, we compared mercury analysis results of the same set of digested soil solutions (with 4 M HNO3 ) immediately upon sample preparation (digestion) and after two months. After two months of storage at room temperature, the mercury concentrations in the digested soil solutions did not significantly change [y (values in Oct.) = 0.996x (values in July) −0.704, R 2 = 0.995]. An average of only 0.4% difference was exhibited between the two measurements as determined by ICP-AES. This indicates that short-term (up to 2 months) storage of acid-extracted soil samples at room temperature is a safe practice for mercury analyses. The set of plant tissues and soil samples (both uncontaminated and contaminated) used throughout this study covered a wide range of mercury concentrations. Mercury concentrations in these sample solutions were digested and then analyzed directly by ICP-AES, without the reduction of Hg2+ to Hg0 in the digested solutions. The advantages with this ICP-AES method as compared to CVAAS are the time and effort saved along with the possibility of simultaneous multi-element

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detection. These advantages are achieved without sacrificing accuracy or precision. Compared to ICP-AES, CVAAS requires the time-consuming reduction process of Hg2+ in solution to Hg0 with SnCl2 before detection. The only advantage CVAAS holds over ICP-AES for the detection of mercury is lower detection limits. Typically, detection limits on the order of 10 to 50 ng/L are acquired by CVAAS (McIntosh and Welz, 1997), however, CVAAS is commonly limited to the detection of one element at a time.

4. Conclusion In summary, we have developed a unique method to rapidly determine mercury concentrations in plant and soil samples by ICP-AES. We chose the 194.2 nm mercury emission line rather than the more intense 253.6 nm line to avoid the spectral interference from iron. Gold solution (1 mg/L Au) was added to the sample solutions to eliminate memory or carryover effects. For the same purpose, we have also extended the wash/rinse time between samples from the typical 45–60 s to 90 s. Under these instrumental conditions, we have achieved a detection limit of 10 μg/L and a linear working range of 20 μg/L to 2 mg/L. By eliminating the reduction step of Hg2+ to Hg0 before detection (as required for CVAAS), our method will save time, costs and effort when compared with traditional CVAAS methods, especially for the analyses of large numbers or batches of environmental samples that often require simultaneous determination of multi-elements.

Acknowledgments This work was supported by funding from the U.S. Department of Energy through Cooperative Agreement DE-FC26-98FT-40395. Special thanks are extended to the Perkin Elmer technical center located in Cumming, GA for performing the mercury analyses by CVAA and ICP-MS.

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