- Email: [email protected]
Microwave vacuum drying of marine sediment: determination of moisture content, metals and total carbon
ANALYTICA CHIMICA ACTA ELSEVIER
Analytica
Chimica Acta 342 (1997) 247-252
Microwave vacuum drying of marine sediment: determination of moisture content, metals and total carbon P.A. Tanner*, L.S. Leong Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Received 9 September
1996; received in revised form 15 November 1996: accepted 22 November 1996
Abstract No significant difference was found in the moisture content of marine sediment determined by using traditional oven drying at 105°C and microwave-drying under vacuum. Subsequent determinations of the major and trace metals, Al, Ca, Fe, Cr, Cu, Mn, Ni, Pb, and Zn, gave similar results after employing the different drying procedures. The total carbon contents were also investigated and again no significant difference was found. Vacuum microwave drying requires only 10 min of time compared to more than 8 h for oven-drying. Contamination of samples is also minimized when microwave digestion is adopted in the same system for the subsequent analysis of metal contents. Keywords: Drying; Vacuum; Microwave; Sediment; Metals; Total carbon
1. Introduction
The analyses of heavy metals and total carbon contents in marine sediment are widely used to assess long-term anthropogenic inputs to the marine environment. In any given sediment study program, the storage of large batch of bulk samples can cause congestion of laboratory space. The usual practice involves drying a small portion of the collected samples to remove moisture and then weighing and storing them until the subsequent analyses. Although standard procedures are available for the instrumental analysis [l] and digestion [2] of sediment samples, less attention has been paid to their pre-treatment, especially the drying procedures. In view of this, a comparative *Corresponding author. E-mail: [email protected];
fax: +852
2788 7406. 0003-2670/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOO3-2670(96)00582-X
assessment of the effects of different methods including oven-, air- and freeze-drying upon the obtained moisture and metal content was carried out previously, and we found that oven- and freeze-drying gave similar values for moisture and metal contents of sediment, but that air drying gave significantly different results [3] under the ambient laboratory conditions. In the past few years, there has been a growing interest in the use of microwave radiation in analytical and environmental chemistry. Zlotorzynski [4] reviewed the latest advances in the application of microwave energy to analytical chemistry including digestion, extraction, chemical reaction, pre-concentration, and desorption of samples. With respect to soil and sediment studies, the adoption of microwave oven for sample digestion prior to the analysis of metal contents is well documented [5-81. However we are unaware of the use of the microwave vacuum techni-
248
PA. Tanner; L.S. Leong/Anctlvtica
que for marine sediment drying, although it has been applied to the drying of foodstuffs [9], wood [IO], and cotton [ll]. Under reduced pressure the sediment water is lost at lower temperatures so that the deconposition or loss of volatile organic matter occurs less and local hot spots are avoided. Furthermore, sample oxidation is minimized. This paper documents a comparison of traditional oven drying at 105°C with microwave drying of sediment under vacuum prior to the analyses of metals using inductively coupled plasma atomic emission spectrometry (ICP-AES) and flame atomic absorption spectrometry (FAAS), as well as total carbon content using a CHN analyzer.
Chimica Actu 342 (1997) 247-252
ment: Aberdeen Typhoon Shelter and Shuen Wan. At Shuen Wan, the major intrusive igneous rock formation is granodiorite, whereas sedimentary and waterlaid volcaniclastic rocks occur near the former sampling site. Aberdeen Typhoon Shelter sediment, collected by a Ponar grab, was deeply situated beneath seawater (>lO m), highly anoxic, finely particle-sized, and heavily polluted by industrial and domestic sewage discharges. Samples were collected by using a plastic scoop from the shallow coastal region in Shuen Wan, where the sediment was oxic, subjected to tidal flushing and currents, coarsely particle-sized, and less prone to direct pollution sources. 2.3. Reagents
2. Experimental 2.1. Apparatus Traditional drying was conducted at 105°C at the mid-upper shelf of a She1 Lab oven (range: 50°C to 2OO”C), with ceramic crucibles used as sample holders. No microwave vacuum drying equipment was available at City University of Hong Kong (CityU) so that this operation was performed by Milestone S.r.1. in Italy. The unit employed was a Milestone LAVIS-1000 multiMOIST system with micro-processor-controlled power up to 1000 W. All sediment samples were digested at CityU in CEM Teflon-lined digestion vessels using a CEM MDS-2000 microwave digester. Perkin-Elmer Plasma 1000 ICP-AES and Shimadzu AA-6501s FAAS instruments were employed for the determination of major and minor metals. A Leco CHN-900 carbon, hydrogen and nitrogen determinator model 600-800-300 was employed for the determination of total carbon content in the samples. All glassware were immersed in a mixture of laboratory grade detergent and bleach for 16 h, then rinsed thoroughly with tap water and immersed in an acid bath (1 : 2 : 9 concentrated nitric : hydrochloric acid : water) for 16 h, then triple-rinsed with deionized water, and finally rinsed with deionized doubledistilled water and dried in air before use. 2.2. Sample collection Bulk sediment samples were collected from two different locations in the Hong Kong marine environ-
Reagent water was prepared firstly by ion-exchange and then double distillation with a Cyclon Fistreem system. AnalaR concentrated nitric acid (69%) for digestion was purchased from BDH. Standard metal stock solutions (1000 ppm) for calibration purpose were purchased from High-Purity Standards. Standard cystine powder (Sigma) was used for single-point calibration of the CHN analyzer. 2.4. Procedures 2.4.1. Determination of moisture content In the laboratory, the silt-clay fraction (<63 urn) was wet-sieved from the bulk sediment with seawater collected in situ, in order to normalize the grain size effects in analyses. The sieved silt-clay slurry was collected in a beaker and mixed thoroughly with a glass rod. Approximately 40 cm3 of this slurry was then transferred to each of the ten 50 cm3 Nelgene tubes and spinned down at 2000 rpm for 10 min with a bench top centrifuge. The supematant seawater was decanted and the residue was homogenized. Subsamples of two 5 g portions were collected from each tube, weighed to three decimal places (mass X), and then subjected to traditional oven drying and microwave drying under vacuum, respectively. For the latter, samples of up to ca. 8.0 g, weighed in 50 cm3 quartz tubes or polypropylene vials, were dried under 650 mbar vacuum for up to 15 min. The routine analytical microwave drying method for subsequent analyses utilized a one-step programme with a time of 10 min using 500 W power. At the end of the drying
l?A. Tanner, LX Leong/Analytica
procedure, the sample mass was E Percent moisture contents were determined as 100(X-Y)/X. All the dried and weighed samples were then ground to fine powder in a mortar and pestle, and stored in a desiccator containing silica gel before chemical analyses. 2.4.2. Determination of metal contents Metals in sediment were leached according to the USEPA Method SW846-3051 [2]. A dried and finely ground sample of 0.500 g was mixed with 10 cm3 Table 1 Instrumental descriptions Leco CHN-900 analyzer Instrumental Microwave
for CEM MDS-2000
microwave
249
Chimicn Actu 342 (1997) 247-252
concentrated nitric acid in a CEM lined digestion vessel and batches of 5 samples plus a blank (without sediment) were digested in a CEM MDS-2000 microwave digester. The digested solutions were then cooled in a fumehood, filtered through Whatman 541 paper, and the washings were made up to 25 cm3 (in 20% nitric acid solution). ICP-AES was used for the determination of Al, Ca, Cr, Cu, Fe, Mn, Ni and Zn, while FASS was used for the determination of Pb in the digested sample solutions. Selection of the appropriate analytical lines for each metal in ICP-AES
digester, Perkin-Elmer
Plasma
1000 ICP-AES,
Shimadzu
AA-6501s
FAAS, and
descriptions digester
1. Specifications 2. Method 3051
FAAS
1. Settings
2. Analyte line ICP-AES
1. Settings
2. Analyte lines
CHN analyzer
Operating
conditions
Magnetron frequency: 2455 MHz Nominal power: 630250 W Power: 100% of nominal value Pressure: 85 psi Time: 20 min Time after pressure: 10 min Temperature: 175°C Number of vessels: 6 (maximum) Gas type: air-acetylene Background correction: Flow: 2.0 1 min-’ Burner height: 7 mm Slit: 0.5 nm Pb: 2 17.0 nm
deuterium lamp
RF power: 1038 W Argon plasma flow: 15.0 1min-’ Auxiliary flow: 1.O 1 min-’ Nebulizer uptake: 1.0 1 min..’ Pump rate: 1.0 cm3 min-’ Viewing height: 15 mm Viewing window: 0.1 nm Background correction: auto Read delay time: 25 s Al: 308.215 nm Ca: 315.887 nm Cr: 357.869 nm Cu: 324.754 nm Fe: 259.940 nm Mn: 257.610 nm Ni: 341.476 nm Zn: 213.856 nm Oxidation furnace temperature: Reduction furnace temperature: Helium flow: 100 cm3 min-’ Helium pressure: 40 psi System pressure: 3 psi
950°C 650°C
250
PA. Tarmel; L.S. L.eong/Analytica
analysis was facilitated by using the wavelength characterization tables and graphics system of the spectrometer [12], and several different wavelengths were used for each metal in trial analyses. The final instrumental conditions of the microwave digester, ICPAES, and FAAS are summarized in Table 1. Dilution of sample solutions was carried out when necessary. Instrumental calibration was performed by 3-point external calibration. Calibrator solutions of each metal were prepared from 1000 ppm stock solutions immediately before analysis, diluted to various concentrations with deionized double-distilled water, and matrix-matched with the same amount of acid as that in the sample solutions. The accuracy and precision of the instrument and calibration curve were evaluated and checked with replicate analysis of control samples at the beginning and subsequently at every tenth or twentieth sample analyzed. Analysis of blank samples was conducted to check the purity of reagents and cleanliness of apparatus. The method recoveries for metals using the microwave digestion method were determined for several certified reference materials, oven dried at 105°C: (a) BCSS-1, coastal marine sediment; (b) PACS- 1, harbour marine sediment (both purchased from National Research Council Canada); and (c) Buffalo River sediment (from NIST). The % recoveries (listed in the order (a), (b), (c); and where ND indicates not determined), were for Al 30,27, ND; Ca 70,38,73; Cr 41,54,53; Cu 92,86,77; Fe 80,68, 71; Mn 91,56,83; Ni 87,85, ND; Zn 97,63,77; Pb 74, 74, ND. 2.4.3. Determination of total carbon content A CHN analyzer was employed for the determination of total carbon content in sediment. Dried and finely ground sediment samples of 2.000f0.200 mg (nominal weight for the CHN analyzer) was weighed using a Leco 650 micro-balance and encapsulated in a tin capsule. Standard cystine (29.99% carbon content), weighed in the same manner as the samples, was used for calibrating the CHN analyzer. Table 1 also summarizes the instrumental conditions for the CHN analyzer. 2.5. Calculations
and statistics
Calculations were performed using an IBM-compatible personal computer (80486DX2- 100 MHz
Chimica Acta 342 (1997) 247-252
micro-processor) installed with Microsoft Excel and SigmaStat. Significant differences of the determined physico-chemical parameters between the two drying methods were compared using Student’s t-test at cc=O.O5 according to Zar [13].
3. Results and discussion 3.1. Moisture content A preliminary experiment to investigate the time required for complete drying using the vacuum microwave system was conducted with 5-replicate sediment samples collected from Aberdeen Typhoon Shelter. The samples were weighed 7 times during the vacuum-drying process of 15 min. The weights of samples were expressed relative to 100% at the initial time. Polynomial regression of the mean weight percent of the samples, W, against time, t min, gave the equation: W = 99.9 - 15.5t + 1.38t2 + 0.0409t3, (IV = 5; R* = 1.00)
(1)
Thus the difference in weight percent from t=lO min to t=l 1 min was found to be 0.06%. The standard deviation of the determined weight percent for the 5 sediment samples, s, was found to decrease with time according to the regression: s=2.06
- 0.267t + 0.00917?,
(N = 6; R* = 0.936). (2)
After a drying time of 10 min the range in moisture content was less than 1% of the mean value (N=5). This precision was considered satisfactory, and since from (1) the loss in mass after 10 min was negligible, the drying operation was set at this time. The results obtained for moisture contents of sediment samples using oven drying and vacuum microwave drying are given in Table 2. No significant difference was found between the moisture contents so determined (Student’s t-test, a=0.05). The precisions of the two drying methods for the determination of moisture content, as indicated by the standard deviations, were found to be comparable (Table 2).
RA. Tame< LX Table 2 Moisture content (%), metals @g g-l), Aberdeen
Moisture content Pb Zn Mn CU Ni Cr Fe Al Ca Total carbon
L.eong/Analytica
251
Chimica Acta 342 (1997) 247-252
and total carbon (%) in marine sediment samples using oven and vacuum microwave typhoon
drying methods
Shuen Wan
shelter
Oven drying
Microwave
drying
59fl (2.4) 95f3 (3.5) 428f14 (3.2) 383flO (2.5) 327f5 (1.5) 29+2 (5.2) 5513 (4.6) 32 8OOf8OO (2.5) 40OOOf7000 (18) 79OOf200 (2.1) 2.6fO.l (3.8)
58.0f0.5 (0.9) 93+5 (4.8) 430f13 (2.9) 382f12 (3.1) 331fll (3.3) 29f2 (6.3) 54+3 (6.3) 32 OOO+lOOO (4.0) 37 000~7000 (20) 76OOf300 a (4.2) 2.5fO.l (4.0)
Oven drying
Microwave
drying
56.8f0.8 (1.4) 128f5 (4.1) 177f4 (2.3) 651f13 (2.0) 430+23 (5.3) 12.3f0.7 (5.7) 17.4zt0.8 (4.6) 27 9OO~t800 (2.8) 22 OOOf8000 (35) 16OOOf200 (1.3) 2.3fO.l (4.3)
56.3f0.8 (1.4) 130f5 (3.6) 173f5 (2.8) 626zt23 (3.7) 427+17 (4.0) 13f2 (13) 19.5zt0.8 (4.1) 27 OOOf 1000 (3.9) 29000f6000 a (19) 15400f400 a (2.5) 2.3f0.2 (8.7)
Values expressed as mean&SD (N=lO), with % coefficients of variation in parentheses. a Indicates significant difference between drying methods (Student’s t-test, a=O.O5).
3.2. Metal contents The amounts of Al, Ca, Cr, Cu, Fe, Mn, Ni, Pb, and Zn determined in the two sets of sediment samples using oven- and microwave-drying prior to digestion and instrumental analysis are given in Table 2. A wide range of analyte concentrations - from Al in the range of 40273 pg g-’ to that of Ni 12.3 pg g-’ - were chosen in order to test the trends in metals obtained for the different drying methods. Except for Al in Shuen Wan sediment, and Ca in sediment from both sites, no significant difference was found between the determined metal contents when using the two drying methods (Student’s t-test, c~=O.05). The differences are not associated with the drying process, but with the non-homogeneous distribution of shell material and aluminosilicates in the sediment samples. This was particularly notable from the visual inspection of the Shuen Wan coastal sediment. The standard deviations of the ten-replicate determinations were small in nearly all cases (Table 2). Aluminium was again an exception. The coefficients of variation (CV) in the determined chemical parameters for the two drying methods ranged between 1.3% and 8.7% (except for Ni from Shuen Wan microwave-dried: 13%, and for all determinations of Al: 18% to 35%). 3.3. Total carbon content Table 2 also presents the results for total carbon content of the two samples using oven- and micro-
wave-drying. Again, no significant difference was found between the results so determined (Student’s t-test, c~=O.05). The carbon content is some 13 times smaller than that of the CHN calibration standard, and in view of this the agreement is good.
4. Conclusions Under vacuum, the boiling point of water is lowered and thus evaporation takes place at a lower temperature. The advantages of using microwave drying of sediment under vacuum conditions include a milder thermal mode of drying, rapid removal of vapours during the shorter analysis time, and the minimization of degradation or oxidation of samples. Traditional techniques like oven- or air-drying are time consuming (>8 h to some days) and microwave vacuum drying of sediment provides a faster turn-around time on a higher number of samples. In the present study, the agreement of the determined moisture content for the two different drying methods employed is good. Considerable error is to be expected from the uncertainty in the water content of the “wet sediment”. In the subsequent analyses of metals and carbon in sediment samples, good agreement has also been found when using the two different sample drying procedures. In this work we have employed two different microwave systems in the analytical process. The sample contamination may be further minimized when the same microwave system and
252
sample holders acid digestion.
EA. Tarmel; L.S. Leong/Anal~tica
are employed
both for drying
and
Acknowledgements The authors wish to thank Mr. Franc0 Visinoni and Mr. Camillo Pirola, Milestone S.r.l., Italy for the microwave drying of the sediment samples.
References [l] APHA, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington DC, 1993. [2] USEPA, SW846 Method 3051, Microwave Assisted Acid Digestion of Sediments, Sludges, Soils, and Oils, 1990.
Chimica Acta 342 (1997) 247-252
[3] P.A. Tanner and L.S. Leong, Mar. Pollut. Bull., 31 (1995) 325. [4] A. Zlotorzynski, Critical Rev. Anal. Chem., 25 (1995) 43. [5] R.A. Nadkami, Anal. Chem., 56 (1984) 2233. [6] A.D. Hewitt and C.M. Reynolds, Atomic Spectroscopy, 11 (1990) 187. [7] J. Nieuwenhuize, C.H. Poley-Vos, A.H. van der Akker and W. van Delft, Analyst, 116 (1991) 347. 181 J.T. Ammons, M.E. Essington, R.J. Lewis, A.O. Gallagher and G.M. Lessman, Common. Soil Sci. Plant Anal., 26 (1995) 831. [9] J.I. Wadsworth, L. Velupillai and L.R. Verma, Trans. of the ASAE, 33 (1990) 199. [lo] Y. Hamano and S. Nishio, Mokuzai Gakkaishi, 34 (1988) 485. [II] W.S. Anthony, Trans. of the ASAE, 26 (1983) 275. [ 121 G.F. Wallace and P. Barrett, Analytical Methods Development for Inductively Coupled Plasma Spectrometry, The PerkinElmer Corporation, Connecticut, 1981. [ 131 J.H. Zar, Biostatistical Analysis, Prentice Hall, New Jersey, 1984.
Analytica
Chimica Acta 342 (1997) 247-252
Microwave vacuum drying of marine sediment: determination of moisture content, metals and total carbon P.A. Tanner*, L.S. Leong Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Received 9 September
1996; received in revised form 15 November 1996: accepted 22 November 1996
Abstract No significant difference was found in the moisture content of marine sediment determined by using traditional oven drying at 105°C and microwave-drying under vacuum. Subsequent determinations of the major and trace metals, Al, Ca, Fe, Cr, Cu, Mn, Ni, Pb, and Zn, gave similar results after employing the different drying procedures. The total carbon contents were also investigated and again no significant difference was found. Vacuum microwave drying requires only 10 min of time compared to more than 8 h for oven-drying. Contamination of samples is also minimized when microwave digestion is adopted in the same system for the subsequent analysis of metal contents. Keywords: Drying; Vacuum; Microwave; Sediment; Metals; Total carbon
1. Introduction
The analyses of heavy metals and total carbon contents in marine sediment are widely used to assess long-term anthropogenic inputs to the marine environment. In any given sediment study program, the storage of large batch of bulk samples can cause congestion of laboratory space. The usual practice involves drying a small portion of the collected samples to remove moisture and then weighing and storing them until the subsequent analyses. Although standard procedures are available for the instrumental analysis [l] and digestion [2] of sediment samples, less attention has been paid to their pre-treatment, especially the drying procedures. In view of this, a comparative *Corresponding author. E-mail: [email protected];
fax: +852
2788 7406. 0003-2670/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOOO3-2670(96)00582-X
assessment of the effects of different methods including oven-, air- and freeze-drying upon the obtained moisture and metal content was carried out previously, and we found that oven- and freeze-drying gave similar values for moisture and metal contents of sediment, but that air drying gave significantly different results [3] under the ambient laboratory conditions. In the past few years, there has been a growing interest in the use of microwave radiation in analytical and environmental chemistry. Zlotorzynski [4] reviewed the latest advances in the application of microwave energy to analytical chemistry including digestion, extraction, chemical reaction, pre-concentration, and desorption of samples. With respect to soil and sediment studies, the adoption of microwave oven for sample digestion prior to the analysis of metal contents is well documented [5-81. However we are unaware of the use of the microwave vacuum techni-
248
PA. Tanner; L.S. Leong/Anctlvtica
que for marine sediment drying, although it has been applied to the drying of foodstuffs [9], wood [IO], and cotton [ll]. Under reduced pressure the sediment water is lost at lower temperatures so that the deconposition or loss of volatile organic matter occurs less and local hot spots are avoided. Furthermore, sample oxidation is minimized. This paper documents a comparison of traditional oven drying at 105°C with microwave drying of sediment under vacuum prior to the analyses of metals using inductively coupled plasma atomic emission spectrometry (ICP-AES) and flame atomic absorption spectrometry (FAAS), as well as total carbon content using a CHN analyzer.
Chimica Actu 342 (1997) 247-252
ment: Aberdeen Typhoon Shelter and Shuen Wan. At Shuen Wan, the major intrusive igneous rock formation is granodiorite, whereas sedimentary and waterlaid volcaniclastic rocks occur near the former sampling site. Aberdeen Typhoon Shelter sediment, collected by a Ponar grab, was deeply situated beneath seawater (>lO m), highly anoxic, finely particle-sized, and heavily polluted by industrial and domestic sewage discharges. Samples were collected by using a plastic scoop from the shallow coastal region in Shuen Wan, where the sediment was oxic, subjected to tidal flushing and currents, coarsely particle-sized, and less prone to direct pollution sources. 2.3. Reagents
2. Experimental 2.1. Apparatus Traditional drying was conducted at 105°C at the mid-upper shelf of a She1 Lab oven (range: 50°C to 2OO”C), with ceramic crucibles used as sample holders. No microwave vacuum drying equipment was available at City University of Hong Kong (CityU) so that this operation was performed by Milestone S.r.1. in Italy. The unit employed was a Milestone LAVIS-1000 multiMOIST system with micro-processor-controlled power up to 1000 W. All sediment samples were digested at CityU in CEM Teflon-lined digestion vessels using a CEM MDS-2000 microwave digester. Perkin-Elmer Plasma 1000 ICP-AES and Shimadzu AA-6501s FAAS instruments were employed for the determination of major and minor metals. A Leco CHN-900 carbon, hydrogen and nitrogen determinator model 600-800-300 was employed for the determination of total carbon content in the samples. All glassware were immersed in a mixture of laboratory grade detergent and bleach for 16 h, then rinsed thoroughly with tap water and immersed in an acid bath (1 : 2 : 9 concentrated nitric : hydrochloric acid : water) for 16 h, then triple-rinsed with deionized water, and finally rinsed with deionized doubledistilled water and dried in air before use. 2.2. Sample collection Bulk sediment samples were collected from two different locations in the Hong Kong marine environ-
Reagent water was prepared firstly by ion-exchange and then double distillation with a Cyclon Fistreem system. AnalaR concentrated nitric acid (69%) for digestion was purchased from BDH. Standard metal stock solutions (1000 ppm) for calibration purpose were purchased from High-Purity Standards. Standard cystine powder (Sigma) was used for single-point calibration of the CHN analyzer. 2.4. Procedures 2.4.1. Determination of moisture content In the laboratory, the silt-clay fraction (<63 urn) was wet-sieved from the bulk sediment with seawater collected in situ, in order to normalize the grain size effects in analyses. The sieved silt-clay slurry was collected in a beaker and mixed thoroughly with a glass rod. Approximately 40 cm3 of this slurry was then transferred to each of the ten 50 cm3 Nelgene tubes and spinned down at 2000 rpm for 10 min with a bench top centrifuge. The supematant seawater was decanted and the residue was homogenized. Subsamples of two 5 g portions were collected from each tube, weighed to three decimal places (mass X), and then subjected to traditional oven drying and microwave drying under vacuum, respectively. For the latter, samples of up to ca. 8.0 g, weighed in 50 cm3 quartz tubes or polypropylene vials, were dried under 650 mbar vacuum for up to 15 min. The routine analytical microwave drying method for subsequent analyses utilized a one-step programme with a time of 10 min using 500 W power. At the end of the drying
l?A. Tanner, LX Leong/Analytica
procedure, the sample mass was E Percent moisture contents were determined as 100(X-Y)/X. All the dried and weighed samples were then ground to fine powder in a mortar and pestle, and stored in a desiccator containing silica gel before chemical analyses. 2.4.2. Determination of metal contents Metals in sediment were leached according to the USEPA Method SW846-3051 [2]. A dried and finely ground sample of 0.500 g was mixed with 10 cm3 Table 1 Instrumental descriptions Leco CHN-900 analyzer Instrumental Microwave
for CEM MDS-2000
microwave
249
Chimicn Actu 342 (1997) 247-252
concentrated nitric acid in a CEM lined digestion vessel and batches of 5 samples plus a blank (without sediment) were digested in a CEM MDS-2000 microwave digester. The digested solutions were then cooled in a fumehood, filtered through Whatman 541 paper, and the washings were made up to 25 cm3 (in 20% nitric acid solution). ICP-AES was used for the determination of Al, Ca, Cr, Cu, Fe, Mn, Ni and Zn, while FASS was used for the determination of Pb in the digested sample solutions. Selection of the appropriate analytical lines for each metal in ICP-AES
digester, Perkin-Elmer
Plasma
1000 ICP-AES,
Shimadzu
AA-6501s
FAAS, and
descriptions digester
1. Specifications 2. Method 3051
FAAS
1. Settings
2. Analyte line ICP-AES
1. Settings
2. Analyte lines
CHN analyzer
Operating
conditions
Magnetron frequency: 2455 MHz Nominal power: 630250 W Power: 100% of nominal value Pressure: 85 psi Time: 20 min Time after pressure: 10 min Temperature: 175°C Number of vessels: 6 (maximum) Gas type: air-acetylene Background correction: Flow: 2.0 1 min-’ Burner height: 7 mm Slit: 0.5 nm Pb: 2 17.0 nm
deuterium lamp
RF power: 1038 W Argon plasma flow: 15.0 1min-’ Auxiliary flow: 1.O 1 min-’ Nebulizer uptake: 1.0 1 min..’ Pump rate: 1.0 cm3 min-’ Viewing height: 15 mm Viewing window: 0.1 nm Background correction: auto Read delay time: 25 s Al: 308.215 nm Ca: 315.887 nm Cr: 357.869 nm Cu: 324.754 nm Fe: 259.940 nm Mn: 257.610 nm Ni: 341.476 nm Zn: 213.856 nm Oxidation furnace temperature: Reduction furnace temperature: Helium flow: 100 cm3 min-’ Helium pressure: 40 psi System pressure: 3 psi
950°C 650°C
250
PA. Tarmel; L.S. L.eong/Analytica
analysis was facilitated by using the wavelength characterization tables and graphics system of the spectrometer [12], and several different wavelengths were used for each metal in trial analyses. The final instrumental conditions of the microwave digester, ICPAES, and FAAS are summarized in Table 1. Dilution of sample solutions was carried out when necessary. Instrumental calibration was performed by 3-point external calibration. Calibrator solutions of each metal were prepared from 1000 ppm stock solutions immediately before analysis, diluted to various concentrations with deionized double-distilled water, and matrix-matched with the same amount of acid as that in the sample solutions. The accuracy and precision of the instrument and calibration curve were evaluated and checked with replicate analysis of control samples at the beginning and subsequently at every tenth or twentieth sample analyzed. Analysis of blank samples was conducted to check the purity of reagents and cleanliness of apparatus. The method recoveries for metals using the microwave digestion method were determined for several certified reference materials, oven dried at 105°C: (a) BCSS-1, coastal marine sediment; (b) PACS- 1, harbour marine sediment (both purchased from National Research Council Canada); and (c) Buffalo River sediment (from NIST). The % recoveries (listed in the order (a), (b), (c); and where ND indicates not determined), were for Al 30,27, ND; Ca 70,38,73; Cr 41,54,53; Cu 92,86,77; Fe 80,68, 71; Mn 91,56,83; Ni 87,85, ND; Zn 97,63,77; Pb 74, 74, ND. 2.4.3. Determination of total carbon content A CHN analyzer was employed for the determination of total carbon content in sediment. Dried and finely ground sediment samples of 2.000f0.200 mg (nominal weight for the CHN analyzer) was weighed using a Leco 650 micro-balance and encapsulated in a tin capsule. Standard cystine (29.99% carbon content), weighed in the same manner as the samples, was used for calibrating the CHN analyzer. Table 1 also summarizes the instrumental conditions for the CHN analyzer. 2.5. Calculations
and statistics
Calculations were performed using an IBM-compatible personal computer (80486DX2- 100 MHz
Chimica Acta 342 (1997) 247-252
micro-processor) installed with Microsoft Excel and SigmaStat. Significant differences of the determined physico-chemical parameters between the two drying methods were compared using Student’s t-test at cc=O.O5 according to Zar [13].
3. Results and discussion 3.1. Moisture content A preliminary experiment to investigate the time required for complete drying using the vacuum microwave system was conducted with 5-replicate sediment samples collected from Aberdeen Typhoon Shelter. The samples were weighed 7 times during the vacuum-drying process of 15 min. The weights of samples were expressed relative to 100% at the initial time. Polynomial regression of the mean weight percent of the samples, W, against time, t min, gave the equation: W = 99.9 - 15.5t + 1.38t2 + 0.0409t3, (IV = 5; R* = 1.00)
(1)
Thus the difference in weight percent from t=lO min to t=l 1 min was found to be 0.06%. The standard deviation of the determined weight percent for the 5 sediment samples, s, was found to decrease with time according to the regression: s=2.06
- 0.267t + 0.00917?,
(N = 6; R* = 0.936). (2)
After a drying time of 10 min the range in moisture content was less than 1% of the mean value (N=5). This precision was considered satisfactory, and since from (1) the loss in mass after 10 min was negligible, the drying operation was set at this time. The results obtained for moisture contents of sediment samples using oven drying and vacuum microwave drying are given in Table 2. No significant difference was found between the moisture contents so determined (Student’s t-test, a=0.05). The precisions of the two drying methods for the determination of moisture content, as indicated by the standard deviations, were found to be comparable (Table 2).
RA. Tame< LX Table 2 Moisture content (%), metals @g g-l), Aberdeen
Moisture content Pb Zn Mn CU Ni Cr Fe Al Ca Total carbon
L.eong/Analytica
251
Chimica Acta 342 (1997) 247-252
and total carbon (%) in marine sediment samples using oven and vacuum microwave typhoon
drying methods
Shuen Wan
shelter
Oven drying
Microwave
drying
59fl (2.4) 95f3 (3.5) 428f14 (3.2) 383flO (2.5) 327f5 (1.5) 29+2 (5.2) 5513 (4.6) 32 8OOf8OO (2.5) 40OOOf7000 (18) 79OOf200 (2.1) 2.6fO.l (3.8)
58.0f0.5 (0.9) 93+5 (4.8) 430f13 (2.9) 382f12 (3.1) 331fll (3.3) 29f2 (6.3) 54+3 (6.3) 32 OOO+lOOO (4.0) 37 000~7000 (20) 76OOf300 a (4.2) 2.5fO.l (4.0)
Oven drying
Microwave
drying
56.8f0.8 (1.4) 128f5 (4.1) 177f4 (2.3) 651f13 (2.0) 430+23 (5.3) 12.3f0.7 (5.7) 17.4zt0.8 (4.6) 27 9OO~t800 (2.8) 22 OOOf8000 (35) 16OOOf200 (1.3) 2.3fO.l (4.3)
56.3f0.8 (1.4) 130f5 (3.6) 173f5 (2.8) 626zt23 (3.7) 427+17 (4.0) 13f2 (13) 19.5zt0.8 (4.1) 27 OOOf 1000 (3.9) 29000f6000 a (19) 15400f400 a (2.5) 2.3f0.2 (8.7)
Values expressed as mean&SD (N=lO), with % coefficients of variation in parentheses. a Indicates significant difference between drying methods (Student’s t-test, a=O.O5).
3.2. Metal contents The amounts of Al, Ca, Cr, Cu, Fe, Mn, Ni, Pb, and Zn determined in the two sets of sediment samples using oven- and microwave-drying prior to digestion and instrumental analysis are given in Table 2. A wide range of analyte concentrations - from Al in the range of 40273 pg g-’ to that of Ni 12.3 pg g-’ - were chosen in order to test the trends in metals obtained for the different drying methods. Except for Al in Shuen Wan sediment, and Ca in sediment from both sites, no significant difference was found between the determined metal contents when using the two drying methods (Student’s t-test, c~=O.05). The differences are not associated with the drying process, but with the non-homogeneous distribution of shell material and aluminosilicates in the sediment samples. This was particularly notable from the visual inspection of the Shuen Wan coastal sediment. The standard deviations of the ten-replicate determinations were small in nearly all cases (Table 2). Aluminium was again an exception. The coefficients of variation (CV) in the determined chemical parameters for the two drying methods ranged between 1.3% and 8.7% (except for Ni from Shuen Wan microwave-dried: 13%, and for all determinations of Al: 18% to 35%). 3.3. Total carbon content Table 2 also presents the results for total carbon content of the two samples using oven- and micro-
wave-drying. Again, no significant difference was found between the results so determined (Student’s t-test, c~=O.05). The carbon content is some 13 times smaller than that of the CHN calibration standard, and in view of this the agreement is good.
4. Conclusions Under vacuum, the boiling point of water is lowered and thus evaporation takes place at a lower temperature. The advantages of using microwave drying of sediment under vacuum conditions include a milder thermal mode of drying, rapid removal of vapours during the shorter analysis time, and the minimization of degradation or oxidation of samples. Traditional techniques like oven- or air-drying are time consuming (>8 h to some days) and microwave vacuum drying of sediment provides a faster turn-around time on a higher number of samples. In the present study, the agreement of the determined moisture content for the two different drying methods employed is good. Considerable error is to be expected from the uncertainty in the water content of the “wet sediment”. In the subsequent analyses of metals and carbon in sediment samples, good agreement has also been found when using the two different sample drying procedures. In this work we have employed two different microwave systems in the analytical process. The sample contamination may be further minimized when the same microwave system and
252
sample holders acid digestion.
EA. Tarmel; L.S. Leong/Anal~tica
are employed
both for drying
and
Acknowledgements The authors wish to thank Mr. Franc0 Visinoni and Mr. Camillo Pirola, Milestone S.r.l., Italy for the microwave drying of the sediment samples.
References [l] APHA, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington DC, 1993. [2] USEPA, SW846 Method 3051, Microwave Assisted Acid Digestion of Sediments, Sludges, Soils, and Oils, 1990.
Chimica Acta 342 (1997) 247-252
[3] P.A. Tanner and L.S. Leong, Mar. Pollut. Bull., 31 (1995) 325. [4] A. Zlotorzynski, Critical Rev. Anal. Chem., 25 (1995) 43. [5] R.A. Nadkami, Anal. Chem., 56 (1984) 2233. [6] A.D. Hewitt and C.M. Reynolds, Atomic Spectroscopy, 11 (1990) 187. [7] J. Nieuwenhuize, C.H. Poley-Vos, A.H. van der Akker and W. van Delft, Analyst, 116 (1991) 347. 181 J.T. Ammons, M.E. Essington, R.J. Lewis, A.O. Gallagher and G.M. Lessman, Common. Soil Sci. Plant Anal., 26 (1995) 831. [9] J.I. Wadsworth, L. Velupillai and L.R. Verma, Trans. of the ASAE, 33 (1990) 199. [lo] Y. Hamano and S. Nishio, Mokuzai Gakkaishi, 34 (1988) 485. [II] W.S. Anthony, Trans. of the ASAE, 26 (1983) 275. [ 121 G.F. Wallace and P. Barrett, Analytical Methods Development for Inductively Coupled Plasma Spectrometry, The PerkinElmer Corporation, Connecticut, 1981. [ 131 J.H. Zar, Biostatistical Analysis, Prentice Hall, New Jersey, 1984.